Colossal comet surprises scientists

Comet McNaught seen over the Pacific Ocean in 2007.

Comet McNaught seen over the Pacific Ocean in 2007.

Ulysses spacecraft data has been used to measure the region of solar wind disturbed by a comet. The comet, C/2006 P1 McNaught, is now known to hold the record for the largest effect on its surrounding space.

Comets have three parts: the core, or nucleus, made of ice and rock; the coma or head, which is a vast cloud of gas and dust surrounding the nucleus; and the tail, which is often split in two (one part composed of gas, the other of dust).

Scientists usually measure a comet by the size of its nucleus or the length of its tail(s).

But now, a group led by Dr Geraint Jones of University College London’s (UCL) Mullard Space Science Laboratory, has used data from the Ulysses spacecraft to measure the region of space disturbed by Comet McNaught’s presence.

Artist's impression of the Ulysses spacecraft flying through the tail of a comet.

Artist's impression of the Ulysses spacecraft flying through the tail of a comet.

Magnetic field measurements show evidence of a shockwave surrounding the comet, formed when ionised gas emitted from the nucleus joined the fast-flowing particles of the solar wind, causing the wind to slow down abruptly.

In early 2007, Comet McNaught became the brightest comet visible from Earth in 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 spacecraft’s 19-year mission. (The other encounters included Comet Hyakutake in 1996, the current record-holder for the comet with the longest measured tail.)

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

Ulysses took an incredible 18 days to travel through the solar wind affected by Comet McNaught, compared to just 2.5 days for Comet Hyakutake.

“This shows that the comet was not only spectacular from the ground; it was a truly immense obstacle to the solar wind,” says Dr Jones.

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, while the famous Comet Halley took only a few hours.

Adapted from information issued by the RAS. Image credits: Sebastian Deiries, ESO / NASA / ESA.

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