Phobos and Deimos

Nagging wives are not usually spoken about in high regard but Angelina Hall was different. Her nagging not only caused the discovery of two worlds it led to the major feature of one being named after her. Her husband Asaph Hall was a nineteenth centaury American astronomer. He sought to try and prove what had been, quite bizarrely predicted by Jonathan Swift a hundred years before in his famous novel “Gulivers Travels” that Mars had two moons. Quite why Jonathan Swift thought this is odd in itself. Swift was a bright well-informed man, but was no scientist and certainly not an astronomer. When he wrote Gulivers Travels in 1776 no telescope on Earth was anything like powerful enough for anybody to deduce anything much about Mars. It would be a centaury before a telescope powerful enough to see the moons would be built. But there in volume two he says “In the floating aerial island of Laputa. They have extended their discoveries much further than our astronomers in Europe. They have likewise discovered two lesser stars or satellites that revolve about Mars” he then goes on to describe the distances of these satellites from Mars and how long they take to orbit the planet. Astonishingly given that it was just a guess, Swift was correct there are two moons of Mars and his predicted measurements weren’t far out either.

 

Almost exactly 100 years later a telescope big enough to be able to see Mars in enough detail was built in Washington state USA. The astronomer in residence was none other than Asaph Hall. Looking for a discovery worthy of the magnificent new telescope he decided to put Jonathan Swift to the test. Swift was not the only one to be tested. The distance was testing the scope to its limits, and it turned out testing Asaph too. He spent night after night trying to spot moons around Mars. It was dull and frustrating work. He found nothing and when he arrived home each morning would moan to his wife about the futility of his task. “Don’t you give up” she is reported to have said on many occasions. Eventually Asaph really had had enough. “I’m stopping” he told her. “You get right back there tonight and carry on” she is reported to have shouted. Like most nagged husbands Asaph did what he was told, and that very night saw a speck circling Mars. A few nights later he spotted a second. He had discovered what came to be called Phobos (picture left ) and Deimos (picture right) the two battered and scarred rocks that circle Mars. They are odd indeed the two moons are no bigger than the two islands that make up Malta. They are perhaps as old as the solar system, probably captured asteroids that got ensnared in Mars gravitational field. The larger moon Phobos is particularly war torn and shows the scars of violent impacts. Two craters dominate, the smaller one named after Asaph is Hall crater, and the larger Strickner crater named after Angelina’s maiden name. A testament to nagging. And on Phobos's sister moon a testament to brilliant gueswork, the dominant feature is Swift crater.