Finally, a much-anticipated astronomical event that didn’t leave us disappointed! The close encounter of Comet Siding Spring with Mars on 19th October 2014 resulted in some great imagery and, hopefully, some interesting science to come. Siding Spring passed a mere 87,000 miles above the red planet: less than half the distance between the Earth and Moon, and about 90% closer to the martian surface than any Earth bound comet within human history.
Another unique event unfolded when Mars passed (almost immediately) through the tail of the icy visitor. Known as a ‘meteor storm’, the event would have been spectacular viewed from the surface of Mars. The tail – or coma – is the result of vaporisation of the nucleus; a conglomerate of ice and rock described by the American astronomer, Fred Whipple, as a “dirty snowball”.
When we see meteors from Earth (often called shooting stars), we imagine them to be huge lumps of rock – or even real stars – blazing across the night sky. Well, apart from the recent Russian meteor event (better described as a fireball), the majority of meteors are about the size of sand grains, and are the tiny remnants of dust left behind from a comet’s tail. The dust is left hanging in empty space until it interacts with the atmosphere of a planet whereupon it burns up, resulting in a fairytale-like meteor shower. The air temperature just in front of a meteor particle can be about 4000 degrees Celcius!
Siding Spring (‘C/2013 A1 Siding Spring’ to give it its proper name) is a long-period comet which has travelled across space and time from the Oort Cloud – a spherical cloud, the outer edge of which marks the boundary between the Solar System and interstellar space. The region contains trillions of frozen objects and is thought to be the long-term home of comets like Siding Spring, ISON, and 67P/Churyumov-Gerasimenko.
An array of Mars orbiters and rovers captured data as the comet brushed past. Orbiting space craft were moved to the lee side of the planet for safety, until the threat of interaction with fine particles emanating from the coma had passed.
“This historic event allowed us to observe the details of this fast-moving Oort Cloud comet in a way never before possible using our existing Mars missions,” said Jim Green, director of NASA’s Planetary Science Division at the agency’s headquarters in Washington. “Observing the effects on Mars of the comet’s dust slamming into the upper atmosphere makes me very happy that we decided to put our spacecraft on the other side of Mars at the peak of the dust tail passage and out of harm’s way.”
Comets are the rubble left over after the formation of the major planets, and are of particular interest to scientists studying planetary evolution. They are thought to reside in the cold backyard of the Oort Cloud for billions of years until a large body (Neptune perhaps) gravitationally tugs them sunward – a journey lasting about a million years.
Look out for these upcoming meteor showers: the dazzling Geminids in December, Quadrantids in January, and Lyrids in April.