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.
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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.
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"In order to perform the Phobos flyby radio science measurements, the spacecraft needed to have its high gain antenna dish pointed at Earth for the entire duration of the flyby operations," said Scuka. "This meant that we were not able to conduct observations with any of the other instruments (which would need to be pointed at Mars)."
However, the operation allowed the spacecraft to beam back an extra 200 Gigabits of observational data, including imagery of Phobos during an earlier 500 kilometer pass of the moon.
The successful flyby marks the end of months of planning by the ESA team managing Mars Express that, as of last week, has been in Mars orbit for 10 years. This flyby opportunity will hopefully provide further clues to the origin of the knobbly 13.4 kilometer (8.3 mile) wide moon that orbits Mars only 6,000 kilometers (3,700 miles) from the planet's surface.
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By precisely measuring the gravitational influence on a spacecraft during flyby, planetary scientists will better understand the moon's composition. During previous flybys, scientists calculated that the moon must be one-third empty space, which means the object may be a "rubble pile" - an agglomeration of smaller rocks hold together under a mutual gravity. But did the material come from a cataclysmic Mars impact? Or was Phobos, and its smaller satellite sibling Deimos, once an asteroid that got captured by Mars' gravity?
Those answers may not come until we can carry out a dedicated sample return mission of the moon's regolith, but the flyby will certainly aid our understanding of Phobos' internal structure.