British scientists reckon they've identified the largest comet yet, using data from the ESA/NASA Ulysses spacecraft to measure the region of space disturbed by the its presence.
In January and February 2007, Comet C/2006 P1 McNaught became the brightest comet visible from Earth for 40 years. Quite by chance, Ulysses crossed its tail - as it did Comet Hyakutake's tail in 1996, the longest measured tail until now.
Ulysses encountered McNaught’s tail at a distance way beyond the spectacular dust tail that was visible from Earth in 2007.
Magnetometer data reveals a decayed shockwave surrounding the comet, created when ionized gas emitted from its nucleus joined the fast-flowing solar wind and slowed it down.
"Ulysses took just two and a half days to traverse the shocked solar wind surrounding Comet Hyakutake, compared to an incredible 18 days in shocked wind surrounding Comet McNaught," says Dr Geraint Jones of University College London.
"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. Giotto's encounter with Comet Grigg-Skjellerup in 1992 took less than an hour; crossing 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.”