Dear Unidawntified Flying Objects,
Flying silently and smoothly through the main asteroid belt between Mars and Jupiter, Dawn emits a blue-green beam of high velocity xenon ions. On the opposite side of the sun from Earth, firing its uniquely efficient ion propulsion system, the distant adventurer is continuing to make good progress on its long trek from the giant protoplanet Vesta to dwarf planet Ceres.
This month, let’s look ahead to some upcoming activities. You can use the sun in December to locate Dawn in the sky, but before we describe that, let’s see how Dawn is looking ahead to Ceres, with plans to take pictures on the night of Dec. 1.
The robotic explorer’s sensors are complex devices that perform many sensitive measurements. To ensure they yield the best possible scientific data, their health must be carefully monitored and maintained, and they must be accurately calibrated. The sophisticated instruments are activated and tested occasionally, and all remain in excellent condition. One final calibration of the science camera is needed before arrival at Ceres. To accomplish it, the camera needs to take pictures of a target that appears just a few pixels across. The endless sky that surrounds our interplanetary traveler is full of stars, but those beautiful pinpoints of light, while easily detectable, are too small for this specialized measurement. But there is an object that just happens to be the right size. On Dec. 1, Ceres will be about nine pixels in diameter, nearly perfect for this calibration.
The images will provide data on very subtle optical properties of the camera that scientists will use when they analyze and interpret the details of some of the pictures returned from orbit. At 740,000 miles (1.2 million kilometers), Dawn’s distance to Ceres will be about three times the separation between Earth and the moon. Its camera, designed for mapping Vesta and Ceres from orbit, will not reveal anything new. It will, however, reveal something cool! The pictures will be the first extended view for the first probe to reach the first dwarf planet discovered. They will show the largest body between the sun and Pluto that has not yet been visited by a spacecraft, Dawn’s destination since it climbed out of Vesta’s gravitational grip more than two years ago.
This will not be the first time Dawn has spotted Ceres. In a different calibration of the camera more than four years ago, the explorer descried its faint destination, far away in both time and space. Back then, still a year before arriving at Vesta, Dawn was more than 1,300 times farther from Ceres than it will be for this new calibration. The giant of the main asteroid belt was an indistinct dot in the vast cosmic landscape.
Dawn’s first photo of Ceres, taken on July 20, 2010. Image credit: NASA/JPL-Caltech/MPS/DLR/IDA
Now Ceres is the brightest object in Dawn’s sky save the distant sun. When it snaps the photos, Ceres will be as bright as Venus sometimes appears from Earth (what astronomers would call visual magnitude -3.6).
Dawn’s first extended picture of Ceres will be only slightly larger than this image of Vesta taken on May 3, 2011, at the beginning of the Vesta approach phase. The inset shows the pixelated Vesta, extracted from the main picture in which the overexposed Vesta can be seen against the background of stars. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
To conserve hydrazine, a precious resource following the loss of two reaction wheels, Dawn will thrust with its ion propulsion system when it performs this calibration, which requires long exposures. In addition to moving the spacecraft along in its trajectory, the ion engine stabilizes the ship, enabling it to point steadily in the zero-gravity of spaceflight. (Dawn’s predecessor, Deep Space 1, used the same trick of ion thrusting in order to be as stable as possible for its initial photos of comet Borrelly.)
As Dawn closes in on its quarry, Ceres will grow brighter and larger. Last month we summarized the plan for photographing Ceres during the first part of the approach phase, yielding views in January comparable to the best we currently have (from Hubble Space Telescope) and in February significantly better. The principal purpose of the pictures is to help navigators steer the ship into this uncharted, final port following a long voyage on the interplanetary seas. The camera serves as the helmsman’s eyes. Ceres has been observed with telescopes from (or near) Earth for more than two centuries, but it has appeared as little more than a faint, fuzzy blob farther away than the sun. But not for much longer!
merest whisper of thrust, the ion engine allows Dawn to maneuver in ways entirely different from conventional spacecraft. In January, we presented in detail Dawn’s unique way of slipping into orbit. In September, a burst of space radiation disrupted the thrust profile. As we saw, the flight team responded swiftly to a very complex problem, minimizing the duration of the missed thrust. One part of their contingency operations was to design a new approach trajectory, accounting for the 95 hours that Dawn coasted instead of thrust. Let’s take a look now at how the resulting trajectory differs from what we discussed at the beginning of this year.The only spaceship ever built to orbit two extraterrestrial destinations, Dawn’s advanced ion propulsion system enables its ambitious mission. Providing the
In the original approach, Dawn would follow a simple spiral around Ceres, approaching from the general direction of the sun, looping over the south pole, going beyond to the night side, and coming back above the north pole before easing into the targeted orbit, known by the stirring name RC3, at an altitude of 8,400 miles (13,500 kilometers). Like a pilot landing a plane, flying this route required lining up on a particular course and speed well in advance. The ion thrusting this year had been setting Dawn up to get on that approach spiral early next year.
The change in its flight profile following the September encounter with a rogue cosmic ray meant the spiral path would be markedly different and would require significantly longer to complete. While the flight team certainly is patient -- after all, Earth’s robotic ambassador won’t reach Ceres until 213 years after its discovery and more than seven years after launch -- the brilliantly creative navigators devised an entirely new approach trajectory that would be shorter. Demonstrating the extraordinary flexibility of ion propulsion, the spacecraft now will take a completely different path but will wind up in exactly the same orbit.
The spacecraft will allow itself to be captured by Ceres on March 6, only about half a day later than the trajectory it was pursuing before the hiatus in thrust, but the geometry both before and after will be quite different. Instead of flying south of Ceres, Dawn is now targeted to trail a little behind it, letting the dwarf planet lead as they both orbit the sun, and then the spacecraft will begin to gently curve around it. (You can see this in the figure below.) Dawn will come to 24,000 miles (38,000 kilometers) and then will slowly arc away. But thanks to the remarkable design of the thrust profile, the ion engine and the gravitational pull from the behemoth of rock and ice will work together. At a distance of 41,000 miles (61,000 kilometers), Ceres will reach out and tenderly take hold of its new consort, and they will be together evermore. Dawn will be in orbit, and Ceres will forever be accompanied by this former resident of Earth.
In this view, looking down on the north pole of Ceres, the sun is off the figure to the left and Ceres' counterclockwise orbital motion around the sun takes it from the bottom of the figure to the top. Dawn flies in from the left, traveling behind Ceres, and then is captured on the way to the apex of its orbit. The white circles are at one-day intervals, illustrating how Dawn slows down gradually at first. (When the circles are closer together, Dawn is moving more slowly.) After capture, both Ceres' gravity and the ion thrust slow it even more before the craft accelerates to the end of the approach phase. (You can think of this perspective as being from above. Then the next figure shows the view from the side, which here would mean looking toward the action from a location off the bottom of the graphic.) Image credit: NASA/JPL
If the spacecraft stopped thrusting just when Ceres captured it, it would continue looping around the massive body in a high, elliptical orbit, but its mission is to scrutinize the mysterious world. Our goal is not to be in just any arbitrary orbit but rather in the particular orbits that have been chosen to provide the best scientific return for the probe’s camera and other sensors. So it won’t stop but instead will continue maneuvering to RC3.
Ever graceful, Dawn will gently thrust to counter its orbital momentum, keeping it from swinging up to the highest altitude it would otherwise attain. On March 18, nearly two weeks after it is captured by Ceres’ gravity, Dawn will arc to the crest of its orbit. Like a ball thrown high that slows to a momentary stop before falling back, Dawn’s orbital ascent will end at an altitude of 47,000 miles (75,000 kilometers), and Ceres’ relentless pull (aided by the constant, gentle thrust) will win out. As it begins descending toward its gravitational master, it will continue working with Ceres. Rather than resist the fall, the spacecraft will thrust to accelerate itself, quickening the trip down to RC3.
There is more to the specification of the orbit than the altitude. One of the other attributes is the orientation of the orbit in space. (Imagine an orbit as a ring around Ceres, but that ring can be tipped and tilted in many ways.) To provide a view of the entire surface as Ceres rotates underneath it, Dawn needs to be in a polar orbit, flying over the north pole as it travels from the nightside to the dayside, moving south as it passes over the equator, sailing back to the unilluminated side when it reaches the south pole, and then heading north above terrain in the dark of night. To accomplish the earlier part of its new approach trajectory, however, Dawn will stay over lower latitudes, very high above the mysterious surface but not far from the equator. Therefore, as it races toward RC3, it will orient its ion engine not only to shorten the time to reach that orbital altitude but also to tip the plane of its orbit so that it encircles the poles (and tilts the plane to be at a particular orientation relative to the sun). Then, finally, as it gets closer still, it will turn to use that famously efficient glowing beam of xenon ions against Ceres’ gravity, acting as a brake rather than an accelerator. By April 23, this first act of a beautiful new celestial ballet will conclude. Dawn will be in the originally intended orbit around Ceres, ready for its next act: the intensive observations of RC3 we described in February.
North is at the top of this figure and the sun is far to the left. Ceres orbital motion around the sun carries it straight into the figure. The original approach took Dawn over Ceres' south pole as it spiraled directly into RC3. On the new approach, it looks here as if it flies in over the north pole, but that is because of the flat depiction. As the previous figure shows, the approach takes Dawn well behind Ceres in their progression around the sun. The upper part of the green trajectory is not in the same plane as the original approach and RC3; rather, it is in the foreground, "in front of" the graphic. As Dawn flies to the right side of the diagram, it also moves back into the plane of the figure to align with the targeted RC3. As before, the circles, spaced at intervals of one day, indicate the spacecraft's speed; where they are closer together, the ship travels more slowly. (You can think of this perspective as being from the side and the previous figure as showing the view from above, off the top of this graphic.) Image credit: NASA/JPL
Dawn’s route to orbit is no more complex and elegant than what any crackerjack spaceship pilot would execute. However, one of the key differences between what our ace will perform and what often happens in science fiction movies is that Dawn’s maneuvers will comply with the laws of physics. And if that’s not gratifying enough, perhaps the fact that it’s real makes it even more impressive. A spaceship sent from Earth more than seven years ago, propelled by electrically accelerated ions, having already maneuvered extensively in orbit around the giant protoplanet Vesta to reveal its myriad secrets, soon will bank and roll, arc and turn, ascend and descend, and swoop into its planned orbit.
Illustration of the relative locations (but not sizes) of Earth, the sun, and Dawn in early December 2014. (Earth and the sun are at that location every December.) The images are superimposed on the trajectory for the entire mission, showing the positions of Earth, Mars, Vesta, and Ceres at milestones during Dawn’s voyage. Image credit: NASA/JPL
As Earth, the sun, and the spacecraft come closer into alignment, radio signals that go back and forth must pass near the sun. The solar environment is fierce indeed, and it will interfere with those radio waves. While some signals will get through, communication will not be reliable. Therefore, controllers plan to send no messages to the spacecraft from Dec. 4 through Dec. 15; all instructions needed during that time will be stored onboard beforehand. Occasionally Deep Space Network antennas, pointing near the sun, will listen through the roaring noise for the faint whisper of the spacecraft, but the team will consider any communication to be a bonus.
Dawn is big for an interplanetary spacecraft (or for an otherworldly dragonfly, for that matter), with a wingspan of nearly 65 feet (19.7 meters). However, more than 3.8 times as far as the sun, 352 million miles (567 million kilometers) away, humankind lacks any technology even remotely capable of glimpsing it. But we can bring to bear something more powerful than our technology: our mind’s eye. From Dec. 8 to 11, if you block the sun’s blazing light with your thumb, you will also be covering Dawn’s location. There, in that direction, is our faraway emissary to new worlds. It has traveled three billion miles (4.8 billion kilometers) already on its extraordinary extraterrestrial expedition, and some of the most exciting miles are still ahead as it nears Ceres. You can see right where it is. It is now on the far side of the sun.
This is the same sun that is more than 100 times the diameter of Earth and a third of a million times its mass. This is the same sun that has been the unchallenged master of our solar system for more than 4.5 billion years. This is the same sun that has shone down on Earth all that time and has been the ultimate source of so much of the heat, light and other energy upon which the planet’s residents have been so dependent. This is the same sun that has so influenced human expression in art, literature, mythology and religion for uncounted millennia. This is the same sun that has motivated scientific studies for centuries. This is the same sun that is our signpost in the Milky Way galaxy. And humans have a spacecraft on the far side of it. We may be humbled by our own insignificance in the universe, yet we still undertake the most valiant adventures in our attempts to comprehend its majesty.
