Catching Up With A Comet

Photo by ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Ten years ago the European Space Agency launched a rocket into the sky. It whizzed around the sun a few times, speeding up as it went, and then around a few planets, including the Earth three times, on a cleverly planned path to hit a very small target a long way away. Clever is not really a big enough word - you try throwing something at target so far away that it won’t hit it until ten years later! The spacecraft at the top of this rocket is called Rosetta.

At the beginning of this month, August 2014, Rosetta reached its target, a comet speeding its way towards our Sun that at the moment is somewhere between the orbits of Mars and Jupiter. This comet has the rather long and difficult name of 67/P Churyumov-Gerasimenko, because of the two people who discovered it. For the sake of the rest of this blog post, and mostly because I can’t pronounce it properly, let’s just call it comet 67/P. Here is a map of Rosetta’s journey:

Source: ESA

Comets are big dirty snowballs of ice and rock that sometimes come from very far out in space and loop around the sun. We think there are countless numbers of them surrounding our solar system in a dark region of space far away from the light of our star called the Oort Cloud, where they have been for a very long time not doing very much. Comets are different from asteroids - asteroids are made mostly of rock and are only found inside our solar system. Sometimes something, maybe a bump with a neighbour, causes a comet to fall in towards the Sun from the Oort cloud and it begins a long journey slowly picking up speed - it can take millions of years until it reaches the inner solar system. Three or four of these long-period comets fall in towards us like this each year. When they arrive they sometimes loop around the Sun and whizz off again never to return, sometimes they get too close and burn up and sometimes they get ‘captured’ by the Sun’s gravity and start to make much smaller circles inside the solar system where we can see them buzzing past us at regular intervals thereafter - we call these ones short-period comets.

This comet, 67/P, is a short-period comet that goes around the Sun every six and a half years or so and is about 3 kilometres by 5 kilometres in size. Since it was discovered it has been around the sun seven times. It isn’t a perfect sphere as you might imagine it should be. Far from it, as you can see from the photo at the beginning of this post, it is an odd shape.

Of course you already know that comets are things that sometimes are bright enough to be seen in the night sky and they have long tails behind them. So where is the tail on this one? The tails are caused when comets get closer to the sun and some of the ice begins to melt. Comet 67/P is on its way toward the sun right now and it is going to be Rosetta’s job to follow it closely for the next 18 months and watch what happens to it as it starts to melt and its tail begins to grow. In fact it is already starting to melt as this next photo shows:

Photo by ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Unfortunately, like most comets, 67/P isn’t close enough or big enough to be so bright that we can see it from Earth with our own eyes, although it can be seen with a telescope. But you don’t need a telescope. The cameras on Rosetta are sending back the most incredible pictures we have ever seen of a comet close up.

This event is important because this is the closest we have ever been to a comet before. The Rosetta spacecraft is less than 100 kilometres from the comet right now and soon will be about 30 kilometres away. In November, it is going to launch a small landing craft, called Philae, onto the surface of the comet. We’ve never tried anything like this before. We’ve seen plenty of comets in telescopes but have never touched one until now.

Knowing more about comets is important because we think they are very old - as old as our Sun and the planets spinning around it. Learning more about comets may mean we learn more about how our solar system was created 4500 million years ago. When Rosetta finds out some more interesting things I’ll let you know.

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