Astronomers using the Hubble Space Telescope (HST) continue to discover surprises abound throughout the Solar System, some 23 years after it launched into orbit on NASA’s now retired Space Shuttle Discovery on mission STS-31. This week, NASA announced that astronomers using the HST have observed—for the first time—an object in the asteroid belt that is spouting six comet-like tails.
The asteroid, designated as P/2013 P5, was only recently discovered last August by astronomers using the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) survey telescope in Hawaii. At the time, the unusually fuzzy-looking object puzzled astronomers. P/2013 P5 does not behave like most asteroids, and its appearance is quite unusual when compared to other asteroids observed throughout the asteroid belt.
The intriguing discovery can only be researched so much by ground-based telescopes and observations; it was time for the powerful Hubble to have its turn. Hubble turned toward P/2013 P5 on Sep. 10 and began imaging the asteroid with its Wide Field Camera 3, and that is when the multiple comet-like tails were discovered.
Hubble positioned again to image the asteroid for a second time on Sep. 23. Not only did the asteroid’s appearance totally change, but P/2013 P5 looked as if it had swung around entirely.
“We were literally dumbfounded when we saw it,” said lead investigator David Jewitt of the University of California at Los Angeles. “Even more amazing, its tail structures change dramatically in just 13 days as it belches out dust. That also caught us by surprise. It’s hard to believe we’re looking at an asteroid.”
Jewitt believes P/2013 P5 is actually a fragment of a much larger asteroid that broke apart, most likely due to a collision, some 200 million years ago (collision fragments are common in orbits similar to P/2013 P5’s). Unlike comets, which are dirty, ice-filled snowballs from the deepest depths of the outer Solar System, asteroid P/2013 P5 is likely composed of metamorphic rocks.
The asteroid’s six tails are actually made of dust radiating from the object, like spokes on a wheel, and the research team believes P/2013 P5 has ejected between 100 and 1,000 tons of dust for at least the last five months. Although not for certain, astronomers believe the asteroid’s rate of rotation increased enough to actually cause the surface to begin flying apart. With a nucleus measured to be only 1,400 feet wide, the object’s weak gravity, according to the leading theory, simply could not hold it together.
An impact event with another asteroid has been ruled out as the cause of the dusty tails—had an impact event occurred a large quantity of dust would have been blasted into space all at once. Hubble’s observations do not support that theory.
Jessica Agarwal of the Max Planck Institute for Solar System Research in Lindau, Germany, conducted some careful modeling based off the observations and analysis of the tail structure. Her work showed that the six tails could have formed by a series of impulsive dust-ejection events, which she calculated to have occurred April 15, July 18, July 24, Aug. 8, Aug. 26, and Sept. 4. The radiation pressure from the Sun stretched the dust into tails and could have also increased the spin rate of the object; the gentle push of sunlight in the vacuum of space has literally (according to the leading theory) accelerated the asteroid’s spin.
Observations over the coming months will help astronomers determine whether or not the asteroid’s dusty tails are the cause of a rotational breakup; dust leaving P/2013 P5 in the equatorial plane would strongly support the rotational breakup theory. In addition, if the theory proves to be true, it would also suggest that rotational breakups are quite common throughout the asteroid belt and may even be the way small asteroids die.
“In astronomy, where you find one, you eventually find a whole bunch more,” Jewitt said. “This is just an amazing object to us, and almost certainly the first of many more to come.”