Dawn is 780,000 miles (1.3 million kilometers) from Ceres, or 3.3 times the average distance between Earth and the moon. It is also 3.77 AU (350 million miles, or 564 million kilometers) from Earth, or 1,525 times as far as the moon and 3.82 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and three minutes to make the round trip.
Farther from Earth and from the sun than it has ever been, Dawn is on course and on schedule for its March 2015 arrival at Ceres, an enigmatic world of rock and ice. To slip gracefully into orbit around the dwarf planet, the spacecraft has been using its uniquely capable departing the giant protoplanet Vesta in Sep. 2012, the stalwart ship has accomplished 99.46 percent of the planned ion thrusting.
What matters most for this daring mission is its ambitious exploration of two uncharted worlds (previews of the Ceres plan were presented from December 2013 to August 2014), but this month and next, we will consider that 0.54 percent of the thrusting Dawn did not accomplish. We begin by seeing what happened on the spacecraft and in mission control. In November we will describe the implications for the approach phase of the mission. (To skip now to some highlights of the new approach schedule, click on the word "click.")
The story begins with radiation, which fills space. Earth's magnetic field deflects much of it, and the atmosphere absorbs much of the rest, but there is no such protection for interplanetary spacecraft. Some particles were energized as recently as a few days earlier on the sun or uncounted millennia ago at a supernova far away in the Milky Way galaxy. Regardless of when and where it started, one particle's cosmic journey ended on Sep. 11 at 2:27 a.m. PDT inside Earth's robotic ambassador to the main asteroid belt. The particle penetrated one of the spacecraft panels and struck an electrical component in a unit that controls the ion propulsion system.
At the time the burst of radiation arrived, Dawn was thrusting as usual, emitting a blue-green beam of high velocity xenon ions from engine #1. Ten times as efficient as conventional chemical propulsion, ion propulsion truly enables this unique mission to orbit two extraterrestrial destinations. With its remarkably gentle thrust, it uses xenon propellant so frugally that it takes more than three and a half days to expend just one pound (0.45 kilograms), providing acceleration with patience.
Dawn's electronics were designed to be resistant to radiation. On this occasion, however, the particle managed to deposit its energy in such a way that it disrupted the behavior of a circuit. The control unit used that circuit to move valves in the elaborate system that transports xenon from the main tank at a pressure of 500 psi (34 times atmospheric pressure) to the ion engine, where it is regulated to around two millionths of a psi (ten million times lower than atmospheric pressure), yielding the parsimonious expenditure of propellant. The controller continued monitoring the xenon flow (along with myriad other parameters needed for the operation of the ion engine), but the valves were unable to move in response to its instructions. Thrusting continued normally for more than an hour as the xenon pressure in the engine decreased very gradually. (Everything with ion propulsion is gradual!) When it reached the minimum acceptable value, the controller executed an orderly termination of thrust and reported its status to the main spacecraft computer.
When the computer was informed that thrust had stopped, it invoked one of Dawn's safe modes. It halted other activities, reconfigured some of the subsystems and rotated to point the main antenna to Earth.
The events to that point were virtually identical to a radiation strike that occurred more than three years earlier. Subsequent events, however, unfolded differently.
In normal circumstances, the mission control team would be able to guide the spacecraft back to normal operations in a matter of hours, as they did in 2011. Indeed, the longest part of the entire process then was simply the time between when Dawn turned to Earth and when the next scheduled tracking session with NASA's worldwide Deep Space Network (DSN) began. Most of the time, Dawn operates on its own using instructions stored in its computer by mission controllers. The DSN is scheduled to communicate with it only at certain times.
Dawn performs a carefully choreographed 2.5-year pas de trois from Vesta to Ceres. Celestial navigators had long known that the trajectory was particularly sensitive to glitches that interfere with ion thrusting during part of 2014. To ensure a prompt response to any interruptions in thrust, therefore, the Dawn project collaborated with the DSN to devise a new method of checking in on the spacecraft more frequently (but for short periods) to verify its health. This strategy helped them detect the condition soon after it occurred.Dawn from Vesta to Ceres
When an antenna at the DSN complex near Madrid, Spain, received the explorer's radio signal that morning, it was apparent that Dawn was neither in exactly the configuration to be expected if it were thrusting nor if it had entered one of its safe modes. Although they did not establish until later in the day what was happening, it turns out that not one but two anomalies occurred on the distant spacecraft, likely both triggered by particles in the radiation burst. Dawn encountered difficulty controlling its attitude with its usual exquisite precision. (Engineers use "attitude" to refer to the orientation of the craft in the zero-gravity conditions of spaceflight. In this case, the spacecraft's orientation was not controlled with its usual precision, but the spacecraft's outlook was as positive and its demeanor as pleasant as ever.) Instead of maintaining a tight lock of its main antenna on faraway Earth, it was drifting very slightly. The rate was 10 times slower than the hour hand on a clock, but that was enough to affect the interplanetary communication. Ultimately one of the onboard systems designed to monitor the overall health and performance of all subsystems detected the attitude discrepancy and called for another, deeper safe mode.
In this safe mode, Dawn further reconfigured some of the subsystems and used a different part of the attitude control system to aim at the solar system's most salient landmark: the sun. It switched to one of its auxiliary antennas and transmitted a wide radio beam.
Meanwhile, the operations team began working with the DSN and other missions to arrange for more time to communicate with Dawn than had previously been scheduled. Projects often collaborate this way, making adjustments for each other in the spirit of shared interest in exploring the solar system with the limited number of DSN stations. Later in the day on Thursday, when an antenna near Goldstone, Calif., was made available to point at Dawn, it was stable in safe mode.
The team decided to aim for resuming thrusting on Monday, Sep. 15. They had already formulated a detailed four-week sequence of commands to transmit to the spacecraft then, so this would avoid the significant complexity of changing the timing, a process that in itself can be time-consuming. This plan would limit the duration of the missed thrust during this sensitive portion of the long flight from Vesta to Ceres. Time was precious.
While it was in safe mode, there were several major challenges in investigating why the spacecraft had not been able to point accurately. The weak radio signal from the auxiliary antenna allowed it to send only a trickle of data. Readers who have heard tales of life late in the 20th century can only imagine what it must have been like for our ancestors with their primitive connections to the Internet. Now imagine the Dawn team trying to diagnose a very subtle drift in attitude that had occurred on a spacecraft 3.2 AU (almost 300 million miles, or 480 million kilometers) from Earth with a connection about one thousand times slower than a dial-up modem from 20 years ago. In addition, radio signals (which all regular readers know travel at the universal limit of the speed of light) took 53 minutes to make the round trip. Therefore, every instruction transmitted from JPL required a long wait for a response. Combined with the intermittent DSN schedule, these conditions greatly limited the pace at which operations could proceed.
To improve the efficiency of the recovery, the DSN agreed to use its newest antenna, known as Deep Space Station 35 (DSS-35), near Canberra, Australia. DSS-35 was not quite ready yet for full-time operational use, and the DSN postponed some of the planned work on it to give Dawn some very valuable extra communications opportunities. It's impressive how all elements of NASA work together to make each project successful.DSN with cranes
Engineers hypothesized that the reconfigurations upon entering safe mode might have rectified the anomaly that prevented the spacecraft from maintaining its characteristic stability. While some people continued the previously planned work of finalizing preparations for Ceres, most of the rest of the operations team split into two shifts. That way, they could progress more quickly through the many steps necessary to command the spacecraft out of safe mode to point the main antenna to Earth again so they could download the large volume of detailed data it had stored on what had occurred. By the time they were ready late on Friday night, however, there was a clear indication that the spacecraft was not ready. Telemetry revealed that the part of the attitude control software that was not used when pointing at the sun in safe mode - but that would be engaged when pointing elsewhere - was still not operating correctly.
Experts at JPL, along with a colleague at Orbital Sciences Corporation in Dulles, VA, scrutinized what telemetry they could receive, performed tests with the spacecraft simulator, and conducted other investigations. The team devised possible explanations, and one by one they tested and eliminated them. Their intensive efforts were powered not only by their skill and their collective experience on Dawn and other missions but also by plenty of pizza and fancy cupcakes. (The cupcakes were delivered in a box lovingly decorated with a big heart, ostensibly by the young daughter of the team member who provided them, but this writer suspects it might have been the team member himself. Regardless, embedded in the action, your correspondent established that the cupcakes were not only a yummy dessert after a pizza lunch but also that they made a terrific dinner. What a versatile and delectable comestible!)
Despite having all the expertise and creativity that could be brought to bear, by Saturday afternoon nothing they had tried had proven effective, including restarting the part of the software that seemed to be implicated in the pointing misbehavior. Confronting such an unyielding situation was not typical for such an experienced flight team. Whenever Dawn had entered one of its safe modes in the preceding seven years of flight, they had usually established the cause within a very few hours and knew precisely how to return to normal operations quickly. This time was different.
The team had still more ideas for systematically trying to fix the uncooperative pointing, but with the clock ticking, the mission director/chief engineer, with a conviction that can only come from cupcakes, decided to pursue a more dramatic course. It would put the spacecraft into an even deeper safe mode, and hence would guarantee a longer time to restore it to its normal operational configuration, but it also seemed a more likely solution. It thus appeared to offer the best possibility of being ready to start thrusting on schedule on Monday, avoiding the difficulty of modifying the four-week sequence of commands and minimizing the period of lost thrust. The idea sounds simple: reboot the main computer.
Rebooting the computer on a ship in deep space is a little bigger deal than rebooting your laptop. Indeed, the last time controllers commanded Dawn to restart its computer was in April 2011, when they installed a new version of software. Such a procedure is very delicate and is not undertaken lightly, given that the computer controls all of the robot's functions in the unforgiving depths of space. Nevertheless, the team made all the preparations that afternoon and evening, and the computer rebooted as commanded two minutes after midnight.
Engineers immediately set about the intricate tasks of verifying that the probe correctly reloaded all of its complex software and was still healthy. It took another 12 hours of reconfiguring the spacecraft and watching the driblet of data before they could confirm around noon on Sunday that the attitude control software was back to its usual excellent performance. Whatever had afflicted it since the radiation burst was now cured. After a brief pause for the tired team members on shift in Dawn mission control to shout things like "Yes!" "Hurray!" and "Time for more cupcakes!" they continued with the complex commanding to point the main antenna to Earth, read out the diagnostic logs, and return each subsystem to its intended state. By Monday afternoon, they had confirmed that hundreds upon hundreds of measurements from the spacecraft were exactly what they needed to be. Dawn was ready to resume ion thrusting, heading for an exciting, extended exploration of the first dwarf planet discovered.
Throughout the contingency operations, even as some people on the team delved into diagnosing and recovering the spacecraft and others continued preparing for Ceres, still others investigated how the few days of unplanned coasting would affect the trajectory. For a mission using ion propulsion, thrusting at any time is affected by thrusting at all other times, in both the past and the future. The new thrust profiles (specifically, both the throttle level and the direction to point the ion engine every second) for the remainder of the cruise phase and the approach phase (concluding with entering the first observation orbit, known as RC3) would have to compensate for the coasting that occurred when thrusting had been scheduled. The flight plans are very complicated, and developing them requires experts who apply very sophisticated software and a touch of artistry. As soon as the interruption in thrust was detected on Thursday, the team began formulating new designs. Initially most of the work assumed thrusting would start on Monday. After the first few attempts to correct the attitude anomaly were unsuccessful, however, they began looking more carefully into later dates. Thanks to the tremendous flexibility of ion propulsion, there was never doubt about ultimately getting into orbit around Ceres, but the thrust profiles and the nature and timeline of the approach phase could change quite a bit.
Once controllers observed that the reboot had resolved the problem, they put the finishing touches on the Monday plan. The team combined the new thrust profile with the pre-existing four-week set of commands already scheduled to be radioed to the spacecraft during a DSN session on Monday. They had already made another change as well. When the radiation burst struck the probe, it had been using ion engine #1, ion engine controller #1, and power unit #1. Although they were confident that simply turning the controller off and then on again would clear the glitch, just as it had in 2011 (and as detailed analysis of the electrical circuitry had indicated), they had decided a few days earlier that there likely would not be time to verify it, so prudence dictated that near-term thrusting not rely on it. Therefore, following the same strategy used three years earlier, the new thrust profile was based on controller #2, which meant it needed to use ion engine #2 and power unit #2. (For those of you keeping score, engine #3 can work with either controller and either power unit, but the standard combination so far has been to use the #1 devices with engine #3.) Each engine, controller, and power unit has been used extensively in the mission, and the expedition now could be completed with only one of each component if need be.
