Image of Vesta taken by the Hubble Space Telescope in 2007.
Credit: Hubble Space Telescope
MARYLAND: After three and a half years of thrusting silently through the void, NASA's Dawn spacecraft is on the threshold of a new world. Deep in the asteroid belt, it is less than four months away from the giant asteroid Vesta.
Dawn will enter orbit around Vesta in July 2011, becoming the first spacecraft ever to orbit a body in the asteroid belt. After conducting a detailed study of the uncharted alien world for a year, the spacecraft will pull off an even more impressive first. It will leave Vesta, fly to dwarf planet Ceres, and enter orbit there.
"We're closing in," said Marc Rayman, Dawn's chief engineer and mission manager. “This is unprecedented. No spacecraft has ever orbited two target bodies, much less worlds in the asteroid belt. A few probes have passed through this vast region of space, but not one could stop and develop an intimate portrait of its residents."
Very fuel efficient
A conventional spacecraft gets a boost from a big rocket, then coasts to its target. Carrying enough fuel for making significant changes in speed or direction along the way would make it too heavy to launch.
Dawn is far more fuel efficient. Spanning almost 20 m, its solar arrays collect power from the Sun to ionise atoms of xenon gas. These ions are expelled silently out the back of the spacecraft by a strong electric field, producing a gentle thrust.
The weightless, frictionless conditions of space flight allow this gossamer force effect to build up, so the spacecraft continuously gains speed.
"This spacecraft ultimately achieves fantastically high velocity while consuming very little propellant - using only a kilogram of xenon every four days, though its engines are almost constantly active."
No one big maneuver
With this system Dawn has been quietly, gradually reshaping its orbit around the sun, slowly spiraling out to its target, getting closer and closer as it loops around.
"By the time the spacecraft is in the vicinity of Vesta, its orbit will be very much like the asteroid's," explained Rayman. "So upon arrival, Dawn can slip into orbit as gently as it's been moving for three and a half years."
A conventional spacecraft screeches into orbit in a single dramatic, nail biting instant. The mission team is usually gathered in the mission control room with their eyes riveted on the telemetry to see that the final critical maneuver goes smoothly.
"With Dawn, there is no one big maneuver, no fiery burn, no single critical moment. Dawn's entry into orbit will be no different from what the spacecraft does almost all the time, what it's doing as you read this article. In fact, when Dawn sidles into orbit, I might be asleep. Or if it's Friday night I'll be dancing, or if it's Saturday I might be out taking pictures of dragonflies,” said Rayman.
