Obviously the Moon is our nearest neighbour in the solar system. But which is the next nearest? Venus? Mars? Or Mercury, perhaps? Yes, any one of these at times – but not all the time.*
Relative orbital positions are changing continuously, of course. But when appropriately aligned, and on fairly rare occasions, the second nearest heavenly body to Earth is in fact Deimos, one of the two moonlets of Mars, the other being Phobos. So, considering their relative proximity, why do we hear so little about them?
Is it because they’re just plain boring? Compared to the massive, swirling, Pollock-esque globe of Jupiter or the spectacularly pure tones of the other gas giants, Saturn, Uranus and Neptune, these two lumps of rock are little more than king size boulders. They seem utterly insignificant in the grand scheme of things planetisimal.
Yes, no doubt one of the reasons is that they’re so tiny. The smaller of the two, Deimos (pictured below), has a mean radius of only 3.9 miles. Phobos, although its radius is 7 miles, has a mass – the amount of material of which it is composed – seven times bigger than that of Deimos. Compare their diminutive dimensions with those of our own Moon with its radius about 1,080 miles.
Deimos is further out from the Martian surface than Phobos, at some 14,500 miles. Time and the gravitational pull of Mars have kneaded it into a rather odd shaped object, a bit reminiscent of some kinds of modernist sculpture.
Deimos is 56% of the size of Phobos. Deimos’ orbit is fairly regular, whereas that of Phobos is very rapid and eccentric. To an observer on Mars, Phobos (pictured below) would rise in the west and set in the east twice a day.
Actually, it’s grossly untrue to say that Phobos and Deimos are forgotten – certainly not by the scientific establishment. Just yesterday, for instance, a group of Japanese and American scientists published a research paper which concludes that “space weathering” on Phobos, in tandem with its eccentric orbit, has caused its surface to be divided into two distinct geologic units, known as the red and blue units.
They believe that much could be learned from exploration of Phobos. “With a better understanding of the surface and subsurface conditions on the Martian moon, the Phobos surface can become a ‘Rosetta-stone’ for understanding space weathering elsewhere in the solar system”, they suggest. The team has added one more theory to the wealth of speculation about the origins of the little moon. (Sometimes, the theories have seemed as eccentric as the Phobos orbit …).
There have been many attempts and proposals to mount dedicated missions to these two moons. None of the three dedicated return sample missions to Phobos was successful. So far, the most valuable observations of Phobos and Deimos were made in 1971 by NASA‘s Mariner IX spacecraft, which sent back a total of 7,000 images of both Mars and its two moons.
The most highly-anticipated mission to Phobos and Deimos is MMX, though this isn’t planned to launch for another six years – its scheduled launch date is September 2024. A very ambitious mission, MMX (see illustration below) will launch from Tanegashima Space Center in Japan and consist of an orbiter and lander, aiming to return surface samples from both moons, with the development of one of the spacecraft’s suite of seven science instruments being supported by NASA.
I wonder what it would be like to stand on Phobos (in particular) or Deimos and see almost the whole sky filled with a close-up view of the Red Planet? Only time will tell … but it seems reasonable to assume that space tourists will some day, in the next hundred years or so, use one, or both, of the moons as a viewing platform to look down on the Red Planet and observe its geographical features, its weather systems and the human settlements that have sprung up and are spreading across its surface.
Then again, the future is a process, not an event. It won’t take long to extract anything of geological value from these two little bodies. And once man-made structures swarm around the skies of Mars, offering scientists and holidaymakers alike a wide range of accommodation options, Phobos and Deimos may seem slightly irrelevant. Maybe they will be put to use by the military; maybe they’ll become private real estate owned by a Martian oligarch; or maybe they’ll be pushed away into the vastnesses of the universe, to clear a safer path for interplanetary transport?
Image credits (all): NASA
* Thanks to Mike Yamiolkoski for pointing out an error in the first version of this section