By the time Dawn was once again perched atop its blue-green pillar of xenon ions on Monday, it had missed about 95 hours of thrusting. That has surprising and interesting consequences for the approach to Ceres early next year, and it provides a fascinating illustration of the creativity of trajectory designers and the powerful capability of ion propulsion. Given how long this log is already, however, we will present the details of the new approach phase next month and explain then how it differs from what we described last December. For those readers whose 2015 social calendars are already filling up, however, we summarize here some of the highlights.
Throughout this year, the flight team has made incremental improvements in the thrust plan, and gradually the Ceres arrival date has shifted earlier by several weeks from what had been anticipated a year ago. Today Dawn is on course for easing into Ceres' gravitational embrace on March 6. The principal effect of the missed thrust is to make the initial orbit larger, so the spaceship will need more time to gently adjust its orbit to RC3 at 8,400 miles (13,500 kilometers). It will reach that altitude on about April 22 which, as it turns out, differs by less than a week from the schedule last year.Hubble images of Ceres
During the approach phase, the spacecraft will interrupt thrusting occasionally to take pictures of Ceres against the background stars, principally to aid in navigating the ship to the uncharted shore ahead. Because arrival has advanced from what we presented 10 months ago, the schedule for imaging has advanced as well. The first "optical navigation" photos will be taken on about Jan. 13. (As we will see next month, Dawn will glimpse Ceres once even sooner than that, but not for navigation purposes.) The onboard camera, designed for mapping Vesta and Ceres from orbit, will show a fuzzy orb about 25 pixels across. Although the pictures will not yet display details quite as fine as those already discerned by Hubble Space Telescope, the different perspective will be intriguing and may contain surprises. The pictures from the second approach imaging session on Jan. 26 will be slightly better than Hubble's, and when the third set is acquired on Feb. 4, they should be about twice as good as what we have today. By the time of the second "rotation characterization" on about Feb. 20 (nearly a month earlier than was planned last year), the pictures will be seven times better than Hubble's.
While the primary purpose of the approach photos is to help guide Dawn to its orbital destination, the images (and visible and infrared spectra collected simultaneously) will serve other purposes. They will provide some early characterizations of the alien world so engineers and scientists can finalize sensor parameters to be used for the many RC3 observations. They will also be used to search for moons. And the pictures surely will thrill everyone along for the ride (including you, loyal reader), as a mysterious fuzzy patch of light, observed from afar for more than two centuries and once called a planet, then an asteroid and now a dwarf planet, finally comes into sharper focus. Wonderfully exciting though they will be, the views will tantalize us, whetting our appetites for more. They will draw us onward with their promises of still more discoveries ahead, as this bold adventure into the unknown begins to reveal the treasures we have so long sought.
Dawn is 1.2 million miles (1.9 million kilometers) from Ceres. It is also 3.65 AU (339 million miles, or 546 million kilometers) from Earth, or 1,475 times as far as the moon and 3.67 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and one minute to make the round trip.
Dr. Marc D. Rayman
5:00 p.m. PDT October 31, 2014
P.S. While Dawn thrusts tirelessly, your correspondent is taking the evening off for Halloween. No longer able to fit in his costume from last year (and that has nothing to do with how many cupcakes he has consumed), this year he is expanding his disguise. Expressing the playful spirit of the holiday, he will be made up as a combination of one part baryonic matter and four parts nonbaryonic cold dark matter. It's time for fun!
On the seventh anniversary of embarking upon its extraordinary extraterrestrial expedition, the Dawn spacecraft is far from the planet where its journey began. While Earth has completed its repetitive loops around the sun seven times, its ambassador to the cosmos has had a much more varied itinerary. On most of its anniversaries, including this one, it reshapes its orbit around the sun, aiming for some of the last uncharted worlds in the inner solar system. (It also zipped past the oft-visited Mars, robbing the red planet of some of its orbital energy to help fling the spacecraft on to the more distant main asteroid belt.) It spent its fourth anniversary exploring the giant protoplanet Vesta, the second most massive object in the asteroid belt, revealing a fascinating, complex, alien place more akin to Earth and the other terrestrial planets than to typical asteroids. This anniversary is the last it will spend sailing on the celestial seas. By its eighth, it will be at its new, permanent home, dwarf planet Ceres.
The mysterious world of rock and ice is the first dwarf planet discovered (129 years before Pluto) and the largest body between the sun and Pluto that a spacecraft has not yet visited. Dawn will take up residence there so it can conduct a detailed investigation, recording pictures and other data not only for scientists but for everyone who has ever gazed up at the night sky in wonder, everyone who is curious about the nature of the universe, everyone who feels the burning passion for adventure and the insatiable hunger for knowledge and everyone who longs to know the cosmos.
Dawn is the only spacecraft ever to orbit a resident of the asteroid belt. It is also the only ship ever targeted to orbit two deep-space destinations. This unique mission would be quite impossible without its advanced ion propulsion system, giving it capabilities well beyond what conventional chemical propulsion provides. That is one of the keys to how such a voyage can be undertaken.
For those who would like to track the probe’s progress in the same terms used on previous (and, we boldly predict, subsequent) anniversaries, we present here the seventh annual summary, reusing text from last year with updates where appropriate. Readers who wish to reflect upon Dawn’s ambitious journey may find it helpful to compare this material with the logs from its first, second, third, fourth, fifth and sixth anniversaries. On this anniversary, as we will see below, the moon will participate in the celebration.
In its seven years of interplanetary travels, the spacecraft has thrust for a total of 1,737 days, or 68 percent of the time (and about 0.000000034 percent of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn’s wont. All this thrusting has cost the craft only 808 pounds (366 kilograms) of its supply of xenon propellant, which was 937 pounds (425 kilograms) on Sep. 27, 2007.
The thrusting so far in the mission has achieved the equivalent of accelerating the probe by 22,800 mph (10.2 kilometers per second). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft’s propulsive work. Having accomplished about seven-eighths of the thrust time planned for its entire mission, Dawn has already far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.)
Since launch, our readers who have remained on or near Earth have completed seven revolutions around the sun, covering 44.0 AU (4.1 billion miles, or 6.6 billion kilometers). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 31.4 AU (2.9 billion miles, or 4.7 billion kilometers). As it climbed away from the sun to match its orbit to that of Vesta, it continued to slow down to Vesta’s speed. It has been slowing down still more to rendezvous with Ceres. Since Dawn’s launch, Vesta has traveled only 28.5 AU (2.6 billion miles, or 4.3 billion kilometers), and the even more sedate Ceres has gone 26.8 AU (2.5 billion miles, or 4.0 billion kilometers). (To develop a feeling for the relative speeds, you might reread this paragraph by paying attention to only one set of units, whether you choose AU, miles, or kilometers. Ignore the other two scales so you can focus on the differences in distance among Earth, Dawn, Vesta and Ceres over the seven years. You will see that as the strength of the sun’s gravitational grip weakens at greater distance, the corresponding orbital speed decreases.)
Another way to investigate the progress of the mission is to chart how Dawn’s orbit around the sun has changed. This discussion will culminate with a few more numbers than we usually include, and readers who prefer not to indulge may skip this material, leaving that much more for the grateful Numerivores. (If you prefer not to skip it, click here.) In order to make the table below comprehensible (and to fulfill our commitment of environmental responsibility), we recycle some more text here on the nature of orbits.
Orbits are ellipses (like flattened circles, or ovals in which the ends are of equal size). So as members of the solar system family follow their paths around the sun, they sometimes move closer and sometimes move farther from it.
In addition to orbits being characterized by shape, or equivalently by the amount of flattening (that is, the deviation from being a perfect circle), and by size, they may be described in part by how they are oriented in space. Using the bias of terrestrial astronomers, the plane of Earth’s orbit around the sun (known as the ecliptic) is a good reference. Other planets and interplanetary spacecraft may travel in orbits that are tipped at some angle to that. The angle between the ecliptic and the plane of another body’s orbit around the sun is the inclination of that orbit. Vesta and Ceres do not orbit the sun in the same plane that Earth does, and Dawn must match its orbit to that of its targets. (The major planets orbit closer to the ecliptic, and part of the arduousness of the journey is changing the inclination of its orbit, an energetically expensive task.)
Now we can see how Dawn has been doing by considering the size and shape (together expressed by the minimum and maximum distances from the sun) and inclination of its orbit on each of its anniversaries. (Experts readily recognize that there is more to describing an orbit than these parameters. Our policy remains that we link to the experts’ websites when their readership extends to one more elliptical galaxy than ours does.)
The table below shows what the orbit would have been if the spacecraft had terminated ion thrusting on its anniversaries; the orbits of its destinations, Vesta and Ceres, are included for comparison. Of course, when Dawn was on the launch pad on Sep. 27, 2007, its orbit around the sun was exactly Earth’s orbit. After launch, it was in its own solar orbit.
|Minimum distance from the Sun (AU)||Maximum distance from the Sun (AU)||Inclination|
|Dawn’s orbit on Sep. 27, 2007 (before launch)||0.98||1.02||0.0°|
|Dawn’s orbit on Sep. 27, 2007 (after launch)||1.00||1.62||0.6°|
|Dawn’s orbit on Sep. 27, 2008||1.21||1.68||1.4°|
|Dawn’s orbit on Sep. 27, 2009||1.42||1.87||6.2°|
|Dawn’s orbit on Sep. 27, 2010||1.89||2.13||6.8°|
|Dawn’s orbit on Sep. 27, 2011||2.15||2.57||7.1°|
|Dawn’s orbit on Sep. 27, 2012||2.17||2.57||7.3°|
|Dawn’s orbit on Sep. 27, 2013||2.44||2.98||8.7°|
|Dawn’s orbit on Sep. 27, 2014||2.46||3.02||9.8°|
For readers who are not overwhelmed by the number of numbers, investing the effort to study the table may help to demonstrate how Dawn has patiently transformed its orbit during the course of its mission. Note that three years ago, the spacecraft’s path around the sun was exactly the same as Vesta’s. Achieving that perfect match was, of course, the objective of the long flight that started in the same solar orbit as Earth, and that is how Dawn managed to slip into orbit around Vesta. While simply flying by it would have been far easier, matching orbits with Vesta required the exceptional capability of the ion propulsion system. Without that technology, NASA’s Discovery Program would not have been able to afford a mission to explore it in such detail. But now, Dawn has gone even beyond that. Having discovered so many of Vesta’s secrets, the stalwart adventurer left the protoplanet behind. No other spacecraft has ever escaped from orbit around one distant solar system object to travel to and orbit still another extraterrestrial destination. A true interplanetary spaceship, Dawn is enlarging, reshaping and tilting its orbit again so that in 2015, it will be identical to Ceres’.
It may surprise you that if Dawn stopped thrusting today, it would sail out farther from the sun than where it is headed, as shown in the table. We can understand that, however, by thinking carefully about how the craft reaches its target. It has been propelling itself up the solar system hill so it can fly to the vicinity of Ceres, and its own momentum now is sufficient to carry it even beyond. This is little different from driving to a destination with the recognition that near the end of your trip, you need to slow down or you will overshoot. While trajectories that use ion propulsion are much more complicated, that fundamental principle applies. Indeed, Dawn’s speed toward Ceres has been declining since December 2013. In addition to the recent and future ion thrusting that guides the ship smoothly into its new port, the gravity of Ceres itself will help tug Dawn in. We will see more about that next month when we present the revised approach plan.
On Sep. 11, as the spacecraft was engaged in routine ion thrusting, a high-energy particle of space radiation struck an electrical component onboard. That triggered a chain of events that halted thrusting and required the team of flight controllers on distant Earth to leap into action to resume normal operations. Their swift and expert response was successful, and by Sep. 15 the robot was back on duty. In the next log, we will describe what happened on the spacecraft and in mission control. We will also see how navigators take advantage of the tremendous flexibility provided by ion propulsion to devise a new path into Ceres orbit following this interruption in thrust. (As we also will see, the rest of the intricate plans for exploring the dwarf planet will be unchanged. The logs from December 2013 through last month have previews of those plans.)
