The complex and intricate steps necessary for Dawn to reach space continue as its launch date grows near.
Workers have begun assembling Dawn’s launch vehicle at Cape Canaveral’s Space Launch Complex 17. Shunning the banal names used by fictional (and some actual) rockets for many decades, the real thing carries an appellation that evokes the true passion of our species for exploring the cosmos. Readers here on Earth (and on most other planets with comparable or greater gravity) are sure to be stirred by the name Delta II 7925H-9.5, capturing everything that’s cool about rockets. Regardless of what it is called, United Launch Alliance’s family of Delta II rockets has a remarkable record of success in delivering spacecraft for NASA and other organizations to space.
As spectacular as a launch is, it represents only the beginning of what is far more exciting -- Dawn’s interplanetary journey of exploration. Launch depends upon many prosaic (but important!) accomplishments, one of which did not go according to plan recently. A crane malfunctioned on pad B at Cape Canaveral’s Space Launch Complex 17, where Dawn’s launch vehicle is being erected. The rocket consists of 9 solid rocket motors and 3 stages.
The 6 motors that will be ignited at liftoff to augment the first stage weigh nearly 18,900 kg (more than 41,600 pounds) each and are 14.66 meters (48 feet 1 inch) tall. The other 3 motors, to be ignited about 79 seconds after liftoff, weigh nearly 19,100 kg (more than 42,000 pounds) each and are 15.06 meters (49 feet 5 inches) tall. (The 3 “air-start” solid motors are taller than the “ground-start” motors because their nozzles are longer.) Given both the great power and tremendous importance of the solid rocket motors, they have to be handled carefully.
Together reaching to a height of 29.4 meters (96 feet 5 inches), the first stage and the interstage (the section between the first and second stages) were placed on the launch pad first. Following that, 3 solid motors were erected on the pad. Then, on May 30, when the first one was being positioned to mate it to the first stage, the crane encountered a problem. No launch vehicle components were damaged.
It took about a week to restore the crane to health, and that delay has necessitated a change in Dawn’s launch date. As recalled from tales told throughout the halo of the Milky Way galaxy since the very first of these logs was written, the extraordinary capability of its ion propulsion system gives Dawn much greater flexibility in when it can launch than interplanetary missions that use conventional chemical propulsion have. The most significant constraint now on Dawn’s launch date is the more limited time during which another interplanetary probe can be launched from a nearby pad. Now in preparation for a thrilling mission at Mars, Phoenix is scheduled for an August departure from pad A. Because of some shared systems and other considerations, some time is needed between launches from these adjacent pads.
Dawn’s new launch period opens on July 7. The launch window that day is from 4:09:31 to 4:36:22 pm EDT. (We apologize for the conflict with the 350,000th Event Horizon Games in the Virgo cluster of galaxies.) In case launch does not occur then because of unfavorable weather or some other problems, here are the windows on the subsequent few days:
July 8: 4:04:49 - 4:33:02
July 9: 3:56:15 - 4:25:23
July 10: 3:53:32 - 4:22:25
July 11: 3:45:13 - 4:14:44
Windows have been computed for still more days, and if launch does not happen by July 11, readers may be assured they can find later windows posted in a future log or in the on-screen captions of the Daily Asteroid Report broadcast on the Interstellar News Channel.
In preparation for launch, the spacecraft now has a full supply of 425 kg (937 pounds) of xenon propellant for its ion propulsion system. The tank already had almost 15 kg (33 pounds) of the noble gas that had been loaded in February 2005 at JPL. It took about 25 hours to load the rest of the xenon this week. While that may seem slow to fill a 272-liter (71.9-gallon) tank, it is worth recalling that more than 5 years of ion thrusting will be required to empty the tank.
The reaction control system, used as one of the means to rotate the spacecraft in the zero-gravity of spaceflight, was given its complete provision of 45.6 kg (101 pounds) of hydrazine. The propellant is highly toxic, so engineers and technicians loading it in the Hazardous Propellant Facility at Astrotech Space Operations wore today’s most fashionable protective garments with self-contained air supplies.
The operations team spent long hours the last 6 days conducting a set of operational readiness tests (ORTs) known affectionately as the ORTathon. The hub for the ORTs is mission control at JPL, but participants were not only there but also at Orbital Sciences Corporation, Astrotech, all 3 Deep Space Network complexes (in Goldstone, California; Canberra, Australia; and Madrid, Spain), and the European Space Operations Centre in Darmstadt, Germany (the control center for a receiving station in Perth, Australia to be used for a few hours after launch). The operations team had to diagnose and resolve many guileful problems created by the simulation supervisor (known as “sim sup” as well as various other sobriquets, depending upon how imaginatively diabolical he is). The ORTs used a sophisticated combination of hardware and software to simulate the spacecraft.
The next log will continue with the progress in preparing to separate Dawn from Earth’s grasp.
Dr. Marc D. Rayman
June 10, 2007
Dawn has been greatly enjoying its stay in the Cape Canaveral area, literally the last place on Earth it will be. Following its arrival in April, the spacecraft and other equipment were unpacked and verified to be in good condition after the long drive from Washington. (Note that “long” is a relative term. Dawn’s space voyage will cover 3.8 million times greater distance and last 3900 times longer.)
The spacecraft has not visited most of the popular sites in its vicinity, but it still has had a very successful stay in the Sunshine State. (Ironically, it has not been exposed to any sunshine there, but it will be see plenty of sunshine at its next destination.)
One of the major accomplishments at Astrotech Space Operations was the successful completion of the final set of comprehensive performance tests (CPTs). It took about two weeks to run these tests on the hardware and software subsystems. Following that, comparison with the results from earlier CPTs verified that the long series of environmental tests and work on the spacecraft did not introduce any unexpected changes that might compromise its operation in space.
