However, these bits would not be very clear and be barely visible to the naked eye. The best time for viewing them would be between May 12 and 14, with the aid of binoculars or a small telescope," he said.

"At thist time, the fragments of the Comet will be in the constellations Cygnus (Hansa) and Pegasus (Khagashwa). All this astral phenomenon will be visible in the East from around midnight till the early hours of the morning in the West. There is a small chance that the sighting of the Comet will be accompanied by meteor showers due to its debris, but it seems unlikely," Dr Sidharth added.

The break-up of a comet is a common phenomenon and sometimes fragments may even crash into the earth, as happened in Tunguska (Siberia) in 1908, he said, adding that fortunately, the site of the impact was uninhabited. It is believed that this devastation was cuaused by the debris of the Comet Encke.

However, Dr Siddarth assured that there is no chance of frgaments from this particular comet hitting the Earth.

The object, called 60558 Echeclus, was discovered in 2000 and is a "centaur" - part rocky asteroid and part icy comet. Its new activity, revealed in images taken on 2 April, makes it look "really strange", says William Romanishen of the University of Oklahoma, US, one of the team that took the images. "The first thing that came to mind was a collision."

Earlier observations showed Echeclus rotates about once every 26 hours, so a fragment would need a push to escape its gravity, says Paul Weissman of the Jet Propulsion Laboratory, who spotted the original cloud of gas and dust around Echeclus on 30 December 2005.
Explosive sublimation

Such an impact on a comet-like body has not been observed before. But there are other possibilities, says Steve Tegler of Northern Arizona University, US, who works with Romanishen. He says it is most likely that the event was caused by explosive sublimation of volatile ices in Echeclus, resulting in material being blasted off.

Tegler says the evaporating ice is probably carbon monoxide, with vaporises at about that distance from the Sun, where the object's temperature is about 80 degrees Kelvin, close to the sublimation temperature. No one has yet analysed the gas composition.

Another puzzle is the difference in activity between the main nucleus and the fragment. Freshly exposed ices normally sublimate, so "you'd expect equal activity from both pieces", Wiessman says. But the nucleus does not look very active.
Unstable orbits

Echeclus was discovered by the Spacewatch telescope in 2000, and at first looked like an asteroid. Then Weissman found it was surrounded by a coma, so astronomers also classed it as a periodic comet, 174P. The photos from 2 April show the coma has now spread out.

Echeclus belongs to a group of more than 100 centaurs with orbits well outside the main asteroid belt. Although originally from the distant Kuiper belt, they now orbit the Sun between Jupiter and Neptune, but will be ejected from those unstable orbits within tens of millions of years. Cometary clouds have been reported around three other centaurs too.

Echeclus is currently moving towards the Sun on its 35-year orbit, and will pass closest to our star - about 880 million km - in April 2015. Other centaurs have become active as they moved inward, Tegler says. But none have shown such dramatic activity.

Scientists once believed the icy comets from the outer regions of the solar system were the most likely source of the water that transformed the early Earth from a dry, barren world. But chemical analyses of comet water - carried out from a distance - ruled out the possibility.

Another possibility was asteroids. But it had seemed impossible to study the water content of asteroids since most of their water appears to have dissipated or is now buried too deeply to observe.
Window to the past

Now Jewitt says this new population within the asteroid belt may provide a way to sample the chemicals in water on or near the surface of these objects. And he says the main-belt comets hold promise for future study as components of the protoplanetary disc that surrounded the Sun - the disc from which the planets formed. "They're a window to some early epoch, back when objects were accreting," he says.

The new study underscores the increasingly hazy distinction between comets and asteroids. "There are different definitions of comet used by different people at different times," Jewitt told New Scientist.

The two traditionally recognised comet reservoirs are the Kuiper Belt, a frigid region beyond Neptune's orbit, and the even more distant Oort Cloud. One definition describes a comet as an object following a highly elliptical, often inclined orbit with origins in one of these two reservoirs.
Carbonaceous covering

But another definition involves what an observer sees either with the naked eye or through a telescope - a comet's streaming gassy tail as it loses ice and other volatile materials through being warmed by the Sun.

Jewitt says based on their nearly circular, stable orbits, the main belt comets are "completely asteroidal". You would never guess that they were anything but asteroids." But in terms of appearance, with their long-lasting tails, he says "they're definitely comets".

