There is only about three quarters of a revolution remaining around the Sun before Dawn leaves Earth to travel on its own to distant worlds. Meanwhile, the project team continues to prepare the spacecraft for its mission. This work has proceeded smoothly despite the chaos of planets apparently coming and going from our solar system.
As readers in other solar systems have no doubt followed with some detached amusement, the definition of “planet” was in the news in this solar system this summer. While much of the focus was on whether Pluto should be considered a planet, Dawn’s second destination, Ceres, also was subjected to this linguistic turmoil. The International Astronomical Union (IAU) adopted a definition of “dwarf planet” that includes Ceres, Pluto, Eris, and perhaps more bodies yet to be characterized sufficiently or even discovered. Ceres is the largest member of the asteroid belt, residing between Mars and Jupiter; the other dwarf planets are part of the Kuiper belt, spending most or all of their time beyond the most distant planet, Neptune.
Resolution 5A passed by the 26th General Assembly of the IAU describes the attributes a body must have to qualify as a dwarf planet. Like a planet, it must orbit the Sun and have sufficient mass for its own gravity to make it nearly spherical. (Vesta, the first stop on Dawn’s interplanetary itinerary, might satisfy the definition of dwarf planet, but not enough is known yet about its gravity and shape.) Unlike planets however, dwarf planets are characterized by not having cleared away other objects from their part of the solar system through the effects of their gravity. This bars any resident of the asteroid belt or the Kuiper belt from membership in the planet club. (Another criterion, that the body not be a satellite, excludes some of the moons of planets from being designated as planets themselves.)
The definition is not widely accepted by the community of planetary scientists, and it remains to be seen how the definition might be changed. Ceres and Vesta were considered planets for half a century following their discoveries in 1801 and 1807 respectively. All scientific evidence indicates that with all the names humans have applied to them, including planets, asteroids, minor planets, protoplanets, and dwarf planets, they have steadfastly remained above the controversy, leading their stately lives without apparent interest.
The Dawn team has never wavered about what to call these bodies; with the utmost clarity and consistency, they have always been known as “Ceres” and “Vesta.” Team members continue to look forward to the wealth of information the spacecraft will return from its orbits around these fascinating places. In continuing to prepare for that, engineers are completing another set of the comprehensive performance tests (as explained in previous logs) to verify that the subsystems on the spacecraft can fulfill the required functions.
Loyal readers will come to be familiar with Dawn’s subsystems as we take it through the rest of its prelaunch preparations and we join it, in spirit if not in person, on its cosmic travels. [Editor’s note: “Loyal readers” is redundant; our recent surveys show 100% of readers in the targeted galaxies are loyal.] As we shall see over the coming years, there is nothing like guiding a spacecraft through the forbidding depths of space to understand how it really works. But now let us have a very very brief introduction to the engineering subsystems that allow Dawn to conduct its mission. In a future log, we will describe the scientific instruments, which will help reveal the natures of Ceres and Vesta.
The command and data handling subsystem includes the main computers that operate the probe along with most of the other electronics. As with most Dawn subsystems, the design includes primary and backup components so that even if a failure occurs far from Earth, the spacecraft can continue to fulfill its scientific mission. This subsystem keeps the spacecraft functioning smoothly as it operates on its own in space. Running in its three primary computers is the master software for the spacecraft, consisting of more than 400,000 lines of C and assembly code. In addition to its own orchestrations of spacecraft activities, it processes commands sent by the mission operations team and issues them when required to other subsystems. It stores the scientific data acquired by the instruments and collects information on the performance of the spacecraft, all to be reported back to Earth. Some engineers would consider this to be the most important subsystem on the spacecraft.
The electrical power subsystem (OK, I know you’re ahead of me on this one) provides the power needed by all electrical components onboard. Its solar arrays convert light from the Sun into electricity, and the subsystem delivers high voltage to the ion propulsion subsystem and lower voltage to all the other subsystems. Because Dawn will need high electrical power for its ion propulsion subsystem even when far from the Sun, the solar arrays are very large for a planetary spacecraft. Each of the two solar array wings is almost 8.3 meters (more than 27 feet) long, and when they are extended shortly after launch, the overall craft will be about 19.7 meters (nearly 65 feet) from wing tip to wing tip. This subsystem includes a powerful battery whose primary purpose is to allow Dawn to operate while on the rocket and during the time immediately after separation when it needs to perform a number of critical functions to deploy its arrays and point them at the Sun. The arrays will generate more than 10 kilowatts at Earth’s distance from the Sun (enough to power 10 average households in the US). This is far more power than Dawn can use, but when it has receded to 3 times Earth’s distance from the Sun, every watt it can yield will be of great value to the spacecraft, with its power-hungry ion propulsion subsystem. Some engineers would consider this to be the most important subsystem on the spacecraft.
