Hubble leads investigation of Mars-bound comet’s threat to orbiting spacecraft

In a little over six months from now, Comet C/2013 A1 (also called Comet Siding Spring) will be making an extremely close pass by the planet Mars. In an effort to figure out if this close encounter is going to be a problem for the various spacecraft we have orbiting the planet, astronomers have been pointing the Hubble Space Telescope at it, specifically to study the dust streaming off the comet's nucleus.

Comet C/2013 A1 was discovered in January of 2013, by astronomer Robert H. McNaught, at Australia's Siding Spring Observatory. Right off the bat, this comet looked like it was going to be an interesting one, because the path that was computed from observations showed that it was going to come very close to Mars — so close that, at least at first, it looked like it might hit the planet! That caused a lot of excitement about the potential to observe such a cataclysmic event from a safe distance, while at the same time, there was plenty of concern about the spacecraft in orbit around the planet and the rovers we have on the surface. Over time and with more observations, the potential for an impact lessened to the point where it's fairly certain that the comet will miss by at least 130,000 kilometres — a little over one third the distance between the Earth and the moon.

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However, while the comet's nucleus should pass by the planet safely, by the time Siding Spring reaches Mars, the coma of dust and gas surrounding it will likely be big enough to encompass the entire planet and a good portion of space around it. Astrophysicist Karl Battams, who was part of the Comet ISON observation team, wrote about Siding Spring on the Planetary Society's expert blog, saying "there's a good chance that we are about to witness the first recorded instance in history of a planet passing through the coma of a comet!"

This has the potential to give Opportunity and Curiosity a spectacular meteor storm in the Martian sky, as the stuff streaming off the comet slams into the top of Mars' atmosphere. At the same time, though, all of that same stuff could have a much-less-spectacular effect on the spacecraft in orbit. The problem comes from something called 'hypervelocity cratering' — where everything coming off the comet (down to the tiniest grains of dust) is turned into a rain of bullets on any of the hardware we have trying to fly through it, since it's all traveling at over 200,000 kilometres per hour.

So, this is where Hubble's images of the comet are coming into play now. The comet nucleus is too small for Hubble to resolve it, but by observing exactly how the comet's coma is developing, and using the comet's relative position to the Earth and the sun (especially when we crossed its orbital plane on Jan. 21), astronomers can tell just how fast the particles are moving.

"This is critical information that we need to determine whether, and to what degree, dust grains in the coma of the comet will impact Mars and spacecraft in the vicinity of Mars," Jian-Yang Li, of the Planetary Science Institute in Tucson, Arizona, said in a statement.

How much of a threat is the comet?

"It's way too early for us to know how much of a threat Siding Spring will be to our orbiters," Soren Madsen, Mars Exploration Program chief engineer at NASA's Jet Propulsion Laboratory, said in a statement. "It could go either way. It could be a huge deal or it could be nothing — or anything in between."

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So, what happens if the scientists figure out that the comet will be a threat to all their valuable spacecraft orbiting Mars? They'll end up using a spacecraft version of the old 'duck and cover' method of protection, which should be far more effective than the Cold War version would have been, since the spacecraft will be using the planet Mars itself for protection.

Unlike Comet ISON, Comet Siding Spring isn't going to be coming anywhere near Earth or the sun. The closest it gets to Earth is about 130 million kilometres (for comparison, the Earth is roughly 150 million kilometres from the sun), at the beginning of September, and the closest it comes to the sun is over 200 million kilometres away, in the days just after it passes Mars. Still, with the right telescopes, astronomers should still be able to get some interesting views of the comet, and the spacecraft around Mars will be doing their best to capture all the pictures and data they can as it swings by. To help with this, NASA's Comet ISON Observing Campaign (CIOC) is getting back together, and turning their attention to Siding Spring for the rest of the year.

(Images courtesy: NASA/ESA/J.-Y. Li (Planetary Science Institute), Hubblesite.org)

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