Tuesday, August 31, 2010

Moon of the Month: Phobos

Mars has two moons. The planet isn't very big, so these are tiny things. It's been suggested that they're not much more than captured asteroids; they don't look like much more than that.

Asaph Hall discovered Phobos--along with Deimos, Mars' other moon--at the United States Naval Observatory using their 26-inch refractor, the most powerful telescope in the world at the time.

Because he was the first guy to find Mars' moons, he was the first guy to discover Mars' mass. This is because the mass of an object determines how strong its gravitational pull is; gravitational pull determines how fast an object moves around a larger one. Scientists still use this technique to measure the mass of objects with deep space probes and groups of stars--as the object pulls on the probe or companion star, it moves differently. The change in motion--the acceleration--created by the object tells us the mass.

There's a big hole in Phobos--a crater that covers a large area of Phobos' surface--called Stickney. It's named for Asaph Hall's wife who apparently encouraged him in his search for satellites around Mars. The crater itself wasn't named until imaged by Mariner 9.

A few of Phobos' properties make it particularly unique. It is very porous. Some astronomers long ago suspected that it was hollow--perhaps built by Martians to protect them from radiation--but this has since been discredited. It wasn't a terrible hypothesis at the time. The idea that there such things as Martians was popular. It's also incredibly close to the planet--moving from rise to set in four-and-a-half.

Phobos is, however, slowly falling into Mars. Due to atmospheric drag, its orbit decays, spiraling over millions of years into the planet. This yields two possibilities. Phobos may fall directly into Mars in eleven million years, or it may break-up to form a ring in around seven million, the remaining pieces pelting the planet for millions of years after.

Phobos may prove useful in the future. Considering how close it is to Mars and how small the moon itself is, it may make a good substation for astronauts trying to land on Mars or return to Earth. It may also be the gateway to hell.


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