Marc Rayman • Dec 05, 2012
Dawn Journal: Hydrazine Haste Makes Waste
Dawn is continuing to gently and patiently change its orbit around the sun. In September, it left Vesta, a complex and fascinating world it had accompanied for 14 months, and now the bold explorer is traveling to the largest world in the main asteroid belt, dwarf planet Ceres.
Dawn has spent most of its time since leaving Earth powering its way through the solar system atop a column of blue-green xenon ions emitted by its advanced ion propulsion system. Mission controllers have made some changes to Dawn's operating profile in order to conserve its supply of a conventional rocket propellant known as hydrazine. Firing it through the small jets of the reaction control system helps the ship rotate or maintain its orientation in the zero-gravity of spaceflight. The flight team had already taken some special steps to preserve this precious propellant, and now they have taken further measures. If you remain awake after the description of what the changes are, you can read about the motivation for such frugality.
Dawn's typical week of interplanetary travel used to include ion thrusting for almost six and two-thirds days. Then it would stop and slowly pirouette to point its main antenna to Earth for about eight hours. That would allow it to send to the giant antennas of NASA's Deep Space Network a full report on its health from the preceding week, including currents, voltages, temperatures, pressures, instructions it had executed, decisions it had made, and almost everything else save its wonderment at operating in the forbidding depths of space so fantastically far from its planet of origin. Engineers also used these communications sessions to radio updated commands to the craft before it turned once again to fire its ion thruster in the required direction.
Now operators have changed the pace of activities. Every turn consumes hydrazine, as the spacecraft expels a few puffs of propellant through some of its jets to start rotating and through opposing jets to stop. Instead of turning weekly, Dawn has been maintaining thrust for two weeks at a time, and beginning in January it will only turn to Earth once every four weeks. After more than five years of reliable performance, controllers have sufficient confidence in the ship to let it sail longer on its own. They have refined the number and frequency of measurements it records so that even with longer intervals of independence, the spacecraft can store the information engineers deem the most important to monitor.
Although contact is established through the main antenna less often, Dawn uses one of its three auxiliary antennas twice a week. Each of these smaller antennas produces a much broader signal so that even when one cannot be aimed directly at Earth, the Deep Space Network can detect its weak transmission. Only brief messages can be communicated this way, but they are sufficient to confirm that the distant ship remains healthy.
In addition to turning less often, Dawn now turns more slowly. Its standard used to be the same blinding pace at which the minute hand races around a clock (fasten your seat belt!). Engineers cut that in half two years ago but returned to the original value at the beginning of the Vesta approach phase. Now they have lowered it to one quarter of a minute hand's rate. Dawn is patient, however. There's no hurry, and the leisurely turns are much more hydrazine-efficient.
With these two changes, the robotic adventurer will arrive at Ceres in 2015 with about half of the 45.6-kilogram (101-pound) hydrazine supply it had when it rocketed away from Cape Canaveral on a lovely September dawn in 2007. Mission planners will be able to make excellent use of it as they guide the probe through its exploration of the giant of the main asteroid belt.
Any limited resource should be consumed responsibly, whether on a planet or on a spaceship. Hydrazine is not the only resource that Dawn's controllers manage carefully, but let's recall why this one has grown in importance recently.
The spacecraft can stabilize or change its orientation using the hydrazine powered jets or reaction wheels. By electrically changing a wheel's spin rate, Dawn can start or stop rotating. When it is relying principally on these gyroscope-like devices, it still occasionally has to expend a little hydrazine to keep them from spinning too fast, as explained nearly four years ago. While thrusting (which is most of the time), the ion thruster works in concert with one of those other actuators to control the orientation.
For an ambitious and complex eight-year interplanetary expedition, Dawn's builders equipped it with backup systems. The craft was designed to use three reaction wheels at a time for normal operations, so it is outfitted with four. One of them encountered increased friction in June 2010. To preserve the life of the remaining wheels, engineers flew the spacecraft with all the wheels turned off from August 2010 until the Vesta approach phase began in May 2011, and they are doing the same during the flight from Vesta to Ceres.
As soon as the wheel had difficulty in 2010, Orbital Sciences Corporation and JPL began working on a method to operate with fewer than three, in case another one faltered. They developed software to operate in a "hybrid" mode with two wheels plus the hydrazine jets and installed it in the robot's main flight computer in April 2011 so it would be available at Vesta if needed.
The exploration of that alien orb, which exceeded all expectations not only for productivity but also for pure awesomeness, went very smoothly with the three operational wheels. As Dawn was spiraling away from the rocky behemoth in August 2012, however, another one experienced the same peculiar friction. Because the wheels had already been scheduled to be powered off shortly thereafter, the flight team continued the departure with them turned off, and it proceeded without further interruptions. With their typical swift professionalism, they immediately began working on the long-term ramifications of two wheels being unavailable in case the devices could not be recovered.
Because the hybrid control scheme uses more hydrazine than three wheels would, and using the hydrazine jets by themselves with no wheels consumes still more, operators undertook the new campaign to conserve the propellant during the journey to Ceres. Ever resourceful, engineers now anticipate that regardless of how healthy the wheels are, the probe will be able to conduct an exciting and rewarding exploration there.
Dawn will arrive at the distant and mysterious Ceres in 2015, and that allows plenty of time for the terrestrial members of the team to complete the exquisitely detailed plans for its adventures there. While that work is underway, the intrepid ship continues forging silently through the vast emptiness of space, distant and alone, patient and persistent. Despite its remoteness, the robot remains tightly bound to its human colleagues, for it is on their behalf and under the power of their ingenuity, thirst for knowledge, and hunger for adventure that it sails deeper into uncharted cosmic seas.
Dawn is 1.5 million kilometers (960 thousand miles) from Vesta and 62 million kilometers (38 million miles) from Ceres. It is also 1.59 AU (238 million kilometers or 148 million miles) from Earth, or 590 times as far as the moon and 1.61 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 26 minutes to make the round trip.
Dr. Marc D. Rayman
11:00 p.m. PST November 30, 2012
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