The alignment of spacecraft components was verified and finalized, ensuring that antennas, ion thrusters, scientific instruments, and other devices are properly oriented.
The huge solar arrays, the largest used for any NASA interplanetary mission, were reinstalled, and the deployment system was given one final test. The last time the two wings, each the width of a singles tennis court, were attached to the spacecraft was December. Each wing consists of 5 panels, and hinges allow the system to be folded for launch, so the spacecraft can fit comfortably in the rocket’s nose cone (known to engineers and perhaps some otorhinolaryngologists as the “payload fairing”).
The next time the arrays are opened will be when Dawn is in space, where its 11,480 solar cells will provide the spacecraft with electrical power. A battery will power the spacecraft from liftoff until it is able to extend the arrays and point them at the Sun. When it does, the full length of the spacecraft from wing tip to wing tip will be 19.7 meters (almost 65 feet), which is greater than the distance from the pitching mound to home plate on Earth’s major league baseball fields. In response to many inquiries we have received, we should point out that it truly is purely coincidental that Dawn’s arrays span exactly the same size as the famously profound sculpture “Tribute to Coincidence,” a popular site for visitors to the Small Magellanic Cloud.
While some team members have been preparing the spacecraft, others have been working with equal diligence to be ready to operate it during its mission. Many tests have been conducted both with the spacecraft and with simulators to verify that all systems onboard and on the ground are ready.
Mission scenario tests (MSTs) (initiated last autumn) have continued, with the final one on the spacecraft taking place on May 20 in a successful simulation of launch. Others have demonstrated the capabilities needed to diagnose and recover from problems during launch or during interplanetary flight. Some MSTs concentrated on the methods that could be used during the mission if it were necessary to reload software in the central computers, the computers in the scientific instruments, or the computers in the star trackers. Installing new software when a probe is far from Earth has proven to be a vital ingredient in the successes of many missions.
Dawn also passed a series of radio communications tests with MIL-71, the facility at the nearby Kennedy Space Center that mimics all of the essential characteristics of the much larger Deep Space Network (DSN) stations. This work verified that Dawn’s systems are fully compatible with the DSN, which, apart from happy memories and fond thoughts, will provide its only link with distant Earth when it is otherwise isolated in the forbidding depths of interplanetary space.
The Dawn project also has been conducting operational readiness tests. (These are known quite unimaginatively as ORTs; and even less cleverly, the acronym is spoken letter by letter and not pronounced as “ort” might sound. Our readers on icy moons of gas giants certainly will recognize a thought-provoking concept herein, although it likely will escape readers elsewhere.) Some ORTs have used the spacecraft and others have relied on simulators, as the focus is less on the spacecraft and more on the team members and the processes, procedures, software, and hardware (including the selection of snacks in mission control -- kudos to the unofficial but vital mission control nutrition engineer!) they will use during operations. ORTs of launch and some of the activities that will be conducted to check out the spacecraft during its first weeks in space have been completed, and more are planned.
On May 28, Dawn was moved to the Hazardous Propellant Facility at Astrotech where xenon and hydrazine will be loaded. The complex procedures of pumping these propellants into the spacecraft tanks have not begun yet, but relocating the spacecraft allows engineers to make preparations.
As work here on Earth has continued to ready Dawn for its flight, scientists took advantage of a favorable opportunity in May to study the explorer’s first destination, asteroid Vesta. During portions of 7 orbits of Earth, the Hubble Space Telescope observed Vesta, the first of Dawn's two destinations. Even this fantastically capable observatory cannot detect the kind of detail Dawn will find after its 4 year, 3.0 billion kilometer (1.9 billion mile) voyage to Vesta, but the data from Hubble will aid scientists as they plan for Dawn’s detailed observations.
While Dawn may get as close as 200 km (about 120 miles) to Vesta, Earth’s closest distance in many years occurred on May 31, at a range of about 171 million kilometers (106 million miles). Vesta is the only resident of the asteroid belt that occasionally is bright enough to see with naked eyes, although good observing conditions are required. Visit http://dawn.jpl.nasa.gov/feature_stories/Vesta_nightsky.asp to learn more, including how you can spot this intriguing asteroid this month.
Reports from the near future reveal that the next log will include news about the propellant loading and related work as well as the status of Dawn’s rocket and the plans for using it to leave the Sunshine State.
Dr. Marc D. Rayman
June 2, 2007
The Dawn spacecraft has completed its longest terrestrial journey on its path to asteroid Vesta and dwarf planet Ceres. While it will be propelled by exotic ion propulsion during most of its mission, this segment of its travels was accomplished using decidedly more conventional chemical propulsion. After being packed with great care at the Naval Research Laboratory (NRL) in Washington, DC, the spacecraft and a great deal of additional equipment left on a truck a few hours before dusk on April 9. Less than 18 hours later, a few hours after dawn, it arrived at its home for the next two months, Astrotech Space Operations in Titusville, Florida, near Cape Canaveral.
When last we checked in with the spacecraft, it had completed an extensive series of tests in a thermal vacuum chamber at NRL. The pace of activities has not let up since then, with engineers and technicians rarely letting the spacecraft have a rest. Myriad tasks are being completed and checked off the long and carefully planned list of steps necessary before the probe may begin its ambitious mission in harsh and remote parts of the solar system. For example, thorough checks for any possible leaks in the ion propulsion system and the reaction control system (the system that uses small conventional thrusters to aid in orienting the spacecraft in the zero-gravity of spaceflight) verified their integrity, certifying them for many years of operation in space. More tests have been conducted to confirm the flow of information between the many elements of the mission control systems and all of the computers onboard the spacecraft.
