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.
Adrian Jannetta writes...
The brightest comet in recent months has been Comet Lovejoy (or 2013 R1 Lovejoy to be precise). Here's a picture I took on the morning of January 5th at around 6.45am.
Comet Lovejoy was closest to the Earth in November and reached its closest distance from the Sun in late December. Although Comet Lovejoy is beginning to fade now, it remains an easy object for binoculars in the sky before sunrise for the next couple of weeks.
The following star chart shows the steady progress of the comet as it leaves the inner solar system on a path taking it below the orbit of the Earth and into the southern sky.
By the end of January the comet will probably have slipped beyond the light grasp of binoculars. Observation will also be hampered by the presence of the moon from January 10th until the last few days of the month.
Comet Lovejoy has been a wonderful comet to observe and I've learned a lot about imaging along the way. The other heartening thing about this comet is that it came out of nowhere! It was only discovered in September - while the world was getting hyped up about ISON - and was always the better of the two comets through the telescope eyepiece. As we start 2014 with two or three comets expected to become binocular bright - there is always the chance that something as beautiful as Comet Lovejoy will be found tomorrow.
Adrian Jannetta writes...
Dramatic images from NASA's SOHO mission show the fate of much heralded Comet 2012S1 ISON. Basically, it's all but gone.
For many months ISON didn't brighten as much as astronomers had hoped. The images show that ISON brightened abruptly and then faded a few hours before reaching perihelion (closest point to the Sun). What emerged on the other side may have fragments of the original nucleus, sublimating and crumbling into a cloud expanding particles. The rapidly changing perspective of the moving cloud created a brightening event which some hoped was a sign that part of the original comet nucleus had survived, and might yet put on a show in the morning sky this week. But it's unlikely there will be anything to see by the time that cloud of material moves away from the Sun.
It's disappointing that we won't see a dramatic comet any time soon. On the plus side, astronomers have studied a comet from Oort Cloud - a pristine sample of the early solar system - and have been able to study it with observatories on Earth and orbiting other planets. It just might be that ISON's legacy as one of the most observed comets in history will contribute much more to our understanding of comets than many other "great" comets have in the past.
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.