Astronomers on Friday said a vast cemetery of comets lies in the asteroid belt between Mars and Jupiter, some revived by a nudge from the Sun after millions of years of dormancy, a finding that would overturn conventional thinking about these wanderers of the Solar System.

"We found a graveyard of comets," said Ignacio Ferr�n of the University of Antioquia in Medellin, Colombia.

"Imagine all these asteroids going around the Sun for aeons, with no hint of activity."

"We have found that some of these are not dead rocks after all, but are dormant comets that may yet come back to life if the energy that they receive from the Sun increases by a few per cent."

Dubbed "dirty snowballs" by US astronomer Fred Whipple in 1949 for their loose assembly of primeval dust and ice, comets are traditionally thought to come from two locations -- the Kuiper Belt and the Oort Cloud -- that lie at extreme distances from the Sun.

This explains why comets can take thousands of years to complete their elliptical circuit around the Sun -- and why such rare visitors have never been recorded in human history.

Around 500 comets are so-called short-period comets, whose orbit has been deflected by the gravitational pull of Jupiter, and they return at times ranging from several years to a couple of centuries.

Writing in the Monthly Notices of Britain's Royal Astronomical Society, Ferrin and colleagues followed an intriguing discovery made by other astronomers -- the detection of at least 12 active comets in the past decade in the main asteroid belt.

Until now, this region has been mainly considered to a dumping ground for space rocks -- rubble from an unborn planet.

Not so, says the new study, which describes it as containing "an enormous graveyard of ancient dormant and extinct rocky comets" whose surface ice has been stripped away by years of exposure to solar rays.

The comets can get reactivated when they pass relatively close to Jupiter, the biggest planet of the Solar System, and the shape of their orbits is tugged.

This can decrease the distance between the comet and the Sun, resulting in a tiny rise in average temperature which in turn warms the subsurface ice and the gases it contains.

These are then disgorged with the dust into space to create the "tail," reflected in the Sun's rays, that so distinguishes comets.

"These objects are the 'Lazarus comets', returning to life after being dormant for thousands or even millions of years," said Ferrin, referring to the Biblical character said to have been brought back from the dead by Jesus.

"Potentially any one of the many thousands of their quiet neighbours could do the same thing."

Little is known about asteroid-belt comets, especially about how they got there. The few visual sighting of them suggest that they move within the herd of asteroids, rather than swing out individually, and the "tail" may only visible when they are closest to the Sun.

"There is no possibility of a potential collision with Earth. The orbits of the Lazarus comets are quite circular to keep them inside the main belt of asteroids," Ferrin said in an email exchange with Earth.

"Although they are relatively near, the fact that they are quite old makes them faint and difficult to spot and to discover. That is why we only know 12 of them," he said.

Additionally their interval of activity is quite short, much shorter than normal comets. A normal comet may be active for months or years. A Lazarus comet may be active for days or weeks."