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Astronomical Observatory: Cool Images

The Outburst of Asteroid (596) Scheila

Asteroid (596) Scheila: Asteroid Scheila was discovered in 1906 and orbits the Sun in 5.01 years in the outer portion of the main asteroid belt, about halfway between the orbits of Mars and Jupiter. (You can visualize the orbit with a NASA java applet.) Physically it is one of the larger asteroids (about 110 km across) and has a dark surface (reflecting back only 4% of the sunlight it receives). In one hundred years of observing it, astronomers found it not unlike the hundreds of thousands of other space rocks we call asteroids.

Asteroid (596) Scheila in outburst

An Outburst: On 12 December 2010, S. Larson (University of Arizona) reported that images of Scheila taken for the Catalina Sky Survey the previous day showed the object to be in outburst: exhibiting structure like a comet. Upon reviewing earlier images taken over the previous month, he determined that the outburst began about 3 December. Furthermore, Warner and Harris have shown that Scheila's observed brightness has been a magnitude brighter (about 2.5 times brighter) than expected since the beginning of the outburst.

The image above was obtained with the Calvin-Rehoboth telescope on the morning of 14 December and shows the comet-like structure. The contrast is set to show clearly the tails extending from the top and bottom of the asteroid, which appears as a large white circle due to overexposure. The image is the average of 104 exposures taken over six hours, so that background stars appear as streaks as the asteroid moves past them at 0.11 degrees per day. The portion of the image shown is 2.6 x 3.1 arcminutes, with north up and east to the left. The Sun was 119 degrees away off to the left. The asteroid was 374 million km from Earth at the time. Hence the tail, which can be clearly seen out to at least 1.6 arcminutes west of the asteroid, has a projected length of 170,000 km, or 1700 times the size of the asteroid.

Questions raised by the discovery:

What caused the outburst? There are two quite distinct possibilities: Scheila is a main belt comet and therefore burst due to internal stresses or evaporation of ices, or Scheila collided with another asteroid.

There are a handful of objects in the main asteroid belt that have exhibited comet behavior (and hence are called "main belt comets". These objects are made of rock and ice, and each time their elliptical orbits bring them close to the Sun, solar heating causes ice to vaporize and a stream of gas and dust to flow away forming a tail. (A good example is 133P/Elst-Pizarro.) Until recently there was no plausible model for how such icy objects (which would have formed far beyond the asteroid belt) could find themselves in the asteroid belt today. However, the most recent models of how Jupiter and Saturn interacted with the protosolar disk in the earliest years (the first few million) of solar system formation provide a natural explanation: the disk caused Jupiter to move radially inward, then outward, in the process scrambling the locations of many asteroids and comets.

Is Scheila a main belt comet? Its outburst occurred when it was far from the Sun, so it may not be. But not all the evidence is in yet. We need spectra of the tail to see if it contains gas (as a comet must) or only dust.

On the other hand, earlier this year an asteroid was observed for the first time just following a collision: P2010/A2 (LINEAR). In this case the debris kicked up will be whatever the asteroid is made of (rock for sure, ice optional), and the outburst will be a onetime thing. Given the large number of asteroids in the main belt, it is estimated that on average one should experience a detectable collision each year.

Is Scheila's outburst due to a collision? Additional evidence of several sorts may be able to provide the proof. If the tail turns out to have no gas in it, the comet outburst idea would be ruled out, leaving the collision idea the last one standing. Alternatively, if observations in the coming months showt that Scheila's orbit or rotation have slightly changed, we would have the smoking gun proving the collision model. But if the impactor was very small, these changes may be too subtle to detect.

Why did Scheila more than double in brightness? Here again there are two possibilities:the added contribution of sunlight reflected off material surrounding the solid object (as in a cometary outburst) or increased reflectivity of the object's surface (caused by a coating of lighter material). This is a question we can answer by observing how the brightness changes as Scheila rotates on its axis. One Scheila orbit ago Brian Warner (2006) showed that the brightness varies with a period of 15.848 hours, its rotation period. In particular, he found that it became fainter by 0.09 magnitudes (about 9%) at one point in each rotation, likely due to small deviations in the shape from spherical. We repeated this experiment with data taken on 14 and 15 December with the Calvin-Rehoboth Observatory. The figure below plots the brightness observed as a function of Warner's rotational cycle.

Lightcurve of Asteroid Scheila

If the enhanced brightness is due to material surrounding the asteroid, this would dilute the brightness variation due to rotation of the solid object. On the other hand, if the brightness is due to a more reflective surface (say reflecting 10% of the light on average iinstead of 4%), then the variations due to shape would still be present. As the Figure shows, we found a variation with a single minimum with amplitude 0.09 magnitude, the same amplitude as before the outburst. We therefore favor the changed reflectivity model. A fraction of the ejected material that went to form the new tail must have coated some portion of the surface. This could be gas that formed a thin frost or simply dust that was slightly lighter than the dark material previously there.

So far, so good. But is there a further prediction that we can test to confirm this interpretation is correct? Yes! If the enhanced brightness is due to greater reflectivity, it should persist even after the tail from the outburst has dispersed. Additional brightness observations in the next few months will provide a decisive test. Stay tuned!

References:

  • S. Larson, 2010 Dec. 12, CBET 2583, "(596) SCHEILA"
  • L. Molnar, 2010 Dec. 18, CBET 2592, "(596) SCHEILA"
  • B. Warner 2006, Minor Planet Bulletin, 33, 58-62 "Asteroid lightcurve analysis at the Palmer Divide Observatory - late 2005 and early 2006".
  • B. Warner and A. Harris, 2010 Dec. 15, CBET 2590, "(596) SCHEILA"
  • Wikipedia, "596 Scheila"

posted 12/20/2010 by L. A. Molnar

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