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The shocking size of Comet McNaught

Instead of using the length of the tail to measure the scale of the comet, scientists used data from the Ulysses spacecraft to gauge the size of the region of space disturbed by the comet's presence.
Provided by the Royal Astronomical Society, United Kingdom
Comet McNaught viewed over the Pacific in 2007.
Sebastian Deiries/ESO
April 13, 2010
British scientists have identified a new candidate for the biggest comet measured to date.

Instead of using the length of the tail to measure the scale of the comet, the group has used data from the European Space Agency (ESA)/NASA Ulysses spacecraft to gauge the size of the region of space disturbed by the comet's presence. Analysis of magnetometer data shows evidence of a decayed shockwave surrounding the comet created when ionized gas emitted from the comet's nucleus joins the fast-flowing particles of the solar wind, causing the wind to slow down abruptly.

In January and February 2007, Comet C/2006 P1 McNaught became the brightest comet visible from Earth for 40 years.

Serendipitously, Ulysses made an unexpected crossing of Comet McNaught's tail during this time, one of three unplanned encounters with comet tails during the 19-year mission. The other encounters included Comet Hyakutake in 1996, the current record-holder for a comet with the longest measured tail.

Ulysses encountered McNaught's tail of ionized gas at a distance downstream of the comet's nucleus more than 1.5 times the distance between Earth and the Sun. This is far beyond the spectacular dust tail that was visible from Earth in 2007.

"It was very difficult to observe Comet McNaught's plasma tail remotely in comparison with the bright dust tail, so we can't really estimate how long it might be," said Geraint Jones of University College London's Mullard Space Science Laboratory in the United Kingdom. "What we can say is that Ulysses took just 2.5 days to traverse the shocked solar wind surrounding Comet Hyakutake, compared to an incredible 18 days in shocked wind surrounding Comet McNaught. This shows that the comet was not only spectacular from the ground, but also it was a truly immense obstacle to the solar wind."

A comparison with crossing times for other comet encounters demonstrates the huge scale of Comet McNaught. The Giotto spacecraft's encounter with Comet Grigg-Skjellerup in 1992 took less than an hour from one shock crossing to another — to cross the shocked region at Comet Halley took a few hours.

"The scale of an active comet depends on the level of out-gassing rather than the size of the nucleus," Jones said. "Comet nuclei aren't necessarily active over their entire surfaces; what we can say is that McNaught's level of gas production was clearly much higher than that of Hyakutake."

Candidate shock features had been found in Ulysses magnetometer data from the Hyakutake encounter in 1996 but their identification was tentative, especially so far downstream from the comet's head. The discovery of similar features at McNaught suggests that this interpretation is correct.
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