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Artist concept of NASA's Dawn spacecraft

Dear Dawnitsways,

The Dawn project welcomes you to deep space! Dawn is operating smoothly on the fourth day of its 8-year adventure. Like new parents, its extremely proud and greatly sleep-deprived Earthbound mission operations team is carefully monitoring its every move.

Launch had been targeted for September 26, but during its last few days on Earth, Dawn continued to be subjected to the vagaries of the weather on that dynamic planet. The Delta II 7925H-9.5 rocket had been scheduled to have its second stage filled with propellants on September 23. The nitrogen tetroxide was pumped in before bad weather prevented further activities at Cape Canaveral's Space Launch Complex 17B, so Dawn waited patiently and safely inside the protective payload fairing, or nose cone, of the rocket. On September 24 a delicious blend of hydrazine and unsymmetrical dimethylhydrazine (together known as Aerozine-50) was loaded as the countdown resumed, targeted for launch on September 27 at the 7:20 am EDT opening of the launch window.

This writer arrived at JPL at 11:30 pm PDT on September 26. The security guards, although recognizing him (and his car), diligently verified his identification in the chilly autumn evening and received his enthusiastic greeting, “We're going to the asteroid belt tonight!” Upon hearing “All right!!” your loyal correspondent was ready to head into mission control.

The countdown continued smoothly until shortly before launch when a ship was discovered to have entered a restricted zone in the waters east of the launch site. This required an unplanned hold.

The Delta rocket does not account for the changing position of the launch pad in space as Earth rotates, so a launch delay would place the spacecraft on a different trajectory. Most interplanetary missions have launch windows of only 1 second because they have too little maneuvering capability to compensate for the altered trajectory of the rocket. Dawn's ion propulsion system gives it much greater flexibility, so its launch window on September 27 was 29 minutes long. That proved to be more than enough to allow the Coast Guard to invite the ship to depart and then continue to ensure that no one would be at risk of being harmed as the launch vehicle flew overhead.

The countdown resumed, no other glitches occurred, the rocket roared to life, and Dawn's voyage began at 7:34:00.372 am EDT. It was propelled off the launch pad not only by nearly 890,000 pounds of thrust (which grew within 1 second to about 1,070,000 pounds) but also by the enthusiasm of the people who designed and built it, those who will fly it and will analyze the data it returns, and the vastly greater number of people who share in the yearning to know the cosmos.

The rocket and all downrange tracking systems performed extremely well, and Dawn's ride to space was very much what had been foretold in prophecy. This was the 76th consecutive successful launch of a Delta II. Following separation from the third stage at 8:36 am, Dawn went to work, and the Deep Space Network at Goldstone, California began receiving its radio transmissions at about 9:43 am.

Since then, the mission operations team at JPL has kept it company constantly, albeit from an increasingly remote location. Even as the cheers of hearing from the probe were echoing in mission control, the team began a prompt assessment of Dawn's health. It was evident quickly that it was in good condition, and operators were pleased to see that the myriad problems they had trained to handle were now little more than a fond recollection from simulations.

Upon conducting more detailed analyses of Dawn's telemetry, engineers found that it handled itself quite admirably, operating completely on its own, in space for the first time. As it was programmed to do, it dealt with the few minor unexpected conditions it encountered with the skill of a seasoned pro.

Over the subsequent days, the team gradually reconfigured the spacecraft subsystems to prepare for the extensive testing and checkout scheduled to conclude in mid-December. By the time this report was filed, the team had sent 148 sets of commands to Dawn and had scrutinized thousands of measurements of temperatures, pressures, voltages, currents, data buffer volumes, valve and switch positions, and many many other parameters. Now the spacecraft is ready to be put through its paces before it begins its ion propelled voyage past Mars and then on to the uncharted and distant worlds Vesta and Ceres.

After years of planning, designing, building, and testing, the Dawn mission is underway. While the fulfillment of its scientific objectives remains well in the future, the craft finally is in space, and a far far more exciting and challenging phase of the project is beginning.

Dawn is 1,158,000 kilometers (720,000 miles) from Earth or 3 times farther than the moon. Radio signals, traveling at the universal limit of the speed of light, take almost 8 seconds to make the round trip.

Dr. Marc D. Rayman
8:30 pm PDT September 30, 2007

› Learn more about the Dawn mission

TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

  • Marc Rayman
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Artist concept of NASA's Dawn spacecraft

Dear Countdawns,

The countdown is underway for Dawn’s liftoff on September 26 at 7:25:00 am EDT.