As expected, some of the thermal vacuum tests had revealed the need to make some minor changes in a few of the 9000 wires connecting different elements of the spacecraft. As these updates were in progress, the device that controls the high voltage, high power electricity from Dawn’s large solar arrays was removed from the spacecraft and shipped to JPL. There is always a risk of accidentally damaging hardware or introducing an error, even in ways that may not be noticed immediately. Therefore, after this unit was modified, it was subjected to additional vibration testing as well as operation in a thermal vacuum chamber. These tests showed the complex assembly to be in fine health and ready for flight, and it was returned to the spacecraft in March.
In the same vein, to ensure that no subtle problems crept in as a consequence of the work to remove or reinstall this device, the spacecraft underwent another acoustic test at NRL similar to one it experienced in November 2006. The spacecraft will be subjected to deafening sound waves during its climb to space. At the beginning of this month, Dawn had another preview of this reverberant environment in a test that demonstrated the entire system was intact and ready for a rocket trip to space (or an evening in a mosh pit).
Following its outstanding performance, the spacecraft was rewarded, as had been promised nearly a year ago, with an all-expense-paid spring vacation in Florida. Dawn is now in the perfect location, near sandy beaches, warm ocean waters, facilities for loading hazardous fuels, and other attractions.
Just as the spacecraft has been following a rigorous schedule of building, testing, checking, and rechecking, the many elements of its Delta II 7925H-9.5 rocket have been undergoing similarly demanding procedures. This version of the venerable Delta series of rockets has not been launched since 2004, but now it is nearly ready again to make the brief but arduous flight from Cape Canaveral to outer space.
To accommodate a change in the schedule for readying Dawn’s rocket, the planned launch date has been shifted from June 20 to June 30. This change will have no significant effect on the plans for the mission, including when the spacecraft will arrive at its celestial targets. The timetable at Space Launch Complex 17 allows Dawn to launch as late as July 19, with the exact date of liftoff depending on the weather as well as the cooperation of millions of components of hardware and software on the rocket, the spacecraft, mission control, range safety, communications systems around the world, and more.
Dawn’s launch will occur around 5:00 pm EDT, but the precise times that are possible will not be determined until early June. Readers may find launch times down to the second in print, on the web, or, to our embarrassment, on graffiti in the asteroid belt, but those times were based on preliminary estimates and will change. Engineers now are working through the complex analyses necessary to establish the exact times the launch window will open and close on each day of Dawn’s 20-day launch period. These analyses incorporate refinements and updates such as the spacecraft’s mass at launch, the thrust and efficiency of the ion propulsion system, the power that will be generated by the solar arrays and consumed by all spacecraft subsystems, and many many other parameters. All of these affect how the spacecraft will use its ion propulsion system to travel through the solar system, so they determine the preferred trajectory as it departs from Earth and hence the guidance information to be loaded in the rocket’s computer and the timing of the launch.
Were Dawn to have relied on ion propulsion for its trip to Florida, it’s easy even for our nonmathematical readers to estimate how long it would take. This remarkable system, known from ancient legends told for eons in most ultra-luminous infrared galaxies (and described in the previous two logs), cannot operate in our planet’s relatively dense air, nor could it overcome the friction and gravity most residents of planetary surfaces are accustomed to. Therefore, the thrust would have been exactly 0. With no thrust, the spacecraft would not have moved toward Florida any faster than the blossoming cherry trees the truck left behind in Washington.
When it is in space, 18 hours of ion thrusting would propel the spacecraft 170 kilometers (slightly more than 100 miles). That’s far less than the nearly 1400 kilometers (about 850 miles) required for last week’s drive. After 18 hours of powered flight in space, Dawn would be streaking along at the incredible speed of nearly 5 meters/second (over 11 miles/hour). The secret of ion propulsion however is that it can accelerate the spacecraft for months or years, eventually yielding much greater changes in speed than can be achieved with chemical propulsion. (We recognize that this is now a secret only to the few sentient species in our audience who did not receive the last two logs because of disputes over subscription fees. Our position remains clear: payment may not be made with dark matter.)
Dawn’s itinerary allocates enough time to accomplish the required thrusting. The explorer will reach Vesta, its first destination in the main asteroid belt, between Mars and Jupiter, about 4.5 years from today. After examining the enormous asteroid with its scientific instruments, Dawn will leave it 5 years from now to propel itself silently, gently, and patiently to Ceres. It will arrive at the dwarf planet (the first spacecraft to visit one) in 2015 to perform detailed studies of that world.
With such a short summary of the agenda, it may be easy to forget that undertakings such as this include many challenges. Dawn hopes to uncover the nature of unfamiliar targets far far from Earth, where humans have never ventured, countless mysteries lurk, and the environment is inhospitable and rarely forgiving. But where there may be great rewards, also there be great risks. Dawn seeks great rewards.
As preparations for launch and mission operations continue, future logs are likely to be shorter. Beginning in June, we hope to exchange prolixity for frequency, and readers everywhere are encouraged to join in the drama of humankind’s next venture into the solar system (and to pay their subscription fees).
Dr. Marc D. Rayman
April 15, 2007
The Dawn spacecraft has just completed the final and most challenging of the environmental tests needed to prepare for its launch and travels through space. During the past month, it has endured the extreme heat and cold of spaceflight in a large vacuum chamber at the Naval Research Laboratory (NRL) in Washington, DC.
In the last few months of 2006, the spacecraft underwent a broad range of tests at Orbital Sciences Corporation in Dulles, Virginia. It passed all of them, and for graduation the spacecraft, along with its retinue of mechanical and electrical test and support equipment, was sent on a pleasingly uneventful drive to NRL during the first weekend in January. Once it arrived, preparations began immediately for the next set of tests in NRL’s thermal vacuum chamber.