The attitude control subsystem (despite the name, this subsystem is as delightful to work with and is as enthusiastic about the mission as all other subsystems) is responsible for controlling the orientation (which engineers refer to as “attitude”) of the craft in the zero-gravity of spaceflight. This subsystem can orient the probe so that it points an ion thruster in the direction required to reach its cosmic destinations, directs an antenna to distant Earth, or aims the camera or other instruments so they may observe their targets. It also will keep the solar arrays pointed at the Sun. To determine its attitude, Dawn uses “star trackers” (again, two are onboard, although only one is needed), cameras that recognize star patterns and thereby reveal the direction they are pointed. (For readers who accompanied Deep Space 1 on its voyage, it was the failure of the sole star tracker during the extended mission that led to the need to conduct the spectacular rescue of the spacecraft. That is described in the logs of 2000, available at http://nmp.jpl.nasa.gov/ds1/archives.html and in late night reruns on most planets not in synchronous rotation around their stars.) The subsystem also carries gyroscopes to improve the accuracy of the pointing. For emergency use, Sun sensors can help the spacecraft establish its approximate attitude when a star tracker is temporarily off-line. Devices known as reaction wheels are electrically spun faster or slower to rotate the spacecraft. Some engineers would consider this to be the most important subsystem on the spacecraft.
For technical reasons, the reaction wheels are not sufficient for all the pointing control Dawn will need during its long mission, so another means is required. In addition to the reaction wheels, which are considered part of the attitude control subsystem, there are two other subsystems that attitude control uses to achieve the orientations it needs. The reaction control subsystem includes 12 small thrusters that use a conventional rocket propellant known as hydrazine; you may not be surprised to know that only 6 thrusters are needed, so even if an entire group of 6 failed, the mission would not be lost. Each brief pulse of a thruster causes the spacecraft to change how fast or in what direction it rotates. This subsystem will be loaded with about 45 kilograms (100 pounds) of hydrazine, although it likely will use much less than that during the mission. Some engineers would consider this to be the most important subsystem on the spacecraft.
Most interplanetary spacecraft use hydrazine-based propulsion not only to turn but also to change their trajectories through space. Dawn is able to undertake its detailed exploration of the most massive bodies in the asteroid belt because it uses a more capable form of propulsion. The ion propulsion subsystem accomplishes this by ionizing xenon gas; that is, it gives it a small positive electrical charge by removing a negatively charged electron from each neutral xenon atom. Once the xenon is ionized, the subsystem can electrically accelerate the ions and emit them at very high speed from any 1 of the 3 ion thrusters. The action of each xenon ion as it is shot from a thruster at up to 40 kilometers per second (89,000 miles per hour) causes a reaction that pushes the spacecraft in the other direction. Dawn will launch with 425 kilograms (937 pounds) of xenon -- more than enough to allow it to travel to and orbit its targets while setting some remarkable records to be described in future logs. Because ion propulsion is so different from conventional propulsion systems, it leads to many differences in the way we design and conduct the mission, and later logs will describe this in more detail (once our attorneys prove their case that the copyright infringement claims by the self-proclaimed Ionic Potentate of Xenon are invalid). In addition to its role in propelling Dawn to Vesta and Ceres, in some cases the ion propulsion subsystem (instead of the reaction wheels or the reaction control subsystem) is used by attitude control to help control the direction the spacecraft points. While this subsystem obviously is important, some engineers would consider the next one to be the most important on the spacecraft.