As Dawn begins the eighth year of its trek through the solar system, Earthlings have a convenient opportunity today to locate the distant spacecraft thanks to the moon. That celestial orb serves as a guidepost to Dawn, which will be well over one thousand times farther away. When the moon rises in the United States later this morning, it will be leading Dawn by less than nine lunar diameters. The sun, moon, planets and stars all appear to move west as Earth rotates on its axis, but the moon itself travels eastward in its orbit quickly enough that it falls behind noticeably over the course of a day. Throughout most of the day today, our natural satellite’s progression will slowly shrink the distance to Dawn. For observes in the eastern part of the country, by the time the moon sets, it will be about one lunar diameter from the spacecraft. For those on the west coast, the moon will be less than its own width from Dawn around sunset. By the time they see the moon setting, Dawn will have passed it and will lead it down to the horizon, the pair still within two lunar diameters of each other. The details are not so important, however. For observers anywhere today, the moon allows us to get a sense of where in the vast sky our faithful explorer is.
Of course Dawn is much, much, much too far away to be seen with our humble eyes. The spacecraft is more than 1.2 million times farther from Earth than the International Space Station is. It is more remote than Mars ever is. The most powerful optical telescopes on high mountains or in orbit could not detect anything nearly as faint as Dawn in the depths of space. Yet readers have ready access to vision far more acute. We can turn our mind’s eye to that part of the sky near the moon. Out there, in that direction, is a probe from Earth, an emissary to the cosmos, silently streaking through the distant void, conducting an ambitious and exciting mission of discovery on behalf of curious and ingenious humans who yearn for new knowledge and new insight and who have an insatiable passion for grand adventures.
Dawn is 2.1 million miles (3.5 million kilometers) from Ceres. It is also 3.37 AU (313 million miles, or 504 million kilometers) from Earth, or 1,290 times as far as the moon and 3.36 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 56 minutes to make the round trip.
Dr. Marc D. Rayman
4:34 a.m. PDT September 27, 2014
Dear Omnipodawnt Readers,
Dawn draws ever closer to the mysterious Ceres, the largest body between the sun and Pluto not yet visited by a probe from Earth. The spacecraft is continuing to climb outward from the sun atop a blue-green beam of xenon ions from its uniquely efficient ion propulsion system. The constant, gentle thrust is reshaping its solar orbit so that by March 2015, it will arrive at the first dwarf planet ever discovered. Once in orbit, it will undertake an ambitious exploration of the exotic world of ice and rock that has been glimpsed only from afar for more than two centuries.
An important characteristic of this interplanetary expedition is that Dawn can linger at its destinations, conducting extensive observations. Since December, we have presented overviews of all the phases of the mission at Ceres save one. (In addition, questions posted by readers each month, occasionally combined with an answer, have helped elucidate some of the interesting features of the mission.) We have described how Dawn will approach its gargantuan new home (with an equatorial diameter of more than 600 miles, or 975 kilometers) and slip into orbit with the elegance of a celestial dancer. The spacecraft will unveil the previously unseen sights with its suite of sophisticated sensors from progressively lower altitude orbits, starting at 8,400 miles (13,500 kilometers), then from survey orbit at 2,730 miles (4,400 kilometers), and then from the misleadingly named high altitude mapping orbit (HAMO) only 910 miles (1,470 kilometers) away. To travel from one orbit to another, it will use its extraordinary ion propulsion system to spiral lower and lower and lower. This month, we look at the final phase of the long mission, as Dawn dives down to the low altitude mapping orbit (LAMO) at 230 miles (375 kilometers). We will also consider what future awaits our intrepid adventurer after it has accomplished the daring plans at Ceres.
It will take the patient and tireless robot two months to descend from HAMO to LAMO, winding in tighter and tighter loops as it goes. By the time it has completed the 160 revolutions needed to reach LAMO, Dawn will be circling Ceres every 5.5 hours. (Ceres rotates on its own axis in 9.1 hours.) The spacecraft will be so close that Ceres will appear as large as a soccer ball seen from less than seven inches (17 centimeters) away. In contrast, Earth will be so remote that the dwarf planet would look to terrestrial observers no larger than a soccer ball from as far as 170 miles (270 kilometers). Dawn will have a uniquely fabulous view.
As in the higher orbits, Dawn will scrutinize Ceres with all of its scientific instruments, returning pictures and other information to eager Earthlings. The camera and visible and infrared mapping spectrometer (VIR) will reveal greater detail than ever on the appearance and the mineralogical composition of the strange landscape. Indeed, the photos will be four times sharper than those from HAMO (and well over 800 times better than the best we have now from Hubble Space Telescope). But just as in LAMO at Vesta, the priority will be on three other sets of measurements which probe even beneath the surface.
All of the mass within Ceres combines to hold Dawn in orbit, exerting a powerful gravitational grip on the ship. But as the spacecraft moves through its orbit, any variations in the internal structure of Ceres from one place to another will lead to slight perturbations of the orbit. If, for example, there is a large region of unusually dense material, even if deep underground, the craft will speed up slightly as it travels toward it. After Dawn passes overhead, the same massive feature will slightly retard its progress, slowing it down just a little.
Dawn will be in almost constant radio contact with Earth during LAMO. When it is pointing its payload of sensors at the surface, it will broadcast a faint radio signal through one of its small auxiliary antennas so exquisitely sensitive receivers on a planet far, far away can detect it. At other times, in order to transmit its findings from LAMO, it will aim its main antenna directly at Earth. In both cases, the slightest changes in speed toward or away from Earth will be revealed in the Doppler shift, in which the frequency of the radio waves changes, much as the pitch of a siren goes up and then down as an ambulance approaches and then recedes. Using this and other remarkably powerful techniques mastered for traveling throughout the solar system, navigators will carefully plot the tiny variations in Dawn’s orbit and from that determine the distribution of mass throughout the interior of the dwarf planet.
The spacecraft will use its sophisticated gamma ray and neutron detector (GRaND) to determine the atomic constituents of the material on the surface and to a depth of up to about a yard (a meter). Gamma rays are a very, very high frequency form of electromagnetic radiation, beyond visible light, beyond ultraviolet, beyond even X-rays. Neutrons are very different from gamma rays. They are the electrically neutral particles in the nuclei of atoms, slightly more massive than protons, and in most elements, neutrons outnumber them too. It would be impressive enough if GRaND only detected these two kinds of nuclear radiation, but it also measures the energy of each kind. (Unfortunately, that description doesn’t lend itself to such a delightful acronym).
Most of the gamma rays and neutrons are byproducts of the collisions between cosmic rays (radiation from elsewhere in space) and the nuclei of atoms in the ground. (Cosmic rays don’t do this very much at Earth; rather, most are diverted by the magnetic field or stopped by atoms in the upper atmosphere.) In addition, some gamma rays are emitted by radioactive elements near the surface. Regardless of the source, the neutrons and the gamma rays that escape from Ceres and travel out into space carry a signature of the type of nucleus they came from. When GRaND intercepts the radiation, it records the energy, and scientists can translate those signatures into the identities of the atoms.
The radiation reaching GRaND, high in space above the surface, is extremely faint. Just as a camera needs a long exposure in very low light, GRaND needs a long exposure to turn Ceres’ dim nuclear glow into a bright picture. Fortunately, GRaND’s pictures do not depend on sunlight; regions in the dark of night are no fainter than those illuminated by the sun.
For most of its time in LAMO, Dawn will point GRaND at the surface beneath it. The typical pattern will be to make 15 orbital revolutions, lasting about 3.5 days, staring down, measuring each neutron and each gamma ray that encounters the instrument. Simultaneously, the craft will transmit its broad radio signal to reveal the gentle buffeting by the variations in the gravitational field. On portions of its flights over the lit terrain, it will take photos and will collect spectra with VIR. Then the spacecraft will rotate to point its main antenna to distant Earth, and while it makes five more circuits in a little more than a day, it will beam its precious discoveries to the 230-foot (70-meter) antennas at NASA’s Deep Space Network.
Dawn will spend more time in each successive observational phase at Ceres than the ones before. After two months in HAMO, during which it will complete about 80 orbits, the probe will devote about three months to LAMO, looping around more than 400 times. That is more than enough time to collect the desired data. Taxpayers have allocated sufficient funds to operate Dawn until June 2016, allowing some extra time for the flight team to grapple with the inevitable glitches that arise in such a challenging undertaking. As in all phases, mission planners recognize that complex operations in that remote and hostile environment probably will not go exactly according to plan, but even if some of the measurements are not completed, enough should be to satisfy all the scientific objectives.
The indefatigable explorer will work hard in LAMO. Aiming its sensors at the surface beneath it throughout its 5.5-hour orbits does not happen naturally. Dawn needs to keep turning to point them down. When it is transmitting its scientific bounty, it needs to hold steady enough to maintain Earth in the sights of its radio antenna. An essential element of the design of the spacecraft to achieve these and related capabilities was the use of three reaction wheels. By electrically changing the speed at which these gyroscope-like devices rotate, the probe can turn or stabilize itself. Because they are so important, four were included, ensuring that if any one encountered difficulty, the ambitious mission could continue with the other three.
As long-time readers know, one did falter in June 2010. Another stopped operating in August 2012. The failure of two such vital devices could have proven fatal for a mission, but thanks to the expertise, creativity, swiftness, and persistence of the members of the Dawn flight team, the prospects for completing the exploration of Ceres are bright.
As long-time readers know, one did falter in June 2010. Another stopped operating in August 2012. The failure of two such vital devices could have proven fatal for a mission, but thanks to the expertise, creativity, swiftness, and persistence of the members of the Dawn flight team, the prospects for completing the exploration of Ceres are bright. The two remaining reaction wheels are powered off now and will not be used for the higher altitude orbits. Rather, the conventional rocket propellant hydrazine, squirted out through the tiny thrusters of the reaction control system, controls the ship’s orientation. It is quite remarkable that the team was able to stretch the small supply to cover all the activities needed from departure from Vesta in 2012 to the end of the mission in LAMO nearly four years later.
When Dawn arrives in LAMO, operators will power the two operable wheels on and use them for as long as the pair lasts. Given the unexpectedly early loss of the other two (as well as the failures of similar units on other spacecraft), engineers do not have high confidence that will be very long. But LAMO is the most hydrazine-expensive part of the mission, so any useable lifetime will lower (but not stop) the hydrazine expenditure. Regardless, with or without functioning reaction wheels, the reliable Dawn spacecraft should be able to conduct a fully rewarding, exciting campaign at the enigmatic world.
What fate awaits our stalwart adventurer following the completion of its primary assignment? There are several possibilities, but they all conclude the same way. If hydrazine remains at the end, and if the spacecraft is still healthy, NASA will decide whether to invest further in Dawn. NASA has many exciting and important activities to choose among — after all, there’s a vast universe to explore! If it provides further funds, Dawn will perform further investigations in LAMO, making GRaND’s gamma ray and neutron pictures even sharper, refining the gravitational measurements, collecting still more photos of the expansive surface, and acquiring even more spectra with VIR.
There is no intention to fly to a lower orbit. Even if the two remaining reaction wheels operate, hydrazine will be running very low, so time will be short. Following another spiral to a different altitude would not be wise. There will be no below-LAMO (BLAMO) or super low altitude mapping orbit (SLAMO) phase of the mission.
There is another issue as well. As we will describe in December, there is good reason to believe Ceres has a substantial inventory of water, mostly as ice but perhaps some as liquid. The distant sun and the gradual decay of radioactive elements provide a little warmth. Telescopic studies suggest the probable presence of organic chemicals. As a result of these and other considerations, scientists recognize that Ceres might display “prebiotic chemistry,” or the ingredients and conditions that, on your planet, led to the origin of life. This could present important clues to help advance our understanding of how life can arise.
We want to protect that special environment from contamination by the great variety of terrestrial materials in the spacecraft. As responsible citizens of the solar system, NASA conforms to “planetary protection” protocols which specify that Dawn may not reach the surface for at least 50 years after arrival. (The reasoning behind the limited duration is that if our data indicate that Ceres really does need special protection, half a century would be long enough to mount another mission if need be.) Extensive analyses by engineers and scientists show that for any credible detail of the dwarf planet’s gravitational field, the orbit will remain relatively stable for much longer than that, perhaps even millennia. The ship will not make landfall.
