As scientists watch, distant asteroid disintegrates

By Will Dunham WASHINGTON (Reuters) - Patsy Cline's classic country song "I Fall to Pieces," has nothing on this one. Scientists said on Thursday they have observed for the first time an asteroid breaking apart, crumbling into at least 10 pieces in sort of a celestial, slow-motion train wreck. The rocky asteroid, named P/2013 R3, was one of the innumerable objects populating the crowded asteroid belt located between the orbits of Mars and Jupiter, roughly three times further away from the sun than Earth. Asteroids have broken apart many times over the eons, but never before have scientists been able to witness it. This time, however, scientists first noticed the dramatic events using ground-based telescopes in Arizona and Hawaii and then got a better look using the orbiting Hubble Space Telescope. "After looking at the asteroid belt for a couple of hundred years - the first one was discovered in 1801 - to find a new thing like this is really exciting," David Jewitt, a UCLA astronomer who led the research, said in a telephone interview. The findings were published in the scientific publication Astrophysical Journal Letters. The asteroid was probably around 2,000 feet in diameter, and no more than about 3,280 feet in diameter before it began to disintegrate, Jewitt said. The break-up unfolded over a period of several months last year, he added. The Hubble telescope detected at least 10 fragments - each having comet-like dust tails. The four largest pieces each had a diameter of up to about 1,300 feet. The scientists do not think the asteroid was destroyed in a collision with another object in part because the way it is breaking apart - fragments drifting slowly at around one mile per hour - does not suggest a violent impact. In addition, the 10 fragments did not all emerge at one time, as they would in an impact, with their appearance staggered over many months, Jewitt said. They also think it is unlikely the asteroid fell to pieces due to the pressure of interior ices warming and vaporizing because at 300 million miles (480 million km) away from the sun it simply would be too cold for that to occur. Instead, they said the break-up was probably the result of the subtle but inexorable effect of sunlight over many, many years causing the asteroid to spin at a slowly increasing rate until it became unstable and ruptured. This phenomenon, known as the YORP effect, has been debated by scientists, but never previously reliably observed. "This is a really bizarre thing to observe - we've never seen anything like it before," added Jessica Agarwal of the Max Planck Institute for Solar System Research in Germany. "The break-up could have many different causes, but the Hubble observations are detailed enough that we can actually pinpoint the process responsible." When hit with sunlight, objects radiate heat back into space. If an object is perfectly round, that phenomenon would not affect its structural stability. But the irregular shape of asteroids - often shaped like a big potato tumbling through space - means that when sunlight is radiated back into space, it exerts a torque on them, leading to a spin. "That net force due to sunlight is very, very weak. But on long time scales, it can push asteroids around," Jewitt said. "So this is probably the way asteroids die in many cases. They spin up and blow themselves apart. And in the process, they make dust and debris that populates the inner solar system." (Reporting by Will Dunham, editing by G Crosse)