The crumbling asteroid, designated P/2013 R3, was first noticed as an unusual, fuzzy-looking object by ground-based telescopes in Arizona September 15, 2013, the researchers reported in the US journal Astrophysical Journal Letters, according to Xinhua.
A follow-up observation on Oct 1 with the W.M. Keck Observatory on the summit of Mauna Kea, a dormant volcano on the island of Hawaii, revealed three bodies moving together in an envelope of dust nearly the diameter of Earth.
Astronomers then looked at it with the Hubble Space Telescope, which soon showed there were really 10 embedded objects, each with comet-like dust tails, said David Jewitt of the University of California at Los Angeles, who led the astronomical forensics investigation.
The four largest rocky fragments are up to 365.76 metre in diameter, about four times the length of a football field, Jewitt said. "Seeing this rock fall apart before our eyes is pretty amazing."
The asteroid began coming apart early last year, and the researchers said the fragments are drifting away from each other at a leisurely pace of about 1.6 km per hour.
The researchers said the rare event is not caused by a collision with another asteroid, or the pressure of interior ices warming and vaporizing.
They suspected the break-up is a result of sunlight, which causes the rotation rate of the asteroid to gradually increase. Eventually, its component pieces succumb to centrifugal force and gently pull apart, "like grapes on a stem", they said.
The possibility of disruption in this manner, known as the YORP effect, has been discussed by scientists for several years, but never reliably observed.
The asteroid's remnant debris, weighing about 200,000 tonnes, will in the future provide a rich source of meteoroids, Jewitt said.
Most will eventually plunge into the sun, but a small fraction of the debris may one day blaze across our skies as meteors, he added.