This is the second time our hero has been within a few days of launch, and with a full 20-day launch period still ahead of it, confidence is high the mission will get underway soon. Now the Dawn project is ready with a new flight profile to allow the probe to leave Earth months later than planned and yet still keep its interplanetary appointments on schedule.

Because this new flight plan begins with a different launch, we present here an update to the July 5 log, accounting for the changes. Dawn’s intention was to launch in June or July, and the postponement was because of circumstances beyond our control; nevertheless, we understand the difficulties this can cause our readers. Therefore, for those readers who have the July 5 log tattooed on either themselves or a relative, we have arranged with our favorite fine tattoo and taxidermy emporium for a discount on this log. (Certain restrictions may apply; this offer void in galaxies with less than the cosmic abundance of deuterium or tattoo ink.) If weather or other minor glitches delay the launch by a few days, we will not publish another update.

In the last log we began on the launch pad with the entire Delta II 7925H-9.5 rocket, including its passenger. Together, they are 285,581 kilograms (629,592 pounds), and we followed the plan for the delivery of the 1218-kilogram (2685-pound) Dawn to space. For a launch prior to October 10 (a date chosen based on the sophisticated mathematics of interplanetary trajectory design, not it being your correspondent’s birthday), the rocket and spacecraft will spend about 62 minutes flying together. For launches on October 10 or 11, one phase of the launch, the coast in Earth orbit, will be 2 minutes 40 seconds longer. Should launch need to occur on October 12 - 15, the coast will be 4 minutes 16 seconds longer than for launches in the beginning of the launch period. In all cases however, the relative timing of other events during the flight of the Delta rocket will remain as described in the previous log.

During their shared flight, the rocket is in control. Following separation from its conveyance to space, Dawn has three primary objectives: 1) get sunlight on its solar 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 also 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, detaching 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.

The Delta does not provide electrical power to the spacecraft (even though it rides in the first-class section), so Dawn carries a large battery. While on the rocket, as few of the probe’s components as possible are turned on. Its computer and a few other devices are operating, heaters are activated as needed, and some data are recorded, but mostly the craft simply waits for the signal that indicates it and the third stage have parted ways. Conserving energy (a responsibility familiar to readers on Earth) is vitally important.

Now one might be tempted to conclude that with the longer time from liftoff to separation for an autumn launch than a summer launch, Dawn’s power reserves will be more critical. Readers are urged to avoid this temptation with their utmost resolve! When the two solar arrays are folded, the outermost panel on each side is oriented so the solar cells point out. The arrays are so powerful that even with only 1 of the 10 panels exposed to the Sun, enough electricity is generated to satisfy all of Dawn’s needs (except thrusting with the ion propulsion system) when at Earth’s distance from that brilliant orb. While the battery will have been partially drained during the last few minutes on the launch pad and during ascent, the intermittent exposure to the Sun in the course of the “barbecue roll” during the quiet coast in Earth orbit will provide sufficient power for all systems that are running and still have enough extra to recharge the battery. By the time the barbecue ends and the second stage begins preparing for its second burn, Dawn’s battery should be fully charged.

Because the craft will be returning a tremendous bounty of rich scientific information from distant Vesta and Ceres, its radio system is powerful. It does not have a mode in which it can transmit at low power, so 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. After 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 commands 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 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 a short extension from each corner of the boxy body of the spacecraft is a pair of 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 again, 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 well 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 are 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 (DSN) complex in Goldstone, California will be ready to detect Dawn’s transmissions and pass the data on to JPL. Had the launch occurred in the summer, Dawn would have begun transmitting its signals in view of DSN and European Space Agency antennas in Australia. Now, following its longer travel time from Florida, the coast in orbit will carry it farther east, so Goldstone has the privilege of being the first to communicate with the spacecraft.

The DSN station 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 35 minutes or as long as 2 hours 50 minutes even if Dawn encounters no surprises along the way, and more than 4 hours 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. One of them will try to find the time to write another of these logs as well.

