Free Astronomy Magazine May-June 2015

SPACE CHRONICLES Unusual asteroid suspected of spinning to explosion by Keck Observatory collision- or rotation-driven activity. These four freakish asteroids are all very small, at a kilometer or less, which makes them unimaginably faint when viewed from a typical distance of a couple hundred million miles. Despite prior attempts, the tiny size of the objects kept scien- tists from determining some of the large centrifugal forces produced ex- ceed the object’s own gravity, caus- ing it to break apart. Rotational disruption is the expected final state of what is called the YORP effect – a slow evolution of the rotation rate due to asymmetric emission of heat. To date, astronomers have identi- fied four objects suspected of either A team led by astronomers from the Jagiellonian Univer- sity in Krakow, Poland, recent- ly used the W. M. Keck Observatory in Hawaii to observe and measure a rare class of “active asteroids” that spontaneously emit dust and have been confounding scientists for years. The team was able to measure the rotational speed of one of these objects, suggesting the asteroid spun so fast it burst, ejecting dust and newly discovered fragments in a trail behind it. The findings are be- ing published in The Astrophysical Journal Letters on March 20, 2015. Unlike the hundreds of thousands of asteroids in the main belt of our so- lar system, which move cleanly along their orbits, active asteroids were discovered several years ago mimick- ing comets with their tails formed by calm, long lasting ice sublima- tion. Then in 2010 a new type of active asteroid was discovered, which ejec- ted dust like a shot without an ob- vious reason. Scientist gravitated around two possible hypotheses. One states the explosion is a result of a hypervelocity collision with an- other minor object. The second pop- ular explanation describes it as a consequence of “rotational disrup- tion”, a process of launching dust and fragments by spinning so fast, the A ctive asteroid P/2012 F5 captured by Keck II/DEIMOS in mid-2014. Top panel shows a wide-angle view of the main nucleus and smaller fragments embedded in a long dust trail. Bottom panel shows a close-up view with the trail numerically removed to enhance the visibility of the fragments. [M. Drahus, W. Waniak (OAUJ) / W. M. Keck Observatory]