Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.
Just after the previous log was posted, further predictions of poor weather at Cape Canaveral and difficulties with a downrange launch vehicle tracking system required a launch postponement. We will provide an update when the countdown is ready to resume.
Dr. Marc D. Rayman
July 6, 2007
The countdown is underway for Dawn’s liftoff on July 8 at 4:04:49 pm EDT.
Launch had been planned for July 7, but unfavorable weather at Cape Canaveral led to the postponement today of the planned liftoff from July 7. The open and close times of Dawn’s daily launch windows for the first few days of its launch period are in a previous log and are transmitted daily in the Telepath Report.
In the last log we followed the plan for what the Delta launch vehicle will do as it delivers its passenger, the Dawn spacecraft, to space. During the 28.5 minutes of their shared flight, the rocket is in control. The timeline is the same for a launch on July 8 as on July 7. For later launches, the coast in Earth orbit will be a little longer, but events before and after will not change.
Following separation, Dawn has three primary objectives: 1) get sunlight on the arrays, 2) establish contact with mission control at JPL, and 3) revel in the beginning of a remarkable mission of exploration. Most of what it does to accomplish the first two steps will be standard procedure for the spacecraft throughout the mission when it encounters a problem and needs to enter “safe mode,” in which it will await instructions from Earth. Of course, separation from the launch vehicle is anything but a problem. Engineers have taken advantage of their extensive work developing the directions Dawn will follow to reach its safe configuration by having it execute nearly the same program as soon as it is flying independently in space. Future logs are sure to have reason to discuss safe mode again.
As few of Dawn’s components as possible are turned on during launch, because with its large solar wings folded against its side (and a segment of the flight in Earth’s shadow), power is provided by a large battery on the spacecraft. Conserving energy (a responsibility familiar to readers on Earth) is vitally important. While on the rocket, Dawn’s computer and a few other devices are operating, heaters are activated as needed, and some data are recorded, but mostly the probe simply waits for the signal that indicates it and the third stage have parted ways.
Because the craft will be returning a tremendous bounty of rich scientific information from distant Vesta and Ceres, its radio system is powerful. Therefore, the transmitter remains off until the solar arrays can provide essentially endless power.
When the third stage releases Dawn, it will leave the spacecraft spinning slowly, with xenon propellant spinning inside in the opposite direction. In addition, the springs that push the spent stage and the eager spacecraft apart are likely to impart a slightly unbalanced push, so Dawn is expected to be turning slowly around all axes. When the computer determines that Dawn has separated, it waits 8 minutes 20 seconds for the friction between the xenon and the spacecraft to lower the spacecraft’s spin rate enough that it can be stabilized by the attitude control system. Known to its friends as ACS, this system is responsible for controlling the spacecraft’s orientation.
After waiting the prescribed time, software directs ACS to begin using its sensors to determine the direction and rate of the spin. Then ACS will command the small rocket thrusters of the reaction control system to fire, gradually stopping the unwanted rotations. The process of bringing the attitude under control can take as little as 1 minute or as long as 15 minutes, depending upon the imbalance in the separation forces and details of the xenon behavior.
Once the spin is fully controlled, it is safe for Dawn to deploy its large solar arrays. Each wing is divided into 5 panels, which are stacked against each other and secured to the spacecraft by cables during launch. To release the wings, small heaters press against the cables, causing them to weaken and break. When they are no longer restrained by the cables, the wings unfold under the gentle urging of springs. With its wings folded, the spacecraft is 1.84 meters (6 feet 1 inch) wide. When they open, the two wings span 19.74 meters (64 feet 9 inches) tip to tip. The software provides 12 minutes 47 seconds to allow the cables to release and the arrays to extend to their full reach.
Although ACS remains in control throughout the solar array deployment, after the computer has allowed for the programmed time to elapse, it requests ACS to perform another stabilization, now with the new, much larger configuration of the spacecraft. ACS may report back that this is complete in as little as 1 minute or as long as 15 minutes.
