17 October 2014 15:34

You’d be forgiven for thinking that comets are a bit like buses - you wait ages for one to arrive and then two come along at once - but 2014 is turning out to be a great year for comet hunters.

For months, the world has been captivated by the steady stream of breath-taking images of comet 67P/Churyumov–Gerasimenko taken by ESA’s Rosetta spacecraft. This audacious mission is the first in history to intercept and escort a comet as it orbits the Sun. It also includes a lander called “Philae” that will descend to the surface of the comet, landing on 12 November 2014. The mission is already revolutionising our understanding of the solar system and, as excitement and anticipation mount in the run-up to the landing, ESA have even launched a competition to name the landing site.

Closer to home, our planetary next-door neighbour is about to receive a weekend visitor from the outer solar system. On Sunday (19 October) Comet C/2013 A1 will swing by Mars in what astronomers would call a near-miss. The comet, also known as “Siding Spring” for the Australian observatory from where it was first spotted, is moving at over 50 kilometres per second and will pass within 132,000 kilometres of Mars. To put that into context, that’s about one-third of the distance between the Earth and the Moon.

While it won’t hit Mars, material from the comet’s coma (the cloud of gas and dust that envelope the solid nucleus) is expected to engulf the red planet. This event therefore offers scientists the chance to explore the interactions between a comet’s atmosphere and a planet’s – a once-in-a-lifetime opportunity. It’s not clear what effects to expect but the flow of fast moving material into the planet’s upper atmosphere could trigger martian aurorae.

Although there is likely to be a spectacular show from Mars, and southern hemisphere astronomers with telescopes have been seeing it for some time, Siding Spring is too faint to be visible from on Earth with the naked eye. The Hubble Space Telescope will be keeping a close eye on the encounter and the international fleet of satellites orbiting Mars have been preparing for Siding Spring’s arrival. NASA’s Mars Atmosphere and Volatile EvolutioN (MAVEN) mission has arrived just in time. Launched in November 2013, the mission is specifically designed to explore the red planet’s upper atmosphere, ionosphere and interactions with the sun and solar wind. It entered Mars orbit on 21 September and after an initial check-out period now has a front-row seat for the cometary fly-by. MAVEN isn’t alone though. NASA has been operating two other probes at Mars, the Mars Reconnaissance Orbiter and Mars Odyssey spacecraft, for some time. ESA’s Mars Express mission has been in orbit around Mars for over a decade and India’s Mars Orbiter Mission, the first Asian mission to the red planet, arrived at Mars just days after MAVEN.

All of these spacecraft are ready to measure the effect of Siding Spring’s close encounter with Mars, but their operators are taking precautions to keep the spacecraft safe. The high speed of the comet relative to Mars means that tiny grains of dust from the comet could damage the spacecraft. Over recent months some of their orbits have been tweaked to place the spacecraft behind Mars during the expected period of greatest risk – the orbital equivalent of a duck-and-cover. On the martian surface, the Opportunity and Curiosity rovers’ cameras might be able to observe the comet as it sweeps overhead and monitor the atmosphere for meteors while the planet encounters the comet’s dust trail.

What will happen? Nobody knows, but this weekend all eyes are on the red planet. Welcome to Mars Comet C/2013 A1!

What do you think? Share your comments with us below.

Professor Jim Wild teaches on our BSc Physics, Astrophysics and Cosmology programme.

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