Meanwhile, Dawn will continue racing away from Earth. In less than 2 hours 45 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 leaving the launch pad, as it travels 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 imaged 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 portrait ever made of that craft.) It would be very faint, perhaps no more than a speck amidst a sea of distant stars between the constellations Auriga and Gemini near right ascension 6 hours 20 minutes and declination +28.5°. [Note to self: before this is posted, remember to insert a wonderfully clever remark here that connects Dawn to a charioteer and the twins, the figures represented by these constellations.] These approximate coordinates will change if Dawn’s launch does not occur on September 26 at the opening of the 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 a little more complex. Throughout the launch period, the farthest from this location would occur for a liftoff at the end of the launch window on October 15. That would shift the coordinates to approximately 7 hours 28 minutes and +26°, within Gemini. For anyone interested in trying to observe the spacecraft, please visit JPL’s HORIZONS system, and change the target body to (no surprise here) “Dawn” to find its exact location.

Even before the navigation team gets a good fix on Dawn after launch and enters the trajectory data into HORIZONS, observers in Hawaii may get a view of Dawn's early light. With a launch on September 26 at the opening of the launch window, the spacecraft will exit the shadow of Earth 1 hour 19 minutes after liftoff (2:44 am Hawaii-Aleutian Standard Time, or HST). At that time, the spacecraft will not yet have deployed its solar arrays, so it may not be very bright, but its relatively small size at that time should be somewhat compensated for by its relative proximity to Earth. It will be about 68° above the horizon when it comes into the sunlight, and will pass directly overhead 13 minutes later.

Dawnophiles in Hawaii, Alaska, and the Pleiades may be treated to a particularly attractive alignment shortly after that. As viewed from the first two of those locations, Dawn will appear to pass less than 1.5° north of the center of that familiar star cluster at about 3:14 am HST when it is less than 18,000 kilometers (about 11,000 miles) from the surface. (Note: “it” refers to Dawn. The Pleiades, in contrast, will be more than 430 light years from Earth, or more than 200 billion times farther than Dawn.) By then it likely will have opened its solar arrays, presenting a much larger target for the Sun to illuminate.

Observers are advised however that, depending upon the spacecraft's progress in the many steps described above, the arrays may already be pointed straight at the Sun by the time it transits the Pleiades, so the reflection would not be directed toward Earth. The Dawn project is confident no one's eyes will be damaged from direct exposure to this view; indeed, the spacecraft may be quite dim. It is possible however that before the spacecraft has completed aiming its panels at the Sun, terrestrial spectators could see a brief bright reflection or “flare,” a phenomenon familiar to amateur satellite observers. We will not know until we receive reports from witnesses.

If liftoff is delayed to later in the launch window, the views described here will occur later by less than the change in launch time. More details on where to look are posted here, and the Dawn navigation and outreach teams will be standing by to update the information as soon as possible after liftoff. We will do our best to give some fortunate observers the opportunity to see Dawn as it recedes into the depths of space. If you obtain any images, we will be interested in seeing them and would appreciate your sending them to the Dawn Education and Public Outreach Team.

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
September 21, 2007

› Learn more about the Dawn mission

TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

  • Marc Rayman
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Artist concept of NASA's Dawn spacecraft

Dear Dawntastics,

Now less than two weeks from its planned September 26 launch, Dawn is eagerly awaiting the beginning of its fantastic adventure. It has been here before, but the opportunity to take the fast track out of Florida in June or July was consumed by delays in the readiness of the launch vehicle, adverse weather conditions, and problems with launch vehicle tracking facilities. After spending a very quiet, if not boring, 7 weeks at nearby Astrotech, the spacecraft returned to the launch pad on September 11.

While Dawn remains unwaveringly committed to its mission to explore Vesta and Ceres, the first part of the trajectory from Cape Canaveral’s Space Launch Complex 17B to the alien worlds is different for an autumn departure from what it would have been in the summer. Lacking an interplanetary version of mapquest.com, the Dawn team has developed a new route. It begins with a different launch profile, so right now -- at this very instant! -- you are reading an updated version of the June 23 log accounting for those changes. (The preceding sentence may not apply to readers capable of advanced time travel, but this sentence does -- or did -- or perhaps will.)

In addition to some changes in times, altitudes, and velocities, readers who have the details of the earlier version of this log committed to memory will notice that the mass of the third stage is different. As with most missions, the Dawn team finished designing the launch trajectory before the spacecraft was fully assembled. To ensure that the parameters loaded into the rocket’s guidance computer would be correct, engineers planned for the maximum possible spacecraft mass. As explained at the beginning of the log we are updating, when Dawn was tested for its stability at 50 rpm, it was better balanced than had been expected. That meant it was not necessary to add as much mass to it as had been considered possible, so Dawn’s final mass was less than anticipated. By then, it would have been too time consuming to update the Delta’s computer parameters, so ballast masses were installed on the third stage. This brought the combined third stage and spacecraft mass to the value used in all the trajectory calculations.