The thermal control subsystem keeps all of Dawn’s subsystems operating within their required temperature ranges as the craft travels from Earth past Mars to Vesta and then continues on to Ceres, reaching 3 times Earth’s distance from the Sun. The temperatures of delicate electronics, precisely aligned structural elements, sensitive mechanical devices and materials, lubricants, adhesives, hydrazine, xenon, and more all must be controlled. This subsystem must ensure that units stay cool even when they experience direct exposure to the searing Sun while being warmed still more by their own electrical activity and stay warm even when they face the paralyzing cold of darkest space. Louvers on some parts of the spacecraft open or close in response to temperature to let heat radiate away or be trapped on the spacecraft as necessary. Some of the spacecraft panels are embedded with tubes of ammonia to help distribute the heat more uniformly, carrying excess heat from electrically powered devices to others that are powered off or otherwise in need of additional heat. The subsystem also includes more than 140 heaters and is one of the largest consumers of electrical power on the spacecraft. While this subsystem obviously is important, some engineers would consider the ion propulsion subsystem to be the most important on the spacecraft.
The telecommunications subsystem allows Dawn to exchange information with Earth, even at enormous distances. The spacecraft’s main antenna is 1.52 meter (5 feet) in diameter, and 3 smaller antennas allow communications when it is not possible or not convenient to point the large dish at Earth. Dawn will communicate with mission controllers through the 34-meter (112-foot) or 70-meter (230-foot) antennas of NASA’s Deep Space Network (DSN) in California, Spain, and eastern Australia. While Dawn is returning scientific data from Ceres at maximum range, the 100-watt radio signal it transmits, after traversing the vast distance to Earth, will be less than one tenth of one millionth of one billionth of a watt when it is received by a 34-meter antenna. If this energy were collected for the age of the universe, it would be enough to illuminate a refrigerator light bulb for 1 second, yet it is sufficient to carry all the images and other rich scientific data to Earth. Dawn’s receiver, always alert for faint whispers from home, can make sense of a signal weaker than one billionth of one billionth of a watt. Some engineers would consider this to be -- well, you get the message.
After this brief overview of the subsystems, it would be easy to lose sight of what some engineers would consider to be more important than any subsystem: the system. All subsystems have to work together for the spacecraft work. Besides the instruments, some essential parts of that spacecraft are missed in this description of active subsystems, such as the structure upon which everything is built. In addition, to connect the many elements of the subsystems to each other, Dawn includes 9000 wires with a total length of about 25 kilometers (15 miles). The cables and their connectors account for more than 83 kilograms (183 pounds) of the mass that will travel to Vesta and Ceres. When fully assembled and loaded with its propellants, Dawn will be somewhat more than 1200 kilograms (2650 pounds).
Some engineers would consider there to be a larger system, still more important than the entirety of the spacecraft, that is needed to make Dawn a success. Indeed, the full system is not only what flies in space; the complete Dawn system has many elements that remain on Earth, including networks of computers, extensive software, antennas, transmitters, receivers, and a team of dedicated and inquisitive people who recognize their good fortune to participate in this grand adventure.
Now strange as it may seem, there seems to be some evidence that 2 of our readers, despite being loyal, have not yet submitted their names to be carried on the spacecraft. The end of the last log described our plans to include the names of all members of what really is the largest and most important system: the people whose spirits are carried aloft by humankind’s efforts to know the cosmos. Don’t be the last one to add your name to the spacecraft at http://www.dawn-mission.org/DawnCommunity/Sendname2asteroid/nameEntry.asp.
Dr. Marc D. Rayman
September 17, 2006
Dawn continues to keep its human handlers very busy as preparations continue on schedule to meet the planned opening of the launch period on June 20, 2007.
Much of June 2006 was devoted to conducting the comprehensive performance tests (CPTs) described in the previous log. In these tests, most of the hardware and software subsystems already on the spacecraft were exercised to help uncover problems ranging from incomplete solder connections on a microchip or a broken wire, to software bugs, to unexpected interactions between subsystems that must work together. Of course, each subsystem was tested extensively as it was being built, but some afflictions may remain hidden until the subsystem is operating on the spacecraft with other subsystems. As thorough as the CPTs are, testing will become more and more demanding over the coming months as the spacecraft is asked to perform in ways progressively more like what it will encounter during its voyage to the asteroid belt and its explorations of Ceres and Vesta, the most massive bodies in that region of the solar system. The upcoming tests will be described in future logs.
