“This is a rock. Seeing it fall apart before our eyes is pretty amazing,” said David Jewitt of the University of California, Los Angeles.
Scientists first noticed the crumbling asteroid as an unusual fuzzy-looking object on September 15, 2013, by the Catalina and Pan-STARRS sky surveys. Follow-up observations October 1 with the Keck Telescope on Mauna Kea, Hawaii, revealed three co-moving bodies embedded in a dusty envelope that is nearly the diameter of Earth.
“Keck showed us that this thing was worth looking at with Hubble,” Jewitt said. With their superior resolution, the space-based Hubble observations soon showed that there were really 10 distinct objects, each with comet-like dust tails. The four largest rocky fragments are up to 700 feet (200 meters) in radius.
The Hubble data showed that the fragments are drifting away from each other at a leisurely 0.9 mph (1.5 km/h) — slower than the speed of a strolling human. The asteroid began coming apart early last year, but the latest images show that pieces continue to emerge.
“This is a really bizarre thing to observe. We’ve never seen anything like it before,” said Jessica Agarwal of the Max Planck Institute for Solar System Research in Germany. “The breakup could have many different causes, but the Hubble observations are detailed enough that we can actually pinpoint the process responsible.”
The ongoing discovery of more fragments makes it unlikely that the asteroid is disintegrating due to a collision with another asteroid, which would be instantaneous and violent in comparison to what has been observed. Scientists would also expect some of the debris from such a high-velocity smashup to travel much faster than they have observed.
It is also unlikely that the asteroid is breaking apart due to the pressure of interior ices warming and vaporizing. The object is too cold for ices to significantly sublimate, and it has presumably maintained its nearly 300-million-mile (480 million kilometers) distance from the Sun for much of the age of the solar system.
This leaves a scenario in which the asteroid is disintegrating due to a subtle effect of sunlight that causes the rotation rate to slowly increase over time. Eventually, its component pieces gently pull apart due to centrifugal force. Scientists have discussed the possibility of disruption by this phenomenon — known as the YORP effect —for several years but, so far, they have never reliably observed it.
For breakup to occur, P/2013 R3 must have a weak fractured interior, probably the result of numerous ancient and non-destructive collisions with other asteroids. Most small asteroids probably have been severely damaged in this way, giving them a “rubble pile” internal structure. P/2013 R3 itself is probably the product of collisional shattering of a bigger body some time in the past billion years.
“This is the latest in a line of weird asteroid discoveries, including the active asteroid P/2013 P5, which we found to be spouting six tails,” said Agarwal. “This indicates that the Sun may play a large role in disintegrating these small solar system bodies, by putting pressure on them via sunlight.”
P/2013 R3’s remnant debris, weighing in at 200,000 tons, will provide a rich source of meteoroids in the future. Most will eventually plunge into the Sun, but a small fraction of the debris may one day blaze across our sky as meteors.