Credit: Antti Kemppainen
Comet McNaught, or the “Great Comet of 2007,” was the brightest to appear in sky for 40 years. It blazed across the sky of the Southern Hemisphere, visible to the unaided eye even in daylight. Astronomers had to caution viewers to protect their vision when viewing it near the Sun, since it could even be seen close to our dazzling home star. Its huge tail could be glimpsed as far north as Colorado.
That tail looked spectacular – but we weren’t seeing the half of it, quite literally. On Feb. 3, 2007, the Ulysses spacecraft, a probe orbiting the Sun to study its atmosphere and solar wind, fortuitously happened to pass through the tail of the comet. Scientists seized on the opportunity to measure the region of space disturbed by the comet’s presence, looking for cometary ions, or electrically charged atoms and molecules, and studying their interaction with the solar wind. A comet’s tail of ionized gas is separate from its dust tail and reacts to the solar wind, changing speed or even bending. In fact, ion tails from comets are how scientists first discovered that the Sun was emitting a solar wind.
Comet McNaught over the Pacific Ocean, as seen by the European Southern Observatory's Paranal Observatory
Credit: ESO/Sebastian Deiries
What they found when they looked at the Ulysses data was the equivalent of one of those cartoon tails that unspools endlessly while an increasingly alarmed character reels it in. It took Ulysses 18 days to travel across the area of solar wind affected by the comet. During an earlier encounter that Ulysses had with Comet Hyakutake, that trip had taken just two and a half days. And Comet Hyakutake was no unimpressive comet itself, having earned the moniker “Great Comet of 1996,” a designation that we can only imagine causes Comet McNaught to pat it gently on the head during comet meet-and-greets in the Oort Cloud. We can also imagine comet meet-and-greets. No one’s ever been to or seen the Oort Cloud, so we can imagine comet rugby matches and comet ice cream socials as well. They’re arranged by dwarf planets, who are known for their organizational skills. Where were we?
Oh, yes. Ions. When Ulysses encountered Comet McNaught’s tail of ionized gas, the spacecraft was downstream of the comet’s nucleus – the chunk of ice and rock that makes up the solid part of a comet -- by over 1.5 times the distance between the Earth and the Sun. The solar wind, filled with comet-generated molecules, had slowed down from 435 miles (700 km) per second to 249 miles (400 km) per second.
No one can say exactly how long the comet’s tail of ions was, but that’s pretty far away for the solar wind to still be affected by a snowball just 6-12 miles (10-20 km) wide. Comet McNaught, with its enthusiastic and expansive release of ionized gas, still holds the current record for “longest comet tail.”