Washington, June 28 (IANS) The mysterious interstellar rock 'Oumuamua could actually be a comet, not an asteroid as earlier believed, suggests new research.

The cigar-shaped object has perplexed scientists since it was first spotted whipping past Earth in October 2017. It is the first object ever seen in our solar system that is known to have originated elsewhere.

At first, scientists assumed 'Oumuamua was a comet, a small icy body that, when heated by the Sun, develops a coma -- a fuzzy atmosphere and tail made of volatile material vaporising off the comet body.

But because 'Oumuamua appears in telescope images as a single point of light without a coma, scientists then concluded it was an asteroid.

The new study, published in the journal Nature, again suggests that the object could be a comet.

Using observations from NASA's Hubble Space Telescope and ground-based observatories, the scientists confirmed 'Oumuamua got an unexpected boost in speed and shift in trajectory as it passed through the inner solar system last year.

The scientists realised that a coma and jets might not be visible to the telescopes used to observe it.

"Our high-precision measurements of 'Oumuamua's position revealed that there was something affecting its motion other than the gravitational forces of the Sun and planets," said lead study author Marco Micheli of European Space Agency's (ESA) Space Situational Awareness Near-Earth Object Coordination Centre in Frascati, Italy.

Analysing the trajectory of the interstellar visitor, the scientists found that the speed boost was consistent with the behaviour of a comet.

"This additional subtle force on 'Oumuamua likely is caused by jets of gaseous material expelled from its surface," said co-author Davide Farnocchia of NASA's Jet Propulsion laboratory (JPl).

"This same kind of outgassing affects the motion of many comets in our solar system."

Comets normally eject large amounts of dust and gas when warmed by the Sun. But according to team scientist Olivier Hainaut of the European Southern Observatory, "there were no visible signs of outgassing from 'Oumuamua, so these forces were not expected".

The team estimates that 'Oumuamua's outgassing may have produced a very small amount of dust particles -- enough to give the object a little kick in speed, but not enough to be detected.

'Oumuamua, less than half a mile in length, now is farther away from our Sun than Jupiter and travelling away from the Sun at about 70,000 mph as it heads toward the outskirts of the solar system.

In only another four years, it will pass Neptune's orbit on its way back into interstellar space, the study said.

--IANS

gb/vd