This orrery was built for NASA/JPL by Pre-Mec Engineering, Inc. and was designed by JPL engineer Raymond A. McCreary (Design Section, 356 – part of the Engineering Mechanics Division).
The scale of Earth and its moon was approximately 1 cm = 6000 km, but the scale of orbits, the Sun, and other moons varied.
Computer animations did not exist in the early 1960s, and like a trajectory model, this orrery helped engineers plan, visualize, and demonstrate the expected flight path, flyby, or landing to be made by a spacecraft. Missions in development at this time were Ranger and Surveyor (lunar missions), Mariner 2 to Venus, and Mariner 4 to Mars.
For more information about the history of JPL, contact the JPL Archives for assistance. [Archival and other sources: Section 321 photo album and index, and JPL/Caltech phone directories
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.
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.
“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’.
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
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. 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
This artist's conception of the Magellan spacecraft was created in about 1983, when it was known as Venus Radar Mapper (VRM). This kind of artwork was usually based on reports and drawings provided to the artist by the project staff. By the time Magellan was launched in May 1989 aboard the space shuttle Atlantis, the configuration had changed. It was not an uncommon occurrence for the design of a spacecraft to evolve over a period of months or years, based on input from the various instrument teams and engineers working on the project. It also happened when projects encountered funding problems and were scaled down in order to meet a budget.
One 1984 VRM project document explained, "The details of the configuration of the VRM spacecraft are changing continually as the spacecraft design matures. This illustration [a line drawing that matches the configuration shown in this artwork] shows the general configuration of the VRM spacecraft .... However several details of this illustration are out of date (such as the FEM length, altimeter antenna design and placement, and the amount of STAR-48 support structure retained after VOl)." Other, less detailed drawings were quickly added to the report to show the recent updates.