With the delay in the launch and the need to redesign the guidance program for the different launch conditions anyway, engineers were able to take advantage of their knowledge of the final spacecraft mass. So the 5.34 kg (11 pounds 12 ounces) of ballast were removed from the third stage, and the new plan benefits a little from the lower mass by commanding the rocket to impart slightly more energy to the spacecraft.

With that background, 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 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 our friends 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 a previous 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 less than 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 77 seconds, and when they are released at L + 2 minutes 39.5 seconds, the rocket will be nearly 73 kilometers (117 miles) high and traveling 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 almost 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 continues 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 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 optimal 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 strategies (modified somewhat from the mathematically perfect solutions) include times that Dawn will thrust with its ion propulsion system and times that it will coast. As reported in another 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 27 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 before October 10, the pause in the second stage’s firing will last about 42 minutes 37 seconds. (Because the solar system is constantly rearranging itself, launches near the end of the launch period will slightly require longer intervals. Maintaining a constant interval for most of the launch period is a degree of flexibility enabled by the ion propulsion system and was chosen to reduce the vast volume of work required to design the autumn launch trajectories in the short time available.)

Had the launch occurred in June or July, Dawn would have departed in a very different direction, which would have been reached with a much shorter pause in Earth orbit. To deliver its precious payload to the new starting point for the interplanetary journey, the Delta rocket now needs 33 minutes 30 seconds longer to travel from the launch pad to its target than it would have on July 8. To keep its temperature comfortable during this extra time, the rocket performs a “barbecue roll,” allowing all parts to receive equal exposure to the hot Sun, warm Earth, and cold space. After turning to the programmed orientation, the second stage begins rolling at the lazy rate of 1 revolution every 6 minutes. During the 29 minutes 50 seconds of barbecuing, Dawn basks in the beautiful glow of its home planet for the last time. These are some of the final quiet moments before it goes to work on a journey of nearly 8 years and 5.1 billion kilometers (3.2 billion miles).

The second stage engine reignites at L + 51 minutes 35 seconds while at an altitude of 179 kilometers (111 miles) and operates for 2 minutes 39 seconds. Fifty seconds later, to finish its contribution to Dawn’s mission, the second stage fires 4 small rockets pointed around its circumference to spin the third stage and spacecraft to 48 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 + 55 minutes 8 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 275 kilometers (171 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 about 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 11.46 kilometers per second (7.12 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 48 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 recognize 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 these “yo-yo” weights are chosen carefully and are accurate to about 1 gram (0.04 ounces) in order to achieve the required change in spin rate. The slight reduction in mass of the third stage because of the removal of ballast required a tiny change in the yo weights. Eschewing both diet and exercise, technicians opted for surgical removal of 7 grams (0.2 ounces) from each one.

Even with a 204-kilogram (450-pound) third stage (which was 2224 kilograms, or 4903 pounds, before it began expending its propellant) and a 1218-kilogram (2685-pound) spacecraft, the 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 62 minutes 1 second after liftoff, while 1021 kilometers (635 miles) above their home world, 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 inside spinning at 39 rpm in the opposite -- the original -- 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. That update of the July 5 log also will have a special suggestion for our readers in Hawaii, Alaska, and near the Pleiades on how to catch Dawn’s early light less than 2 hours after liftoff.

Dr. Marc D. Rayman
September 12, 2007

› Learn more about the Dawn mission

TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

  • Marc Rayman
READ MORE

Artist concept of NASA's Dawn spacecraft

Dear Dawnbassadors,

NASA is preparing (again) to bring Dawn to the Florida skies as all systems are gearing up for a September 26 launch. This new date is later than had been planned just a few months ago; nevertheless, as we shall see, in the most important sense, this genuinely is not a delay for Dawn’s mission of adventure, discovery, and the search for answers to exciting and important scientific questions. Earth’s next interplanetary ambassador remains on schedule for its engagements.