Just as when a teneral dragonfly spreads wide its new wings for the first time, these intricately patterned marvels must be pointed at the Sun. Up to this time, Dawn has paid attention only to itself, without regard to the external universe. (Of course, it continues coasting away from Earth with the energy given to it by its recent companion, the Delta rocket.) Supported on small extensions from each corner of the boxy body of the spacecraft are solar cells, just like those on the arrays. But these cells are not intended to meet Dawn’s electrical needs; instead, ACS uses them to find the location of the Sun. This is not very different from using your eyes to find the Sun, a particularly appropriate analogy both for dragonflies and for those readers who have eyes that allow them to see in all directions simultaneously. Once it has established where the Sun is, it rotates with its thrusters to point the arrays in that direction. Depending upon the orientation the probe happens to be in prior to this activity, it can take as little as 1 minute and as long as 18 minutes to locate the Sun and complete the turn.
As soon as light from the solar system’s master, the star at the center, reaches the arrays, the battery begins to recharge, and all of Dawn’s electrical needs for the rest of its 8-year mission will be satisfied by the energy the solar cells receive from the Sun.
The computer waits another 4 minutes after the arrays are fully illuminated by the Sun to make sure all systems remain stable, and then it activates its power-hungry radio transmitter. It should take about 4 minutes 30 seconds for the transmitter to warm up and begin sending radio signals, reporting on the status of all systems.
The spacecraft is well prepared to resolve a wide range of problems as it progresses through the list of tasks to complete between separating from the Delta and powering on its radio. If it has not been delayed by correcting any anomalies, the entire sequence could take as little as 32 minutes 37 seconds and as long as 77 minutes 37 seconds; otherwise, this could stretch to over 3 hours. In mission control at JPL, the operations team, taking a cue from one of the virtues Dawn will display as it traverses the solar system, will remain patient. Nevertheless, everyone will look forward to verifying that it is starting its long journey in good health.
But Dawn’s radio signals may not reach Earth quite yet. Without information on where that planet is, the spacecraft cannot know where to point its antenna. (For most of the mission, Dawn will know where it is in relation to Earth and other solar system bodies, but at this early stage, having just begun its flight, such information will not yet be available onboard.)
After it has finished directing its solar arrays at the Sun, the spacecraft begins a roll around the line between it and the Sun, turning once per hour, perhaps appearing like an exotic and lazy windmill. Given the direction of its departure from home, the Sun and Earth will be at about right angles from Dawn’s perspective. So as it makes its slow spin, it uses an antenna pointed at the same right angle to the solar arrays. The antenna sweeps out a broad beam, like a wide searchlight sending its signal out to anyone who happens to see it.
Antennas at the Deep Space Network complex near Canberra, Australia and at the European Space Agency’s facility in Perth, Australia will be ready to detect Dawn’s transmissions and pass the data on to JPL. These stations should be able to receive signals during about half of each rotation of the spacecraft, or about 30 minutes every hour.
It is impossible to predict where Dawn’s antenna will be pointed when it begins transmitting, so it might be aimed at Earth immediately, or it could take as long as 30 minutes until the spacecraft’s rotation brings it around to start the half hour of terrestrial coverage.
With all these steps, the time from liftoff to the receipt of the first radio signal may be as little as about 1 hour 1 minute or as long as 2 hours 16 minutes even if Dawn encounters no surprises along the way, and more than 3 hours 30 minutes if it does. If you are entering your planet’s friendly betting pool on when Dawn’s data first will light up the computers in mission control, you are advised to consider that the likelihood that all circumstances will conspire to yield the shortest possible time is extraordinarily low. That time is more a theoretical minimum than a practical guide, and although mission control will be ready, no one there will be expecting signals that early.
Once controllers see the data, they will begin evaluating the spacecraft’s condition. Over the course of the subsequent few days, they also will review the data it stored during launch and begin configuring it for further operations.
Meanwhile, Dawn will continue racing away from Earth. In less than 2 hours 15 minutes from liftoff, it will be more than 35,800 kilometers (22,200 miles) high, passing the ring of satellites in geosynchronous orbit, and thus will be more remote than the great majority of spacecraft launched in Earth’s half century of probing and utilizing space. It will go beyond the most distant point in the moon’s elliptical orbit less than 29 hours after launch, traveling farther from home than humans have ever ventured. Yet that is but the very beginning of Dawn’s journey.
