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A flyby of fear


It was scheduled for eight zero nine hours of Europe, Sunday, December 29th, the closest approach ever to Phobos. Touching upon the surface, the European probe Mars Express collected basic data on the origin of the enigmatic larger of the two Martian moons.

A flyby of fear

Image source: www.jpl.nasa.gov

Just 45 km away. That was the distance that separates the Martian moon Phobos by the ESA’s Mars Express probe at dawn on December 29. A quick flyby so close that it was impossible even to take a picture. But is that really the closeness record to allow scientists to trace in an indirect way – by measuring from Earth, with four tracking stations, the imperceptible slowdown Mars Express, the expected order of a few centimeters per second – the parameter that raises the most their curiosity: what is the force of gravity exerted by Phobos?

The reason is obvious. While the surface of Phobos, thanks to repeated passes made ​​by the Mars Express itself in recent years , we know pretty much everything (for example, that is lumpy so as to have earned the nickname of “potato”), it remains a mystery what we lies beneath. Or rather, that there is. Yeah, because scientists are now convinced that at least a quarter or even a third of the volume of the moon is occupied by empty spaces. And its interior is therefore a sort of rubble, with large gaps between a rock and the other block.

How to verify this? The ideal would be to dig, or at least analyze a bit of the referred material Phobos is formed. It was the task set for itself: the unfortunate Russian mission Phobos -Grunt. But the attempt has failed miserably in the bud, just two years ago, due to a failure to substitute a few hours after launch. And to try again because you just do not talk about it, at least not before 2020, we have to sharpen our wits. Here, then, the idea of measuring the force of gravity, and therefore the mass, by exploiting the close passage of the probe. The trick, without the proper proportions, is similar to that used with great success by another European probe – the ESA’s GOCE satellite – to reconstruct the detailed map of the Earth’s gravitational field. In this way, having already measured with good accuracy the surface (1548 square kilometers, roughly corresponding to the extension of the province of Milan) and the volume of the moon (27 x 22 x 18 km), scientists are planning to get their self an idea about the distribution of density, so that they can speculate about the mysterious origin of this mysterious celestial body.

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