© Patrick Boomer
Patrick Boomer, captured these photographs Southwest of Red Deer, Alberta, Canada. These comparison photos of Comet McNaught, taken on Jan. 6th, Jan. 7th and Jan 9th (2007) shows the difference a day makes. Both photos were taken at 5:36pm local time.
Comet McNaught, the so-called Great Comet of 2007, has been identified as the biggest comet measured to date, according to scientists, whose calculations were based on the comet's overall influence in space.

Instead of using the length of the comet's tail to measure the scale of the comet, astronomers used data from the ESA/NASA Ulysses spacecraft to determine the size of the region of space disturbed by the comet's presence - a cosmic wake across the solar system.

Through analysis of magnetometer data, scientists found evidence of a decayed shockwave surrounding the comet, which was created when ionized gas emitted from the comet's nucleus joined the fast-flowing particles of the solar wind. That, in turn, caused the solar wind around the comet to abruptly slow down.

Great Comet McNaught

Comet C/2006 P1 McNaught was discovered by astronomer Robert H. McNaught in August 2006. In January and February 2007, Comet McNaught became the brightest comet visible from Earth for the past 40 years. [Photos of Comet McNaught.]

Serendipitously, the robotic Ulysses spacecraft made an unexpected crossing of Comet McNaught's tail in 2007, which allowed astronomers to collect valuable and pertinent data.

Ulysses encountered McNaught's tail of ionized gas downstream of the comet's nucleus, at a distance of more than 1.5 times the distance between the Earth and the sun. That's about 139 million miles (224 million km). The average distance between the Earth and sun, called 1 astronomical unit, is about 93 million miles (150 million km).

This was far longer than the spectacular dust tail that was visible from Earth in 2007.

This image from the SECCHI/HI-1B instrument on the NASA STEREO-B (Behind) spacecraft was taken on January 11, 2007 just after the door covering the instrument was opened for the first time after the STEREO launch on October 26, 2006. The image is dominated by a spectacular view of Comet C/2006 P1 (McNaught).
Ulysses' 2007 encounter with McNaught was one of three unplanned encounters that Ulysses made with comet tails during its 19-year mission, which ended in 2009. The other encounters included Comet Hyakutake in 1996. Hyakutake is the current record-holder for the comet with the longest measured tail.

Measuring a comet

But the data collected from Ulysses' encounter with McNaught allowed scientists to gauge the size of that comet.

"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," said Dr. Geraint Jones of University College London's Mullard Space Science Laboratory, who will present the findings at the Royal Astronomical Society National Astronomy Meeting in Glasgow, Scotland, on April 13.

"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," Jones said. "This shows that the comet was not only spectacular from the ground, it was a truly immense obstacle to the solar wind."

Scientists also compared the crossing times for other comet encounters, which further revealed the huge scale of Comet McNaught. The Giotto spacecraft's encounter with Comet Grigg-Skjellerup in 1992, for example, took less than an hour from one shock crossing to another. Similarly, it took only a few hours to cross the shocked region of the famed Halley's Comet.

In 2007, the nucleus of Comet McNaught was estimated to be less than 15 miles (25 km), but Jones used a different measure to calculate the comet's scale.

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