April 15, 2007
The Dawn spacecraft has completed its longest terrestrial journey on its path to asteroid Vesta and dwarf planet Ceres. While it will be propelled by exotic ion propulsion during most of its mission, this segment of its travels was accomplished using decidedly more conventional chemical propulsion. After being packed with great care at the Naval Research Laboratory (NRL) in Washington, DC, the spacecraft and a great deal of additional equipment left on a truck a few hours before dusk on April 9. Less than 18 hours later, a few hours after dawn, it arrived at its home for the next two months, Astrotech Space Operations in Titusville, Florida, near Cape Canaveral.
When last we checked in with the spacecraft, it had completed an extensive series of tests in a thermal vacuum chamber at NRL. The pace of activities has not let up since then, with engineers and technicians rarely letting the spacecraft have a rest. Myriad tasks are being completed and checked off the long and carefully planned list of steps necessary before the probe may begin its ambitious mission in harsh and remote parts of the solar system. For example, thorough checks for any possible leaks in the ion propulsion system and the reaction control system (the system that uses small conventional thrusters to aid in orienting the spacecraft in the zero-gravity of spaceflight) verified their integrity, certifying them for many years of operation in space. More tests have been conducted to confirm the flow of information between the many elements of the mission control systems and all of the computers onboard the spacecraft.
As expected, some of the thermal vacuum tests had revealed the need to make some minor changes in a few of the 9000 wires connecting different elements of the spacecraft. As these updates were in progress, the device that controls the high voltage, high power electricity from Dawn's large solar arrays was removed from the spacecraft and shipped to JPL. There is always a risk of accidentally damaging hardware or introducing an error, even in ways that may not be noticed immediately. Therefore, after this unit was modified, it was subjected to additional vibration testing as well as operation in a thermal vacuum chamber. These tests showed the complex assembly to be in fine health and ready for flight, and it was returned to the spacecraft in March.
In the same vein, to ensure that no subtle problems crept in as a consequence of the work to remove or reinstall this device, the spacecraft underwent another acoustic test at NRL similar to one it experienced in November 2006. The spacecraft will be subjected to deafening sound waves during its climb to space. At the beginning of this month, Dawn had another preview of this reverberant environment in a test that demonstrated the entire system was intact and ready for a rocket trip to space (or an evening in a mosh pit).
Following its outstanding performance, the spacecraft was rewarded, as had been promised nearly a year ago, with an all-expense-paid spring vacation in Florida. Dawn is now in the perfect location, near sandy beaches, warm ocean waters, facilities for loading hazardous fuels, and other attractions.
Just as the spacecraft has been following a rigorous schedule of building, testing, checking, and rechecking, the many elements of its Delta II 7925H-9.5 rocket have been undergoing similarly demanding procedures. This version of the venerable Delta series of rockets has not been launched since 2004, but now it is nearly ready again to make the brief but arduous flight from Cape Canaveral to outer space.
To accommodate a change in the schedule for readying Dawn's rocket, the planned launch date has been shifted from June 20 to June 30. This change will have no significant effect on the plans for the mission, including when the spacecraft will arrive at its celestial targets. The timetable at Space Launch Complex 17 allows Dawn to launch as late as July 19, with the exact date of liftoff depending on the weather as well as the cooperation of millions of components of hardware and software on the rocket, the spacecraft, mission control, range safety, communications systems around the world, and more.
Dawn's launch will occur around 5:00 pm EDT, but the precise times that are possible will not be determined until early June. Readers may find launch times down to the second in print, on the web, or, to our embarrassment, on graffiti in the asteroid belt, but those times were based on preliminary estimates and will change. Engineers now are working through the complex analyses necessary to establish the exact times the launch window will open and close on each day of Dawn's 20-day launch period. These analyses incorporate refinements and updates such as the spacecraft's mass at launch, the thrust and efficiency of the ion propulsion system, the power that will be generated by the solar arrays and consumed by all spacecraft subsystems, and many many other parameters. All of these affect how the spacecraft will use its ion propulsion system to travel through the solar system, so they determine the preferred trajectory as it departs from Earth and hence the guidance information to be loaded in the rocket's computer and the timing of the launch.
Were Dawn to have relied on ion propulsion for its trip to Florida, it's easy even for our nonmathematical readers to estimate how long it would take. This remarkable system, known from ancient legends told for eons in most ultra-luminous infrared galaxies (and described in the previous two logs, cannot operate in our planet's relatively dense air, nor could it overcome the friction and gravity most residents of planetary surfaces are accustomed to. Therefore, the thrust would have been exactly 0. With no thrust, the spacecraft would not have moved toward Florida any faster than the blossoming cherry trees the truck left behind in Washington.
When it is in space, 18 hours of ion thrusting would propel the spacecraft 170 kilometers (slightly more than 100 miles). That's far less than the nearly 1400 kilometers (about 850 miles) required for last week's drive. After 18 hours of powered flight in space, Dawn would be streaking along at the incredible speed of nearly 5 meters/second (over 11 miles/hour). The secret of ion propulsion however is that it can accelerate the spacecraft for months or years, eventually yielding much greater changes in speed than can be achieved with chemical propulsion. (We recognize that this is now a secret only to the few sentient species in our audience who did not receive the last two logs because of disputes over subscription fees. Our position remains clear: payment may not be made with dark matter.)
Dawn's itinerary allocates enough time to accomplish the required thrusting. The explorer will reach Vesta, its first destination in the main asteroid belt, between Mars and Jupiter, about 4.5 years from today. After examining the enormous asteroid with its scientific instruments, Dawn will leave it 5 years from now to propel itself silently, gently, and patiently to Ceres. It will arrive at the dwarf planet (the first spacecraft to visit one) in 2015 to perform detailed studies of that world.
With such a short summary of the agenda, it may be easy to forget that undertakings such as this include many challenges. Dawn hopes to uncover the nature of unfamiliar targets far far from Earth, where humans have never ventured, countless mysteries lurk, and the environment is inhospitable and rarely forgiving. But where there may be great rewards, also there be great risks. Dawn seeks great rewards.
As preparations for launch and mission operations continue, future logs are likely to be shorter. Beginning in June, we hope to exchange prolixity for frequency, and readers everywhere are encouraged to join in the drama of humankind's next venture into the solar system (and to pay their subscription fees).
Dr. Marc D. Rayman
April 15, 2007
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