After the spacecraft and special monitoring equipment were installed in the chamber, plates whose temperature could be individually controlled were positioned around the spacecraft to ensure the desired thermal test conditions could be achieved. Pumps began removing air early in the evening of January 23, and by the next morning, the interior of the chamber was more than 100 million times below normal atmospheric pressure. Dawn experienced the same vacuum condition last summer, but that was to drive off contaminants. This time, the goal was to operate the spacecraft in ways similar to what it will face during its mission. By subjecting it to heat and cold, and testing the performance of its subsystems under both extremes, the engineering team could verify that the critically important thermal control subsystem will be able to keep temperatures within desired limits throughout the full range of conditions Dawn will encounter in space.
The enormous solar arrays had been removed at Orbital and will not be reconnected to the spacecraft until April. The vacuum chamber at NRL is not large enough to accommodate the arrays when they are open, extending 19.7 meters (almost 65 feet) tip-to-tip. If they had been in their stowed, or folded, position for these tests, as they will be when on the rocket, they would have covered parts of the structure that are supposed to be exposed in space. That would have retained heat inside the probe and prevented these tests from accurately duplicating the temperatures it will experience in space.
After the chamber pressure was reduced, the temperature was raised gradually to 45°C (113°F) and held there for almost a week while engineering and science subsystems were put through their paces. Following that, quite unaware of the hibernal, but comparatively balmy, conditions outside NRL, the spacecraft was brought to -25°C (-13°F) for several more days of tests. Although it is designed to operate under these conditions, few of the Dawn team members are well suited either to working in the absence of air or at such temperatures. Therefore, on February 8, when the time came to make planned changes in the test configuration, the chamber was brought back to normal atmospheric pressure and temperature so people could enter.
Tests resumed the next day under vacuum. Among other activities in this second phase, some mission scenario tests were conducted. Unlike the many tests focused on individual subsystems, these were designed to make the subsystems work together as they must when Dawn is operating in orbit around distant Vesta, the first of its mysterious and enticing destinations. In fact, the mission scenario tests provide an opportunity to assess even more than the collective performance of the subsystems; the spacecraft and some of the systems in mission control operate together in much the same way they will during the mission.
On February 14, Dawn’s ion propulsion system was powered on for its long-awaited “hot fire test.” (Your correspondent -- ever the romantic -- conducted what proved to be an unsuccessful search for a Valentine’s Day card that appropriately expressed the sentiments associated with such an experience.) The ion propulsion system cannot operate in normal atmospheric pressure, so although its individual components had been tested quite extensively, this was the only opportunity to test them all together. The digital control interface units, power processor units, xenon feed system, and thrusters all performed beautifully. The extremely gentle thrust, as described in the last log, caused no more movement of the spacecraft than if a piece of paper had been lain on it, but the team certainly felt a powerful boost to see the bluish glow of the thruster and gain one more indication that Dawn is getting close to flight.
The spacecraft has 3 thrusters, with only 1 to be operated at a time in the mission. In this test, one of them could not be fired because it was blocked by hardware supporting the spacecraft. That thruster still had a nearly full test. It ionized xenon but did not apply the voltage needed to accelerate the ions. Between the other 2 thrusters, the ion propulsion system operated at 5 different throttle levels for a total of 34 minutes of thrusting.
To generate propulsion, the thrusters emit high velocity xenon ions. The impingement of those ions on nearly any object, including the chamber wall, could erode it, blasting off contaminants that could settle on the spacecraft. Therefore, specially designed targets were positioned about 2 meters (almost 7 feet) from the thrusters. Myriad specially oriented fibers of carbon on the targets captured most of the materials that the ions would otherwise have liberated. Sensors located in the vicinity proved that this system did indeed prevent adverse levels of contamination from accumulating. In fact, it worked so well that the hot fire test could have continued longer than planned, but there was no need for an extension.
The thermal vacuum testing concluded on February 17, and the spacecraft was removed from the chamber two days later. As the operations in the vacuum chamber were so intensive, with the team working around the clock, a tremendous amount of data was collected, and it will require several weeks for engineers to analyze all of them. Serving its purpose well, however, the test has already pointed to several changes that will bring the maturing space probe closer to its final readiness for flight, such as adjusting the size of some heaters, the amount of insulation over certain components, and the values of parameters in software that control temperatures. While this and other work is being conducted on the spacecraft, the unit that governs the delivery of electrical power from the solar arrays to the onboard subsystems will be removed and returned to JPL where engineers will make some modifications.
With the thermal vacuum work having been completed, representing the end of Dawn’s many months of environmental testing, this would conclude our update on Dawn’s progress, were it not for an item we report on with some pride. The last log was chosen by readers on a majority of planets surveyed as being among the 1000 “Most Interesting Dawn Articles Written on December 28, 2006.” Lest this be misinterpreted as being even more prestigious than it is, it should be revealed that this recognition applies only in the category of articles of 1900 - 2000 words (in the original language). Nevertheless, it is this kind of appreciation that makes the many seconds of writing seem worthwhile. This surge in interest may be attributed to the inclusion of an explanation of the principles underlying the ion propulsion system (IPS), the remarkable technology that enables Dawn to undertake its unique and exciting mission. Thus, to build upon the success of the previous log (and, by the way, to fulfill the promise in that log’s final paragraph), it may be interesting to explore how the IPS is used and why it makes operating Dawn different from deep space missions with conventional propulsion. We’ll see much much more about this as we join Dawn on its long flight through space, but for now, let’s take a very brief look at how spacecraft reach their extraterrestrial destinations and see some of the differences when ion propulsion is used.
The physics that explains the complex beauty of orbital ballet tells us that the velocity of any object in orbit around the Sun, be it one of humankind’s interplanetary robotic explorers or the solar system’s natural residents of planets, asteroids, and comets, depends upon the exact shape and size of its orbit and where it is in the orbit. All orbits are ellipses (like squashed circles, or ovals in which the ends are the same size), but the degree of flattening and the overall size allow for an infinite range of theoretically possible orbits (including perfect circles). Because of our understanding of the mathematical principles, if we know the orbit, we can calculate the velocity at any position in the orbit; if we know the velocity at any one position, we can calculate the entire orbit. We also know that if we change the velocity at any position, the overall shape and size of the entire orbit will change.