Despite some romantic notions of a controlled landing, it would not be physically possible, even if there were no planetary protection prohibitions. Ceres is entirely unlike the little chunks of rock most people think of as asteroids. The behemoth’s surface gravity is nearly three percent of Earth’s. At 800 kilograms, Dawn would weigh the equivalent of about 50 pounds there. The famously efficient but gentle thrust of the ion propulsion system, providing a force equivalent to the weight of a sheet of paper on Earth, would be quite insufficient for slowing the spaceship down as it approached the hard ground.
The best place for Dawn, should it be asked to continue its work, will be in LAMO. And when the last puffs of hydrazine are expelled, it will no longer be able to aim its instruments at the surface, any of its ion engines in the direction required to maneuver, its antenna at Earth, or its solar arrays at the sun. The battery will be depleted in a matter of hours. The spacecraft will remain in orbit as surely as the moon remains in orbit around Earth, but it will cease operating.
Long after its final controlled actions, indeed long after you, faithful reader, and your correspondent and everyone else involved in the mission (whether directly or by virtue of sharing in the excitement and the wonder of such a grand undertaking) are gone, Dawn’s successes will still be important. Its place in the annals of space exploration will be secure: the first spacecraft to orbit an object in the asteroid belt, the first spacecraft to visit a dwarf planet (indeed, the first spacecraft to visit the first dwarf planet that was discovered), the first spacecraft to orbit a dwarf planet, the first spacecraft to orbit any two extraterrestrial destinations, and more. And with Ceres and Vesta being, by far, the two most massive of the millions of objects between Mars and Jupiter, Dawn will have single-handedly examined about 40 percent of the mass of the main asteroid belt. Its scientific legacy will be secure, having revealed myriad fascinating and exciting insights into two such different and exotic alien bodies, introducing Earth to some of the last uncharted worlds in the inner solar system. Leaving the remarkable craft in orbit around the distant colossus will be a fitting and honorable conclusion to its historic journey of discovery. This interplanetary ambassador from Earth will be an inert celestial monument to the power of human ingenuity, creativity, and curiosity, a lasting reminder that our passion for bold adventures and our noble aspirations to know the cosmos can take us very, very far beyond the confines of our humble home.
Dawn is 3.0 million miles (4.8 million kilometers) from Ceres. It is also 3.07 AU (285 million miles, or 459 million kilometers) from Earth, or 1,185 times as far as the moon and 3.04 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 51 minutes to make the round trip.
Dr. Marc D. Rayman
6:00 p.m. PDT August 31, 2014
Dear Studawnts and Teachers,
Patient and persistent, silent and alone, Dawn is continuing its extraordinary extraterrestrial expedition. Flying through the main asteroid belt between Mars and Jupiter, the spacecraft is using its advanced ion propulsion system to travel from Vesta, the giant protoplanet it unveiled in 2011 and 2012, to Ceres, the dwarf planet it will reach in about eight months.
Most of these logs since December have presented previews of the ambitious plan for entering orbit and operating at Ceres to discover the secrets this alien world has held since the dawn of the solar system. We will continue with the previews next month. But now with Dawn three quarters of the way from Vesta to Ceres, let's check in on the progress of the mission, both on the spacecraft and in mission control at JPL.
The mission is going extremely well. Thank you for asking.
For readers who want more details, read on ...
The spacecraft, in what is sometimes misleadingly called quiet cruise, has spent more than 97 percent of the time this year following the carefully designed ion thrust flight plan needed to reshape its solar orbit, gradually making it more and more like Ceres' orbit around the sun. This is the key to how the ship can so elegantly enter into orbit around the massive body even with the delicate thrust, never greater than the weight of a single sheet of paper.
The probe is equipped with three ion engines, although it only uses one at a time. (The locations of the engines were revealed shortly after launch when the spacecraft was too far from Earth for the information to be exploited for tawdry sensationalism.) Despite the disciplined and rigorous nature of operating a spaceship in the main asteroid belt, the team enjoys adding a lighthearted touch to their work, so they refer to the engines by the zany names #1, #2, and #3.
Darth Vader and his Empire cohorts in "Star Wars" flew TIE (Twin Ion Engine) Fighters in their battles against Luke Skywalker and others in the Rebel Alliance. Outfitted with three ion engines, Dawn does the TIE Fighters one better. We should acknowledge, however, that the design of the TIE Fighters did appear to provide greater agility, perhaps at the expense of fuel efficiency. Your correspondent would concur that when you are trying to destroy your enemy while dodging blasts from his laser cannons, economy of propellant consumption probably shouldn't be your highest priority.
All three engines on Dawn are healthy, and mission controllers consider many criteria in formulating the plan for which one to use. This called for switching from thruster #2 to thruster #1 on May 27. Thruster #1 had last been used to propel the ship on Jan. 4, 2010. After well over four years of inaction in space, it came to life and emitted the famous blue-green beam of high velocity xenon ions right on schedule (at 4:19:19 pm PDT, should you wish to take yourself back to that moment), gently and reliably pushing the spacecraft closer to its appointment with Ceres.
Without the tremendous capability of ion propulsion, a mission to orbit either Vesta or Ceres alone would have been unaffordable within NASA's Discovery program. A mission to orbit both destinations would be altogether impossible. The reason ion propulsion is so much more efficient than conventional chemical propulsion is that it can turn electrical energy into thrust. Chemical propulsion systems are limited to the energy stored in the propellants.
Thanks to Dawn's huge solar arrays, electrical energy is available in abundance, even far from the brilliant sun. To make accurate predictions of the efficiency of the solar cells as Dawn continues to recede from the sun, engineers occasionally conduct a special calibration. As we described in more detail a year ago, they command the robot to rotate its panels to receive less sunlight, simulating being at greater solar distances, as the ion propulsion system is throttled to lower power levels. Following the first such calibration on June 24, 2013, we assured readers (including you) that we would repeat the calibration as Dawn continued its solar system travels. So you will be relieved to know that it was performed again on Oct. 14, Feb. 3, and May 27, and another is scheduled for Sept. 15. Having high confidence in how much power will be available for ion thrusting for the rest of the journey allows navigators to plot the best possible course. Dawn is on a real power trip!
The reason for going to Ceres, besides it being an incredibly cool thing to do, is to use the suite of sophisticated sensors to learn about this mysterious dwarf planet. (In December, we will describe what is known about Ceres, just in time for it to change with Dawn's observations.) Controllers activated and tested the cameras and all the spectrometers this summer, verifying that they remain in excellent condition and as ready to investigate the uncharted lands ahead as they were for the fascinating lands astern. The engineers also installed updated software in the primary camera in June and are ready to install it in the backup camera next month to enhance some of the devices' functions. All of the scientific instruments are normally turned off when Dawn is not orbiting one of its targets. They will be powered on again in October for a final health check before the approach phase, during which they will provide our first exciting new views of Ceres.
To achieve a successful mission at Ceres, in addition to putting the finishing touches on the incredibly intricate plans, the operations team works hard to take good care of the spacecraft, ensuring it stays healthy and on course. In the remote depths of space, the robot has to be able to function on its own most of the time, but it does so with periodic guidance and oversight by its human handlers on a faraway planet. That means they need to stay diligent, keep their skills sharp, and remain watchful for any indications of undesirable conditions. On July 22, the team received information showing that Dawn was in safe mode, a special configuration invoked by onboard software to protect the spacecraft and the mission, preventing unexpected situations from getting out of control.
As engineers inspected the trickle of telemetry, they began to discover that this was a more dire situation than they had ever seen for the distant craft. Among the surprises was an open circuit in one of the pressurized cells of the nickel-hydrogen battery, a portion of the reaction control system that was so cold that its hydrazine propellant was in danger of freezing, temperatures elsewhere on the spacecraft so low that the delicate cameras were at risk of being damaged, and a sun sensor with degraded vision. To make it still more complicated, waveguide transfer switch #5, used to direct the radio signal from the transmitter inside the spacecraft to one of its antennas for beaming to Earth, was stuck and so would not move when software instructed it to. Other data showed that part of the computer memory was compromised by space radiation. As if all that were not bad enough, one of the two star trackers, devices that recognize patterns of stars just as you might recognize constellations to determine your orientation at night without a compass or other aids, was no longer functional. Further complicating the effort to get the mission back on track was an antenna at the Deep Space Network that needed to be taken out of service for emergency repairs. And the entire situation was exacerbated by Dawn already being in its lowest altitude orbit around Ceres (the subject of next month's log), so for part of every 5.5-hour orbital revolution, it was out of contact as the world beneath it blocked the radio signal.
Confronted with an almost bewildering array of complex problems, the team of experts spent three days working through them with their usual cool professionalism, ultimately finding ways to overcome each obstacle to continue the mission. It would be extraordinarily, even unbelievably, unlikely for so many separate problems to stack up so quickly, even for a ship in the severe conditions of deep space, more than 232 million miles (374 million kilometers) from Dawn mission control on the top floor of JPL's building 264. However, it easily can happen in an operational readiness test (ORT, pronounced letter by letter and not as a word, for those readers who want to conduct their own ORTs). The telemetry came from the spacecraft simulator, just down the hall from the mission control room, and the problems were the fiendishly clever creations of the ORT mastermind. (So now you may calm down, reassured that the scenario just described did not actually happen.)
While mission controllers exercised their skills in the ORT, the real spacecraft continued streaking through the asteroid belt, its interplanetary travels bringing it 45 thousand miles (73 thousand kilometers) closer to Ceres each day. But it is not only the Dawn team members who are part of this adventure. The stalwart explorer is transporting everyone who ever gazes in wonder at the night sky, everyone who yearns to know what lies beyond the confines of our humble home, and everyone awed by the mystery, the grandeur, and the immensity of the cosmos. Fueled by their passionate longing, the journey holds the promise of exciting new knowledge and thrilling new insights as a strange world, glimpsed only from afar for more than two centuries, is soon to be unveiled.
Dawn is 4.2 million miles (6.7 million kilometers) from Ceres. It is also 2.67 AU (248 million miles, or 399 million kilometers) from Earth, or 995 times as far as the moon and 2.63 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 44 minutes to make the round trip
Dr. Marc D. Rayman
6:00 p.m. PDT July 31, 2014
Deep in the main asteroid belt, between Mars and Jupiter, far from Earth, far from the sun, far now even from the giant protoplanet Vesta that it orbited for 14 months, Dawn flies with its sights set on dwarf planet Ceres. Using the uniquely efficient, whisper-like thrust of its remarkable ion propulsion system, the interplanetary adventurer is making good progress toward its rendezvous with the uncharted, alien world in about nine months.
Dawn’s ambitious mission of exploration will require it to carry out a complex plan at Ceres. In December, we had a preview of the “fapproach phase,” and in January, we saw how the high velocity beam of xenon ions will let the ship slip smoothly into Ceres’s gravitational embrace. We followed that with a description in February of the first of four orbital phases (with the delightfully irreverent name RC3), in which the probe will scrutinize the exotic landscape from an altitude of 8,400 miles (13,500 kilometers). We saw in April how the spacecraft will take advantage of the extraordinary maneuverability of ion propulsion to spiral from one observation orbit to another, each one lower than the one before, and each one affording a more detailed view of the exotic world of rock and ice. The second orbit, at an altitude of about 2,730 miles (4,400 kilometers), known to insiders (like you, faithful reader) as “survey orbit,” was the topic of our preview in May. This month, we will have an overview of the plan for the third and penultimate orbital phase, the “high altitude mapping orbit” (HAMO).
(The origins of the names of the phases are based on ancient ideas, and the reasons are, or should be, lost in the mists of time. Readers should avoid trying to infer anything at all meaningful in the designations. After some careful consideration, your correspondent chose to use the same names the Dawn team uses rather than create more helpful descriptors for the purposes of these logs. What is important is not what the different orbits are called but rather what amazing new discoveries each one enables.)
It will take Dawn almost six weeks to descend to HAMO, where it will be 910 miles (1,470 kilometers) high, or three times closer to the mysterious surface than in survey orbit. As we have seen before, a lower orbit, whether around Ceres, Earth, the sun, or the Milky Way galaxy, means greater orbital velocity to balance the stronger gravitational grip. In HAMO, the spacecraft will complete each loop around Ceres in 19 hours, only one quarter of the time it will take in survey orbit.
In formulating the HAMO plans, Dawn’s human colleagues (most of whom reside much, much closer to Earth than the spacecraft does) have taken advantage of their tremendous successes with HAMO1 and HAMO2 at Vesta. We will see below, however, there is one particularly interesting difference.
