Astronomers Spy Out Active Asteroids

In the “Department of Unusual Solar System Activities” today we have a close look at a type of asteroid that may spin so fast that it literally explodes itself as it ejects dust out to space. This comes to us courtesy of the W.M. Keck Observatory in Hawai’i, and a team of astronomers at the Jagellonian University of Krakow, Poland. To date, astronomers know of a handful of these busy objects and are seeking to understand why they do what they do.

The main asteroid belt of our solar system is likely a pretty orderly place, with asteroids moving along in their orbits without too many collisions, sort of like a well-run superhighway. But, sometimes things happen, even on a smoothly moving roadway. One car runs into another and that sets one or both of them spinning. Or, a truck has a blowout on its tire, and that sends it weaving all over the road.

In the case of the asteroid belt, which is home to the very fascinating Ceres dwarf planet (currently being studied by the Dawn spacecraft), we’re talking about “active asteroids” that are posing some interesting challenges to astronomers studying them. These are asteroids that eject steams of ice particles out as they orbit, most of the time in a steady jet. But, in 2010, astronomers found a more “explody” version of these objects. They eject material in shots, sort of like that tire blowout. The reasons they do this aren’t clear yet, but there are two ideas to explain them. One is that two objects—one moving very fast (say, at hypervelocity (which means really high velocity)) running into another. That causes one or both to catastrophically spew ice particles and dust to space in giant spurts.

The other idea is that an active asteroid experiences “rotational disruption”. That literally means it rotates so fast and wildly that it alters the shape of the asteroid, introducing cracks and crevices and fragmenting the body. As the active asteroid spins, it launches dust and ice chunks, which further unbalancing itself. Eventually, it can break apart.

The astronomers on the team using Keck focused in on a specific one that had caught their interest to see if they could capture a view of one of these tiny objects and figure out what makes it so unusual. The object they chose is called P/2012 F5, which had a big dust outburst in 2011. They wanted to measure its rotation rate (how fast it spins on its axes), and whether or not it was fragmented and broken apart. They found four large fragments in the object, which is rotating once every 3.2 hours. That’s fast enough to cause these impulsive explosions that result in dust and ice outbursts. For now, this object is still one big fragmented body. As it rotates, it is heated by the Sun on different sides and it’s possible that one day it will rotate itself apart, sending more ice and dust and fragments into space.

Interestingly, P/2012 F5 was first thought to be a comet due to its outbursts. Astronomers now know that it’s a fragmented asteroid, acting like a comet. Why it’s rotating and how it got fragmented—those are questions still to be answered.

Back in graduate school, we often talked about “asteroids acting like comets”, although they were quite rare. Now, what was once rare is an observable object, giving us another look at how amazing varied the places in our solar system can be.