In a matter of days now, on the 20th of October, a rare astronomical event will take place when comet C/2013 A1, otherwise known as comet Siding Spring, has a very close encounter with the planet Mars.

Comet Siding Spring was discovered in 2013 by Astronomer Robert McNaught at the Siding Springs Observatory in New South Wales, Australia. This is far from McNaught’s first discovery. He has in fact discovered or co-discovered 82 comets in his life thus-far, as well as 475 asteroids. Comet Siding Springs though, is certainly one of his more interesting finds.

Initial observations of the comet’s trajectory suggested it had a small chance of impacting Mars. How powerful would such an impact be?

We know roughly how large the comet is, about 700 m in diameter and we can assume it is made largely of ice like we believe most comets to be. A ball of ice, 700 m in diameter would have a mass of around 180 billion tonnes so this should be approximately the mass of the comet Siding Springs. It is travelling at around 200,000km/hr (around 56 km/s) relative to Mars. If you think back to high school physics, you might remember a formula…

                E = ½mv­2  

…which is used to figure out how much kinetic energy something has. If we plug our mass and velocity into this equation we find out that if Siding Springs struck Mars, it would hit with energy of about 280,000,000,000,000,000,000 joules. This is about the same about of energy you would get from detonating 60 billion tonnes of TNT or 8,000 times more energy than you’d get from detonating every nuclear weapon on earth.

After longer observations of the comet’s trajectory, we now know that the comet will fortunately (or perhaps unfortunately if you like big explosions) not strike Mars. It will however, pass very close; missing the red planet by perhaps as little as 140,000 km. This is only about one third of the distance between the Earth and the Moon. Although the comet won’t hit Mars the close encounter should still have some effects, especially the planet’s atmosphere. Fortunately a new space probe, MAVEN, arrived at Mars a few days ago. The mission of this probe? To study Mars’s atmosphere. The fact that the arrival of MAVEN and the passage of Siding Springs happen at around the same time was not planned, it is simply a lucky coincidence but one which will hopefully yield fascinating insight into how close passes of comets can affect a planet’s atmosphere