Distant though it will be, it may be possible for terrestrial observers with capable telescopes to glimpse the probe in the first week or two of its travels. (Other spacecraft have been observed not long after they left Earth. See http://www.jpl.nasa.gov/releases/98/ds1palomar.html for what this former member of the Deep Space 1 team considers to be the best image ever taken of that spacecraft.) It would be very faint, perhaps no more than a speck amidst a sea of stars in the constellation Cetus near right ascension 0 hours 52 minutes and declination -18°. (These approximate coordinates will change by a few degrees if Dawn’s launch does not occur on July 8 at the opening of its window. For a launch at a later time that day, the position will move to slightly higher right ascension. The dependence upon the day in the launch period is more complex, but in general, if the launch takes place on a later day, the location will shift to slightly higher right ascension and higher declination.) For anyone interested in trying to observe the spacecraft, please visit http://ssd.jpl.nasa.gov/horizons.cgi and change the target body to (no surprise here) “Dawn” to find its exact location.
If all goes according to plan, this will be the last log written when Dawn is bound to Earth. We hope readers throughout the cosmos join in wishing the explorer well as it gets underway for a journey that offers new knowledge, excitement, the rewards -- and the risks -- of facing the unknown, and the spirit of adventure that compels humankind to undertake such bold quests.
Dr. Marc D. Rayman
July 5, 2007
Now only two weeks away from its planned launch, Dawn is eagerly awaiting the beginning of its cosmic adventure.
Once the xenon and hydrazine propellants were loaded, as described in the last log, the spacecraft was ready for its final balancing and weighing. As we will see below (that direction applies only for those of you reading this in a gravitational field), during part of its flight on the Delta 7925H-9.5 rocket, the spacecraft will be spun, and it is crucial that it satisfy certain requirements on how well balanced it is so the rocket remains stable. In addition, to help the rocket’s guidance system deliver it to the correct target in space, an accurate measurement of the spacecraft’s weight is important.
The spacecraft was designed so that it would be balanced, but minor adjustments in individual components during assembly and test can alter the balance. So the spacecraft was placed on a rig that measured how stable it was as it spun at 50 revolutions per minute (rpm). After each spin, engineers calculated how to improve the balance. Small weights then were attached to mounting fixtures on the spacecraft that had been included for just such a possibility. Then the spin was repeated to verify the predicted improvement. By the end of the fourth spin, 7 tungsten weights had been added and thin sheets of brass were included for fine adjustments. The spacecraft was balanced better than needed by the rocket, and fewer weights were used than had been expected.
When technicians were attaching the spacecraft to the spin assembly, a wrench slipped and made inadvertent contact with one of the solar panels. The arrays are folded for launch, and in that configuration, the back (the side without the solar cells) of one panel was close to the attachment point for the spin rig. The tool made a minor dent and no cells were affected. The panel was repaired easily.
With a few other final preparations, such as installing a delicate sunshade on the spacecraft’s 1.52-meter (5-foot) main antenna to keep its temperature within acceptable limits during spaceflight, the spacecraft finally was ready to be introduced to its rocket. Dawn was firmly attached to the third stage of the Delta at Astrotech, and it won’t be separated until both are in space. Not far away, at Cape Canaveral’s Space Launch Complex 17B, the second stage was hoisted atop the first stage.
Now that launch is so close, let’s have a preview of what is planned during this important event. Much of the work on the design of the spacecraft has focused on ensuring that it is prepared for the acceleration, vibration, noise, heat and cold, and other conditions it will experience during the ride to space. And yet for all that effort, as well as the spectacular sights and sounds for observers, this is the shortest phase of the mission. During it, Dawn will be a polite passenger, patiently recording data and awaiting its chance to begin flying on its own in space to undertake its mission of discovery deep in the solar system.
This log has many more numbers (readers are encouraged to quantify this) than most, and hence will be of special interest to new members of our audience, the Numerivores who reside in the “quadruple quasar” Q2237+0305. Others need only follow well enough to gain a sense of how dynamic Dawn’s departure from home will be, in great contrast to the more leisurely pace of its interplanetary flight.