The team believes in order to survive at such proximity to the Sun, the volatiles in the main belt comets would have to be covered by a layer of possibly carbonaceous material. They say an impact event could then uncover some of the volatiles, allowing the Sun's heat to trigger the observed outgassing.
Activated asteroids

Asteroid expert Richard Binzel at MIT questions the need for the new classification. "I prefer to think of them as activated asteroids," he told New Scientist. "It's no surprise if some asteroids have some water content, particularly in the outer asteroid belt."

He says volatiles have been measured to make up about 10% of some carbonaceous meteorites that are thought to come from the region.

Jewitt says potentially tens of thousands of main belt objects contain ice and have simply not been observed during their active period. In order to be seen spewing dust, the objects would have to have been hit by a meteor size boulder within the last thousand years or so, he adds.

Journal reference: Science (DOI: 10.1126/science.1125150)

But now, one astronomer has come up with a theory that might tie some of the loose ends together.

Instead of the conventional view of a comet's nucleus as a solid, several-miles-wide rubble pile or dirty snowball, Michael Belton, a lead scientist for last year's Deep Impact comet mission, suggests that the nucleus may be more like a lump of papier mache -- built up from a random assortment of irregular sheets of varying thickness.

''The presence of layers is ubiquitous" in the nuclei seen so far, Belton said, ''and may be an essential element of their internal structure." In his view, the nuclei were built up gradually as hundreds of smaller bodies smashed together over time, each flattening out and sticking to the growing body, forming one layer after another.

Astronomers were startled and confused by the dramatic and unexpected differences between the nuclei of Tempel 1 (seen by last year's Deep Impact mission), Wild 2 (as seen by the Stardust mission in 2004) and Borrelly (seen by deep Space 1 in 2001).

Belton's new theory, which he outlined at a conference in Houston last month, identifies all the varied and unexplained features seen on these comets -- including supposed craters on Wild 2, mesa-like plateaus on Borrelly, and distinctly different, overlapping surface textures on Tempel 1 -- as different aspects of the layered model he nicknamed Talps (for ''splat" spelled backwards).

Clark Chapman, a specialist in asteroids and comets at the Southwest Research Institute in Boulder, Colo., agrees with Belton that ''it looks like comets have layers in them," but he said the theory is still untested. ''It's a first step toward trying to understand comets differently."

The new model would have significant implications for the life cycle of comets and for how we might attack a comet headed for Earth. Pushing aside a solid ball with a huge rocket or nuclear blast might make sense, but using the same approach against a ball of many layers might cause the comet to splinter and could magnify the damage rather than avert it, Belton suggests.

The find of a new type of comet -- the third known -- adds a lot of new questions to comet research and possibly helps answer a longstanding mystery: How the Earth has so much water when models suggest it shouldn't.

As the solar system's inner planets coalesced from the cloud of gas and dust swirling around the sun, the sun's heat caused water to evaporate. The new discovery suggests that Earth's water supply might have been replenished by some comets or asteroids that initially formed just a bit farther out and so might have retained their ice as they hurtled around the sun and eventually smashed into our planet.

Astronomers Henry Hsieh and David Jewitt of the University of Hawaii announced late last month that they have found comets with asteroid-like orbits -- circling the sun as planets do, between Mars and Jupiter, instead of the very elongated orbits characteristic of all previously known comets.

Finding comets like these suggests that there could be icy asteroids or comets that formed much closer to the sun than previously thought. They would have replenished Earth's water supply when they crashed into its surface.

''I think it's very significant," Jewitt said, to find such a fundamentally different group of comets, which must have formed separately from all the others.

But it will take more study to figure out how this new population will compare to the others and what kind of structure they might have. Being born in a hotter region of the growing solar system, for example, might have produced a different kind of layering, if any.

Belton, president of Belton Space Exploration Initiatives in Tucson, said he'd like to have a chance to prove his model by getting a closer look at some of these comets, particularly with a radar analysis -- which past missions couldn't perform -- that could clearly show whether the orb is layered deep down.

It may be a while before he gets that wish, but the European Space Agency's Rosetta mission will provide close-up views in 2014 of another comet nucleus and will use microwaves to probe its inner structure. Other comet missions have been proposed.

''The reconnaissance is over," Belton said. ''It's time to get into the detailed exploration phase."
© Copyright 2006 Globe Newspaper Company.