It's green, about 300,000 miles wide and some 38 million miles away, and tonight a comet called Lulin could be visible to the naked eye.

At around midnight, UK space scientists and amateur stargazers will be looking due south for a glimpse of the unusual celestial body as it reaches its nearest point to Earth.

Discovered a year ago, the comet gets its green colour from a poisonous, cyanide-like gas in its atmosphere. It will appear close to Saturn in the sky during its first visit to the Earth's inner solar system and University of Leicester experts hope the close encounter will give them a valuable insight into the comet.

They are using Nasa's Swift satellite to monitor Lulin as it nears Earth - the spacecraft has recorded simultaneous ultraviolet and X-ray images of the comet.

To the naked eye, it will appear like a small green cloud creeping across the night sky.

University of Leicester researcher Jenny Carter said: "The closest it will be is tonight when it moves close to Saturn in the sky, around half-past midnight. It's almost due south at that time.

"It will be better, obviously, if you have binoculars or a telescope and find a clear, dark sky."

A comet is a clump of frozen gases mixed with dust. These "dirty snowballs" cast off gas and dust whenever they venture near the sun. Formally known as C/2007 N3, Comet Lulin was discovered last year by astronomers at Taiwan's Lulin Observatory.