The CPTs yield a tremendous volume of data, and engineers are still analyzing the details of their subsystems' performance, but all indications are that the tests went extremely well. As our humble human readers know, some errors are inevitable in a design as intricate and complex as one of Earth's interplanetary spacecraft. So far it appears that all such flaws are easily correctable.
Some subsystems have not yet had their first CPT. The ion propulsion subsystem and the instruments for collecting scientific data are awaiting their tests in August. We will introduce each of Dawn's subsystems in the next log.
Some of the ion propulsion subsystem's individual components received some extra testing recently before being mated to the spacecraft in June. The power processing units have to provide the properly controlled voltages and currents to different elements of the ion thrusters, which apply the electrical power to xenon gas to produce fantastically efficient propulsion, without which Dawn's ambitious mission would be quite impossible. Each unit will process up to 2500 watts (much more than the average house consumes), and we wanted special assurance that these devices would perform reliably on the mission. So in addition to the testing they received at the company that manufactured them for Dawn, each one was subjected to further trials in one of JPL's laboratories. The devices were operated for about 20 days in vacuum chambers. During most of that time, the units were pushed to the highest temperature they will experience on the spacecraft of 35°C (95°F). Both units passed with flying colors (what other kind of colors would you expect for space hardware?), adding to the confidence that they are up to the rigors of Dawn's mission.
While operating in their thermal vacuum chambers, the power processing units were under the control of the same software that runs in the ion propulsion control unit on the spacecraft. So this work provided a bonus opportunity to test the software that operates this complex subsystem on its travels through deep space.
Each of the three ion thrusters will be mounted on a mechanism that allows its pointing direction to be fine-tuned by other software on the spacecraft. As we will see in the next log, this accurate aiming is essential, so if one of these mechanisms fails, the attached thruster would be useless. To verify the robustness of the design for the mechanism, a test unit was subjected to 10 times the amount of work the ones to be flown on Dawn will have to provide. The performance was flawless.
After the ion propulsion subsystem testing and the first set of CPTs were completed in June, the focus of the Dawn team's activity in July was on what nontechnical readers might think of as baking the spacecraft. The technical term used by the engineering team was -- well, baking out the spacecraft. This was not a test; rather, bake-out was intended to heat the spacecraft to drive off contaminants it might have collected, despite the assembly having been conducted in a specially controlled "clean room" at Orbital Sciences Corporation.
The first step in the bake-out was to clean the oven in which the baking would occur, a cylinder 3.7 meters high and 4.9 meters long. (Ever poetic, team members fondly refer to this facility as the "12 by 16 foot chamber.") For several days, the chamber was heated to 95°C (203°F). This ensured that its interior would be free of chemical residue that might contaminate Dawn.
With the chamber certified to be clean, the spacecraft was moved in. As it is not completely assembled yet, some of the flight equipment was simply placed in the chamber with the spacecraft. After the chamber was sealed, it took 7 hours to reduce the pressure to about 100 million times lower than atmospheric pressure. Then the temperature was raised gradually over more than a day and a half to bring the spacecraft to a toasty 53°C (127°F). As with most other details of the design, assembly, test, and operations in flight, the temperature was selected after a substantial amount of careful analysis. It had to be high enough to force the contaminants off in a reasonable amount of time without the heat endangering the spacecraft. As Dawn will go farther from the hot Sun than Earth is, accommodating very high temperatures was not among its design criteria.
Accompanying the flight hardware in the chamber were sensors to allow the operators to monitor temperatures, and team members babysat the spacecraft around the clock to ensure its safety. Contamination monitors permitted the operators to observe the rate at which material was being driven from the spacecraft, and the bake-out was scheduled to continue until the rates reached a predetermined low value. Although it had been expected this would require nearly a week of baking, it turned out to require less than two days at the maximum temperature. The team was pleased to conclude that the spacecraft must have started out cleaner than had been anticipated.
In addition to the sensors that provided feedback during the bake-out, other contamination monitors were included that will be analyzed in laboratories to understand more about the kinds of contaminants that were present. Ultraclean silicon wafers were intended to collect tiny fibers, many times thinner than human hair, and aluminum plates can reveal evidence of films just a few molecules thick.
