March 6, 2014. On this date, The Astrophysical Journal Letters published a study of the first-ever-seen breakup of an asteroid. The asteroid had been discovered in September, 2013, in the Catalina and Pan-STARRS sky surveys. It was designated P/2013 R3. Shortly after its discovery, observations by the Keck Telescope in Hawaii showed not one, but three bodies traveling together. They were surrounded by a dusty envelope nearly as wide as the Earth. Subsequent observations by the Hubble Space Telescope showed that P/2013 R3 consisted of at least 10 separate objects. Like comets, each of the pieces had a tail. The four biggest pieces were each up to to 660 feet (200 meters) wide. Together, the space rocks were estimated to weigh a total of 200,000 tons.
Fragile icy comet nuclei have been seen to fall apart as they approach the sun. But asteroids are not, by definition, fragile icy bodies. They are chunks of rock or metal. David Jewitt of UCLA, who led the astronomical investigation, said at the time:
This is a rock. Seeing it fall apart before our eyes is pretty amazing.
Jewitt and the other scientists who studied P/2013 R3 pointed out that, because the asteroid seemed to disintegrate slowly, over a period of months, it was unlikely a violent and sudden collision with another asteroid caused the break up.
They said it was also unlikely that the asteroid broke apart due to the pressure of interior ices warming and vaporizing, as the asteroid drew near the sun. Icy comets do become active for this reason, and do sometimes disintegrate, but P/2013 R3 was believed to be too cold for such a process, plus it had presumably maintained its nearly 480-million-kilometer distance from the sun for much of the age of our solar system.
That left a possible scenario in which the asteroid disintegrated due to a subtle effect of sunlight, known as the YORP effect — which would cause the rotation rate to increase slowly over time. Eventually, the component pieces of the asteroid would gently pull apart due to centrifugal force.
For break-up to occur in this way, P/2013 R3 would have needed to have a weak, fractured interior. That fractured interior might have been the result of numerous ancient and non-destructive collisions with other asteroids, early in the solar system’s history. Most small asteroids are thought to have been severely damaged in this way, giving them a “rubble pile” internal structure.
Thus P/2013 R3 itself was probably the product of collisional shattering of a bigger body sometime in the last billion years.
Astronomers have said that most of P/2013 R3’s remnant debris will eventually plunge into the sun. But some will a rich source for future meteor showers and may even one day cross Earth’s sky as meteors.
By the way, P/2013 R3 is one of several objects known as active asteroids (called main-belt comets by some astronomers). They have the orbits of asteroids – inside the orbit of the giant planet Jupiter – but they look like comets in that they show tails. Some astronomers call them main-belt comets, but, according to David Jewitt:
… this was misinterpreted by many people to mean that the active objects are necessarily icy. In most cases, we don’t know if they are icy. In some cases we know that they are not.
The dwarf planet 1 Ceres – first asteroid ever to be discovered on January 1, 1801 – is known to have hydrated minerals on its surface, and thus is considered by Jewitt’s team to be an active asteroid. The Dawn spacecraft, arriving at Ceres on March 6, 2015, should help shed some light on this object, which might consist of a silicate core surrounded by an icy mantle.
Meanwhile, the object known as 3200 Phaethon – parent body of the Geminid meteor shower – has long been considered unusual, because, although it looks like an asteroid, it’s known to lose mass from its surface (hence the meteors). However, Jewitt’s team doesn’t consider 3200 Phaethon to be an active asteroid, because it’s not in the main asteroid belt. Instead, 3200 Phaethon sweeps in very near the sun, closer than any other named asteroid. That’s why astronomers gave it its name; it was named after the Greek myth of Phaëton, son of the sun god Helios.
Bottom line: This is the publication date of the study of the asteroid known as P/2013 R3. Over a period of months, it was seen to break up into as many as 10 smaller pieces. It was the first asteroid studied to break apart in this way.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.