Phobos

Yes, the weekly theme is phobias, but I'm giving you the slightly unpredictive text version: i.e. Phobos.

It's not another (bloody) Greek island, but the innermost moon of Mars (the outermost being Deimos). The idea came to me in the gym the other morning as I watched the European Space Agency's Rosetta spacecraft descending to a "controlled impact" on comet 67P. The ESA registered that Rosetta's solar batteries were failing and it wouldn't manage the journey back to Earth so they opted to crash-land it onto the comet it had been circling and investigating for the last two years, a fitting mechanical suicide.

The root of the word is the same - Φόβος in Greek meaning fear or horror, so a phobia is a horror of something and Phobos is a horror of a heavenly body (as the photograph illustrates), not even spherical but lumpy, pock-marked and dull.

Discovered in 1877 by an American astronomer, and named after a son of Ares, god of war (Mars being the warlike planet), Phobos is only about 25 kilometres across and therefore has just too little mass and gravitational force to maintain a rounded shape. It also has a very low albedo (look it up - it's my word of the week!) and as a result is one of the least reflective moons in the solar system. If that doesn't make you feel sorry for the mis-shapen, ugly little thing, then consider its inevitable fate, for Phobos' days are numbered.

Phobos orbits just 6,000 kilometers above the surface of Mars, closer than any other moon around any other planet. It also orbits Mars faster than that planet rotates (circling it twice per Martian day) and the effect of its orbital period being shorter than the planet's rotation actually decelerates Phobos in its orbit, causing it to gradually spiral closer to Mars at the rate of 2 metres every hundred years. Eventually it is going to either crash into the planet's surface or disintegrate under the tidal pull into fragments which will circle Mars rather like the rings of Saturn, (as depicted in the graphic below). Okay, it's days are numbered -but it's a pretty big number, somewhere in the region of 50 million years. We shouldn't worry.

This week's poem is something I've just cobbled together in anticipation of Phobos' demise...

Orbituary
Poor pock-marked wedge of heavenly pumice,
dull, mis-shapen offspring of the god of war,
your lot, to orbit in decreasing circles,
played fast and loose by tidal forces,
slowly fragmenting to the core.
In fifty million years or more
your day of destiny will surely come.
When all of this sad spiralling is done,
then rest in pieces, tiny moon.

Thanks for reading. Have a good week, S ;-)