In early 2007 Comet C/2006 P1 McNaught became the brightest comet visible from Earth for 40 years, and now, according to new data, is also the largest comet measured to date.

Instead of using a standard method of measuring the length of the comet’s tail to determine its size, a group lead by Geraint Jones of UCL’s Mullard Space Science Laboratory used data from the ESA/NASA Ulysses spacecraft as it passed through the comet’s ionized gas tail in 2007. It crossed the comet’s tail at a distance downstream of its nucleus more than 1.5 times the distance between the Earth and the Sun, a much greater distance than the spectacular dust tail that was visible to observers on the Earth in 2007.

The data provided estimates of the size of the region of space disturbed by the comet's presence, showing evidence of a decayed shockwave surrounding the comet created when ionized gas emitted from the comet's nucleus joined the fast-flowing particles of the solar wind, causing the wind to slow down abruptly.

"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, " says Jones. "What we can say is that Ulysses took just 2.5 days to traverse the shocked solar wind surrounding Comet Hyakutake [whose tail it crossed in 1996], 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."

Other comet-crossing spacecraft, such as Giotto, encountered Comet Grigg-Skjellerup in 1992 and Comet Halley in 1986, and took less than one hour and several hours respectively to cross the shocked region, demonstrating further the huge scale of Comet McNaught.

"The scale of an active comet depends on the level of outgassing rather than the size of the nucleus," adds Jones. "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."