Free Astronomy Magazine July-August 2019

37 JULY-AUGUST 2019 SPACE CHRONICLES force that can cause the asteroid to spin faster. If this centrifugal force eventually overcomes gravity, the asteroid becomes unstable. Land- slides on the object can release rub- ble and dust into space, leaving behind a tail of debris, as seen here with asteroid Gault. “This self-destruction event is rare” explained Olivier Hainaut (Euro- pean Southern Observatory, Ger- many). “Active and unstable as- teroids such as Gault are only now being detected by means of new survey telescopes that scan the en- tire sky, which means asteroids such as Gault that are misbehaving can- not escape detection any more.” As- tronomers estimate that among the 800,000 known asteroids that oc- cupy the Asteroid Belt between Mars and Jupiter, YORP disruptions occur roughly once per year. The di- rect observation of this activity by the Hubble Space Telescope has pro- vided astronomers with a special opportunity to study the composi- tion of asteroids. By researching the material that this unstable asteroid releases into space, astronomers can get a glimpse into the history of planet formation in the early ages of the Solar System. Understanding the nature of this ac- tive and self-destructive object has been a collaborative effort involv- ing researchers and facilities around the world. The asteroid’s debris tail was first detected by the University of Hawai i/NASA ATLAS (Asteroid Terrestrial-Impact Last Alert System) telescopes in the Hawaiian Islands on 5 January 2019. Upon review of archival data from ATLAS and UH/NASA Pan-STARRS (Panoramic Survey Telescope and Rapid Re- sponse System), it was found that the object’s larger tail of debris had been observed earlier in December 2018. Shortly thereafter, in January 2019, a second, shorter tail was seen by various telescopes, including the Isaac Newton, William Herschel, and ESA OGS Telescopes in La Palma and Tenerife, Spain; the Himalayan Chandra Telescope in India; and the CFHT in Hawai i. Subsequent analy- sis of these observations suggested that the two events that produced these debris trails occurred around 28 October and 30 December 2018, respectively. These tails will only be visible for only a few months, after which the dust will have dispersed into interplanetary space. Follow-up observations were then made by various ground-based tel- escopes. These data were used to deduce a two-hour rotation period for Gault, which is very close to the critical speed at which material will begin to tumble and slide across the asteroid’s surface before drift- ing off into space. “Gault is the best ‘smoking-gun’ example of a fast rotator right at the two-hour limit”, explained lead author Jan Kleyna (University of Hawai i, USA). “It could have been on the brink of instability for 10 million years. Even a tiny distur- bance, like a small impact from a pebble, might have triggered the recent outbursts.” Hubble’s sharp imaging provided valuable detail regarding the aster- oid’s activity. From the narrow width of the streaming tails, re- searchers inferred that the release of material took place in short episodes lasting from a few hours to a couple of days. From the absence of excess dust in the immediate vicinity of the aster- oid, they concluded that the aster- oid’s activity was not caused by a collision with another massive ob- ject. Researchers hope that further observations will provide even more insight into this rare and cu- rious object. ! T he asteroid 6478 Gault is seen with the Hubble Space Telescope, show- ing two narrow, comet-like tails of debris that tell us that the asteroid is slowly undergoing self-destruction. The bright streaks surrounding the as- teroid are background stars. The Gault asteroid is located between the orbits of Mars and Jupiter. [NASA, ESA, K. Meech and J. Kleyna (University of Hawaii), O. Hainaut (European South- ern Observatory)]