In december 2011, the Catalina all-sky survey discovered that asteroid (596) Scheila had suddenly increased in brightness, and that it developed a two large comet-like tails. Using Swift’s Target of Opportunity program, we were able to observe the comet from space only a few days after the discovery of the outburst. We discovered that Scheila’s plumes contained no water. Instead, the 110 km sized asteroid was hit by a much smaller asteroid [1].

Update: Follow-up observations showed that the impact changed the lightcurve of Scheila significantly. To understand what effect to 70-meter impactor may have had on Scheila’s surface, we compared the impact event with close-up images of asteroids, acquired by spacecraft like Rosetta. The collision may have created a bright spot on Scheila between 7 to 20 km in diameter - more than 10 times larger than the estimated size of the impact crater. This spot would consist of a thin layer of 2 mm to 2 cm thick at most, and it might be mixed with bright material from the impactor [2].


[1] “Collisional Excavation of Asteroid (596) Scheila” - D. Bodewits, M.S. Kelley, J.Y. Li, W.B. Landsman, S. Besse and M.F. A’Hearn. Published in The Astrophysical Journal Letters 733, L3, 2011 (ArXiv here)

[2] “Scheila’s Scar: Direct Evidence of Impact Surface Alteration on a Primitive Asteroid.” - D. Bodewits, J.-B. Vincent, and M.S.P. Kelley. Published in Icarus 229, p190, 2014 (ArXiv here).

Left: Swift observed asteroid Scheila to determine if the asteroid was a giant comet. The plumes contained no water but were the result of an impact by a 100m sized rogue asteroid.

ABOVE: Swift observations of Asteroid (596) Scheila showed two dust plumes containing 100 million kilograms of dust (click image for a you-tube video explaining the impact).