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!
The only spaceship ever built to orbit two extraterrestrial destinations, Dawn’s advanced ion propulsion system enables its ambitious mission. Providing the 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.
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.
Several spacecraft were built for the Mariner Mars 1964 mission. The ones that were actually launched were referred to as Mariner C-2 and Mariner C-3 until they were renamed Mariner 3 and Mariner 4, respectively. There was also a Proof Test Model (PTM, or Mariner C-1) and a Structural Test Model (STM). This photo shows Mariner C-2 configured for system tests in May 1964. It appears to be in the Spacecraft Assembly Facility, with the observation area at the top of the photo.
Mariner 3 was launched November 5, 1964, but the shroud did not fully eject from the spacecraft, the solar panels did not deploy, and the batteries ran out of power. The problem was fixed on Mariner 4, which began its successful journey to Mars on November 28, 1964.
Documentation found in the Archives does not identify the purpose of the sphere covering the magnetometer during this test.This post was written for “Historical Photo of the Month,” a blog by Julie Cooper of JPL’s Library and Archives Group.
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 here)
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!
“Most space projects live nine lives on the test bench before they are allowed one life in flight.”* The Mariner Mars mission was on a tight schedule in 1964, so testing was not quite as extensive as it was for other missions. A full-size temperature-control model and a proof-test model went through a series of environmental and vibration tests in the 25-foot space simulator at NASA’s Jet Propulsion Laboratory and other test facilities. This photo was taken in June 1964, outside of the Spacecraft Assembly Facility at JPL. In this unusual outdoor setting, the solar panel test took place in a large plastic tent.
After testing was completed, two spacecraft and a spare (the proof-test model) were partly disassembled, carefully packed and loaded on moving vans for a trip to the Air Force Eastern Test Range in Cape Kennedy, Florida. They were inspected, reassembled, and tested again before launch.
*To Mars: the Odyssey of Mariner IV, TM33-229, 1965.
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’.
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.
Before there was email, the JPL intranet, or streaming video to keep employees informed, Dr. Al Hibbs hosted a bi-weekly internal TV show to provide mission and technology updates, and discuss how current events affected JPL and NASA. It was shown on closed circuit televisions in the two cafeterias during breaks and lunch. At the time, the most common way of reaching all employees was to distribute hard copies of Universe, This Week, Director’s Letters, project status reports, and flyers.
Hibbs had worked at JPL since 1950 and was well known as the “Voice of JPL,” using his knowledge of engineering and science to explain complex concepts to the public during many of JPL’s planetary missions. In this 1980 photo, Hibbs (at left) talks to Rep. Don Fuqua of Florida, a member of the House of Representatives Science and Technology Committee.
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
Several different full-size and scale models were made of the Ranger spacecraft (Block I, II, and III configurations). Scale models were used by the projects at NASA's Jet Propulsion Laboratory at a time when there was no computer animation. Engineers and scientists used them to visualize the spacecraft and its orientation as it reached the moon or a planet.
Three members of the Ranger 7 television experiment team stand near a scale model and lunar globe. From left: Ewen Whitaker, Dr. Gerard Kuiper, and Ray Heacock. Kuiper was the director of the Lunar and Planetary Laboratory (LPL) at the University of Arizona. Whitaker was a research associate at LPL. Heacock was the Lunar and Planetary Instruments section chief at JPL.
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.