NASA’s Dawn spacecraft passed a crucial waypoint on the way to the asteroid belt Tuesday, whizzing 340 miles above Mars to use the planet’s gravity to set up for the probe’s arrival at its first target in 2011.
Dawn’s closest approach occurred at about 7:28 p.m. EST, effectively changing the probe’s velocity by more than 5,800 mph.
“The navigators were dead on and the thing flew past and got the energy assist that it needed,” said DC Agle, NASA spokesman.
The fly-by put Dawn on track for its intercept of Vesta, a rocky asteroid that could harbor clues about the formation of the planets.
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,” Marc Rayman, the mission’s chief engineer, said before the encounter.
The gravity assist was critical to the mission because it saves the weight of additional fuel that would be needed to propel the spacecraft. That extra fuel could have made the $425 million mission unaffordable, officials said.
Dawn’s highly efficient ion propulsion system uses small amounts of xenon gas to gradually guide the spacecraft across the solar system. The probe’s three thrusters have already fired for 270 days, consuming 158 pounds of xenon and changing the spacecraft’s speed by about 4,050 mph.
Scientists are taking advantage of the Mars fly-by to calibrate Dawn’s suite of instruments before they are employed in the asteroid belt.
The sensors are collecting test data for comparison with information gathered by a fleet of orbiters stationed at Mars. The instruments analyze neutrons, gamma rays, and ultraviolet, visible and infrared light.
“Any time you compare data sets, there is the potential to find something new,” said Tom Prettyman, the lead investigator for Dawn’s gamma ray and neutron detector.
Observations began early Tuesday and ground controllers were only able to monitor the fly-by through low-gain auxiliary antennas. Dawn will turn its primary antenna toward Earth Thursday morning and capture a final set of images of Mars Friday morning.
All data from the fly-by should be sent back to Earth by next week, according to Rayman.
Dawn’s space age propulsion system will remain silent for the next four months as Dawn coasts in its new orbit that stretches further from the sun.
Engineers will fire up the engines again in the middle of June to begin driving Dawn toward Vesta.
The oval-shaped asteroid has an average diameter of about 320 miles, or about the size of Arizona. Vesta’s surface is coated with hardened lava that seeped from its surface shortly after its formation.
Officials estimate Dawn will need to fire its ion propulsion system for about 720 more days before reaching Vesta. Arrival at the asteroid is targeted for around Sept. 1, 2011.
The arrival date could change based on the Dawn’s final trajectory, according to a NASA spokesman.
Dawn will spend up to nine months studying Vesta before beginning a three-year chase of asteroid Ceres, a spherical icy dwarf planet.
Ceres arrival is scheduled for February 2015, and officials tentatively plan to retire Dawn later that year.