Ceres and other forgotten worlds

Most people know that our solar system has eight planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.  Controversy surrounds Pluto, from it’s discovery in 1930 until it’s relegation from the premier planet league to more lowly the status of dwarf planet in 2006 it was our ninth planet. Its new lowly status restores the planetary numbers back to what we had between the (mid nineteenth century) discovery of Neptune and Pluto.

So it is rather odd to see in the London Science museum a model solar system (an Orrery) built in 1813 for the British astronomer William Pearson, It has not, as you might expect, seven, but eleven planets. No he wasn’t guessing or predicting what would be discovered, he was representing the orthodox view of the solar system at the time. There amongst the familiar innermost seven planets between the orbits of Mars and Jupiter are four we’ve almost forgotten Ceres, Vesta, Pallas and Juno. These are real worlds, although like Pluto they would by the mid nineteenth century be relegated, and their relegation was more severe than that meted out to Pluto condemned to virtually non-league status as asteroids.

Yet these four worlds are no mere fragments of rock. Ceres is a spherical world nearly 600 miles across with a light atmosphere and probably a substantial quantity of water. With a good eye and a dark night you can even see it from Earth without a telescope. It has been ignored by space exploration and barely mentioned in text books on our solar system for century and a half. It’s sisters have fared no better, although they are rather more potato shaped than spherical and look a bit more like what you might expect an asteroid to look like nobody much talks about Vesta, Pallas and Juno either.

In fact there are many many more. Hygiea an ovoid world the size of Belgium, Eunomia a rock world the size of Wales, and bizarrely a binary asteroid Antiope that is made up of two equal sized parts orbiting around each other. There are over two hundred more asteroids known to measure more than 70 miles across. In total there is thought to be over a million asteroids forming the asteroid belt.

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.