Celestial navigators have developed remarkably sophisticated methods of using these seemingly simple principles to achieve astronomical accuracy in flying throughout the solar system. To deliver a spacecraft to its remote destination, engineers use the nature of orbits to choreograph the perfect cosmic dance, ensuring that the individual performers (such as the spacecraft and the planet it will explore) arrive at the same spot at the same time.
When the paths of the spacecraft and the target cross, the laws of celestial motion dictate that the objects following the orbits travel at very different velocities, generally many kilometers per second (many thousands of miles per hour) for interplanetary missions. If the goal is to fly by the target, the spacecraft conducts its observations during the brief time they are near each other; no additional orbit changes are needed. Suppose instead the objective is to go into orbit around the destination. That means the spacecraft will join the target as the latter follows its own orbit around the Sun, just as the moon and satellites in Earth orbit accompany our planet on its annual heliocentric loop. To accomplish this, the spacecraft must swerve from its original solar orbit in order to match the speed and direction of its new solar orbit, which is precisely the same as the target’s orbit around the Sun. (For our purposes here, we will not attend to the details of the spacecraft’s orbit around its destination. We shall return to that in a future log however, as it is an important part of Dawn’s story. You are encouraged simply to accept that it is adequate to consider only orbits around the Sun for now.)
Perhaps imagining this, as one gazes thoughtfully into the cosmic void, it becomes clear why spacecraft usually have to execute a large burn of their propulsion systems to get into orbit around another solar system body. That maneuver accomplishes the swerve to change the craft’s path around the Sun. (If the objective of the mission is to slam into the target or its atmosphere, the energy of the collision changes the orbit by just the amount that otherwise would be effected by the spacecraft’s thrusters.)
Interplanetary missions with conventional chemical propulsion rely on a powerful rocket to be thrown from Earth into orbit around the Sun, after which they spend months or years following that orbit, coasting to their targets. On occasion, the flight may be interrupted by a brief firing of the spacecraft’s engine to adjust its course or a more dramatic passage by a planet whose gravity alters the orbit, thereby boosting the probe on its way, but for the most part, the journey is a very passive one, with the craft doing little to help itself along. Upon reaching its target, it fires up its engine again to veer into its new orbit. In effect, most missions floor it very briefly and coast most of the time.
The use of the IPS creates a very different situation. The rocket does not place Dawn into an orbit that will intersect the orbit of its target. Dawn is so much more capable of its own maneuvering, that it relies on the rocket only to propel it away from Earth. Once its journey has begun, it steers its own course. By thrusting gently but persistently for years, Dawn constantly reshapes its orbit around the Sun. The flight profile -- the direction and timing of the thrusting -- is calculated to smoothly sculpt Dawn’s orbit, gradually changing the trajectory so that it is identical to that of its quarry. With its amazingly low rate of fuel consumption, Dawn will spend most of its mission with a light touch on the accelerator.
Contrary to common intuition, unlike missions with chemical propulsion, Dawn will not have to execute a special, dedicated ion thrusting maneuver to get into orbit around Vesta or Ceres. Indeed, the thrusting to arrive in orbit will be no different from the years of thrusting that precede it. With the utmost elegance, Dawn will approach each target very slowly because, under the influence of its IPS, its orbit around the Sun will slowly take the required shape. Instead of veering and swerving, Dawn’s maneuvering will be characterized more by grace and delicacy. As it creeps up on an asteroid, it will slip into orbit so gently that a casual observer would not even notice the transition.
One of the many consequences of the whisper-like force of the IPS is that engineers must ensure that the flight profile allows enough time to accomplish the needed thrusting. Because of the rigors of space travel and the complexity of spacecraft engineering, all probes experience the occasional unexpected event that interferes with planned activities, and much sophisticated work is devoted to developing systems to safeguarding the lonely craft when such anomalies arise. So the Dawn mission must be designed to account for the inevitable glitches that will interrupt thrusting, whether they be from a burst of cosmic radiation, a software bug, or a balky component.
For a conventional planetary orbit insertion, if the maneuver of a few tens of minutes were missed, the objectives of the entire mission would be lost -- there can be no second chance. Propulsion then is truly vital, so most missions have very short periods of extremely high vulnerability and long periods of no vulnerability at all. A great deal of effort is devoted to protecting the critical minutes. With ion propulsion, missions generally have very long intervals of low vulnerability. Part of the arcane science of formulating Dawn’s flight profile is ensuring that the mission can tolerate weeks of missed thrusting at any time and still make its way to the distant worlds Vesta and Ceres to help unlock the secrets they hold.
Dawn’s next destination is Cape Canaveral. We will check in again in April once the spacecraft has arrived at that familiar part of the solar system to begin final preparations for launch.
Dr. Marc D. Rayman
February 19, 2007
The Dawn spacecraft has made its new year's resolution: to leave Earth behind in 2007 and embark upon its celestial voyage of adventure and discovery. (Actually, it was either that or spend more quality time with friends and family. As much as we all like Dawn, I think we can be grateful it made the choice it did.) The spacecraft is well on its way to achieving its goal.
Over the past few months, Dawn has completed all of the demanding environmental tests planned for it at Orbital Sciences Corporation. In the last log, we saw why such tests are so important. Since then, Dawn has been spun, vibrated, and blasted by noise, and careful testing afterwards has verified that it can withstand these insults and still operate as planned.