Following the decision in July to reschedule the launch, a complex story described in the previous log and occasionally told with other scary stories around campfires or microscopic black holes in elliptical galaxies, the first priority was to move Dawn from Cape Canaveral’s Space Launch Complex 17B to a safe location so NASA’s bold new Mars explorer, Phoenix, could launch from nearby 17A. After the protective payload fairing (the nose cone) was removed from the Delta II rocket, the spacecraft and mated third stage were detached from the second stage on July 22. They were transported back to the Hazardous Processing Facility clean room at Astrotech Space Operations for safe storage. Since then, the spacecraft has had a very leisurely summer vacation, with little to do but allow technicians and engineers to maintain its readiness while it enthusiastically anticipates the autumnal beginning of its deep space voyage. Meanwhile, the first and second stages of the rocket remained on the launch pad, where they had a delightfully close view of the launch of Phoenix on August 4. Soon Dawn and the third stage will be reunited with the rest of the launch vehicle.

One of the keys to success in space exploration (and in some other challenging and complex endeavors of the human species) is careful planning and preparation for contingencies. In one example of that, as soon as the crane used to erect Dawn’s launch vehicle malfunctioned on May 30, engineers at JPL, Kennedy Space Center, and United Launch Alliance began detailed preparations for the possibility of a launch in September or October. Had the work not begun as early and as intensively as it did, it is quite likely that it would not have been possible to complete it in time for launching after Phoenix. Even with its uniquely capable ion propulsion system, Dawn cannot conduct its exploration of both Vesta and Ceres if it does not launch before late October 2007.

In working out the complex strategies for planning launches, engineers use a great deal of jargon, such as “C3 = 11.4 km2/s2” (C3 equals 11.4 kilometers squared per second squared), “-2? RLA dispersion for a 95% PCS” (negative two sigma right ascension of the launch asymptote dispersion for a 95% probability of command shutdown), or “This project is so cool, I can hardly believe we get paid to do this” (OK, perhaps this last isn’t really jargon, but it is part of the Dawn parlance). Terms that are particularly pertinent to our discussion here are the “launch period,” the interval of days on which a launch may occur, and the “launch window,” the range of time on any one day in the launch period during which a launch may take place.

Most interplanetary missions have brief launch periods during which they must take off or pay the price of a significant change in their itineraries. As explained in previous logs, Dawn’s travels are unusual because of the extraordinary capability of the ion propulsion system, which provides a thrust of susurrant gentleness that is more than compensated for by its virtually tireless persistence. With so much maneuvering capability, even though its launch was deferred by months, Dawn’s scheduled arrivals at asteroid Vesta and dwarf planet Ceres are effectively unchanged. The delay in launch does not necessitate a delay in accomplishment of Dawn’s goals, so in many important ways, from an overall mission perspective, the postponement of the launch is inconsequential.

Indeed, a complex combination of myriad factors, including the positions of Earth in its orbit on candidate launch dates in 2007, of Mars in its orbit in the first few months of 2009, and of Vesta in its orbit late in 2011, makes the new launch more favorable for the mission. Although still a challenge of astronomical proportion, this will make it slightly easier for Dawn to complete its assignment. This may be translated to slightly greater resilience in keeping its alien appointments should it encounter difficulties on its voyage through the unforgiving and remote depths of space.

Now Dawn will follow a completely different launch trajectory and take a different path to Mars. Had it launched in July, the spacecraft would have used Mars early in April 2009 to boost it along its way to Vesta. In the new plan, it will swing by the red planet in February 2009. Mars is one of the easiest destinations in the solar system to reach, and Dawn could travel there more quickly if that were its sole objective. Much of the ion thrusting prior to Mars however is designed to aim the craft so that when it reaches Earth’s neighbor, the planet’s gravity slings it in the most effective way to help it in its long flight to distant Vesta. Leaving Earth in September or October lets Dawn gain greater benefit from its brief visit to Mars.

Despite remaining with Earth throughout the summer of 2007, Dawn’s new flight profile will allow it to catch up with the old one. The spacecraft will be in the same place in the solar system in the summer of 2009 as it would have been had it launched in June or July. The mission after that will be quite similar to what it would have been with the earlier launch. The principal difference is that to accomplish the mission with the later launch, Dawn will consume a little less of its xenon propellant, so more will be available in case the probe needs to perform unplanned thrusting.

To account for the new launch date and path to Mars, Dawn’s departure from Earth will be very different from what it would have been in June or July. Instead of launch windows in the middle of the afternoon, now Dawn will launch closer to dawn. On September 26, the launch window is 7:25 am EDT to 7:54 am EDT. (To simplify coordination among the many organizations around the world participating in the launch, liftoff is scheduled on the whole minute. The capability to round off the time this way is another benefit of the ion propulsion system’s flexibility, and it should be particularly appreciated by all Dawn enthusiasts on planets whose clocks don’t have second hands, including our newest readers, members of the Honorable Minority of Antipunctualists in the Horologium supercluster of galaxies.)

