BioEd Online

As Comet McNaught makes its closest pass to the Sun today, researchers are using the rare occurrence to take some stunning photos and hopefully learn something new about what comets are made of.

Already the space-bound STEREO mission has snapped the highest-resolution images ever taken of the fine structure of a comet's tail as it passes close to the Sun, just when the dust is being driven off most violently. "We should be able to learn more about how dust breaks off from a comet," says comet expert Geraint Jones, at the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany.

Comet McNaught is of an average size, but is passing almost directly in line between the Earth and Sun. The resulting scattering of sunlight makes it very bright. It has been visible to the naked eye in the Northern Hemisphere's southwestern skies at twilight for the past few days, and there are some reports of it being spotted by telescope even in daylight.

"It's certainly the brightest [comet seen from Earth] in the past 30 years, perhaps even 40 years," says Jonathan Shanklin, director of the British Astronomical Association Comet Section.

The brightness means that researchers should be able to get a lot of spectrographic information about the comet's composition, including for some of the scarcer elements that are usually too dim to investigate.

The information gained this way should complement what we already know about comet composition from projects such as the Stardust mission, which scooped up a bit of a comet's tail and brought it back to Earth. Stardust brought back the more durable elements by and large, says Shanklin, who primarily works as a meteorologist for the British Antarctic Survey in Cambridge, UK. "This comet will show us the more volatile elements as they peel off," he says.

Not just pretty pictures

The comet's pass is also providing a fantastic photo opportunity for space-bound observations by craft such as SOHO and STEREO, which are aimed at the Sun. For eleven-year-old SOHO, this is the brightest comet it has ever seen.

SOHO will take a close look at the comet's ion tail (as opposed to the dust tail seen from Earth), which moves in response to the solar wind. "This can tell us something about how the solar wind behaves, as well as the tail," says Jones.

As for STEREO, which launched just 3 months ago, the comet is ensuring that the craft's SECCHI instrument has a spectacular first light. "Yesterday we opened the one remaining door that had been closed on SECCHI, and the very first image we got was of the comet. That was a wonderful treat for us," says Karl Battams at the Naval Research Laboratory in Washington DC.

Getting a broad survey of comet compositions and behaviour is useful for projects such as Rosetta, which aims to make a soft landing on a comet in 2014.

"We're keeping an eye on all comets. As this is a fairly bright one, you can make some measurements that you can't normally do," says Gerhard Schwehm, Rosetta project scientist. He'll be watching the data stream in with interest but sadly won't be able to observe it with his own eyes. "Unfortunately the weather is too bad in the Netherlands to see it."

Watching out

This comet was first spotted just 5 months ago by Robert McNaught, an amateur-turned-professional astronomer who heads the Siding Spring Survey in Australia. This project is aimed at taking broad surveys of the sky to watch out for near-Earth objects, such as incoming asteroids.

McNaught has found more than 30 comets so far. The record for spotting comets, instrument-wise, goes to SOHO, which is responsible for spying more than 1,200, thanks mainly to amateurs scanning through its public data. Shanklin has so far laid claim to six of these. "It's very exciting when you find one. Unfortunately there's a lot more competition now."

Comet McNaught is on a parabolic orbit that takes it past the Earth only once, from the north to the south. It remains visible to observers in the Northern Hemisphere for another day or so. By Monday it should be on full display in the Southern Hemisphere, where it is expected to stay visible to the naked eye until the end of January.

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