A team of astronomers led by team led by Michal Drahus of the Jagiellonian University (Krakow, Poland) used one of the twin 10-meter (394-inch) telescopes of the W.M. Keck Observatory in Hawaii to study the strange behavior of four comet-like asteroids.
Most of the hundreds of thousands of known asteroids are made of rock and don’t develop fuzzy comas and tails the way similar-sized icy comets do. But since 2010, astronomers have uncovered a small number of oddball, “active asteroids” that mimic comets by releasing clouds of dust as they spin.
One of them, P/2012 F5 (Gibbs), located in the outer zone of the main asteroid belt, spins so fast, it’s literally falling apart. Using the Keck II telescope, Drahus and team discovered at least four fragments flung from the object.
We knew the asteroid was an active one because it had previously released a cloud of dust in a single, quick impulse back around July 1, 2011. Like a comet, the object looked slightly fuzzy and left a left a dust trail; this time around, P/2012 F5 took it to the next level and deposited a string of tiny asteroid satellites.
There are two theories on how otherwise quiet asteroids can suddenly explode to life — through a collision with another smaller asteroid or by “rotational disruption”, which is exactly what it sounds like.
An asteroid can spin so fast that its weak gravity is overwhelmed by centrifugal force, the tendency of material to pull away from a rapidly spinning object. Centrifugal force can cause small objects like P/2012 F5 and its ilk to break apart. No surprisingly, the team measured F5’s rotation rate at just 3.24 hours, fast enough for it to theoretically explode.
So how do you spin up an asteroid until it shoots pieces of itself into space like some hell-bent disk golfer? Just add a little YORP. An acronym for Yarkovsky–O’Keefe–Radzievskii–Paddack effect, heating from the Sun can cause an asteroid’s tilt and rotation rate to change over time.
Sunlight shining on an asteroid warms the rock which releases the energy as heat, giving the object a tiny push. Assuming the asteroid is irregular in shape – and most are because they’re so small – some areas get hotter and give off more heat than others. The imbalance causes a torque on the asteroid, increasing its spin rate. Depending on the shape of the asteroid and variations in the reflectivity of its surface (some areas may be darker or lighter than others), those smidgeons of thrust can add up to twirl an asteroid to the breaking point.
And because many asteroids are little more than rubble piles, breaking up is easy to do.
“This is really cool because fast rotation has been suspected of catapulting dust and triggering fragmentation of some active asteroids and comets. But up until now we couldn’t fully test this hypothesis as we didn’t know how fast fragmented objects rotate,” Drahus said.
Astronomer Alex Gibbs discovered P/2012 F5 on March 22, 2012 with the Mount Lemmon 1.5 meter reflector in Arizona. It was initially classified as a comet, based on its fuzzy look, but two independent teams quickly showed that the dust was blasted out in a single pulse about a year before the discovery – something that doesn’t happen to comets, which continuously emit dust and other materials as the Sun vaporizes ice from their nuclei.
Gibb’s find and the other known active asteroids are all under 0.6 miles or one kilometer across. So tiny they’re incredibly faint. That’s why the biggest telescope on Earth was needed to dig down into the details and uncover the tale of an asteroids torn asunder by nothing more than sunlight.