If unfavorable weather or other fortuities prevent launch (possibilities with which all loyal readers are exceptionally familiar) on September 26, launch windows during the rest of the launch period are:

Sept. 27: 7:20 - 7:49 am EDT
Sept. 28: 7:14 - 7:43
Sept. 29: 7:09 - 7:38
Sept. 30: 7:03 - 7:32
Oct. 1: 7:12 - 7:31
Oct. 2: 6:55 - 7:24
Oct. 3: 6:49 - 7:17
Oct. 4: 6:44 - 7:13
Oct. 5: 6:41 - 7:10
Oct. 6: 6:38 - 7:07
Oct. 7: 6:35 - 7:12
Oct. 8: 6:34 - 7:12
Oct. 9: 6:33 - 7:11
Oct. 10: 5:43 - 6:23
Oct. 11: 5:42 - 6:22
Oct. 12: 5:13 - 5:54
Oct. 13: 5:13 - 5:57
Oct. 14: 5:16 - 5:58
Oct. 15: 5:18 - 6:00

Mortal readers are encouraged not to waste time trying to discern a pattern in either the time of the opening of the launch windows or the window durations. The underlying reasons for these values are manifold and complicated, and to avoid violating statutes in some spiral galaxies on publishing dangerously boring text, the explanations will be omitted. Let’s look briefly at just one relatively simple observation: the 19-minute window on October 1 is shorter than all the others. Under certain circumstances that are unlikely but possible, an earlier launch window opening on that day would make Dawn pass close enough to the moon less than 28 hours later that the gravitational deflection of the spacecraft could only be compensated by significantly more ion thrusting than planned. Rather than take the small risk of incurring this minor complication in the mission, the window was shortened. On all other days in the launch period, as the spacecraft departs Earth in roughly the same direction, the moon will be elsewhere in its orbit so it will not cause as much interference in the trajectory. Still, the moon’s gravity is included in all analyses.

Demanding as it is, there is more to replanning the mission than designing new trajectories for the rocket and the spacecraft. Because the geometry for the departure from Earth has changed so much, the Dawn operations team has had to redesign many of the activities scheduled during the early part of the mission. The location of the spacecraft relative to Earth and the Sun will be quite different from what had been planned, so onboard instructions for how to orient in order to achieve certain objectives must be modified. While Dawn itself has spent an unusually quiet and leisurely time at Astrotech waiting to be reunited with its rocket, mission controllers have been very busy indeed developing new plans and the corresponding sets of commands for the first few months of the mission.

To maintain proficiency for launch, the team also completed another set of simulations of the final 16 hours of countdown, launch, and the first day or so of flight. Most of the week of August 27 was devoted to a slightly shortened version of the ORTathon that was conducted early in June. Differences from the first ORTathon included not only the launch time and trajectory, but also a new set of fiendish surprises injected by the simulation supervisor and a modified (and, in this participant’s careful analysis, a superior) selection of snack food in mission control.

One regrettable consequence of the changed launch conditions is that the timelines presented in the June 23 and July 5 logs will not apply for the new launch period. This threatens a lucrative deal negotiated with importers of the stone-engraved versions of these logs on icy asteroids in most irregular galaxies. Therefore, in the coming weeks, when the relevant analyses are completed and the new data are available, those logs will be reposted with the only changes being those that are essential to bring them up to date for the new launch plan.

Dr. Marc D. Rayman
September 3, 2007

› Learn more about the Dawn mission

TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

  • Marc Rayman
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Artist concept of NASA's Dawn spacecraft

Dear Dawntothegrounds,

There are two ways for a spacecraft to leave its launch pad: climbing on a blazing tower of powerful flames accompanied by a thunderous announcement of its departure or suspended securely and gently on the crane that hoisted it there in the first place. Now Dawn has the opportunity to experience both, with the former to be in September and the latter this month. The spacecraft and third stage of the Delta rocket are being prepared now for removal from the second stage. They will be transported to the clean room at nearby Astrotech Space Operations before being returned to the launch pad in less than two months.

For more than a year, Dawn had been planning for a launch between June 20 and July 10. As is well known to readers of these logs in galaxies of all shapes, Dawn’s ion propulsion system affords it a flexibility in its trajectory unavailable to missions that use conventional propulsion. One of the important consequences of this is that, as we will see in more detail in the next log, the postponement of the launch to September does not change the scientific objectives or plans; even the dates for arriving at Vesta and Ceres are essentially unaffected. Dawn’s launch period was chosen based on the readiness of the project to begin the mission and on the schedule for the use of Cape Canaveral’s Space Launch Complex 17, from which Delta II rockets get their start to space.

