Marc Rayman • Sep 21, 2007
Dawn Journal: Awaiting Launch II
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 1,218-kilogram (2,685-pound) Dawn to space. For a launch prior to October 10 (a date chosen based on the sophisticated mathematics of interplanetary trajectory design, not because it is your correspondent's birthday), the rocket and spacecraft will spend about 62 minutes flying together. For launches on October 10 or 11, one of the launch phases, 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 one 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" (see the previous log for an explanation) 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 one 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 five 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[link "virtues" to the phrase with "the indefatigably patient probe" in the previous log] 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. (At right is 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°. 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 will be posted Monday, 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 Web master.
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
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