One of the tests included attaching Dawn to the structure that will connect it to the upper stage of the Delta II 7925H-9.5 rocket so essential to keeping its new year's resolution. Part of the objective of this test was to verify that the spacecraft and the rocket, although manufactured separately, really will fit together when they meet at Cape Canaveral in June. In addition, this test was used to subject Dawn to another special condition it will experience in its mission. Following the burn of the Delta's third (and last) stage, the rocket will relinquish its firm grasp on the spacecraft. The firing of the release mechanism will cause a shock (certainly physical, possibly emotional) to go through the spacecraft as it is freed to operate in space on its own. Feeling this shock is part of the battery of tests the spacecraft has now completed. Continuing with its perfect record, Dawn passed beautifully, demonstrating that it can tolerate the shock and separate cleanly, with no structures impeding its departure from the rocket.
Following all these tests, the two large solar array wings were extended, allowing engineers another test of the deployment system and the opportunity to verify that the delicate cells were still healthy. Each wing extends 8.3 meters (more than 27 feet) and weighs almost 63 kg (139 pounds). The system is not designed to be strong enough to support them under the strong pull of Earth's gravity; of course, when Dawn is in its natural environment of spaceflight, no such force will be exerted upon the arrays. For working in the exotic conditions here on the surface of our planet, a special structure is erected to bear the weight of the wings yet allow them to unfold smoothly. After the tests, the solar arrays were removed, and they will not be reattached until the spacecraft is in Florida.
Now Dawn is being prepared for its departure from Orbital Sciences in Dulles, VA. Next month it will be transported to the Naval Research Laboratory (NRL) in Washington, D.C. for the final phase of environmental tests, all of which will be conducted with the spacecraft in a vacuum. Orbital has the vacuum facilities to accommodate the spacecraft (see the description in the July 29, 2006 log), but this upcoming series of tests will include a brief firing of the ion thrusters, and that requires a different vacuum system. Because NRL has the needed capability and is near Orbital, it was a natural location for this work. Dawn will spend about 3 months there, and the next log will report on the activities, including the operation of the ion propulsion system.
Devoted readers have asked for more information on ion propulsion. This is only one of the important subsystems onboard (see the overview and relative importance of all the subsystems and systems on September 17, 2006 and October 29, 2006), and Dawn will rely upon all of them in order to explore the remote, alien worlds Ceres and Vesta. Over the many years of the mission, we shall have occasion to learn a great deal more about many facets of the engineering and science of this exceptional adventure, but starting in this log, and continuing in the next, we will take a more detailed look at the ion propulsion system.
While most of our audience is, of course, quite familiar with this topic, we should recall that our readership extends to planetary systems that have had little experience with this technology, and it is to them that this material is directed. Although it may be surprising, apparently there are even some readers who did not follow NASA's Deep Space 1 (DS1) mission, which tested ion propulsion and other high-risk technologies to protect subsequent missions from the risk and cost of being the first users of such advanced systems. Dawn is one of DS1's beneficiaries, and being the first spacecraft ever built to orbit 2 target bodies after leaving Earth, it would be effectively impossible without ion propulsion.
Ion propulsion had its origins in solid science, but despite some scientific and engineering work, it resided principally in the fictional universes of Star Trek, Star Wars, and other fanciful stories. DS1 helped bring ion propulsion from the domain of science fiction to science fact.
First let's recall how a propulsion system works. Most conventional systems use high pressure or temperature to push a gas through a rocket nozzle. The action of the gas leaving the nozzle causes a reaction that pushes the craft in the opposite direction. This is what causes a balloon to fly around when the end is opened and the stretched rubber squeezes the air out. Ion propulsion works on the same principle, but the method of pushing the gas out is unique.
The inert gas xenon, which is similar to helium and neon but heavier, is used as propellant. The composition of xenon is simple: each atom consists of a tiny and dense nucleus surrounded by a cloud of electrons. The nucleus is 54 positively charged protons plus about 76 neutral neutrons. (Xenon gas is a mixture of 9 isotopes, meaning there are 9 different values for the number of neutrons. From a low of 70 to a high of 82, the number of neutrons makes only very modest differences in the behaviors of the atoms.) The 54 positive charges in the nucleus are precisely balanced by 54 negatively charged electrons, rendering the atom electrically neutral -- until the ion propulsion system gets in the act.
Inside the ion thruster, an electron beam, somewhat like the beam that illuminates the screen in a television, bombards the xenon atoms. When this beam knocks an electron out of an atom, the result is an electrically unbalanced atom: 54 positive charges and 53 negative charges. Now with a net electrical charge of 1 unit, such an atom is known as an "ion." Because it is electrically charged, the xenon ion can feel the effect of an electrical field, which is simply a voltage. So the thruster applies more than 1000 volts to accelerate the xenon ions, expelling them at speeds as high as 40 kilometers/second (89,000 miles/hour). Each ion, tiny though it is, pushes back on the thruster as it leaves, and this reaction force is what propels the spacecraft. The ions are shot from the thruster at roughly 10 times the speed of the propellants expelled by rockets on typical spacecraft, and this is the source of ion propulsion's extraordinary efficacy.
All else being equal, for the same amount of propellant, a spacecraft equipped with ion propulsion can achieve 10 times the speed of a craft outfitted with normal propulsion, or a spacecraft with ion propulsion can carry far less propellant to accomplish the same job as a spacecraft using more standard propulsion. This translates into a capability for NASA to undertake extremely ambitious missions such as Dawn.
The rate at which xenon is flowed through the thruster is very low. At the highest throttle level, the system uses only about 3.25 milligrams/second, so 24 hours of continuous thrusting would expend only 10 ounces of xenon. Because the xenon is used so frugally, the corresponding thrust is very gentle. The main engine on some interplanetary spacecraft may provide about 10,000 times greater thrust but, of course, such systems are so fuel-hungry that their ultimate speed is more limited.
