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The Shocking Size of Comet McNaught

Last Updated on Tuesday, 13 April 2010 19:20
Published on Tuesday, 13 April 2010 19:15
mcnaught.jpgBritish scientists have identified a new candidate for the biggest comet measured to date. Dr Geraint Jones of UCL’s Mullard Space Science Laboratory will be presenting the results at the RAS National Astronomy Meeting in Glasgow on Tuesday 13th April.  Instead of using the length of the tail to measure the scale of the comet, the group have used data from the 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 shockwave surrounding the comet created when ionized gas emitted from the comet’s nucleus interacts with fast-flowing particles in 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 the comet with the longest 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 the Earth and the Sun.  This is far beyond the spectacular dust tail that was visible from Earth in 2007.

Dr Jones said, “It’s very difficult to observe Comet McNaught’s plasma tail in comparison with the dust tail, so we can’t really estimate how long it might be.  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; 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 half 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 outgassing rather than the size of the nucleus,” Dr Jones added. “Comet nuclei aren't necessarily active over their entire surfaces, and all we can say is that McNaught's level of gas production was 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.



From the 12th to the 16th April 2010, more than 500 astronomers and space scientists will gather at the University of Glasgow for the RAS National Astronomy Meeting (NAM 2010), held in conjunction with the UK Solar Physics (UKSP) and Magnetosphere Ionosphere and Solar-Terrestrial Physics (MIST) meetings.

NAM 2010 includes 14 plenary lectures and 35 parallel sessions featuring new research from across the fields of astronomy, space science, solar and solar-terrestrial physics, including the evolution of massive stars, dark matter, the role of high-energy particles, explosions on the Sun and in the distant Universe and the prospects for astronomy with a new generation of giant telescopes.

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Ulysses, a joint ESA-NASA deep-space mission, was designed to study the heliosphere - the region of space influenced by the Sun and its magnetic field. The primary scientific goal was to make the first-ever measurements of the unexplored region of space above the Sun's poles. Other areas of investigation include determination of the global properties and behaviour of the solar wind, the study of energetic particles of solar and interplanetary origin, measurement of the magnetic field of the Sun and the heliosphere, study of galactic cosmic rays, investigation of how the heliosphere interacts with interstellar space, and participation in a programme to identify the origin of gamma-ray bursts. Ulysses was launched on 06 October 1990 and ceased operations on 30 June 2009 after becoming the longest running ESA-operated spacecraft.

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Comet McNaught viewed over the Pacific in 2007. Credit: Sebastian Deiries/ESO

Images of the Ulysses spacecraft can be found at:

Videos and animations relating to the Ulysses mission can be found at:


Geraint Jones

Mullard Space Science Laboratory

Holmbury St Mary

Dorking, Surrey

Tel: +44 (0)1483 204 263 (direct) 

Email: ghj<at>