Both before and after the bake-out, the spacecraft was inspected carefully under illumination with ultraviolet light and separately with old-fashioned (but still effective!) visible light. Such inspections will be repeated many more times before launch. Any debris that is found is removed with a brush and small vacuum or with special materials soaked with purified alcohol. As one might imagine, using an inspector's thumb or the corner of his T-shirt to wipe off unwanted material is not part of such a delicate procedure.
While the careful assembly and demanding testing of the spacecraft continues, there remains one piece of essential hardware that will not even begin fabrication until late this year. It is not needed for Dawn to carry out its assignment to explore alien worlds and to reveal clues to the dawn of the solar system, but it is a vital ingredient in another aspect of Dawn's mission. And despite the engineering and scientific breadth and depth of the Dawn team, this component is beyond our means to produce without the help of hundreds of thousands of people, including you, dear reader.
In many ways, we are still in the early stages of humankind's journeys through the solar system. This is evident when we recognize that Dawn is the first spacecraft designed to orbit a target in the main asteroid belt. This vast region of space has been traversed by a few probes that were flung from the inner solar system to reach the outer solar system, but no spacecraft has yet been assigned to stop there and develop an intimate portrait of some of the residents. Indeed, until Dawn, with its ion propulsion system, no spacecraft has had such a capability.
As other explorers, Dawn's mission is about more than seeing what is out there and helping us unravel secrets of the cosmos. Dawn carries with it the spirit of those it represents on its homeworld: a yearning to extend beyond the bounds of our terrestrial experience, a grand quest for ever greater knowledge, an irrepressible drive to understand how our celestial neighborhood fits together in space and time. Missions like Dawn's help fuel the passionate and noble fires that burn in the hearts and minds of people everywhere. It is these shared feelings that make this more than simply another attempt to record scientific data.
To emphasize the nature of this joint participation in the mission and make it more real, more rewarding, and more personal, we invite you to include your name on the spacecraft. Later this year, we will fabricate a chip to be carried on the spacecraft, imprinted with the name of each person who wishes to have a part in this collective adventure of humankind. While the Dawn project can make the chip, it would mean nothing without all the names.
So be among the first on your planet to submit your name by going to http://www.dawn-mission.org/DawnCommunity/Sendname2asteroid/nameEntry.asp.
Dr. Marc D. Rayman
July 29, 2006
Dawn is making good progress in preparing for its 2007 launch. Let's look forward to some of what must happen during the next year on the most visible part of the Dawn project, the spacecraft, to prepare for its launch. We will discuss other tasks, such as training operations team members, formulating details of the science data acquisition plans, finalizing the software to be used by mission controllers, selecting the ascent trajectory for the rocket, designing Dawn pins, shirts, tattoos, etc., in later logs.
In the previous log (which, it may be revealed with pride, has been nominated for Spam of the Year awards on fewer than 10 planets per galaxy averaged over the full spatial range of readership), it was reported that the spacecraft already was about 90% assembled. It may seem surprising then that Dawn still has a very full Earth-bound year ahead of it. One reason is that attaching any of the sophisticated hardware systems to the spacecraft is a very exacting, and thus time consuming, process.
Most of the units on the spacecraft are complex, expensive, custom-built devices that must be handled with extraordinary care to minimize the risk of damage. In some cases, repair or replacement could take months or even years. Unlike production-line products, such as aircraft, cars, computers, and those nifty thought-controlled confectionery machines that are popular in the Large Magellanic Cloud, the Dawn spacecraft design is being assembled for the first time and there won't be an opportunity for a second chance. That calls for extreme care in every step.
Before any electrical device is connected, a painstaking procedure is followed to verify that all wires carry the signals they are supposed to. We cannot risk that some undetected damage to a connector might create a short circuit or that an error in the wiring, or even in the documentation for the wiring, might lead to too much power being delivered to a sensitive electrical component.
All mechanical connections have to be checked carefully as well, to be sure that they do not place undue stress on other parts that could lead to misalignments of components or structural weakening that might compromise the spacecraft. Every device is attached securely enough to survive launch but not so tightly that something is damaged or distorted.
To reduce the possibility of human error, each step in the long process of assembling the spacecraft is planned and documented in detail. The work executed meticulously by one technician or engineer is observed by another who also carefully inspects the workmanship.
