Big name, Small Comet --- 
Small Probe, Big Science

Kamal Lodaya, The Institute of Mathematical Sciences, Chennai

The European spacecraft Rosetta reached the nucleus of the comet Churyumov-Gerasimenko (CG for short) when it was 50 lakh kilometres away, on 6 August last year. We have covered this in Jantar Mantar earlier and showed its peculiar shape. People said it looked "duck-like".
Comet CG is not big or spectacular. The nucleus is just 4 kilometres in size. In a month's time Rosetta went into orbit around it at a distance of 30 km, which is amazing considering how tiny the comet's gravity is. This meant that it was able to take some spectacular pictures. On 12 November a small craft, Philae, detached itself from Rosetta and landed on the comet's surface. Although it bounced away and landed somewhere unknown, it was able to take pictures of the surface and make other measurements.
Over the year CG came closer and closer to the Sun, until on 13 August this year, it reached its closest point to the Sun (perihelion), about 19 lakh kilometres away. Yet as it came closer, it warmed up. When Rosetta reached CG its "daytime" surface temperature was measured to be around minus 70 degrees Celsius. It was above 10 degrees Celsius in June and was forecast to go higher.
So much dust started coming out of the comet that Rosetta had to retreat. It is no longer captured by CG's gravity and is orbiting the nucleus at a safe distance of 300 kilometres. It might retreat farther if things look dangerous.
From the beginning of this year, there has been a steady flow of dust and gas coming out from the comet from the regions which are under the Sun (see back inside cover photo). From the other measurements taken it looks like the gas is largely water vapour.
From May this year we have seen events when a "fountain" of dust suddenly erupts from the comet. By suddenly we mean within 15 or 20 minutes. On 12 August (the day before perihelion) there was a huge "jet" of gas and dust which appeared brighter than the entire nucleus. You can find a video of this jet made from pictures taken every 20 minutes on the internet. Now it looks like this jet has large amounts of other gases, such as hydrogen sulphide and carbon dioxide, which we know are more explosive than water vapour.
When Rosetta orbited the comet last year, detailed maps were made of its 4-kilometre surface. So the observing scientists were able to trace where exactly these fountains were coming from. There they found sinkholes, just as you find sinkholes on a beach full of sand.
The theory built up by the scientists is that CG's nucleus has huge amounts of frozen ices of many compounds, inorganic and organic, in its interior. As the comet nears the Sun its interior gets slowly warmed up. Some of these ices sublimate (convert to gas). These gases push out the surface above them to create fountains of dust. Sometimes they are strong enough to violently throw out the dust on top and escape as a jet. This makes the sinkhole on CG's surface.
Meanwhile amateur astronomers from all over the world are helping photograph and track this faint comet from the Earth. Of course the nucleus is too small, but the dust spreads around it to form a coma (head). Over the last couple of months before perihelion, CG's coma has grown to 50,000 kilometres!
Then, as we have seen with other comets, the gas and dust in the coma gets pushed away from the Sun to form a tail. CG's tail is now several lakh kilometres. This theory of comets is not new. The basic idea of a "dirty snowball" was suggested by American astronomer Fred Whipple in the 1950s. Later models have suggested "icy dirtballs" where the ices are buried inside the dirt. What is astonishing is that, thanks to Rosetta, we can see from close up what is happening on a comet's nucleus.
Provided it does not get damaged, Rosetta will observe the comet for another year and more. We know that the little lander Philae tried to communicate with Rosetta in June this year, when its batteries seem to have been recharged by the Sun. Communication was lost later and it is guessed that this is because of the huge amount of dust surrounding the comet. The hope is that when the dust storm comes down a few months from now and Rosetta is able to go closer to CG, one can get more data from Philae about what things look like on the surface of the nucleus. The scientists are also keen to photograph from Rosetta the changes which have happened there.
Images from European Space Agency: http://www.esa.int/spaceinimages