An ancient comet discovered 110 years ago, thought to have no tail, has been found to have one 20 million miles long.

The discovery of an enormous dust tail on the comet Tempel 2 was made by the earth-orbiting Infrared Astronomical Satellite. A super-sensitive infrared telescope aboard the satellite provides a different view of the sky than optical telescopes because it "sees" the heat celestial bodies give off and not their light. The satellite was built in the Netherlands, was put into orbit by the United States and is controlled by a ground station in Chilton, England.

The discovery comes as a surprise because Tempel 2 is estimated to be hundreds of years old and to have circled the sun so many times that, most astronomers believed, it would have lost its dust tail long ago to solar heat.

Most comets like Tempel 2, which circles the sun every 5.28 years, do not have visible tails and were thought to have none at all.

"We have to change our thinking on this right now," said Ray Newburn of the Jet Propulsion Laboratory in Pasadena, Calif., where the infrared telescope on the satellite was built. "It could be that all comets, even faded and quiet comets, have tails that can be seen in the infrared light."

No tail or debris from Tempel 2 had been seen from the Earth before, presumably because the dust tail is so faint in the visible light that it could not be seen by even the largest telescopes which would require long exposure times to photograph it.

Tempel 2 is one of about 100 "short-period" comets, so-called because they circle the sun so frequently. The comet is now about 100 million miles from Earth, headed for the outer regions of the solar system near Jupiter, where it will change direction to head back toward the sun again.