While each of us is eager to get Dawn on its way, rushing this work is unwise. Once it has embarked upon its cosmic travels, repairing any electrical or mechanical problems generally will be extremely difficult or impossible. (During its voyage, Dawn will be more than 1.5 million times farther from Earth than astronauts who work on the International Space Station; emergency roadside assistance will be quite unavailable.) While many mission control teams have accomplished remarkably innovative repairs on remote spacecraft, or learned to work around irreparable damage, expending the effort before launch to prevent problems after launch is the best recipe for success. That brings us to the work that will be the primary focus of the combined Orbital Sciences Corporation/JPL team between now and launch.
Dawn's mission to explore alien worlds we have only glimpsed from afar will be an extremely arduous one, so we will subject the spacecraft to extensive testing to verify that it is up to the challenge. Each component receives a battery of tests during its own assembly before being brought to the spacecraft, but the majority of the testing that awaits Dawn is on the spacecraft as a whole to make sure that all systems work together correctly and perform as intended in their installed configurations.
Most of the rest of this spring (note: all seasons herein refer to Earth's northern hemisphere -- nonresident readers, consult your almanacs) will be devoted to the first set of comprehensive performance tests, putting hardware and software subsystems now on the spacecraft through their paces. (Following the tradition nearly as ancient and revered as nerdiness itself, these tests are generally referred to by an acronym -- CPTs.) In addition to helping establish that the subsystems perform as they are designed to, the first set of CPTs will establish a reference against which to compare the results of subsequent runs of the same CPTs, thereby showing that other tests performed on the spacecraft did not damage it. The CPTs have already been executed on simulators to make sure that they work correctly so that valuable time with the spacecraft is used effectively.
Although the spacecraft is in an environmentally controlled facility (a "clean room," quite unlike my office) most of the systems on it came with a small inventory of chemicals that could contaminate some of the sensitive surfaces when Dawn is in space. Therefore, during the heat of the Dulles, Virginia summer, the spacecraft will be baked for about a week in a vacuum chamber to drive off these undesirable contaminants. (Note: while the chamber will be much hotter than the outdoors at Orbital Sciences, the vacuum will make it less humid than Dulles. Nevertheless, this environment is not recommended even for those who prefer dry heat.) CPTs will be repeated afterwards to verify that no harm was inflicted during the relocation or the baking.
During the gorgeous Virginia autumn, the Dawn team will conduct a series of tests designed to prove that the spacecraft can withstand the environmental conditions it will face during launch. It will be exposed to the thunderous noise that will rumble around it in the rocket as well as vibration, shock waves, and electromagnetic fields.
Despite the inability to predict weather far in advance, the Dawn team already knows that the winter will be a time of great temperature variation. In preparation for what it will experience during spaceflight, the spacecraft will once again be placed in a thermal vacuum chamber, but for much longer than the bake-out. Over the course of about a month, Dawn will experience sweltering heat and biting cold, and it will have to prove that it can operate as designed throughout the range.
While we do not want there to be problems, finding them here on Earth would be far superior to discovering them when Dawn is in the far reaches of deep space. Although human readers might consider all these tests to be punishing in the extreme, it is worth recalling that much of the work in designing the spacecraft was devoted to ensuring that the system would be able to operate under such harsh conditions. The tests over the coming year will give Dawn just a preview of what it will spend most of its productive life experiencing as it strives to accomplish its raison d'être.
Throughout the coming year, certain components will be removed or installed at times planned carefully to fit in the complex campaign to get Dawn safely to space. One simple example is the ion thrusters, the most salient part of the ion propulsion system. Two of the three thrusters project from the spacecraft (see, for example, http://dawn.jpl.nasa.gov/mission/spacecraft.asp and the thrusters depicted in gray on the lower left and right at http://dawn.jpl.nasa.gov/multimedia/images/cylinder_300.jpg), and these precise and delicate devices could be damaged by the highly skilled, albeit human, workers who are performing other tasks on the spacecraft. So mock-ups with the same mass will be used during some of the tests.
For most of the tests, special sensors, such as contamination monitors or accelerometers, will be installed temporarily. Between environmental tests and after the spacecraft is transported from one facility to another, CPTs or, in some cases, more limited performance tests (you guessed it -- LPTs) will be conducted to aid in the assessment of the effects of the test on our robotic explorer.
