Mars Express Orbiter completes a daring Phobos Flyby at arm’s reach of the mysterious moon of Mars. Completing its ten-year old mission on Dec. 29 2013, the ESA had extra celebrating to do on New Years Eve. During the flyby the orbiter relayed information about its velocity and the mild acceleration caused by the gravity of Phobos via long-range radio signals captured on earth by a joint NASA and ESA radio receiver dish array. ESA scientists used the information to help determine the mass of Phobos, whose structure has been determined to be almost a third empty space. Both Phobos and the smaller moon Deimos share this mysterious structure, and astronomers are eager to get more information about the circumstances of their origins.
A previous flyby completed in March of 2010 was the original closest flyby of 67km and led to the discovery that Phobos was composed of small pieces of debris held together by their mutual gravitational fields. Speculation is rampant over the origin of the debris and the internal structure of Phobos. Some think there was a meteorite impact that smashed a piece of the martian surface into space, tying the force of the impact to the planet’s lack of atmosphere. Others insist that both Phobos and its smaller sibling Deimos are composed of rubble or asteroids that floated past and were trapped in Mars’ orbit. The information being beamed to ESA researchers as the Mars Express Orbiter completes a daring Phobos flyby is being sent by way of a constant radio signal, meaning that the orbiter is unable to take any pictures while it is passing over the surface. By analyzing changes in the signal’s frequency, astronomers can determine the amount of acceleration caused by the gravity of Phobos and use the data to estimate its density based on its size, known to be near 27 x 22 x 18 km.
Focusing its high gain antenna dish back at earth, the Mars Express Orbiter completes a daring Phobos flyby without the ability to take close up pictures of the mysterious martian moon, but is able to relay just enough data to allow minute Doppler changes in its position to be sent back to earth to be used in the most accurate measurement of Phobos gravitational field to date. The Mars Express Orbiter ensured it was on course for the record-breaking flyby with its high-resolution camera, spending weeks on a slow approach while capturing nearly constant photographs of its target to track its movements and ensure it was able to keep itself in position with minimal maneuvering. In the coming weeks it will continue to photograph and track the orbit of Phobos as it plans its next swing around the planet Mars. In the wake of the news from Mars Express, data hungry scientists are turning their thoughts to missions seeking physical samples of the surface of Phobos. This would require actually landing a craft on the surface, something that in all likelihood will take at least another ten years.
By Daniel O’Brien