Asteroid Switches Gears, Becomes Comet

The asteroid/comet 596 Scheila photographed remotely by Australian amateur astronomer Joseph Brimacombe using a telescope in New Mexico. The glowing fuzz or coma around the asteroid is visible in larger amateur telescopes. Click the photo for a short movie. Credit: Joseph Brimacombe

Early yesterday morning, Steve Larson with the Catalina Sky Survey, photographed what was thought to be an ordinary asteroid, (596) Scheila, but when he examined the images, discovered it wore the guise of a comet. Instead of looking like a star-like point, the usual appearance of an asteroid, Scheila was surrounded by a vaguely spiral-shaped cloud of vapor.

The asteroid belt, a broad zone between Mars and Jupiter, is home to at least several millions of asteroids. They range in size from rocks up to 590-mile-diameter Ceres. The new main belt comets orbit among them. Credit: ESO

596 Scheila was discovered in February 1906 by German astronomer August Kopff and named after a student acquaintance. It orbits in the main belt of asteroids between Mars and Jupiter. Currently, Scheila is about 235 million miles from Earth. Nearly all the asteroids in the main belt are rocky bodies, but hidden among them is a newly emerging class of “sometimes asteroids-sometimes comets” called main belt comets.

The first one, 1979 OW7, was discovered in 1979 and designated as an asteroid based on its star-like appearance and orbit. When re-observed in 1996 by Eric Elst and Guido Pizarro, 1979 OW7 had sprouted a small coma (envelope of gas around the head of a comet) and a tail. Now known as Comet 133P/Elst-Pizarro, it was first thought the asteroid suffered an impact from another smaller asteroid, creating the dust tail. But Elst-Pizarro grew a new tail in 2002, making it more likely a dormant comet – one that comes back to life when its orbit brings it near enough to the sun that solar heat vaporizes lingering pockets of ice on or near the surface. Only four or five of these quick-change artists have been discovered to date; Scheila looks to be the most recent member in a rare class.

Split personality Comet Elst-Pizarro in 1996, when it went from being classed as an asteroid to a comet. Credit: NASA

Most comets orbit the sun in highly elliptical or oblong orbits that take them beyond Jupiter and even Neptune into the frozen reaches of the solar system’s Kuiper Belt, a repository of icy asteroids and comets. Many more hail from a spherical halo called the Oort Cloud that’s even further away. Not only do main belt comets come from closer in, their orbits are like those of asteroids, which travel in more circular paths, compared to the stretched out, cigar-shaped orbits of comets.

What’s so surprising about finding dormant comets in the asteroid belt is that logic implies all the ice from these bodies should have vaporized away because of their proximity to the sun’s heat. Yet here they are. Did they migrate there from a more distant, colder region? That’s not likely according to the latest studies, which suggest they’ve been there all along. Another question that scientists will try to answer in the coming days and weeks is what caused Scheila to turn comet in the first place. Was it chance vaporization of ice as in Elst-Pizarro’s case or did the 70-mile diameter asteroid/comet get whacked by another asteroid that happened to cross its path? An impact would release a cloud of water ice and dust that the sun’s light would push back into a tail. Perhaps an impact occurred much earlier and the freshly-exposed ice got jazzed by sunlight at a later date when conditions were right. If icy comets do inhabit the inner solar system, there’s speculation they might have been responsible for delivering water to Earth after our planet cooled down following it fiery hot formation. Catching Schelia in the act of swapping personalities may someday help us learn more about the water we use to make our daily coffee.

Schelia's orbit takes it both above (light blue) the plane of Earth's orbit as well as below (dark blue). The number "596" refers to it being the 596th asteroid to have its orbit determined. Credit: JPL

This morning I got up to have a look at 596 Schelia. To my amazement, I have all ten fingers left to prove I survived the fierce winds and -8 F temperature. The asteroid/comet, located in Leo the Lion, was a faint star of magnitude 14.3. Using a 15-inch telescope and magnification of 145x, I teased out a small puff of vapory light around it. Advanced amateur astronomers can locate the comet using JPL’s Small-Body Database Browser. Scroll down below the orbit diagram and input the asteroid’s orbital elements (numbers describing its orbit) into your planetarium software program. From there, you can create a personalized finder chart. If you don’t have a program that allows you to add or change asteroid elements, you can scroll down a bit further, click on the ephemeris link, select your location, time, etc. and then hand-plot Schelia’s position on a detailed star atlas.

4 Responses

  1. joe adrian

    Could you tell me what bright star appears every night early pm high in sky to south of minnesota. Today 12 Dec 10, the star is to east of moon at 5:20 pm.

  2. Michael Watson

    Just checked the JPL Small-Body Database. They put 596 Scheila’s current (Dec. 17) distance from the Sun as 3.1 AU (~288 million miles) Since this well outside the snow line, I wonder how ice could be subliming from the object and creating a tail and coma.

    1. astrobob

      Good question. It’s possible, as is believed to be the case in the asteroid Ceres, that the ice is beneath an outer layer of dust or debris. That would keep it insulated except in rare circumstances when an impact or fissure might expose ice, which would then sublimate away to form a cloud or coma around the asteroid. This is only one possibility, the other being actual dust from an impact rather than ice. I’ve heard of only one spectrum of the object done by an amateur and his results hint at ice.

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