In the last log, we saw that to leave the launch pad, the Delta rocket will use its liquid-fueled first stage and 6 of the 9 solid rockets strapped to its side. Thirty seconds later (L + 30 seconds) it will exceed the speed of sound. The solid motors burn out at about L + 77 seconds when the rocket is at an altitude of about 24 kilometers (15 miles), and the remaining 3 motors ignite 2 seconds later. Three of the spent motors separate at L + 80.5 seconds, and the other 3 are jettisoned 1 second later as the rocket continues its ascent. The remaining 3 motors burn for 76 seconds, and when they are released at L + 2 minutes 39.5 seconds, the rocket will be 73 kilometers (45 miles) high and traveling about 10 times the speed of sound. The first stage’s main engine continues firing on its own until L + 4 minutes 23 seconds, and then the rocket coasts for 14 seconds. After 8.5 seconds of the coast, having lofted Dawn to 130 kilometers (81 miles), the first stage separates.
When the second stage engine is commanded to life 5.5 seconds later, the rocket is traveling at 6.1 kilometers per second (3.8 miles per second, or nearly 14,000 miles per hour). At an altitude of 135 kilometers (84 miles), the shroud that shielded Dawn from the dense atmosphere below is no longer needed, so it is ejected. Now 4 minutes 41 seconds from liftoff, Dawn has its first view of space. The second stage will continue climbing and accelerating until it reaches the altitude and velocity to be in a low orbit. At L + 8 minutes 58 seconds, the stage stops firing.
Let’s take advantage of the brief hiatus in orbit to consider the timing of all the events during launch. The overwhelming majority of spacecraft our species [Note to extraterrestrial editors who repost these reports: change the previous two words to “humankind.”] sends beyond the atmosphere remain gravitationally tied to Earth. They accompany the planet on its endlessly repetitive travels around the Sun, and except for the few that are designed for scientific observations of the cosmos, the orbits of these satellites are mostly unrelated to the rest of the solar system. Where Earth is in its orbit, and where other members of the Sun’s retinue are, generally do not matter. Such is not the case for Dawn (and other interplanetary probes).
The entire launch sequence is timed so that Dawn will depart Earth at a carefully chosen point in the solar system. For each possible launch day, extensive analysis has established the mathematically best plan for reaching Vesta and Ceres, distant worlds that beckon and that Dawn seeks to unveil. The analyses account for the gravitational effects of the Sun and all planets, and the resulting plans include times that Dawn will thrust with its ion propulsion system and times that it will coast. As reported in a previous log, many years of exquisitely gentle thrusting allows the indefatigably patient probe to reshape its orbit around the Sun to rendezvous with its destinations. As we will see in logs after launch, the first 80 days of the mission will be devoted to checking out the spacecraft systems and preparing for the long journey ahead. At L + 80 days, the thrusting needed to follow the flight plan begins, and the timing of the launch sequence is arranged so that Dawn will be at the correct location in the solar system, about 28 million kilometers (17 million miles) from Earth, at that time.
The second and third stages linger in Earth orbit so that following the ascent from Cape Canaveral, they are properly positioned to propel Dawn to reach its required location nearly 3 months later. If launch occurs on July 7, the pause in the second stage’s firing will last about 9 minutes 7 seconds. (Because the solar system will have rearranged itself a little by the next day, launches on other dates will slightly require longer intervals.) The engine will reignite at L + 18 minutes 5 seconds while at an altitude of 185 kilometers (115 miles) and operate for 2 minutes 38 seconds. Fifty seconds later, to finish its contribution to Dawn’s mission, the second stage will fire 4 small rockets pointed around its circumference to spin the third stage and spacecraft to 50 rpm. (Unlike the first and second stages, the third stage is stabilized by gyroscopic rotation, like a spinning bullet or football.) This is when the spacecraft’s balance becomes most important. The second stage separates at L + 21 minutes 37 seconds.
