Astronomers once thought asteroids were boring, wayward space rocks that simply orbit around the Sun. But recent observations show that asteroids are anything but dull. In reality they are dynamic, active objects that can ultimately disintegrate due to the long-term subtle effects of sunlight, which can slowly spin them up until they begin to shed material. Several ground- and space-based telescopes have caught the gradual self-destruction of the asteroid (6478) Gault, which is located 214 million miles (344 million km) from the Sun, between the orbits of Mars and Jupiter. Images from the NASA/ESA Hubble Space Telescope show two narrow, comet-like tails of dusty debris streaming from the 2.5-mile (4 km) wide asteroid. Each tail represents an episode in which the asteroid gently shed its material — key evidence that Gault is beginning to come apart.

Discovered in 1988, (6478) Gault has been observed repeatedly, but the debris tails are the first evidence of disintegration.

Of the roughly 800,000 known asteroids between Mars and Jupiter, astronomers estimate that this type of event in the asteroid belt is rare, occurring roughly once a year.

Watching an asteroid become unglued gives astronomers the opportunity to study the makeup of these space rocks without sending a spacecraft to sample them.

“We didn’t have to go to Gault. We just had to look at the image of the streamers, and we can see all of the dust grains well-sorted by size,” said ESO astronomer Dr. Olivier Hainaut, a member of the Gault observing team.

“All the large grains (about the size of sand particles) are close to the object and the smallest grains (about the size of flour grains) are the farthest away because they are being pushed fastest by pressure from sunlight.”

Gault is only the second asteroid uncovered whose disintegration is decisively linked to a spin-up process, known as a YORP (Yarkovsky-O’Keefe-Radzievskii-Paddack) torque.

When sunlight heats an asteroid, infrared radiation escaping from its warmed surface carries off momentum as well as heat. This creates a tiny force which can cause the asteroid to spin faster and faster. If this centrifugal force overcomes gravity, the surface becomes unstable, and landslides send dust and rubble drifting into space.

The astronomers estimate that Gault could have been slowly spinning up for more than 100 million years.

Piecing together Gault’s recent activity is an astronomical forensics investigation involving telescopes and astronomers around the world.

The initial clue was the fortuitous detection of the first debris tail, observed on January 5, 2019, by the Asteroid Terrestrial-Impact Last Alert System (ATLAS) telescope in Hawaii.

The tail also turned up in archival data from December 2018 from ATLAS and Pan-STARRS telescopes in Hawaii.

In mid-January, a second shorter tail was spied by the Canada-France-Hawaii Telescope in Hawaii and the Isaac Newton Telescope in Spain, as well as by other observers.

An analysis of both tails suggests the two dust releases occurred around October 28 and December 30, 2018.

Follow-up observations with the William Herschel Telescope and ESA’s Optical Ground Station in La Palma and Tenerife, Spain, and the Himalayan Chandra Telescope in India, measured a two-hour rotation period for the object, close to the critical speed at which a loose ‘rubble-pile’ asteroid begins to break up.

“Gault is the best smoking-gun example of a fast rotator right at the two-hour limit,” said team member Dr. Jan Kleyna, an astronomer at the University of Hawaii.

The asteroid’s narrow streamers suggest that the dust was released in short bursts, lasting anywhere from a few hours to a few days. These sudden events puffed away enough debris to make a dirt ball approximately 500 feet (150 m) across if compacted together. The tails will begin fading away in a few months as the dust disperses into interplanetary space.

The astronomers estimate that the longer tail stretches over half a million miles (800,000 km) and is roughly 3,000 miles (4,800 km) wide. The shorter tail is about a quarter as long.

“Asteroids such as Gault cannot escape detection anymore. That means that all these asteroids that start misbehaving get caught,” Dr. Hainaut said.

The study will be published in the Astrophysical Journal Letters.

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Jan T. Kleyna et al. 2019. The Sporadic Activity of (6478) Gault: A YORP-driven event? ApJL, in press;