Cosmic graveyard of comets discovered

A cosmic graveyard of comets has been discovered by astronomers in the asteroid belt between Mars and Jupiter.
Researchers, led by Ignacio Ferrin from the University of Anitoquia describe how some of these objects, inactive for millions of years, have returned to life leading them to name the group the 'Lazarus comets'.
Most observed comets have highly elliptical orbits, meaning that they only rarely approach the Sun. Some of these so-called long period comets take thousands of years to complete each orbit around our nearest star.
Although uncommon events, comets also collide with Earth from time to time and may have helped bring water to our planet, researchers said.

The new study looked at a third and distinct region of the Solar System, the main belt of asteroids between the orbits of Mars and Jupiter. This volume of space contains more than 1 million objects ranging in size from 1 m to 800 km.
In the last decade 12 active comets have been discovered in the asteroid main belt region. This was something of a surprise and the research team set out to investigate their origin.

"We found a graveyard of comets," said Ferrin.
"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," said Ferrin.

Surprisingly, this can happy fairly easily, as the orbits of many objects in the asteroid belt are nudged by the gravity of Jupiter, researchers said.
The shape of their orbits can then change, leading to a decrease in the minimum distance between the object and the Sun (perihelion) and a slight increase in average temperature.
According to this interpretation, millions of years ago the main belt was populated by thousands of active comets. This population aged and the activity subsided. What we see today is the residual activity of that glorious past.
Twelve of those rocks are true comets that were rejuvenated after their minimum distance from the Sun was reduced a little. The little extra energy they received from the Sun was then sufficient to revive them from the graveyard.

"These objects are the 'Lazarus comets', returning to life after being dormant for thousands or even millions of years. Potentially any one of the many thousands of their quiet neighbours could do the same thing," Ferrin said.