Comet Lulin visits inner

Solar System

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 23 February, 2009

On 24 February Comet Lulin will make its first visit to the inner Solar System, streaking past the Earth at a distance of 38 million miles, or 160 times further than the Moon.

Comet Lulin, formally known as C/2007 N3, was discovered last year by astronomers at Taiwan's Lulin Observatory. Nicknamed the “green comet”, Lulin’s atmosphere contains poisonous cyanogen and diatomic carbon gases. Researchers at the University of Leicester will be using NASA’s Swift satellite to monitor the comet in X-ray, ultraviolet and optical light as it closes in on the Earth this week.

"The wonderful ease of scheduling of Swift and its joint UV and X-ray capability make Swift the observatory of choice for observations like these," says Dr Julian Osborne, leader of the Swift project at Leicester. The University of Leicester played a major role in developing Swift's X-Ray Telescope.

This image of Comet Lulin taken 28 Jan merges data acquired by Swift's Ultraviolet/Optical Telescope (blue and green) and X-Ray Telescope (red). At the time of the observation, the comet was 99.5 million miles from Earth and 115.3 million miles from the Sun. Image: Univ. of Leicester/NASA/Swift/Carter et al.

Comets are thought to represent some of the most pristine ingredients of the Solar System. They are made up of dust, gas and ice, and as they approach the Sun, the frozen components sublime and stream out in tails. Swift observations of the comet on 28 January revealed that it is certainly active. "The UVOT data show that Lulin was shedding nearly 800 gallons of water each second," says team member Dennis Bodewits, a NASA Postdoctoral Fellow at the Goddard Space Flight Center in Greenbelt, which is enough to fill an Olympic-size swimming pool in less than 15 minutes.

Although Swift can’t see water directly, ultraviolet light from the Sun quickly breaks apart water molecules into hydrogen atoms and hydroxyl (OH) molecules, the latter of which Swift's Ultraviolet/Optical Telescopes (UVOT) can detect. "This gives us a unique view into the types and quantities of gas a comet produces," Bodewits explains.

The images taken so far reveal a hydroxyl cloud spanning a distance greater than the distance between Earth and the Moon, and a tail streaming off at an angle. Solar radiation pushes the icy grains away from the comet, and as they gradually evaporate, they create a thin tail of hydroxyl molecules. Farther from the comet, even the hydroxyl molecule succumbs to solar ultraviolet radiation, breaking into its constituent oxygen and hydrogen atoms.

Track Comet Lulin through Virgo and towards Leo over the next week. AN graphic by Greg Smye-Rumsby.

"The solar wind - a fast-moving stream of particles from the Sun - interacts with the comet's broader cloud of atoms," says Stefan Immler, also at Goddard. "This causes the solar wind to light up with X-rays, and that's what Swift's XRT sees." This interaction, called charge exchange, results in X-rays from most comets when they pass within about three times Earth's distance from the Sun. Because Lulin is so active and is losing a lot of gas, its X-ray emitting region extends in a large cloud far sunward of the comet. The team hope that the forthcoming observations of Lulin will reveal more on the comet’s chemistry and enable scientists to build up a three-dimensional picture of the comet during its maiden voyage through the Solar System.

Comet Lulin will be visible to the naked eye, and for most locations in the Northern Hemisphere will be easiest to spot after midnight when it is high in the sky. In small telescopes it will appear as a dim fuzzy ‘star’ - brighter in the centre and more diffuse around the edges. It will fade from view by mid-March.

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