For the next 37 seconds, the spinning assembly continues following the orbit the second stage left it in, and then the final burn of the Delta begins. The third stage fires for 86 seconds, and during that time it exceeds “escape velocity” so that it has enough energy to break free of Earth’s gravitational hold. When the solid motor burns out, it is only at an altitude of 278 kilometers (173 miles), but Earth is too weak to slow the rapidly receding craft enough to bring it back. (Pause here for a moment of awe: 80 days later, the spacecraft will be more than 100 thousand times farther from Earth.) Unlike a ball you might throw that goes up and then comes down, the Delta will have thrown Dawn so hard that it will never fall down. It will be in its own orbit around the Sun, traveling at 11.43 kilometers per second (7.10 miles per second, or 25,600 miles per hour) relative to Earth. With the third stage spent, for the rest of the mission, onboard propulsion will be achieved only with ions.
When the second stage spins the spacecraft, the xenon propellant stored inside does not immediately spin up to 50 rpm, just as when you rotate a glass filled with a liquid, it takes a while for the liquid to catch up with its container. (We know that some readers live on planets without liquids, but the analogy applies to gases as well. In fact, the xenon on Dawn is maintained at a temperature and pressure that create a special state called “supercritical,” in which it bears some similarity to a gas and some to a liquid. Amazing though its properties are, supercritical xenon should not be confused with superheroes that may bear similar names.) The friction between the rapidly spinning spacecraft and the xenon inside it causes the spacecraft’s spin to slow down and the xenon’s spin rate to grow. The Dawn project has invested a great deal of effort over the past 2 years to understand the detailed behavior of the xenon while the spacecraft is spinning. This has involved both sophisticated analysis techniques as well as spin tests with a tank of exactly the same shape and size as Dawn’s filled with a fluid with properties similar to those of xenon’s. Based on this work, engineers can predict how quickly the spacecraft and xenon will change each other’s spin rates.
After the third stage has finished firing, it remains securely attached to Dawn for another 4 minutes 50 seconds. Although the stage is stabilized by spinning, the spacecraft does not operate that way; yet by this time, they would be spinning together at 46 rpm, too fast for the latter’s control system. Therefore, starting 5 seconds before separation, the third stage activates a surprisingly simple system to slow its rotation rate. Wrapped around the Delta are two cables, each 12.15 meters (39 feet 10 inches) long. At the end of each is a 1.44-kilogram (3-pound-3-ounce) weight made of aluminum and tungsten. When the cables are released, the spin causes them to unwind. As they carry the weights farther and farther out, the spin slows down because of the same principle that makes an ice skater spin faster by pulling her arms in or slower by extending them to her sides. After 4 seconds, when they are fully unwound, the cables unhook from the spacecraft. With their weights still attached, they enter independent orbits around the Sun; perhaps one of them will be studied by a future solar system archeologist.
The values of the weights are chosen carefully and are accurate to about 1 gram (0.04 ounces) in order to achieve the required change in spin rate. Even with a 208-kilogram (459-pound) third stage (which was 2230 kilograms, or 4915 pounds, before it began expending its propellant) and a 1218-kilogram (2685-pound) spacecraft, this small “yo-yo” system halts the spin and even reverses it, leaving Dawn rotating at 3 rpm in the opposite direction from its original spin. About 1 second after the cables have separated, the attachment between Dawn and its rocket is severed, and springs push them apart.
Only 28 minutes 30 seconds after liftoff, while 1016 kilometers (631 miles) above their home planet, the Delta bids the spacecraft farewell. The third stage, its raison d'être fulfilled and having no further purpose, continues on its own through the vast emptiness of the solar system. But its disconnection from Dawn triggers sensors on the spacecraft that alert the central computer to the separation.
Spinning slowly at 3 rpm in one direction, with xenon spinning inside in the opposite direction (because the propellant still lags behind its container), Dawn waits for 8 minutes 20 seconds. That is long enough for the spacecraft and xenon each to slow the other down, and after that, Dawn’s systems are ready to go to work.
In the next log, shortly before launch, we will see what the spacecraft plans to do as mission control waits to hear from it.
Dr. Marc D. Rayman
June 23, 2007
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