DEC 30, 2013 06:29 PM ET // BY IAN O'NEILL
On Sunday, at 5:17 p.m. GMT (12:17 p.m. EST), Europe’s Mars Express orbiter successfully completed a daring low-pass of Mars’ largest moon Phobos.
In an effort to precisely measure the gravitational field of the moon, the 10 year-old mission was sent on a trajectory that took it only 45 kilometers (28 miles) from the dusty surface, the closest any spacecraft has ever come to the natural satellite.
At the time of flyby, Mars Express was transmitting a “continuous radio signal across 208 million km of space” to NASA’s radio antennae near Madrid, Spain, wrote Daniel Scuka at ESA’s European Space Operations Centre, Darmstadt, Germany, in a blog update. The 70 meter radio antenna is part of NASA’s Deep Space Network (DSN), which precisely tracked the spacecraft’s signal. Post-flyby, NASA’s 70 meter Goldstone DSN antenna in the Mojave Desert, Calif., and ESA’s 35 meter antenna at New Norcia in Australia continued to track the mission.
During the flyby, DSN operators reported “a slight effect in the Doppler residuals,” meaning that, as expected, Phobos’ gravity had accelerated Mars Express’ orbital velocity very slightly. Through careful analysis of the Doppler shifting of the radio signal, Phobos’ gravity can be measured, allowing scientists to discern its mass and density — the most precise measurement to date.
All focus was on the spacecraft’s ability to send a continuous stream of data back to Earth, so close-up snapshots were not a possibility.