When Dawn passes all of its tests, it will be rewarded in the same way many humans are: it will take a road trip to Florida for spring vacation. Not far from the warm waters and sandy beaches of Cape Canaveral, Dawn will be given the final tests to verify that it was not harmed in shipment. The ion propulsion system's xenon tank will be filled with 425 kg (937 pounds) of xenon, and the reaction control system (used to help rotate the spacecraft, but not to propel it to Vesta and Ceres) will be loaded with about 45 kg (100 pounds) of hydrazine propellant. There will be flurry of other activity as well, as Dawn presents its last opportunity to be tested and readied for flight. Of course, the plan is for Dawn to leave Florida by a very different route from the one by which it arrived.
Dawn will have an exciting adventure after launch as it travels through the solar system, some of the time without company and some with Vesta or with Ceres. But its last trip around the Sun while still on its planet of origin will be no less busy.
Dr. Marc D. Rayman
May 29, 2006
Coming in summer 2007 to a solar system near you (well, near most of you, anyway): the Dawn mission!
NASA’s next planned venture into the solar system, Dawn is a collaborative effort of scientists, engineers, and people in other disciplines at NASA/JPL, UCLA, Orbital Sciences Corporation, the space agencies of Germany and Italy, and other universities and private companies in the United States and elsewhere. But there is more to this mission than the people working directly on it. I view this as an adventure of humankind, with a spacecraft carrying not only a suite of sophisticated scientific instruments and impressive engineering gadgetry, but the dreams, aspirations, and most noble spirit of exploration of our still-young space-faring species. For those of you who are members of that species (and even those of you who aren’t), I invite you to share in this extraordinary adventure.
In what still seems like only yesterday (and note that I didn’t sleep at all last night), I enjoyed giving some of you an inside view of the exciting flight of Deep Space 1, and I’m proud that those reports are still in circulation as a profitable set of late-night reruns throughout much of the Virgo Supercluster of galaxies. Now, by starting more than one year before launch, I am looking forward to this opportunity to involve you in our preparations for dispatching another of our planet’s robotic emissaries. I hope you will join me throughout the rest of Dawn’s residence here on Earth as well as on its journey to worlds we have yet to know.
The Dawn project is on course now for a launch into the cosmic void in 14 months. Most of the project’s work was put on hold in October 2005 while NASA reevaluated it, and last month NASA approved Dawn for continuation. We are reassembling our team and formulating new and detailed plans for completing the myriad tasks necessary to begin a nearly decade-long mission in deep space. While the spacecraft is about 90% assembled in one of Orbital Sciences’ environmentally controlled “clean rooms,” much work remains to finish the delicate job of installing the rest of the components and to conduct extensive and rigorous testing to verify the readiness of the entire spacecraft and the ground operations system (consisting not only of the highly trained people, but also all of their hardware, software, and procedures).
In the next log, I will provide some of the details of our new plan for the next 14 months, but for the first of these logs, it seems more appropriate to devote some attention to the overall mission. I will offer more about this over the coming year, but let’s start with a broad overview of Dawn.
The fascinating process that is science has yielded remarkable insights into the formation of our solar system, but many questions remain unanswered and many details are yet to be filled in. In brief, about 4.6 billion years ago, one of the Milky Way galaxy’s vast nebulae of gas and dust began to collapse. As it did so, most of the material fell to the center of the cloud, eventually forming the Sun, where the majority of the mass in our solar system remains concentrated. But as many residents and visitors to it know, the solar system consists of more than the Sun. Some of the tiny particles of dust accreted elsewhere in the condensing cloud, gradually growing in size to become rocks and eventually building up to planets. There is greater uncertainty about how the largest planets, Jupiter and Saturn, formed, but apparently once Jupiter did achieve its enormous bulk, its powerful gravity halted the assembly of nearby matter into planets. Much of that material, deprived so long ago of the opportunity to continue conglomerating, now forms the asteroid belt, between Jupiter and Mars. The two most massive protoplanetary remnants of that epoch are Ceres and Vesta, and they are Dawn’s destinations.
