February 16, 2009
Launched in September 2007 and propelled by hyper-efficient ion engines, NASA's Dawn spacecraft passed Mars' orbit last summer. At that time, the asteroid belt - where Dawn's two targets, asteroid Vesta and the dwarf planet Ceres, reside - had never been closer. In early July the spacecraft began to lose altitude, falling back towards the inner solar system. Then October 31, 2008, after 270 days of almost continuous thrusting, the ion drive turned off.
"Not only are our thrusters off and we are dropping in altitude, but we are plunging toward Mars," said Marc Rayman, the Dawn project's chief engineer from NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "And everybody here on Dawn could not be happier."
The team's joy at plummeting toward a planet is not unfounded. Mars was always an important waypoint for the Dawn mission. It all has to do with gravity assists.
A gravity assist is the use of the relative movement and gravity of a planet or other celestial body to alter the path and speed of a spacecraft, typically in order to save fuel, time, and expense. A spacecraft traveling to an outer planet (or in this case an asteroid) will decelerate because the incessant tug of the Sun's gravity slows it down. By flying a spacecraft close by a large planet and its large gravity field, some of the planet's speed as it orbits the Sun is transferred to the spacecraft. In Dawn's case, it is using the Red Planet's tremendous angular momentum (the speed at which Mars orbits the Sun) to give it a little extra oomph.
"A big oomph actually," said Rayman. "The gravity of Mars will change Dawn's path, enlarging its elliptical orbit and sending the probe farther from the Sun. It will also change Dawn's orbital plane by more than 5 degrees. This is important because Dawn has to maneuver into the same plane in which Vesta orbits the Sun."
If Dawn had to perform these orbital adjustments on its own with no Mars gravitational deflection, it would have required the spacecraft to fire up its engines and change velocity by more than 5,800 miles per hour (9,300 kilometers per hour). Such velocity changes would have required Dawn to carry an extra 230 pounds (104 kilograms) of xenon fuel.
"Without the gravity assist, our mission would not have been affordable, even with the extraordinary capability of the ion propulsion system," said Rayman. "That's why we are happy Dawn is now plunging toward Mars."
"It is fortuitous that we need Mars to get to Vesta and Ceres," said Carol Raymond, Dawn's deputy principal investigator, from JPL. "Since there are other spacecraft currently operating at Mars with similar instrumentation, we will be able to check our measurements against their knowledge of Mars, and carry that information farther out into the solar system."
But the Mars gravity assist is not the final hurdle on Dawn's road to the asteroid belt. The subsequent 30 months include more than 27 months of ion thrusting to successfully rendezvous with Dawn's first target, Vesta.
While an accurately flown encounter with Mars makes a big difference in the life of NASA's asteroid pioneer, the planet does not come out unscathed. Weighing in at 2,500 pounds (1,134 kilograms), Dawn has its own mass and thereby its own gravitational field. In contrast, the somewhat more massive planet is almost 600 million million million times more substantial than that of the spacecraft.
"The laws of physics tell us that Mars will pay a price for helping Dawn," said Rayman. "The flyby will cause Mars to slow in its orbit enough that after one year, its position will be off by about the width of an atom. If you add that up, it will take about 180 million years for Mars to be out of position by one inch (2.5 centimeters). We appreciate Mars making that sacrifice so Dawn can conduct its exciting mission of discovery in the asteroid belt."
Dawn's 3 billion mile (4.8 billion kilometer) odyssey includes orbiting asteroid Vesta in 2011 and the dwarf planet Ceres in 2015. These two giants of the asteroid belt have been witness to much of our solar system's history. By using Dawn's instruments to study both objects for several months, scientists can more accurately compare and contrast the two. Dawn's science instrument suite will measure geology, elemental and mineral composition, shape, surface topography, geomorphology and tectonic history, and will also seek water-bearing minerals. In addition, the Dawn spacecraft's orbit characteristics around Vesta and Ceres will be used to measure the celestial bodies' masses and gravity fields.