As in all observation phases at Ceres (and Vesta), Dawn’s orbital path will take it from pole to pole and back. It will fly over the sunlit side as it travels from north to south and then above the side in the deep darkness of night on the northward segment of each orbit. This polar orbit ensures a view of all latitudes. As Ceres pirouettes on its axis, it presents all longitudes to the orbiting observer. The mission planners have choreographed the celestial pas de deux so that in a dozen revolutions, Dawn’s camera can map nearly the entire surface.
Rather than mapping once, however, the spacecraft will map Ceres up to six times. One of Dawn’s many objectives is to develop a topographical map, revealing the detailed contours of the terrain, such as the depths of craters, the heights of mountains, and the slopes and variations of plains. To do so, it will follow the same strategy employed so successfully at Vesta, by taking pictures at different angles, much like stereo imaging. The spacecraft will make its first HAMO map by aiming its camera straight down, photographing the ground directly beneath it. Then it will map the surface again with the camera pointed in a slightly different direction, and it will repeat this for a total of six maps, or six mapping “cycles.” With views from up to six different perspectives, the landscape will pop from flat images into its full three dimensionality. (As with all the plans, engineers recognize that complex and challenging operations in the forbidding, unforgiving depths of space do not always go as intended. So they plan to collect more data than they need. If some of the images, or even entire maps, are not acquired, there should still be plenty of pictures to use in revealing the topography.)
In addition to acquiring the photos, Dawn will make other measurements in HAMO. During some of the cycles, the camera will use its color filters to glean more about the nature of the surface. The visible and infrared mapping spectrometer will collect spectra to help scientists determine the composition of the surface, its temperature, and other properties.
Exquisitely accurate radio tracking of the spacecraft in its orbit, as indicated by the Doppler shift (the change in frequency, or pitch, as the craft moves toward or away from Earth) and by the time it takes radio signals to make the round trip from Earth, allows navigators to determine the strength of the gravitational tugging. That can be translated into not only the mass of Ceres but also how the mass is distributed in its interior. In August, when we look ahead to the fourth and final science phase of the Ceres mission, the low altitude mapping orbit, we will explain this in greater detail.
Although still too high for anything but the weakest indication of radiation from Ceres, the gamma ray and neutron detector will measure the radiation environment in HAMO. This will yield a useful reference for the stronger signals it will detect when it is closer.
There is a noteworthy difference between how Dawn will operate in HAMO and how it operated in HAMO1 and HAMO2 at Vesta and even how it will operate in survey orbit at Ceres. In those other orbits, whenever the spacecraft flies above the hemisphere in sunlight, it keeps its sensors pointed at the surface, and whenever it is over the night side, it points its main antenna to Earth. At Vesta, where each HAMO revolution took just over 12 hours, this meant that about every six hours, it had to execute a turn. Were it to follow the same plan at Ceres with a 19-hour HAMO period, when it passed over the north pole, it would begin aiming its scientific instruments at the dwarf planet. When it reached the south pole 9.5 hours later, it would rotate to point its antenna to Earth. Another 9.5 hours after that, when it reached the north pole again, it would pivot to bring the alien terrain back into its sights.
If the robot had its full complement of functioning reaction wheels, that is what it would do in HAMO. Reaction wheels are similar to gyroscopes, and by electrically changing the speed at which they spin, the probe can turn or stabilize itself. The mission was designed to use three reaction wheels, so the ship was outfitted with four. Two are no longer operable. While such a loss could be devastating for some spacecraft, the Dawn flight team has devised highly innovative solutions to accomplish all of the original, ambitious objectives, regardless of the condition of any of the wheels, even the two that are (currently) still healthy. Key to Dawn’s success will be conserving hydrazine, the conventional rocket fuel that it can use to accomplish turns. Dawn’s controllers are taking care with every soupçon of the precious propellant, stretching the supply to allow the mission to complete its bold plans. When the hydrazine is exhausted, Dawn’s expedition will conclude.
Turning so often in HAMO, keeping up with the frequent transitions between flying over the illuminated surface and the surface in the darkness of night, would be unaffordable without reaction wheels, a profligate use of the irreplaceable hydrazine. Instead, it is significantly more efficient to turn less often, allowing the spacecraft sometimes to wait patiently for half an orbit as its instruments stare at the undetectably dark land beneath it and sometimes to maintain its antenna pointing at Earth, even when it is passing over features it otherwise could see. It will see them on other loops however. With this strategy, Ceres can be mapped extensively in HAMO without consuming an excessive amount of hydrazine.
In each mapping cycle, Dawn will make two and a half or three and a half revolutions peering at Ceres, storing images and other valuable data onboard. (The specific duration varies from one cycle to another.) Then, with its memory full, it will turn so it can beam some of its precious findings to distant Earth while it is on the night side of Ceres. That will not be long enough to completely empty the memory but will be sufficient to make room for more data, so after half an orbit, it will turn back to resume its observations. It will follow this pattern for one full cycle, with the dozen passages over the day side providing enough opportunities to complete one map. Then it will devote two and a half revolutions, or two full days, to transmitting the rest of its scientific treasures for the benefit of all those on Earth who ever look to the sky with wonder.
So over the course of 14 complete circuits around Ceres in 11 days, the spacecraft will turn only six or eight times. Ever the responsible conservationists, the team developed all the details of this plan to acquire as much data as possible with the minimum expenditure of hydrazine.
It will take more than two months to carry out all the HAMO activities, with the spacecraft making more than 80 orbital loops. This continues the trend in which the explorer will spend more time in each successive orbital phase than in the ones before. It will complete its assignment in survey orbit in 22 days, during which it will circle Ceres seven times. As we will see in August, the final orbital phase will last even longer than HAMO and include many more revolutions.
Each phase of the mission at Ceres will reveal exciting new insights into a relict from the dawn of the solar system. That same solar system’s complex ballet happens to be playing out now in a way that affords terrestrial observers a nice view of Ceres, Vesta, Mars and the moon. (It also affords Cerean observers a nice view of Vesta, Mars, the moon, and Earth, but that will be described in more detail in the special Cerean local edition of this log.) We wrote in March about the alignment and provided a chart you can still use to locate Vesta and Ceres with a small telescope or even good binoculars. On July 5, Ceres and Vesta will appear to be separated by only one third the diameter of the full moon, even as these distant worlds are 0.57 AU (52 million miles, or 85 million kilometers) from each other. In Earth’s skies, Mars and the moon (both of which are closer to Earth) will not be far away, all of them in Virgo.
Although even the most powerful telescopes are quite insufficient to show it, when we turn our mind’s eye to the sky, with its greater visual acuity, we can discern one more object in this lovely arrangement of gleaming celestial jewels set against the backdrop of the incomparable blackness of the universe. A probe from Earth, a robotic ambassador to the cosmos, on a long and daring expedition, is in transit from Vesta to Ceres. Even as those terrestrial observers enjoy the view, Dawn is patiently making its way through the interplanetary void to a world that has been glimpsed only from afar for more than two centuries. Soon it will undertake a new phase of its extraordinary mission, promising exciting new knowledge and surprising new insights. Engaged in one of humankind’s grand adventures, we extend the best we have within ourselves to reach far, far beyond our humble home.
Dawn is 5.6 million miles (9.0 million kilometers) from Ceres. It is also 2.24 AU (208 million miles, or 335 million kilometers) from Earth, or 825 times as far as the moon and 2.20 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 37 minutes to make the round trip.
Dr. Marc D. Rayman
7:00 p.m. PDT June 30, 2014
Silently streaking through the main asteroid belt, emitting a blue-green beam of xenon ions, Dawn continues its ambitious interplanetary expedition. On behalf of creatures on distant Earth who seek not only knowledge and insight but also bold adventure, the spacecraft is heading toward its appointment with Ceres. In about 10 months, it will enter orbit around the ancient survivor from the dawn of the solar system, providing humankind with its first detailed view of a dwarf planet.
This month we continue with the preview of how Dawn will explore Ceres. In December we focused on the "approach phase," and in January we described how the craft spirals gracefully into orbit with its extraordinary ion propulsion system. The plans for the first observational orbit (with a marvelously evocative name for a first examination of an uncharted world: RC3 — is that cool, or what?), at an altitude of 8,400 miles (13,500 kilometers), were presented in February. Last month, we followed Dawn on its spiral descent from each orbital altitude to the next, with progressively lower orbits providing better views than the ones before. Now we can look ahead to the second orbital phase, survey orbit.
In survey orbit, Dawn will make seven revolutions at an altitude of about 2,730 miles (4,400 kilometers). At that distance, each orbit will take three days and three hours. Mission planners chose an orbit period close to what they used for survey orbit at Vesta, allowing them to take advantage of many of the patterns in the complex choreography they had already developed. Dawn performed it so beautifully that it provides an excellent basis for the Ceres encore. Of course, there are some adjustments, mostly in the interest of husbanding precious hydrazine propellant in the wake of the loss of two of the spacecraft's four reaction wheels. (Although such a loss could have dire consequences for some missions, the resourceful Dawn team has devised a plan that can achieve all of the original objectives regardless of the condition of the reaction wheels.)
We had a preview of survey orbit at Vesta four years ago, and we reviewed the wonderfully successful outcome in September 2011. When we develop the capability to travel backwards in time, we will insert a summary of what occurred in survey orbit at Ceres here: _______…… Well, nothing yet. So, let's continue with the preview.
As in all phases at Ceres (and Vesta), Dawn follows what space trajectory experts (and geeks) call a polar orbit. The ship's course will take it above the north pole, and then it will sail south over the side bathed in the light of the sun. After flying over the south pole, Dawn will head north. Although the surface beneath it will be dark, the spacecraft will be high enough that it will not enter the dwarf planet's shadow. The distant sun will constantly illuminate the large solar arrays.
The leisurely pace in survey orbit allows the explorer to gather a wealth of data during the more than 37 hours on the day side. It will train its science camera and visible and infrared mapping spectrometer (VIR) on the surface lit by the sun. The camera will collect hundreds of images using all seven of its color filters. It will reveal details three times finer than it observed in RC3 orbit and 70 times sharper than the best we have from the Hubble Space Telescope. VIR will acquire millions of spectra to help scientists determine the minerals present as well as the temperature and other properties of the surface. While the sensors are pointed at the surface, the main antenna cannot simultaneously be aimed at Earth, so the robot will store its pictures and spectra.
One Cerean day, the time it takes Ceres to rotate once on its axis, is a little over nine hours. (For comparison, Earth, as some of its residents and visitors know, takes 24 hours. Jupiter turns in just under 10 hours, Vesta in five hours and 21 minutes, and your correspondent's cat Regulus in about 0.5 seconds when chasing a laser spot.) So as Dawn travels from the north pole to the south pole, Ceres will spin underneath it four times. Dawn will be close enough that even the wide field of view of its camera won't capture the entire disc below, from horizon to horizon, but over the course of the seven orbits, the probe will see most of the surface. As in developing the plan for Vesta, engineers (like certain murine rodents and male humans) are keenly aware that as careful, as thorough, and as diligent as they are, their schemes don't always execute perfectly. In the unknown, forbidding depths of space with a complex campaign to carry out, glitches can occur and events can go awry. The plan is designed with the recognition that some observations will not be achieved, but those that are promise great rewards.
Most of the time, the spacecraft will gaze straight down at the alien terrain immediately beneath it. But on the first, second, and fourth passages over the day side of Ceres, it will spend some of the time looking at the limb against the blackness of space. Pictures with this perspective will not only be helpful for establishing the exact shape of the dwarf planet but they also will provide some very appealing views for eager sightseers on Earth.
In addition to using the camera and VIR, Dawn will measure space radiation with its gamma ray and neutron detector (GRaND). GRaND will still be too far from Ceres to sense the nuclear particles emanating from it, but recording the radiation environment will provide a valuable context for the sensitive measurements it will make at lower altitudes.
When Dawn's orbit takes it over the dark side, it will turn away from the dwarf planet it is studying and toward the planet it left in 2007 where its human colleagues still reside. With its 5-foot (1.52-meter) main antenna, it will spend most of the day and a half radioing its precious findings across uncounted millions of miles (kilometers) of interplanetary space. (Well, you won't have to count them, but we will.)
In addition to the instrument data it encodes, Dawn's radio signal will allow scientists and engineers to measure how massive Ceres is. By observing the Doppler shift (the change in frequency caused by the spacecraft's motion), they can determine how fast the ship moves in orbit. Timing how long the signals (traveling at the universal limit of the speed of light) take to make the round trip, navigators can calculate how far the probe is and hence where it is in its orbit. Combining these (and including other information as well) allows them to compute how strongly Ceres pulls on its orbital companion. The strength of its gravitational force reveals its heft.
