Monday, 19 May 2014

Rosetta's Destination Comet Grows a Tail

Comet 67P/Churyumov–Gerasimenko (centre), target for ESA's Rosetta space probe, begins to develop a tail.
Image credit: ESA

Just a short post today, as I'm writing a lot of other stuff at the moment (more on that to follow...).

Back in January I wrote about the European Space Agency's exciting Rosetta mission as it reactivated from a three year hibernation. Last week the mission reached another major milestone, although not one it had any control over.

Rosetta's target, the comet 67P/Churyumov–Gerasimenko, has begun to wake up.

The comet, seen as a fast moving white dot in this series of images taken with Rosetta's main camera, has suddenly exploded into life, beginning to form the huge tail that comets are famous for. As Rosetta closes into orbit with 67P/Churyumov–Gerasimenko in August, we will be able to see this fascinating process unfold up-close for the first time.

Comets actually spend most of their time without tails, orbiting for millions if not billions of years in the far reaches of the Solar System. Occasionally a chance encounter with another object will give them just enough of a gravitational nudge to send them tumbling into towards the Sun.

Once there, the increasing heat from the Sun begins to melt the ice that makes up most of the comet. The ice is embedded and mixed in with rock and dust, so as it sublimates (turns directly from solid to gas) it beings to blast that dust out in volcano-like eruptions. The escaping gas and dust flows out into a huge cloud, or coma, out of which forms the tail. And it really is big: Whilst the nucleus of a comet may only be few kilometers across, the tail can grow to lengths of hundreds of millions of kilometers. That's significantly larger than the Sun, although the coma is much less dense.

Comets actually have two tails, seen clearly in the image of comet Hale-Bopp below (One of my earliest memories is of being taken outside at night to watch this comet as it flew past Earth in 1997). The blue tail is formed of charged particles, or ions, individual charged particles released from the comet. These interact with the magnetic field carried in the solar wind, so stream out from the comet in a straight line directly away from the Sun.

The white tail is formed out of the dust liberated by the escaping water, pushed away from the comet by sunlight. As they fall back from the comet each dust particle moves into a separate orbit, slightly slower than the orbit of the comet thanks to the increased distance from the Sun. This causes the tail stretch out into a curve, as each successive particle out from the comet gets slightly further behind on its orbit.

Thanks to this behavior comet tails are a key tool in our attempts to understand the solar wind, which has a big affect on the satellites and other space technology that modern civilization depends on. One of Rosetta's targets as it closes in on 67P/Churyumov–Gerasimenko is to investigate this relationship between the comet and the solar wind from the inside.
Comet Hale-Bopp, which flew past Earth in 1997. The separate ion (blue) and dust (white) tails can be clearly seen.
Image credit: Nasa. 

Rosetta is closing in on its target, aiming to reach orbit in August. As I hope I've shown you, the show has only just begun...

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