Comets are small astronomical bodies that mostly travel from the far reaches of the solar system inwards. As they move closer to the sun they heat up, evaporating volatiles like carbon-dioxide and water to produce their characteristic tail.
They are quite difficult to study as although their tails can be enormous, the actual comet is relatively small. They are only a few kilometers across and surrounded by a cloud of tail forming particles when they are close to us. There have been a couple of missions to comets. The most recent was the NASA deep impact mission which was a bonus. After visiting the comet Tempel 1 in 2005 they visited the relatively small and unpredictable comet Hartley 2 in 2010. This comet is slightly unusual and spins peculiarly, occasionally showing hiccups in its tail. A team from the University of Maryland have just finished analysing the data from this flyby.
They have found that the comet looks like a giant 1km long dumbbell with jets of material flying out of the ends. At the moment slightly more material is flying out from the small end than the large one. The amount of water compared to carbon dioxide also appears to be lower at the small end than the large one. This might indicate that a comet has a varied structure in its original makeup.
So comets might be a more active and varied type of object than was previously thought. Hartley 2 will certainly continue to be studied from ground based telescopes to see if more can be learnt about its strange behaviour.