By the end of survey orbit, Dawn will have given humankind a truly extraordinary view of a dwarf planet that has been cloaked in mystery despite more than 200 years of telescopic studies. As the exotic world of rock and ice begins to yield its secrets to the robotic ambassador from Earth, we will be transported there. We will behold new landscapes that will simultaneously quench our thirst for exploration and ignite our desire for even more. It is as humankind reaches ever farther into the universe that we demonstrate a part of what it means to be human, combining our burning need for greater understanding with our passion for adventure and our exceptional creativity, resourcefulness and tenacity. As we venture deeper into space, we discover much of what lies deep within ourselves.
Dawn is 7.2 million miles (12 million kilometers) from Ceres. It is also 1.87 AU (174 million miles, or 280 million kilometers) from Earth, or 695 times as far as the moon and 1.84 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 31 minutes to make the round trip.
Dr. Marc D. Rayman
11:00 p.m. PDT May 31, 2014
P.S. This is the 101st Dawn Journal. Only 99 more to go before cake and balloons again!
Dear Compedawnt Readers,
Less than a year from its rendezvous with dwarf planet Ceres, Dawn is continuing to make excellent progress on its ambitious interplanetary adventure. The only vessel from Earth ever to take up residence in the main asteroid belt between Mars and Jupiter, the spacecraft grows more distant from Earth and from the sun as it gradually closes in on Ceres. Dawn devotes the majority of its time to thrusting with its remarkable ion propulsion system, reshaping its heliocentric path so that by the time it nears Ceres, the explorer and the alien world will be in essentially the same orbit around the sun.
In December, we saw what Dawn will do during the "approach phase"; to Ceres early in 2015, and in January, we reviewed the unique and graceful method of spiraling into orbit. We described in February the first orbit (with the incredibly cool name RC3) from which intensive scientific observations will be conducted, at an altitude of 8,400 miles (13,500 kilometers). But Dawn will take advantage of the extraordinary capability of ion propulsion to fly to three other orbital locations from which it will further scrutinize the mysterious world.
Let’s recall how the spacecraft will travel from one orbit to another. While some of these plans may sound like just neat ideas, they are much more than that; they have been proven with outstanding success. Dawn maneuvered extensively during its 14 months in orbit around Vesta. (One of the many discussions of that was in November 2011.) The seasoned space traveler and its veteran crew on distant Earth are looking forward to applying their expertise at Ceres.
As long-time readers of these logs know so well, the ion thrust is uniquely efficient but also extremely low. Ion propulsion provides acceleration with patience. Ultimately the patience pays off, enabling Dawn to accomplish feats far beyond what any other spacecraft has ever had the capability to do, including orbiting two extraterrestrial destinations. The gentle thrust, comparable to the weight of a single sheet of paper, means it takes many weeks to maneuver from one observational orbit to another. Of course, it is worthwhile to spend that much time, because each of the orbital phases is designed to provide an exciting trove of scientific data.
Those of you who have navigated around the solar system, as well as others who have contemplated the nature of orbits without having practical experience, recognize that the lower the orbital altitude, the faster the orbital motion. This important principle is a consequence of gravity’s strength increasing as the distance between the massive body and the orbiting object decreases. The speed has to be higher to balance the stronger gravitational pull. (For a reminder of some of the details, be sure to go here before you go out for your next orbital expedition.)
While Dawn slowly reduces its altitude under the faint pressure of its ion engine, it continues circling Ceres, orbiting in the behemoth’s gravitational grip. The effect of combining these motions is that the path from one altitude to another is a spiral. And as Dawn descends and zips around Ceres faster and faster, the spirals get tighter and tighter.
The first coils around Ceres will be long and slow. After completing its investigations in RC3, the probe will spiral down to”survey orbit,”; about 2,700 miles (4,400 kilometers) above the surface. During that month-long descent, it will make only about five revolutions. After three weeks surveying Ceres from that new vantage point, Dawn will follow a tighter spiral down to the (misleadingly named) high altitude mapping orbit (HAMO) at 910 miles (1,470 kilometers). In the six-week trip to HAMO, the craft will wind around almost 30 times. It will devote two months to performing extensive observations in HAMO. And finally as 2015 draws to a close, it will fly an even more tightly wound course to reach its low altitude mapping orbit (LAMO) at 230 miles (375 kilometers), where it will collect data until the end of the mission. The ship will loop around 160 times during the two months to go from HAMO to LAMO. (We will preview the plans for survey orbit, HAMO and LAMO in May, July and August of this year, and if all goes well, we will describe the results in 2015 and 2016.)
Designing the spiral trajectories is a complex and sophisticated process. It is not sufficient simply to activate the thrust and expect to arrive at the desired destination, any more than it is sufficient to press the accelerator in your car and expect to reach your goal. You have to steer carefully (and if you don’t, please don’t drive near me), and so does Dawn. As the ship revolves around Ceres, it must constantly change the pointing of the blue-green beam of high velocity xenon ions to stay on precisely the desired winding route to the targeted orbit. The mission control team at JPL will program the ship to orient its thruster in just the right direction at the right time to propel itself on the intended spiraling course.
Aiming a thruster in the direction needed to spiral around Ceres requires turning the entire spacecraft. Each thruster is mounted on its own gimbal with a limited range of motion. In normal operation, the gimbal is positioned so that the line of thrust goes through the center of the ship. When the gimbal is swiveled to another direction, the gentle force from the ion engine causes the ship to rotate slowly. This is similar to the use of an outboard motor on a boat. When it is aligned with the centerline of the boat, the craft travels straight ahead. When the motor is turned, it continues to propel the boat but also turns it. In essence, Dawn’s steering of its thrust is accomplished by pivoting the ion engine.
A crucial difference between the boat and our interplanetary ship is that with the former, the farther the motor is turned, the tighter the curving course. (Another difference is that the spacecraft wouldn’t float.) Dawn doesn’t have that liberty. For our craft, the gimballing of the thruster needs to be carefully coordinated with the orbital motion, as if the motorboat operator needed to compensate for a curving current. This has important implications at Ceres. Sophisticated as it is, Dawn knows its own location in orbit only by virtue of information mission controllers install onboard to predict where it will be at any time. That is based on their best computations of Ceres’ gravity, the planned operation of the ion propulsion system, and many other considerations, but it will never be perfectly accurate. Let’s take a look at two of the reasons.
Ceres, like Vesta, Earth, the moon, Mars, and other planets or planetary-type bodies, has a complex gravity field. The distribution of materials of different densities within the interior creates variations in the strength of the gravitational force, so Dawn will feel a slightly changing tug from Ceres as it travels in orbit. But there is a noteworthy difference between Ceres’ gravity field and the fields of those other worlds: Ceres’ field is unknown. We will have to measure it as we go. The subtle irregularities in gravity as Dawn descends will cause small deflections from the planned trajectory. Our ship will be traversing unknown, choppy waters.
Other phenomena will lead to slight discrepancies as well. The ion propulsion system will be responsible for changing the orbit, so even tiny deviations from the intended thrust eventually may build up to have a significant effect. This is no different from any realistic electrical or mechanical system, which is sure to have imperfections. If you planned a trip in which you would drive 60.0 miles (96.6 kilometers) at 60.0 mph (96.6 kilometers per hour), you could expect to arrive in exactly 60.0 minutes. (No surprises there, as it isn’t exactly rocket science.) But even if you maintained the speedometer as close to 60 as you could, it would not be accurate enough to indicate the true speed. If your actual speed averaged 60.4 mph (97.2 kilometers per hour), you would arrive 24 seconds early. Perhaps that difference wouldn’t matter to you (and if it did, you might consider replacing your car with a spaceship), but such minuscule errors, when compounded by Dawn’s repeated spirals around Ceres, would make a difference in achieving its carefully chosen orbit.
As a result of these and other effects, mission controllers will need to adjust the complex flight plan as Dawn travels from one observational orbit to another. So it will thrust for a few days and then stop to allow navigators to get a new fix on its position. When it points its main antenna to Earth, the Doppler shift of its radio signal will reveal its speed, and the time for radio signals (traveling, as all readers know so well, at the universal limit of the speed of light) to make the round trip will yield its distance. Combining those measurements with other data, mission controllers will update the plan for where to point the thruster at each instant during the next phase of the spiral, check it, double check it, and transmit it to the faraway robot, which will then put it into action. This intensive process will be repeated every few days as Dawn maneuvers to lower orbits.
The flight team succeeded brilliantly in performing this kind of work at Vesta, but they will encounter some differences at Ceres. Sunlight is even weaker in that remote part of the asteroid belt. The giant solar arrays will generate less electrical power for the ion propulsion system, so the whisper-like thrust will be even fainter. In addition, Ceres is more massive than Vesta, so its gravitational hold is stronger. Of course, the team has developed plans to account for these and other differences as they guide Dawn from one orbit to another.
The reward for these particularly challenging parts of the mission will be new perspectives on Ceres. The distant landscapes, barely even hinted at by observations for more than two centuries, will come into sharper and sharper focus as Dawn spirals closer. At each new orbital perch, the explorer will reveal exciting new details, allowing new discoveries and new insights. Everyone who is curious about the cosmos is welcome to join the journey as human ingenuity and curiosity take us far, far from home to an uncharted world.
Dawn is 9.2 million miles (15 million kilometers) from Ceres. It is also 1.61 AU (149 million miles, or 241 million kilometers) from Earth, or 620 times as far as the moon and 1.60 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 27 minutes to make the round trip.
Dr. Marc D. Rayman
4:00 p.m. PDT April 30, 2014
P.S. This is the 100th Dawn Journal, so this seems like a good time to end. This will be the last one.
P.P.S. Until next month.
Powering its way through deep space, Dawn draws ever closer to dwarf planet Ceres. To reach its destination, the interplanetary spaceship gently reshapes its path around the sun with its extraordinary ion propulsion system. In about a year, the spacecraft will gracefully slip into orbit so it can begin to unveil the nature of the mysterious world of rock and ice, an intriguing protoplanetary remnant from the dawn of the solar system.
Even as Dawn ascends the solar system hill, climbing farther and farther from the sun, penetrating deeper into the main asteroid belt between Mars and Jupiter, its distance to Earth is shrinking. This behavior may be perplexing for readers with a geocentric bias, but to understand it, we can take a broader perspective.
The sun is the conductor of the solar system symphony. Its gravity dictates the movements of everything that orbits it: Earth as well as the other planets, Vesta, Ceres, and myriad smaller objects, including asteroids and Dawn. (Actually, the gravity of every single body affects how all of the others move, but with more than 99 percent of the entire solar system's mass concentrated in the gargantuan sun, it dominates the gravitational landscape.)
Whether it is for a planet or Dawn orbiting the sun, a spacecraft or moon orbiting a planet, the sun or other stars orbiting the Milky Way (the Milky Way galaxy, that is, not your correspondent’s cat Milky Way), or the Milky Way galaxy orbiting the Virgo supercluster of galaxies (home to an appreciable fraction of our readership), any orbit is the perfect balance between the inward tug of gravity and the inexorable tendency of objects to travel in a straight line. If you attach a weight to a string and swing it around in a circle, the force you use to pull on the string mimics the gravitational force the sun exerts on the bodies that orbit it. The effort you expend in keeping the weight circling serves constantly to redirect its course, forcing it to curve; if you release the string, the weight’s natural motion would take it away in a straight line (we are ignoring here the effect of Earth’s gravity on the weight).
The force of gravity dwindles as the distance increases, so the sun pulls harder on a nearby body than on a farther one. Therefore, to remain in orbit, to balance the relentless gravitational lure, the closer object must travel at higher speed, resisting the stronger attraction. The same effect applies at Earth. Satellites that orbit very close (including, for example, the International Space Station, 250 miles, or 400 kilometers, above the surface) must streak around the planet at about 17,000 mph (7.6 kilometers per second) to avoid being drawn down. The moon, orbiting almost a thousand times farther above, needs only to travel at less than 2300 mph (about 1.0 kilometers per second) to balance Earth’s weaker hold at its remote location.
