Dawn spacecraft to Vesta and Ceres passing by Mars today (February 17 2009)

Vesta, Ceres and the Moon to scale. Vesta will be Dawn's first destination, Ceres the second.

Mars is pretty well-explored so this isn't even as close to as exciting as something like a new asteroid or arriving at Vesta or Mars, but a noteworthy event nevertheless. Spaceflightnow.com has the information:

"A big oomph actually," said Rayman. "The gravity of Mars will change Dawn's path about the sun, 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 (9,330 kilometers per second). 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."

Also happy for the opportunity to fly past the fourth rock from the sun is Dawn's science team. With asteroid Vesta still more than two-and-a-half years away, Mars provides the perfect opportunity to give their highly-tuned instruments a workout.

"It is fortuitous that we need Mars to get out 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."