This image of Comet Lulin, taken on 28 January, merges data from Swift's Ultraviolet/Optical Telescope (blue and green) and X-Ray Telescope (red). The comet's tail extends off to the right, while a cloud of oxygen and hydrogen atoms extends far Sun-ward of the comet. At the time of the observation, the comet was 160 million km from Earth and 185 million km from the Sun (Image: NASA/Swift/U of Leicester/DSS/STScI/ AURUA/Bodewits et al.)
A space telescope that usually studies the most powerful explosions in the universe has set its sights on an approaching comet. Its observations at ultraviolet and X-ray wavelengths should help reveal the comet's composition, structure and its interaction with the solar wind.
Comet Lulin, which was discovered in 2008 by astronomers at the Lulin Observatory in Taiwan, will make its closest pass near Earth on 24 February. At that time, it will come within 61 million kilometres, or 40% the Sun-Earth distance, from our planet.
Amateur astronomers have been watching the approaching comet, which is bright enough to be visible with the naked eye from dark sites (see this image taken by Jack Newton).
Now, NASA's Swift space telescope, designed to study cosmic explosions called gamma-ray bursts, has released an image of the comet.
The icy body is shedding gas and dust as it nears the Sun, whose ultraviolet light breaks apart the comet's water molecules into hydrogen atoms and hydroxyl (OH) molecules. Swift's Ultraviolet/Optical Telescope (UVOT), which can detect the hydroxyl molecules, found that they fill a cloud more than 400,000 km across.
"The comet is quite active," team member Dennis Bodewits of NASA's Goddard Space Flight Center in Maryland said in a statement. "The UVOT data show that Lulin was shedding nearly 800 gallons of water each second" - enough to fill an Olympic-size swimming pool in less than 15 minutes.
Farther from the comet, solar ultraviolet radiation also breaks up hydroxyl molecules - into oxygen and hydrogen atoms.
"The UV will [teach] us about the composition of the comet," Bodewits told New Scientist, adding that such studies are interesting because comets might have brought water to Earth several billion years ago.
Studying the comet in X-rays reveals how it interacts with the solar wind, a stream of charged particles from the Sun. That's because positive ions in the solar wind steal electrons from neutral gases, such as hydroxyl, that they hit.
Since the stolen electrons are in an excited state, "this makes the solar wind glow when it interacts with a comet", says Bodewits.
The observations could lend insight into why Mars has such a thin atmosphere. "The Earth is lucky because we have a magnetic field that protects us from most of the solar wind," he continues. "But Mars, lacking such a shield, might have lost its atmosphere because of the interaction with the solar wind."
Jenny Carter of the University of Leicester in the UK, who is leading Swift's studies of the comet, says the team plans to continue its observations. "We are looking forward to future observations of Comet Lulin, when we hope to get better X-ray data to help us determine its makeup," she said. "They will allow us to build up a more complete 3D picture of the comet during its flight through the solar system."