The night watchman
comet
The peacock-like tail of the comet became clearly
visible once it entered darker skies.
This shot was captured by Rob McNaught on
25 January at Siding Spring.

Most nights, as the southern hemisphere sleeps, Rob McNaught is awake and on guard. He’s part of an international team of astronomers scanning the skies for Near Earth Objects (NEOs), such as asteroids and comets, that could pose a threat to our planet. Scientists believe that large objects colliding with the Earth in the past may have had cataclysmic effects, wreaking destruction at the point of impact, altering global climate patterns and causing mass extinctions. Working with colleagues at the University of Arizona, McNaught has discovered or co-discovered more than 30 comets and thousands of asteroids from his base at Siding Spring in outback New South Wales. So far none of the NEOs appear to be an immediate threat. But very rarely, one of these space travellers does pass close to our planet with spectacular results.

If anything, McNaught’s expectations were lower than usual when he began routine scans at the Uppsala Schmidt telescope on the night of 7 August 2006. This is just one of the optical telescopes housed by the ANU Research School of Astronomy and Astrophysics at the site. The moon was full, and he was due to observe a section of sky close to the edge of the Milky Way where the stars are denser – two factors that add up to poor NEO spotting. But one of the scans showed up a faint point of light emerging out of the brightness of the Milky Way. Further analysis showed this to be a comet headed towards the Sun, which McNaught reported to the Central Bureau of Astronomical Telegrams, the international clearing house for comet discoveries. Tracking the comet over the next few days, he saw that this object was going to pass reasonably close by the Sun, well within the orbit of Earth and even perhaps within that of Mercury. This could translate into quite a show for space spotters, but it could also mean that the comet would become lost in the glare of the Sun.

On hearing of the discovery, the response from the astronomical community was muted. This caution was a lesson learned from the disappointment of Comet Kohoutek in 1973, which was initially hailed as the ‘comet of the century’ before it fizzled out. McNaught says it’s common for a comet’s rate of brightening to diminish as it nears the Sun, so astronomers have learned not to get their hopes up.

“At the time, I wasn’t sure what it would be like,” McNaught says. “My peers thought there was no reason to believe it would be unusual. But after observing it over time, I noticed fairly early on that it had a rapid rate of brightening, which was quite unusual.”

As it journeyed closer to the Sun, the comet continued to grow in luminescence. McNaught became more confident that this was something out of the ordinary. By mid-January, his predictions were confirmed. This ball of ice and dust brightened so dramatically as it approached that it became visible to the naked eye in daylight. Astronomers and amateur stargazers thrilled at the sight of what was soon dubbed ‘the Great Comet of 2007’. This object that shone in the sky brighter than the Venus was designated C/2006 P1, but most people came to know it as ‘Comet McNaught’, after the convention that a comet is named for its finder. And for him, it was the fulfilment of a long-held dream.

“I wanted to see and discover comets ever since I was a kid,” McNaught says. “So to see this one turn out so big and bright was hugely exciting. Just witnessing it with my own two eyes was much more important than being credited with the discovery.”

To ensure he would have a good chance of viewing the comet when it was at its brightest, McNaught booked a flight to Hong Kong. He knew from previous experience that comets near the Sun are more easily seen from altitude, where there is less pollution, fewer clouds and the horizon is much lower. A trip to the Northern Hemisphere would also allow him to view the comet a day before it became visible in Australia.

On 12 January 2007, from the window of a passenger jet, McNaught witnessed his very own comet punch a pin hole in the late-afternoon sky.

“I was really excited,” he says. “It was the first time I’d seen a comet with the naked eye in daylight. It was a special moment.”

His excitement remained high after he returned to Siding Spring, as Comet McNaught remained visible in the southern hemisphere for several weeks. Emails and media requests flooded in from around the world as more people spotted the light in the sky. After reaching perihelion (the point in its orbit that’s closest to the Sun) on 12 January, the comet appeared higher relative to the horizon, placing it in darker skies. Here, as if delivering a parting gift as it began its long journey back to the outer edges of the solar system, C/2006 P1 revealed an incandescent tail. This peacock-like effect is caused by ejected dust being pushed outward from the Sun by the pressure of sunlight, but also lagging behind the comet as it undertakes its tight turn on passing the Sun.

Comet McNaught’s showy pass by Earth allowed scientists to study it in detail. Although the comet’s composition is still largely unknown, it’s believed that this was its first passage through the inner solar system where our planet resides. It’s thought that the comet originated in the Oort Cloud, a dense ring of icy objects orbiting the Sun at the distance of about one light year, or slightly less than ten million kilometres. Even though it made the trip at a fair clip, it won’t be back by Earth for many thousands of years. But there are more comets and asteroids where it came from, and probability says that one of them could intersect Earth’s orbit. When it comes to the prospect of an NEO impact, McNaught says we need to be alert but not alarmed.

“There are many dangers that we treat as simple consequences of living, but are too busy getting on with our lives to worry about,” McNaught says. “I don’t think it would be healthy to worry about the prospect of a comet impact. But it does make sense for us to be aware of the threat, quantify it and prepare to deal with any potential threat.”


Editor's Note:  First published in the Autumn 2007 edition of the ANU Reporter.  For permission to reproduce this article please contact ANU.