Free Astronomy Magazine May-June 2015

SPACE CHRONICLES The astronomers calculat- ed the object’s rotation period by measuring small periodic fluctuations in brightness. Such oscilla- tions occur naturally as the irregular nucleus ro- tates about its spin axis and reflects different amounts of sunlight dur- ing a rotation cycle. “This is a well-established technique but its applica- tion on faint targets is challeng- ing,” said Waclaw Waniak of the Jagiellonian University who pro- cessed the Keck Observatory data. “The main difficulty is the bright- ness must to be probed every few minutes so we don’t have time for long exposures. We needed the huge collecting area of Keck II, which captures a plentiful amount of photons in a very short time.” The photons were then concentrat- ed in the telescope’s light path and sent to the DEIMOS instrument to produce the data that allowed the scientists to determine P/2012 F5’s nature. While monitoring bright- ness in the individual 3-minute ex- posures, scientists also compiled all the data to produce a single ultra- deep image, which revealed the fragments. The success wouldn’t be possible if the selected target, P/2012 F5, were not an ideal candidate for this study. Alex R. Gibbs discovered the object on March 22, 2012 with the Mount Lemmon 1.5 meter reflector. It was initially classified as a comet, based solely on its “dusty” look. But two independent teams quickly have shown all this dust was emitted in a single pulse about a year before the discovery – something that doesn’t happen to comets. When the dust settled in 2013, another team using the University of Hawaii’s 2.2-meter telescope on Mauna Kea detected a star-like nucleus and suggested a maximum size of 2 kilometers. “We suspected that this upper limit was close to the actual size of the object. Consequently, we chose to observe P/2012 F5 because – despite its small size – it appeared to be the largest and easiest to observe active asteroid suspected of rotational dis- ruption,” said Jessica Agarwal of the Max Planck Institute for Solar Sys- tem Research who chose P/2012 F5 as the subject. As a result of the study, P/2012 F5 is the first freshly fragmented object in the solar sys- tem with a well-determined spin rate, and this spin rate turns out to be the fastest among the active asteroids. A careful analysis made by the team shows that these two fea- tures of the object are consistent with the “rotational disruption” sce- nario. But alternative explanations, such as fragmentation due to an im- pact, cannot be completely ruled out. “There are many faster rotators among asteroids which don’t show signs of a recent mass loss. And there are many hypervelocity impac- tors straying out there and looking for targets to hit – be it a fast or slow rotator,” Drahus said. “We’re indebted to the Caltech Op- tical Observatories for generously awarding Keck Observatory time for this program,” said Drahus – for- merly a NRAO Jansky Fellow at Cal- tech. “Without the huge collecting area of Keck II’s 10-meter mirror, we wouldn’t be able to achieve our goals so swiftly.” B rightness fluctua- tions of the nucleus of P/2012 F5 during two consecutive rotation cy- cles. Presented versus time (top panel) and versus the nucleus rota- tion phase (bottom panel). [M. Drahus, W. Waniak (OAUJ) / W. M. Keck Observatory] n key characteristics that could prove or disprove the theories. Until last August, when the team led by Michal Drahus of the Jagiel- lonian University was awarded time at Keck Observatory. “When we pointed Keck II at P/2012 F5 last August, we hoped to measure how fast it rotated and check whether it had sizable frag- ments. And the data showed us all that,” Drahus said. The team discov- ered at least four fragments of the object, previously established to have impulsively ejected dust in mid-2011. They also measured a very short rotation period of 3.24 hours – fast enough to cause the object impulsively explode. “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.