The force of the ion thruster on the spacecraft is comparable to the weight of a single sheet of paper. So here is an ion propulsion experiment you may conduct safely at home: hold a piece of paper in your hand, and you will feel the same force that the ion thruster exerts. Because the fuel efficiency is so great, the thruster can provide its push not for a few minutes, like most engines, but rather for months or even years. In the weightless and frictionless conditions of spaceflight, the effect of this thrust can gradually build up to allow the spacecraft to achieve very very high speed. Ion propulsion delivers acceleration with patience.
Throughout its mission, Dawn will be farther from the Sun than Earth, but as long as it is less than about twice Earth's distance from the Sun, those huge solar arrays will generate enough power to operate the ion propulsion system at its maximum throttle level. At that setting, the acceleration will be equivalent to about 7 meters/second/day, or slightly more than 15 miles/hour/day: one full day of thrusting would change the spacecraft's speed by 15 miles/hour. That means it would take Dawn 4 days to accelerate from 0 to 60 miles/hour. Perhaps this does not evoke the image of a hot rod, but its parsimonious consumption of xenon lets it thrust for much longer than 4 days.
To put this in perspective, consider a greatly simplified example based upon the remarkable probes NASA has in orbit around Mars now. When they arrived at the planet, these spacecraft had to burn their engines to drop into orbit. While each mission is different, such a maneuver might be about 1000 meters/second (2200 miles/hour) and could consume about 300 kilograms (660 pounds) of propellants. With its ion propulsion system, Dawn could accomplish the same change in speed with less than 30 kilograms of xenon. A typical Mars mission might complete its maneuver in less than 25 minutes, while Dawn might require more than 3 months. If one has the patience, the ion propulsion can be very effective. Now for many missions, the greater complexity and cost of ion propulsion is unnecessary, and it is quite clear that we can get into orbit around Mars without it. But as humankind engages in ever more ambitious missions in deep space, opening our frontiers, revealing otherwise inaccessible vistas, and seeking answers to new and more exciting questions about the cosmos, the tremendous capability of ion propulsion will be an essential ingredient.
By the end of its mission, having operated from its maximum throttle level down to lower levels when Dawn was much farther from the Sun, the spacecraft will have accumulated over 5 years of total thrust time, giving it an effective change in speed of 11 kilometers/second, or well over 24,000 miles/hour. That is about the same as the entire Delta rocket with its 9 solid motor strap-ons, first stage, second stage, and third stage, and it is far in excess of what any single-stage craft has accomplished.
In the next log, we will see how the Dawn mission takes advantage of ion propulsion and how its use makes the profile of the mission different from most interplanetary flights. In the meantime, the spacecraft will use conventional transportation technology to travel to NRL for more rigorous tests in preparation for the challenging mission it has resolved to begin in 2007.
Dr. Marc D. Rayman
December 28, 2006
The Dawn spacecraft is in space! Well, not quite, but it is getting a taste of the space environment, courtesy of the team preparing it for its mission.
Although the individual components of the spacecraft have already been tested, the point of the testing in Orbital Sciences Corporation's Environmental Test Facility is to verify that the fully assembled spacecraft will survive the rigors of launch and be able to fulfill its ambitious mission of exploration in deep space.
When Dawn is on the launch pad at Cape Canaveral and during the brief (but exciting!) trip from there to space, many radio signals between systems on the ground and between ground systems and the rocket will impinge upon it. Some of the tests are designed to verify that these signals will have no adverse effects on the spacecraft. Other tests show that Dawn's electronics do not produce signals that might interfere with these other systems.
As one illustration of the importance of such tests, consider the scientists eager for Dawn's intimate portrait of the enormous asteroids Ceres and Vesta, the team members who have invested years of their lives in creating this spaceship and the means to use it to explore distant worlds, all people who thirst for greater knowledge of our solar system and the thrill of discovery, and taxpayers who make it possible. One may reasonably expect members of all of those groups to find it unsatisfying if any of Dawn's electronics produced signals that accidentally activated the rocket's self-destruct system. (That system is designed to be commanded with radio signals transmitted by range safety in the event of a serious malfunction during ascent.) Similarly, if Dawn's radio emissions interfered with the rocket's reception of range safety's self-destruct command, the legions of Dawnophiles throughout the Milky Way Galaxy, and the even greater number elsewhere, would no longer give this project their loyal support.
In addition to testing Dawn's compatibility with other systems, engineers are testing its self-compatibility. It is essential to verify that none of the subsystems, including the radio used for interplanetary communications, emits electromagnetic radiation that might interfere with other subsystems.
Of course, Dawn's designers and builders were well aware of these and other concerns, and they have methods to make the probe satisfy the many associated requirements, but only through testing may we be confident that the work was successful.
With the completion of testing in the electromagnetic interference/electromagnetic compatibility facility, the spacecraft will be prepared for a series of mechanical tests. For the rocket's control systems to remain stable with Dawn perched at the top, it must be accurately balanced and must meet certain criteria for how stable it is when it is spun, and the next set of tests will help prepare for that. (As we will discuss in an upcoming log in more detail, the spacecraft will spin at about 50 rpm during a portion of the time it is on the rocket.) Also on Dawn's agenda during November are deafening noise and powerful shaking that will show its readiness for the ride to space. Sensors to measure the movements of certain parts of the spacecraft will be installed (after the balance measurements are complete) for these tests.
As we know from earlier logs, between many of these environmental tests, other tests will be conducted on the spacecraft to ensure that its systems remain intact, undamaged by previous environments and ready for the next. At each stage, the health of the spacecraft will be verified.
