Press Release

The shocking size of Comet McNaught

13-Apr-2010, Plain html version.

Royal Astronomical Society Press Release
RAS PN 10/24 (NAM 08)

The shocking size of Comet McNaught

British 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 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 the 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 the Earth and the Sun. This is far beyond the spectacular dust tail that was visible from Earth in 2007. Dr Jones said, “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. 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 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; 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.


Geraint Jones
Mullard Space Science Laboratory
University College London
Holmbury St Mary
Dorking, Surrey

Dr Jones can be reached through the NAM press office on Tuesday 13th April.
NAM 2010 Press Office (12th – 16th April only)
Room G358
Gilbert Scott Building
University of Glasgow.
Tel: +44 (0)141 330 7409, +44 (0)141 330 7410, +44 (0)141 330 7411

Anita Heward
Press Officer
Royal Astronomical Society
Mob: +44 (0)7756 034 243

Dr Robert Massey
Press and Policy Officer
Royal Astronomical Society
Mob: +44 (0)794 124 8035



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. The magnetic field investigation on Ulysses was led by a team from Imperial College London. For more details, see:


Image Caption: Comet McNaught viewed over the Pacific in 2007. Credit: Sebastian Deiries/ESO. Original and full size versions.

Images of the Ulysses spacecraft can be found at:

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



The RAS National Astronomy Meeting 2010 will take place from 12-16th April at the University of Glasgow. The conference is held in conjunction with the UK Solar Physics (UKSP) and Magnetosphere Ionosphere and Solar-Terrestrial Physics (MIST) meetings. NAM2010 ( is principally sponsored by the Royal Astronomical Society (RAS) and the University of Glasgow.


The Royal Astronomical Society (RAS:, founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognises outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3000 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.


The University of Glasgow (founded 1451) is one of the world’s top 100 research universities with more than 70 per cent of its research rated as world-leading or internationally excellent. The Physics and Astronomy Department is one of the top four in the UK’s major research-intensive universities, the Russell Group.

The conference comes to Glasgow during the 250th anniversary year of the founding of the Regius Chair of Astronomy at the University of Glasgow, first held by astronomer and meteorologist Alexander Wilson in 1760. The present incumbent is Prof. John Brown, 10th Astronomer Royal for Scotland.