NASA's latest asteroid-chasing spacecraft, named Dawn, was launched in 2007 to explore the dwarf worlds Vesta and Ceres. But if you were aboard that spacecraft right now, you'd be getting a spectacular view of the planet Mars.
That's because, as I type this, Dawn is passing just 341 miles (549 km) from the Red Planet. During this brief encounter Mars's gravity will bend the craft's trajectory by 5° and boost its velocity by about 2,500 miles per hour (1.1 km per second). Had Dawn not encountered Mars en route to the asteroid belt, it would have needed to carry an additional 230 pounds of xenon to fuel its super-efficient ion-powered engines. That would have been too much to bring along, nixing the mission plans.
Mars does more than simply give Dawn a swift kick onward — it provides a handy calibration target for the craft's three scientific instruments: a visible-light camera, a visible and infrared mapping spectrometer, and a gamma-ray and neutron spectrometer nicknamed GRaND. (A press release about GRaND's activities is here.)
Because Dawn's approach path is on the dark side of Mars, the best tourist snapshots will come right after the flyby. (For a complete timeline of what happens when, read the interesting and informative online blog written by project manager Marc Raymond.)
But don't expect to see images beamed to Earth right away. To pan across the planet's amazing ocher plains, eons-old craters, and other features, Dawn has had to pivot away from its alignment with Earth. It'll return to its normal cruise attitude and reestablish high-rate communication in a couple of days, when the pretty pictures will start streaming home.
Then, on February 20th, Dawn swings back to view Mars one more time. The planet will be distant and receding, but it'll give flight controllers a view similar to how Vesta will appear when the spacecraft nears Vesta in 2011. It reaches Ceres in 2015.