2 December 2010
A small and very active comet
“Hartley 2 is the most active comet ever studied in situ,” says Frédéric Merlin, a lecturer and researcher at Paris 7 University supported by CNES. “We have observed chunks of ice spewing out into the coma and a real cosmic snow storm at the surface of its nucleus.”
A spectacular and serendipitous scene indeed.
In fact, the U.S. Deep Impact probe was originally scheduled to end its mission after studying comet Tempel 1 and then just continue its journey wandering into outer space.
But since it was in such good shape and still had fuel on board, scientists decided to send the probe to encounter comet Hartley 2 as part of the extended EPOXI mission.
“Hartley 2 is a small comet 2 km long that has only been transiting in the near neighbourhood of the Sun for a few decades,” explains Frédéric Merlin. “During the flyby, the comet was relatively close to the Sun1 and therefore subjected to pretty high surface temperatures of a few tens of degrees centigrade.”
“So we were expecting to find a very active comet, since being so close to the Sun would cause the icy nucleus to vaporize quickly,” continues Merlin. “And we were right, as more than half of the surface of Hartley 2 showed signs of activity.”
Fuelled by carbon dioxide
From the ground, it is impossible to precisely ascertain the chemical composition of comets, particularly of their nucleus.
Space probes are therefore the only way to study these dirty snowballs—remnants from the formation of the solar system—in detail. And here again, EPOXI has reaped a rich harvest of data.
The instruments on the probe have confirmed the presence of chunks of water ice in the comet’s coma and allowed scientists to estimate their size for the first time. Some of them are as big as basketballs!
The probe has also discovered what is driving all this activity. “Until now, we thought that gas jets and comet activity were being fuelled by the vaporization of water or carbon monoxide ice. But we’re seeing mostly carbon dioxide,” says Frédéric Merlin. “On Hartley 2, jets of carbon dioxide are spraying out chunks of ice to form cometary snow storms2. And we think this could explain the activity on most comets.”
It will take scientists several years to sift through and decipher the 120,000 images and spectra acquired during the flyby of comet Hartley 2.
Meanwhile, the Deep Impact spacecraft’s odyssey continues and it could well have more surprises in store.
1 The comet was a little more than 1 astronomical unit (AU) from the Sun, 1 AU being the Earth-Sun distance.
2 On Hartley 2 it is snowing up mostly into space, rather than down as it does on Earth.