Seeing Lulin, Once in a Million Years

October 07, 2009

Content prepared in collaboration with Discovery Space partner Lowell Observatory.

Lowell Observatory astronomers had a huge responsibility earlier this year: they were planning for the arrival of an ancient VIP (or should I say VIC?). Comet Lulin was making its rapid pass of the inner Solar System, putting on a show for observers. However, to see the "dirty snowball" astronomers had to be patient.

Although Lulin was discovered in 2007, it drifted past the sun in 2008. But astronomers were prepared in January 2009 when it re-started its display in the night sky.

A brief glimpse

"Lulin was a challenging comet to observe," said Matthew Knight, a Postdoctoral Research Scientist at Lowell Observatory. "It was moving rapidly around the sun in the opposite direction as the Earth. As a result, it was behind the sun until early January. When we were first able to observe it in late January it was only visible for a couple of hours just before sunrise. A month later (late February) it was up all night, by late March it was setting shortly after midnight, and by late April it was just visible for a couple of hours after sunset."

"One aspect that made Lulin a lot of fun to observe was that for a brief time in February it was visible with the naked eye. So I would go outside while the telescope was taking an image and look at it with my eye at the same time. You don't get a chance to do that very often in astronomy," added Knight.

Although the observing opportunity was brief, the 1.1 meter John S. Hall Telescope at Lowell's dark sky research site on Anderson Mesa near Flagstaff captured the comet in great detail. The results of the study were presented at the American Astronomical Society's Division of Planetary Sciences meeting in Puerto Rico on Tuesday.

The Comet Lulin corkscrew

The astronomers presented results on how they used the cyanogen gas being vented from near Lulin's poles as a tracer to measure the comet's rotation to a very high precision. Usually cometary dust is studied from ground-based observatories, but in this case the cyanogen had a stronger signal, allowing the researchers to derive more detail from the data. Measuring the corkscrewing gas, it was found that the rotational period of Lulin's nucleus is 42 hours (within an error margin of only 1%).

Now Knight and colleague David Schleicher are working on ways to extract even more information from the observations carried out at the start of the year; they hope to reconstruct the comet's nucleus as a 3D model.

"How do you do 3-D when everything looks like it is in the plane of the sky?," asks Schleicher. "What we're trying to do is create a 3-D model that reproduces not only what we see in the basic corkscrew, but also how the appearance changes from month to month."

Most interestingly, this 3D model should reveal how the "seasons" altered the comet's characteristics as it zoomed through the Solar System.

Not in a million years

Unfortunately, Knight and Schleicher won't get another chance to collect any further data from Comet Lulin. The interplanetary interloper is currently journeying back out of the Solar System toward a mysterious region called the Oort Cloud, a little under 1 light year from the sun. This little fact makes any follow-up observing campaign an impossibility as Comet Lulin is a long-period comet, with an orbital period of 1 million years.

Fair well Lulin, it was a pleasure...

A special thanks to Steele Wotkyns, Public Relations Manager at Lowell Observatory for providing the material for this article.

Image: "False color image of Comet Lulin (2007 N3) as seen March 2, 2009 using the CN filter on the Lowell 1.1-m telescope. The diagonal lines are stars which were streaked during the exposure due to the comet's high rate of motion. The comet looks elongated from the bottom left to top right due to the presence of CN jets which could be seen after more advanced image enhancement techniques were applied." --Matthew Knight. Credit: Lowell Observatory.