asteroid gault
Hubble Space Telescope captured this striking image of asteroid Gault, showing two narrow, comet-like tails of dusty debris. Each tail represents an episode in which the asteroid gently shed its material—key evidence that Gault is beginning to come apart. (Photo credit: NASA, ESA, K. Meech, J. Klenya and O. Hainaut.

Astronomers once thought asteroids were boring, wayward space rocks that simply orbit around the sun. Only in science fiction movies were they dramatic, changing objects.

New observations are turning science fiction into science fact, showing that asteroids are anything but dull. Asteroid Gault, discovered in 1998, has begun to slowly disintegrate. The crumbling was first detected earlier this year on January 5, by the University of Hawaiʻi at Mānoa Institute for Astronomy (IfA) Asteroid Terrestrial-Impact Last Alert System (ATLAS) telescopes on Mauna Loa and Haleakalā. Later investigation proved there were no asteroid collisions.

“Each night the ATLAS survey scans the sky looking for hazardous near-Earth asteroids, and we also observe tens of thousands of known asteroids in the main asteroid belt,” said Larry Denneau, an ATLAS project scientist. “Our collaborator Ken Smith in Belfast, Ireland, found an unusual looking moving object, and he alerted us that it might be a new comet. Instead, it turned out to be an asteroid in the main belt that just developed a comet-like tail. These events are rare and mysterious, and we were lucky to detect the event right after its turn-on.”

Rare asteroid behavior

Gault is a well-known asteroid and the newly found tails are the first evidence of any misbehavior. These new observations suggest that asteroids are dynamic, active worlds that can ultimately disintegrate due to the long-term subtle effect of sunlight, which can slowly spin them up until they begin to shed material.

Astronomers estimate that this type of event is rare, occurring roughly once a year among the 800,000 known asteroids between Mars and Jupiter. That’s why only the latest astronomical surveys—like ATLAS—that map vast swaths of the sky nightly, can catch asteroids as they fall apart.

“Asteroids such as Gault cannot escape detection anymore,” noted Olivier Hainaut, a member of the observing team of the European Southern Observatory in Garching, Germany. “That means that all these asteroids that start misbehaving get caught.”

Observations find archival evidence

Once the new tail was discovered, Denneau and IfA colleague Robert Weryk looked back into archival data from ATLAS and the UH Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) telescopes. The tail also turned up in data taken as far back as December 2018. In mid-January, a second shorter tail was detected by IfA astronomer Jan Kleyna using the Canada France Hawaiʻi Telescope, as well as by other observers. An analysis of both tails suggests the two dust releases occurred last year around October 28 and December 30.

Tantalized by this new discovery, IfA astronomers Kleyna and Karen Meech, along with colleagues worldwide, began to observe Gault with telescopes around the world and in space. Spectacular images of asteroid 6478 Gault from NASA’s Hubble Space Telescope show two narrow, comet-like tails of debris streaming from the 2.5-mile-wide asteroid. The tails are telltale evidence that Gault is beginning to come apart by gently puffing off material in two separate episodes over the past several months.

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 that 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.

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