In addition to Dawn lighting up the Florida skies, NASA has planned to launch an exciting Mars lander, Phoenix, this summer on a Delta II. Phoenix’s launch period starts August 3 and, like most planetary missions, without ion propulsion, if it were unable to launch within a few weeks, it would have to wait a very long time for the correct planetary alignment to recur.

There are two pads at the complex. Phoenix has been scheduled for pad A, and Dawn’s departure will be from pad B. The pads are less than 175 meters (575 feet) apart, and prudence dictates that a rocket not launch from one when a spacecraft -- an extremely precious resource -- is mounted on a rocket at the adjacent one. In addition, there are some shared facilities for the two pads, including the environmental control systems for the protective enclosures for the delicate hardware. (This also is why Dawn must be removed from 17B while Phoenix has its turn at the starting gate.)

These considerations led to the plan to end Dawn’s launch period early enough that there would then be sufficient time and resources to devote to preparing Phoenix for its launch.

Following a delay of 10 days in the production of Dawn’s rocket components earlier this year, the beginning of the launch period was rescheduled for June 30. The time required to repair a crane at 17B at the end of May and beginning of June pushed the earliest attempt to launch Dawn to July 7. There was much less flexibility in the closing of the launch period because of the need to preserve the schedule for Phoenix, but creative launch teams began looking into ways to launch Dawn a bit later than July 10.

Launching a mission into space requires much much more than the simultaneous cooperation of a rocket, a spacecraft, and the weather.

The Delta’s flight is fully controlled by its onboard computers. Even with this launcher’s extraordinarily high record of success, it is important for engineers to receive telemetry from the rocket during the major events of its short mission so they can verify that it performed correctly. This allows every launch not only to benefit from the successes of the previous ones but also, if necessary, to avoid those rare problems that do occasionally affect rockets. Each mission, with its unique trajectory, requires downrange tracking at certain locations. Antennas at many ground sites are available to track the vehicles, but often some portions of the flight are not within view of any of these stations. In such cases, ships or aircraft are used.

For the June 20 - July 10 launch, NASA had planned to use an aircraft (known as a P-3 Orion, a name sure to appeal to readers from Betelgeuse to Rigel) in the southeast Atlantic Ocean to receive data from the second and third stages. As the end of Dawn’s launch period changed, conflicts with prior commitments for that aircraft led to the decision to use a ship instead.

Tracking ships are leased by NASA from private owners and are outfitted with a system for acquiring the signals from the rocket. The principal burden this system carries is its name: ocean-going test and evaluation transportable resource (known to rocket scientists and lutrine dyslexics as OTTR, and pronounced “otter”). Standard telecommunications systems on the ships relay the data collected by the OTTR back to controllers via communications satellites.

After all preparations had been made for its voyage, including installing the OTTR hardware and vaccinating the crew members for yellow fever, the 57-meter (186-foot) vessel left its California port on June 6, as the Dawn operations team was conducting its ORTathon. The next day, the OTTR demonstrated that it was operating as expected by tracking a Delta II carrying an Italian spacecraft from Vandenberg Air Force Base in California to low Earth orbit. The ship was projected to be in position by July 4 to track Dawn’s launch on July 7.

On June 19, the OTTR’s transport reached the entrance to the Panama Canal. While awaiting its turn to transit the canal, the ship’s chief engineer conducted an inspection of the engine and discovered a problem in 1 of the 12 cylinders. The ship had been scheduled to stop in Puerto Rico, so arrangements were made to repair it there. Well before it left harbor at San Juan on June 29, it was apparent the OTTR would not reach its destination by July 7.

As soon as the OTTR’s transportation problems arose, NASA began working on alternate plans. As part of its normal set of equipment, a U. S. Air Force jet known as Big Crow (a modified KC-135, derived from the Boeing 707 design) has systems that could receive the Delta II telemetry. The plane was undergoing scheduled maintenance at that time in preparation for an appointment that would make it unavailable after July 9. Plans were formulated to have Big Crow track the launch should it occur on July 7, 8, or 9, with the expectation that the OTTR would be in position by about July 10.