While they seem to have formed at very similar distances from the nascent Sun, and thus, one might expect, under similar conditions, observations from distant Earth show these two bodies to be very different from each other. Water seems to have played an important role in Ceres’ history, and there is reason to believe it might still harbor a substantial inventory of that precious commodity, never having been hot enough to drive the water away. Vesta, in contrast, displays the signatures of minerals found in lava, indicating that different forces shaped its history. Despite the impressive discoveries made so far, our ability to learn about these asteroids from Earth, hundreds of millions of kilometers away, is very limited indeed. By gathering information about Ceres and Vesta from orbit around them, at distances of only hundreds of kilometers, scientists can learn much much more and retrieve the records the protoplanets hold about the very early solar system.
While some people may think of all asteroids as chips of space rock, Ceres and Vesta in many ways are more like planets -- real worlds. The largest asteroid yet encountered by a spacecraft is Mathilde, which the remarkable NEAR-Shoemaker spacecraft glimpsed as they zipped past each other in 1997. It has a very irregular shape, with its largest dimension being about 66 kilometers (41 miles). In contrast, Vesta’s equatorial diameter is about 580 kilometers (360 miles). That is sometimes compared to the size of Arizona in the United States. A tremendous crater at Vesta’s south pole is about 460 km (285 miles) in diameter. How exciting it will be to use Dawn to see the rugged terrain and complex geology of that enormous excavation, a window provided by nature to let us peer deep into Vesta’s interior. Ceres, which by itself contains one quarter of all the mass in the asteroid belt, is about 975 km (605 miles) in diameter. The only states in the United States that are larger are Texas and Alaska. But comparisons of the protoplanets’ diameters with terrestrial landforms fail to convey their real sizes, because these orbs are three dimensional bodies. The surface area of Vesta is more than three times that of Arizona, and Ceres’ surface is as large as Alaska plus Texas plus California. In fact, it is about one third of the area of the United States, and almost 40% of the area of the contiguous United States. These are big places, and there certainly will be many beautiful and intriguing things to see in their varied and alien landscapes. Part of the allure of Dawn is that it is bound for some of the last unexplored worlds in the inner solar system.
No spacecraft has ever attempted to orbit two targets after leaving Earth. Such a feat would be far beyond our capabilities without the use of ion propulsion, which Deep Space 1 proved to be the fantastically efficient and reliable system generations of science fiction fans have known it to be. Ion propulsion is also what allowed NASA to shift Dawn’s launch date from its original plan of 2006 to 2007 without having to change the plans for the rich scientific investigations to be conducted. Most missions beyond Earth orbit are restricted to short launch periods, usually only a few weeks long. (Engineers distinguish the launch period -- the range of days on which a launch can occur -- from the launch window -- the span of time on any one day in which a launch can take place.) With the extraordinary maneuvering capability of its ion propulsion system, Dawn could conduct its planned mission with a launch any time from May 2006 (or perhaps much earlier) to November 2007. This has given us the flexibility to fit Dawn’s launch in an opening in the schedule at Cape Canaveral. Based upon that, and not the more interesting science of celestial mechanics, we are targeting a launch in June or July 2007.
The flexibility afforded by the ion propulsion system means that the details of Dawn’s itinerary may still change, but in the current plan the spacecraft will fly past Mars in March 2009 on its way to the more distant asteroid belt. Thrusting with its ion propulsion to ever-so-gently shape its trajectory to match Vesta’s path around the Sun, Dawn will ease into orbit around Vesta in September 2011. It will spend about seven months there, subjecting Vesta to intense scrutiny with its scientific sensors. Leaving behind what will then be a familiar world, Dawn will resume its interplanetary travels. Nearly three years later, following its arrival at Ceres in February 2015, it will devote five months to coaxing out the secrets that are stored there. At the end of the mission, Dawn will remain in orbit, accompanying Ceres on its leisurely 4.6-year revolutions around the Sun. Because of its heft, the gravity of Ceres is too high for Dawn ever to make a controlled landing.
Travels far from Earth, exploration of new worlds, ion propulsion, rocket science, amazing feats of engineering, new scientific understandings, probably some disappointments and scares but certainly some drama and thrills -- all this lies ahead on this futuristic mission. As the Dawn team works hard to prepare for next year’s launch and the voyage that follows, I hope you will join me in this exciting journey through space and time as we seek the dawn of the solar system. The future -- and the past -- await us!
Dr. Marc D. Rayman
April 18, 2006