Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.
Adrian Jannetta writes...
It doesn't seem like almost a year since I was outside on cold autumn mornings trying to snap a picture of Mars with a comet. Back then it was soon-t0-disappoint Comet ISON.
When Douglas Adams wrote
...he wasn’t exaggerating. In the solar system there are big planets, small planets, countless asteroids and comets. But the space they share means that actual collisions or near misses are very rare events. In October we'll see a rare event.
Comet 2013 A1 Siding Spring was discovered by veteran comet hunter Robert McNaught at the Siding Spring Observatory in New South Wales, Australia. Preliminary calculations of the orbit suggested that the comet would pass very close to, or possibly impact on the surface of Mars. The energy released by such an event would be the equivalent to an explosion of 20 million tons of TNT – or several good sized nuclear bombs. As is usual in these cases further observations ruled out an impact. Space is indeed big and planets present small targets. However, at around 7.30pm (BST) on October 19th Comet Siding Spring will fly past Mars at a distance of around 23,500 miles above the surface. Is that a safe distance? It sounds like a safe distance until you understand what a comet is.
Comets are affectionately known as “dirty snowballs” to astronomers. They comprise of a small nucleus of ice (water and CO2) and dust (usually particles of carbon and other more complex compounds). The nucleus might be just a mile or two in size but sunlight will heat the surface generating geysers and jets that carry away material from the comet. The ejected material forms a coma – a kind of atmosphere – around the nucleus. The coma dwarfs the nucleus – it might be tens or hundreds of thousands of miles in diameter. In 2007 the coma of Comet Holmes was briefly bigger than the Sun! The action of sunlight and the varying speeds at which material is carried away from the comet combine to create two or more tails pointing away from the Sun. All of these factors mean that although an impact with the solid nucleus is not possible this time the coma and tail of the comet will very likely encounter Mars.
These days Mars is a hub of activity. As I write this article an Indian spacecraft – the Mangalyaan – entered into orbit around Mars and joined the half dozen other orbiters and landers on the surface of the Red Planet. All of these active missions are taking steps to minimise the risk of high speed dust particles from the comet destroying vital components. Some of them began small manoeuvres weeks before so that they’ll be on the opposite side of Mars to the comet at the time of closest approach.
Despite the risks posed by Comet Siding-Spring the encounter promises to be a wonderful opportunity for astronomers to study a new comet from close range using some of the best technology of the space age.
Dr Adrian Jannetta FRAS
Amateur astronomer and mathematics teacher. Guitar strumming explorer of the universe!
Proud nerd and founder of the school space club. A young whippersnapper with a bucket-load of passion.