Dawn's busy autumn includes still another kind of test. Now that it is a complete spacecraft, it is scheduled for tests of its responses to many of the complex sets of commands that will be sent to it while far from Earth. These "mission scenario tests" exercise not only the spacecraft but also many of the software systems used by mission control. While this testing is invaluable, it does have some noteworthy limitations. The spacecraft cannot respond now to all of these commands, because, for example, it cannot rotate itself while on Earth, it cannot see stars to establish its orientation, and the ion propulsion system can thrust only in vacuum. (The ion propulsion system will be operated briefly with the spacecraft in a vacuum chamber early next year.) Sophisticated simulators connected to the spacecraft compensate for these and other limitations of the terrestrial test program.
Important progress has already been made in these tests. The team has demonstrated that commands can move smoothly through the complex path from mission control at JPL, to the spacecraft's main computer and then to its engineering and science subsystems, and that responses can flow back to mission control, to the Dawn Science Center at UCLA, and to the institutions that built the science instruments.
The last log described the spacecraft's engineering subsystems, including a characterization of their relative importance, but what about these science instruments? Some people would say they are more important than any of the engineering subsystems. While others would disagree, everyone would concur that without the science instruments, the long and arduous journey to Vesta and then to Ceres would be of no value without the information these instruments will gather. We'll be seeing much more about them in the years ahead, but let's introduce them now.
Dawn is built and operated by humans who, in contrast to many of our other readers, are very visual creatures. So it is no surprise that the spacecraft carries cameras to share the sights with those who remain at home. (There have been, and will continue to be, many space missions that are fantastically productive and tremendously exciting even without cameras; nevertheless, the visceral appeal of pictures is undeniable.) Besides satisfying our innate curiosity to know what Vesta and Ceres look like, the cameras will provide important data essential to gaining an understanding of the geological and physical properties of these enigmatic bodies. And in the spirit of this mission representing all humankind, and not only those who happen to reside in one portion of one continent, the cameras on Dawn are contributed by Germany. The Max-Planck-Institut für Sonnensystemforschung (Max Planck Institute for Solar System Research) was responsible for their design and fabrication, in cooperation with the Institut für Planetenforschung (Institute for Planetary Research) of the Deutsches Zentrum für Luft- und Raumfahrt (German Aerospace Center) and the Institut für Datentechnik und Kommunikationsnetze (Institute for Computer and Communication Network Engineering) of the Technischen Universität Braunschweig (Technical University of Braunschweig).
Because of the long duration of Dawn's mission and the extraordinarily remote locations in which it will operate (more than one million times farther from Earth than the International Space Station), most of the critical subsystems include backups, thus allowing Dawn to persevere even in the event of a malfunction. The images of Vesta and Ceres are so essential that the probe carries two identical cameras. In addition to the value for science and for the visually oriented fans of the mission, the images are critical for navigating the spacecraft in the vicinity of these bodies.
The cameras incorporate filters in 7 color ranges, chosen principally to help study the minerals on Vesta's surface. In addition to detecting the visible light humans see, the cameras will register near infrared light.
Another scientific instrument, contributed by Italy, covers a still broader range of light, from shorter wavelengths in the ultraviolet through the same wavelengths in which the cameras operate, to farther in the infrared. It is provided by Agenzia Spaziale Italiana (Italian Space Agency), and it was designed and built at Galileo Avionica, in collaboration with the Istituto Nazionale di Astrofisica (National Institute for Astrophysics). Because of its sensitivity across the entire visible (V) spectrum and well into the infrared (IR), the team that designed the instrument has named it VIR ("vir" is Latin for "man").
Each VIR picture records how strong the light is at more than 400 wavelength ranges in every pixel. Instruments such as this are known as imaging spectrometers, and they see the world much as we might if we looked through a prism, which breaks light into its component colors. But this yields more than a beautifully surreal view of extraterrestrial landscapes. When scientists compare VIR's observations of its targets with laboratory measurements of minerals, they can determine what minerals compose the surfaces of Vesta and Ceres.
With the data from the cameras and VIR, scientists will discover much about the nature of those alien worlds, but these highly capable instruments cannot reveal everything we would like to know. To learn still more, Dawn will carry a device that measures the energy of gamma rays and neutrons. Gamma rays are a form of light, not only higher energy than visible, and even higher than ultraviolet, but more energetic even than X-rays. Neutrons are particles that normally reside in the nuclei of atoms (about half of your body weight is neutrons, regardless of how much Halloween candy you eat). Some of the gamma rays and neutrons emitted by Vesta and Ceres are produced by radioactive elements and others are created by the bombardment of the surface material by cosmic rays. As they emanate from the surface and travel into space, some will be intercepted by Dawn's gamma ray and neutron detector (GRaND) which, despite its name, is very humble. (For the sake of having an interesting appellation, it's fortunate that GRaND detects gamma rays and neutrons and not neutrons and gamma rays.) The complex and impressive instrument was designed and built by a team at the Los Alamos National Laboratory.
The gamma rays and neutrons reveal many of the important atomic constituents to a depth of one meter (three feet) or so on Vesta and Ceres, thereby adding to the detailed story Dawn will tell. As we know from the first log, Ceres may be rich in water. If it is, the signature of water may be contained in GRaND's data.
Although there is not a special instrument for it, Dawn will make another set of scientific measurements at its destinations. Using the radio signals exchanged between the telecommunications system (described in the previous log) and NASA's Deep Space Network, scientists can detect subtle variations in the gravitational attraction between each asteroid and Dawn. These variations reflect details of the internal distribution of mass, so these tiny effects allow us to learn about the interior structure of the massive bodies Dawn will orbit.
Dawn is well prepared to help scientists extract a wealth of information about Vesta and Ceres and thus teach us a great deal about the nature of the solar system when planets were forming. In less than 8 months, the craft will be launched on the beginning of its journey to that distant past. While much work remains, each step in the preparations brings us closer to witnessing the thrilling discoveries it will make. I hope you continue to share in the eager anticipation and ultimately in the excitement of the rewards.
Dr. Marc D. Rayman
October 29, 2006
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