Besides the time required to repair the engine, there were two other obstacles to the OTTR’s providing support for Dawn’s launch. Headwinds and rough seas prevented the ship from making progress at its expected speed. In addition, Dawn’s rocket would follow a different trajectory almost every day of the launch period, thus changing the required location for the ship to put the OTTR within view of the rocket’s flight. When the decision was made in May to switch from the Orion to the ship, some of the ascent trajectories were quickly replanned to ensure that the shift from one day to the next was within the ship’s daily travel range. But now the ship, already behind schedule, was fighting the uncooperative conditions of the Atlantic as it chased a target that moved each day.

Meanwhile, for a time, weather seemed to be a threat to Big Crow as well. It had to conduct a flight to verify all systems before it began the multisegment trip to Ascension Island, its base station for the daily flights to the coordinates for tracking Dawn’s launch. Bad weather delayed the check flight, but finally it was completed, and the bird flew to its temporary island roost in good time.

Weather certainly proved to be a hindrance at Cape Canaveral. Predictions of afternoon thunderstorms made Dawn’s July 7 launch appear unlikely, and prospects in subsequent days did not look much better. Formulating an accurate forecast of the weather conditions was essential. Two days before launch, the second stage of the Delta is loaded with its propellants. One of the two propellants is highly corrosive, and once the second stage has been exposed to it, the stage remains useful for only about 37 days. After that, this part of the Delta would have to undergo an extensive refurbishment or replacement, either of which would consume many months and be very expensive. There would not be enough time to have a restored or new second stage before late October, after which the changing alignment of the solar system would no longer allow even Dawn’s powerful ion propulsion system to accomplish the planned mission for many years.

With Dawn’s launch continuing to creep up on the opening of Phoenix’s launch period, it was essential not to load second stage propellants until favorable launch conditions were foreseen. Once the second stage was filled, poor weather, an unauthorized incursion of a boat or aircraft into the launch vehicle safety zone, a balky valve, a misbehaving sensor, or any of the other myriad glitches that can lead to a launch scrub could create a serious dilemma. If NASA made a subsequent attempt to launch Dawn, that would deprive Phoenix of some of its precious launch opportunities. If Phoenix were given priority so that it would have all of the planned chances to launch during its limited period, that would impose a very long and expensive delay on Dawn, after which its scientific goals would be compromised. Therefore, NASA was very diligent in waiting for all conditions to be satisfactory before fueling the second stage.

On July 7, after several days of postponements, with forecasts still showing a high probability of inclement weather at the time of Dawn’s daily launch windows, the projected date for the OTTR being in position continuing to slip, and the Big Crow needing to depart soon for its previously scheduled commitment, the decision became clear. The exploration of the solar system (and taxpayers who fund it) would be best served by not attempting to launch Dawn in July. Dawn has the capability to conduct its mission with a lift off after Phoenix, whereas the Mars lander needs to leave Earth in August.

This complex story would not have been told (even in this version, simplified so that readers of all species may reach the end in less than a generation) had myriad conditions not conspired to prevent the launch during the June - July period. There are too many aspects of reaching space and undertaking ambitious missions there to describe them all. Many are rarely mentioned because, complicated though they may be, they usually work well enough that they blend into the background. Of course, such a rich background is a crucial part of the tapestry we weave in attempting to probe the universe, and without it, the beautiful highlights would not be possible.

Now Dawn is preparing to vacate Space Launch Complex 17 while Phoenix prepares for the opening of its 3-week launch period on August 3. After Phoenix has left Florida for the chillier north pole of Mars, Dawn will once again take its place of honor at the top of the rocket. In the next log, we will see how Dawn gets to spend its unplanned Florida vacation as well as how the change in its launch date affects its mission of exploration far from Earth.

Dr. Marc D. Rayman
July 15, 2007

› Learn more about the Dawn mission

TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

  • Marc Rayman
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Artist concept of NASA's Dawn spacecraft

Dear Dawnpours,

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

› Learn more about the Dawn mission

TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

  • Marc Rayman
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Artist concept of NASA's Dawn spacecraft

Dear Countdawns,

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

› Learn more about the Dawn mission

TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

  • Marc Rayman
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Artist concept of NASA's Dawn spacecraft

Dear Dawnventurers,

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

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TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

  • Marc Rayman
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Artist concept of NASA's Dawn spacecraft

Dear Dawntelligentsia,

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

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TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

  • Marc Rayman
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Artist concept of NASA's Dawn spacecraft

Dear Dawngineers,

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

› Learn more about the Dawn mission

TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

  • Marc Rayman
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