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The comet contains the gases cyanogen and diatomic carbon, which give it its green colour. ((Paolo Candy/Cimini Astronomical Observatory))

A comet is closing in on Earth and may be visible to the naked eye around 3 a.m. ET Tuesday.

When it's able to be seen, the Comet Lulin will be just 61 million kilometres away — about 160 times the distance between the Earth and the moon — which will be its closest approach to our planet, according to NASA.

The comet, whose official name is C/2007 N3, is expected to appear about a third of the way up the southern sky, a few degrees from Saturn in the constellation Leo. It is called "Lulin" after Lulin Observatory in Taiwan, where astronomers discovered it last year.

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This image of Lulin was taken by NASA's Swift Satellite. The blue image was taken by the satellite's ultraviolet telescope, and the red image was taken by a telescope on the satellite that detects X-rays. ((NASA))

The comet is a ball of ice and other frozen compounds that partially vaporize into a tail of gas and dust as the comet approaches the sun. 

Lulin appears green in photographs taken through a telescope because the gases gushing from its surface include two that appear that colour — cyanogen and diatomic carbon. The comet looks like it has two tails, one on each side, because of its orientation relative to the Earth, which creates an optical illusion called an "anti-tail."

Usually both tails, the dust tail and the ion tail, point in the same direction and may appear to be part of the same unit.

Scientists are using NASA's Swift Satellite to track and study the comet as it approaches.

The satellite includes probes that detect ultraviolet and X-ray radiation, which can be used to learn more about its chemistry, said a news release Friday from the Science and Technology Facilities Council in the United Kingdom, which is collaborating with NASA.

The satellite can detect hydroxyl molecules that are produced when water is broken down by ultraviolet light from the sun.

So far, the satellite's data show Lulin was shedding nearly 3,000 litres of water each second, or enough water to fill an Olympic-size swimming pool in less than 15 minutes, the release said.