The Hubble Space Telescope imaged 596 Scheila on Dec. 27, 2010, when the asteroid was about 218 million miles away. The asteroid is surrounded by a C-shaped cloud of particles and displays a linear dust tail in this visible-light picture acquired by Hubble's Wide Field Camera 3. Because Hubble tracked the asteroid during the exposure, star images are trailed. Credit: NASA/ESA/D. Jewitt (UCLA)
Imagine a truck, speeding down the freeway. Flying in the opposite direction is a mosquito. Smack. The mosquito collides with the truck's fender, vaporizing. All that's left is the gooey remains of the mosquito's lunch across the chrome and the rest is lost into the howling wind behind the vehicle.
Apologies for the gory analogy, but this is basically what happened in the asteroid belt late last year. A 100 kilometer-wide asteroid collided with an estimated 30 meter-wide asteroid, creating a short-lived comet-like tail of asteroid innards. (To make the mosquito analogy more accurate, the mosquito would need to have a solid metal body that ripped out a chunk of the truck's fender, too. But you get the picture.)
Originally, it was assumed that the observed brightening and brief formation of a comet-like tail around main-belt asteroid 596 Scheila was most likely caused when the asteroid vented some dust or water vapor into space. Perhaps this large chunk of space rock had some comet-like qualities that made it vent stuff into space every now and again?
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However, after follow-up observations by the Swift Gamma-Ray Burst Mission and Hubble Space Telescope, it is now thought the asteroid was actually hit by another, much smaller asteroid in the belt of solar system debris between the orbits of Mars and Jupiter.
"Collisions between asteroids create rock fragments, from fine dust to huge boulders, that impact planets and their moons," said Dennis Bodewits, an astronomer at the University of Maryland in College Park and lead author of the Swift study. "Yet this is the first time we've been able to catch one just weeks after the smash-up; long before the evidence fades away."
On Dec. 11, 2010, the University of Arizona's Catalina Sky Survey, a project that scans the sky for asteroids that drift close to the Earth, noticed Scheila had inextricably doubled in brightness. Oddly, the survey also revealed a glow surrounding the asteroid.
All indications were that Scheila had undergone some kind of change. Perhaps an area of volatiles (such as water ice) had rotated to face the sun, causing it to heat up and start venting vapor into space?
Fascinated by this odd comet-like behavior, astronomers looked through Catalina's archived photographs of the same asteroid to work out that the outburst began some time between Nov. 11 and Dec. 3, 2010.
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However, more research was needed. Specifically, to prove that this outburst was down to cometary activity, astronomers needed to analyze the spectrum of light being reflected off the "comet" coma (the vapor surrounding the object).
Now we have that data, and it wasn't cometary vapor.
Using the Ultraviolet/Optical Telescope (UVOT) aboard Swift, scientists were unable to detect molecules associated with cometary venting. Usually, molecules of water associated with comets will break down into hydroxyl and hydrogen when exposed to ultraviolet light from the sun. Neither were detected in the halo surrounding Scheila.
Faint dust plumes bookend asteroid 596 Scheila, which is overexposed in this composite image. Visible and ultraviolet images from Swift's UVOT (circled) are merged with a Digital Sky Survey image of the same region. The UVOT images were acquired on Dec. 15, 2010, when the asteroid was about 232 million miles from Earth. Credit: NASA/Swift/DSS/D. Bodewits (UMD)
Hubble then took a look at the asteroid and spotted two plumes, a bright dust plume to the north of the asteroid and a fainter one to the south. Astronomers surmized that the two plumes were most likely caused by a collision with a smaller asteroid striking Scheila's surface at an angle of "less than 30 degrees, leaving a crater 1,000 feet [300 meters] across," according to the press release.
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"The Hubble data are most simply explained by the impact, at 11,000 mph, of a previously unknown asteroid about 100 feet in diameter," said Hubble team leader David Jewitt at the University of California in Los Angeles.
The collision would have blasted 660,000 tons of dust into space (nearly twice the mass of the Empire State Building), producing the comet-like coma surrounding the asteroid.
"The dust cloud around Scheila could be 10,000 times as massive as the one ejected from
comet 9P/Tempel 1 during NASA's Deep Impact mission," said co-author Michael Kelley, also at the University of Maryland. "Collisions allow us to peek inside comets and asteroids. Ejecta kicked up by Deep Impact contained lots of ice, and the absence of ice in Scheila's interior shows that it's entirely unlike comets."
Only last year, Hubble was used to take a closer look at a bizarre "X"-shaped formation in the asteroid belt. Again, it was assumed that the shape may have been caused by a main-belt comet or an asteroid-comet hybrid. In actuality, the shape was caused by an energetic collision between two asteroids, kicking up a tail of debris being swept back by solar wind pressure.
It is estimated that, on average, astronomers should be able to detect one asteroid collision per year, so the Scheila collision appears to confirm this.
Note: It's worth checking out the NASA video accompanying this news, it includes a rather familiar Star Wars scene...
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Tags: Asteroids, Comets, Hubble Telescope, Space Telescopes
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