For that reason, Mercury zips around the sun faster than any of the other planets. Mars travels more slowly than Earth, and the still more distant residents of the asteroid belt, whether natural (all of them but one) or a product of human ingenuity (one: Dawn), proceed at an even more leisurely pace. As Earth makes its relatively rapid annual trip around the sun, the distance to the spacecraft that left it behind in 2007 alternately shrinks and grows.
We can visualize this with one of the popular models of clocks available in the Dawn gift shop on your planet, in which the hour hand is longer than the minute hand. Imagine the sun as being at the center of the clock. The tip of the short minute hand represents Earth, and the end of the hour hand represents Dawn. Some of the time (such as between noon and shortly after 12:30), the distance between the ends of the hands increases. Then the situation reverses as the faster minute hand begins moving closer and closer to the hour hand as the time approaches about 1:05.
Earth and Dawn are exhibiting the same repetitive behavior. Of course, their relative motion is more complicated than that of the clock hands, because Dawn’s ion thrusting is constantly changing its solar orbit (and so the distance and speed at which it loops around the sun), but the principle is the same. They have been drawing closer since August 2013. Earth, coming from behind, is now about to pass Dawn and move ahead. The stalwart probe will not even take note however, as its sights remain firmly set on an unexplored alien world.
On April 10, the separation will be 1.56 AU (1.56 times the average distance between Earth and the sun, which means 145 million miles, or 233 million kilometers), an almost inconceivably large distance (well in excess of half a million times farther than the International Space Station, which orbits Earth, not the sun) but less than it has been since September 2011. (The skeptical reader may verify this by reviewing the concluding paragraph of each log in the intervening months.) Enjoy the upcoming propinquity while you can! As the ship sails outward from the sun toward Ceres, it will never again be this close to its planet of origin. The next time Earth, taking an inside track, overtakes it, in July 2015 (by which time Dawn will be orbiting Ceres), they will only come within 1.94 AU (180 million miles, or 290 million kilometers) of each other.
By the way, Vesta, the endlessly fascinating protoplanet Dawn unveiled in 2011-2012, will be at its smallest separation from Earth of 1.23 AU (114 million miles, or 183 million km) on April 18. Ceres, still awaiting a visitor from Earth, despite having first been glimpsed from there in 1801, will attain its minimum distance on April 15, when it will be 1.64 AU (153 million miles, or 246 million km) away. It should not be a surprise that Dawn’s distance is intermediate; it is between them as it journeys from one to the other.
Not only is each one nearly at its shortest geocentric range, but from Earth’s point of view, they all appear to be near each other in the constellation Virgo. In fact, they also look close to Mars, so you can locate these exotic worlds (and even the undetectably small spacecraft) in the evening sky by using the salient red planet as a signpost. In July, the coincidental celestial alignment will make Vesta and Ceres appear to be separated by only one third the diameter of the full Moon, although these behemoths of the asteroid belt will be 0.57 AU (52 million miles, or 85 million kilometers) from each other.
We mentioned above that by constantly modifying its orbit under the persistent pressure of its ion engine, Dawn complicates the simple clock-like behavior of its motion relative to Earth. On Halloween 2012, we were treated to the startling fact that to rendezvous with Ceres, at a greater distance from the sun, Dawn had to come in toward the sun for a portion of its journey; quite a trick! In that memorable log (which is here, for those readers who didn't find every detail to be so memorable), we observed that it would not be until May 2014 that Dawn would be as far from the sun as it was on Nov. 1, 2012. Sure enough, having faithfully performed all of the complex and intricate choreography since then, it will fly to more than 2.57 AU from the solar system’s star in May, and it will continue heading outward.
With the sun behind it and without regard to where Earth or most other residents of the solar system are in their orbits, Dawn rises to ever greater heights on its extraordinary expedition. Distant though it is, the celestial ambassador is propelled by the burning passion for knowledge, the powerful yearning to reach beyond the horizon, and the noble spirit of adventure of the inhabitants of faraway Earth. The journey ahead presents many unknowns, promising both great challenges and great rewards. That, after all, is the reason for undertaking it, for such voyages enrich the lives of all who share in the grand quest to understand more about the cosmos and our humble place in it.
Dawn is 11 million miles (18 million kilometers) from Ceres. It is also 1.57 AU (146 million miles, or 235 million kilometers) from Earth, or 625 times as far as the moon and 1.57 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 26 minutes to make the round trip.
Dr. Marc D. Rayman
4:00 p.m. PDT March 31, 2014
Dear Ardawnt Readers,
Continuing its daring mission to explore some of the last uncharted worlds in the inner solar system, Dawn remains on course and on schedule for its rendezvous with dwarf planet Ceres next year. Silently and patiently streaking through the main asteroid belt between Mars and Jupiter, the ardent adventurer is gradually reshaping its orbit around the Sun with its uniquely efficient ion propulsion system. Vesta, the giant protoplanet it unveiled during its spectacular expedition there in 2011-2012, grows ever more distant.
In December, and January, we saw Dawn's plans for the "approach phase" to Ceres and how it will slip gracefully into orbit under the gentle control of its ion engine. Entering orbit, gratifying and historic though it will be, is only a means to an end. The reason for orbiting its destinations is to have all the time needed to use its suite of sophisticated sensors to scrutinize these alien worlds.
As at Vesta, Dawn will take advantage of the extraordinary capability of its ion propulsion system to maneuver extensively in orbit at Ceres. During the course of its long mission there, it will fly to four successively lower orbital altitudes, each chosen to optimize certain investigations. (The probe occupied six different orbits at Vesta, where two of them followed the lowest altitude. As the spacecraft will not leave Ceres, there is no value in ascending from its fourth and lowest orbit.) All of the plans for exploring Ceres have been developed to discover as much as possible about this mysterious dwarf planet while husbanding the precious hydrazine propellant, ensuring that Dawn will complete its ambitious mission there regardless of the health of its reaction wheels.
All of its orbits at Ceres will be circular and polar, meaning the spacecraft will pass over the north pole and the south pole, so all latitudes will come within view. Thanks to Ceres's own rotation, all longitudes will be presented to the orbiting observer. To visualize this, think of (or even look at) a common globe of Earth. A ring encircling it represents Dawn's orbital path. If the ring is only over the equator, the spacecraft cannot attain good views of the high northern and southern latitudes. If, instead, the ring goes over both poles, then the combined motion of the globe spinning on its axis and the craft moving along the ring provides an opportunity for complete coverage.
Dawn will orbit in the same direction it did at Vesta, traveling from north to south over the side illuminated by the distant Sun. After flying over the south pole, it will head north, the surface directly beneath it in the dark of night. When it travels over the north pole, the terrain below will come into sunlight and the ship will sail south again.
Dawn's first orbital phase is distinguished not only by providing the first opportunity to conduct intensive observations of Ceres but also by having the least appealing name of any of the Ceres phases. It is known as RC3, or the third "rotation characterization" of the Ceres mission. (RC1 and RC2 will occur during the approach phase, as described in December.)
During RC3 in April 2015, Dawn will have its first opportunity for a global characterization of its new residence in the asteroid belt. It will take pictures and record visible and infrared spectra of the surface, which will help scientists determine its composition. In addition to learning about the appearance and makeup of Ceres, these observations will allow scientists to establish exactly where Ceres's pole points. The axis Earth rotates around, for example, happens to point very near a star that has been correspondingly named Polaris, or the North Star. [Note to editors of local editions: You may change the preceding sentence to describe wherever the axis of your planet points.] We know only roughly where Ceres's pole is from our telescopic studies, but Dawn's measurements in RC3 will yield a much more accurate result. Also, as the spacecraft circles in Ceres's gravitational hold, navigators will measure the strength of the gravitational pull and hence its overall mass.
RC3 will be at an orbital altitude of about 8,400 miles (13,500 kilometers). From there, the dwarf planet will appear eight times larger than the moon as viewed from Earth, or about the size of a soccer ball seen from 10 feet (3.1 meters). At that distance, Dawn will be able to capture the entire disk of Ceres in its pictures. The explorer's camera, designed for mapping unfamiliar extraterrestrial landscapes from orbit, will see details more than 20 times finer than we have now from the Hubble Space Telescope.
Although all instruments will be operated in RC3, the gamma-ray and neutron detector (GRaND) will not be able to detect the faint nuclear emissions from Ceres when it is this far away. Rather, it will measure cosmic radiation. In August we will learn more about how GRaND will measure Ceres's atomic composition when it is closer.
It will take about 15 days to complete a single orbital revolution at this altitude. Meanwhile, Ceres turns on its axis in just over nine hours (more than two and a half times faster than Earth). Dawn's leisurely pace compared to the spinning world beneath it presents a very convenient way to map it. It is almost as if the probe hovers in place, progressing only through a short arc of its orbit as Ceres pirouettes helpfully before it.
When Dawn is on the lit side of Ceres over a latitude of about 43 degrees north, it will point its scientific instruments at the unfamiliar, exotic surface. As Ceres completes one full rotation, the robot will fill its data buffers with as much as they can hold, storing images and spectra. By then, most of the northern hemisphere will have presented itself, and Dawn will have traveled to about 34 degrees north latitude. The spacecraft will then aim its main antenna to Earth and beam its prized findings back for all those who long to know more about the mysteries of the solar system. When Dawn is between 3 degrees north and 6 degrees south latitude, it will perform the same routine, acquiring more photos and spectra as Ceres turns to reveal its equatorial regions. To gain a thorough view of the southern latitudes, it will follow the same strategy as it orbits from 34 degrees south to 43 degrees south.
When Dawn goes over to the dark side, it will still have important measurements to make (as long as Darth Vader does not interfere). While the surface immediately beneath it will be in darkness, part of the limb will be illuminated, displaying a lovely crescent against the blackness of space. Both in the southern hemisphere and in the northern, the spacecraft will collect more pictures and spectra from this unique perspective. Dawn's orbital dance has been carefully choreographed to ensure the sensitive instruments are not pointed too close to the Sun.
Although it is not the primary objective of the measurements, team members are working to determine whether observations from the vantage point of the night side of RC3 might shed more light on the recent fascinating detection of water vapor around Ceres by the Herschel Space Observatory. Whether the water is lofted into space by ice sublimating on the surface or by geysers or cryovolcanoes (“cold volcanoes,” which may be active on this small, frigid world of rock and ice far from the sun) is not yet known. Scientists do not even know whether any water vapor will still be there when Dawn is. Even if it is not, it may be that signs of water will be evident on the surface from other measurements. We will discuss this intriguing possibility more in the December 2014 log.
Dawn’s controllers will take advantage of the flexibility afforded by ion propulsion to guide the spacecraft into whatever part of the RC3 orbit turns out to be most efficient, based on details of the trajectory as it closes in on Ceres. So, for example, if it spirals down to RC3 over the unlit side, its observations of the day hemisphere will first be in the north, then the equator, then the south. But if it arrives in RC3 over the low northern latitudes on the side lit by the sun, it will begin its observations over the equator and then continue in the south. After it flies north over the other side and then returns to the half of Ceres that is in daylight, it will be ready to conclude RC3 by collecting its northern hemisphere data. The flight team has formulated the plan so that the activities can be executed in whatever order is most natural. The schedule will be finalized during the approach phase, and readers may rest assured that the answer will be presented in these logs.
If all goes according to plan, which is never assured when undertaking challenging tasks in a forbidding, distant, alien environment that has never even been visited by a flyby spacecraft for an initial reconnaissance, Dawn will collect in excess of 1,000 pictures and several million spectra in RC3. After that rich bounty is securely on Earth, it will resume ion thrusting to lower its altitude to the next orbit. We will discuss the spiral descent in April and that second observation phase in May.
Dawn’s first inspection of Ceres in RC3 promises both to provide tremendous advancements in our knowledge and whet our appetites for its subsequent examinations. The most massive resident of the main asteroid belt was also the first one to be discovered. Yet for the more than two centuries since then, our glimpses from afar have shown little more than a fuzzy round dot. That distant orb, shining among the stars, has intrigued us for so long. When finally its invitation for an ambassador from Earth is answered next year, the secrets it has held since the dawn of the solar system will begin to be revealed. The rewards for the long and challenging journey will be new insights, new understanding, and new fuel for the fires that burn within everyone who feels the passion to explore.
Dawn is 14 million miles (22 million kilometers) from Ceres. It is also 1.76 AU (163 million miles, or 263 million kilometers) from Earth, or 725 times as far as the moon and 1.77 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 29 minutes to make the round trip.
Dr. Marc D. Rayman
3:00 p.m. PST February 28, 2014