Asteroid (596) Scheila and Analysis of Collision with Another Asteroid

From the article...

Scientists have captured and studied the collision of two asteroids for only the second time in the history of astronomy. In the May 20 edition of the Astrophysical Journal Letters (currently online), UCLA's David Jewitt and colleagues report on observations from NASA's Hubble Space Telescope of a large asteroid that was hit by a much smaller one.

On Dec. 11, 2010, astronomers noticed that an asteroid known as (596) Scheila had unexpectedly brightened and was sporting short-lived dust plumes. Data from NASA's Swift satellite and Hubble Space Telescope showed that these changes likely occurred after Scheila was struck by a much smaller asteroid, probably in late November or early December. The shape, evolution and content of the plumes enabled the scientists to reconstruct what occurred.

The smaller asteroid crashed into Scheila with the energy of at least a 100 kiloton nuclear bomb. The smallest particles from the collision escaped, and the dust formed plumes. These plumes were barely visible when Hubble observed Scheila two weeks later, and within two months, the plumes were gone completely, along with the evidence that Scheila had suffered a violent collision.

"The Hubble data are most simply explained by the impact, at 11,000 miles per hour, of a previously unknown asteroid about 100 feet in diameter," said Jewitt, the Hubble team leader and a UCLA professor of Earth and space sciences and of physics and astronomy.

PAPER REFERENCES:

COLLISIONAL EXCAVATION OF ASTEROID (596) SCHEILA

D. Bodewits1, M. S. Kelley1, J.-Y. Li1, W. B. Landsman2, S. Besse1 and M. F. A’Hearn1
dennis@astro.umd.edu msk@astro.umd.edu jyli@astro.umd.edu sbesse@astro.umd.edu ma@astro.umd.edu Wayne.B.Landsman@nasa.gov
1 Department of Astronomy, University of Maryland, College Park, MD 20742, USA
2 NASA GSFC, Adnet Systems, Mailstop 667, Greenbelt, MD 20771, USA

The Astrophysical Journal Letters Volume 733 Number 1
D. Bodewits et al. 2011 ApJ 733 L3 doi: 10.1088/2041-8205/733/1/L3

Abstract:
We observed asteroid (596) Scheila and its ejecta cloud using the Swift UV-optical telescope. We obtained photometry of the nucleus and the ejecta, and for the first time measured the asteroid's reflection spectrum between 290 and 500 nm. Our measurements indicate significant reddening at UV wavelengths (13% per 103 Å) and a possible broad, unidentified absorption feature around 380 nm. Our measurements indicate that the outburst has not permanently increased the asteroid's brightness. We did not detect any of the gases that are typically associated with either hypervolatile activity thought responsible for cometary outbursts (CO+, CO2 +), or for any volatiles excavated with the dust (OH, NH, CN, C2, C3). We estimate that 6 × 108 kg of dust was released with a high ejection velocity of 57 m s–1 (assuming 1 μm sized particles). While the asteroid is red in color and the ejecta have the same color as the Sun, we suggest that the dust does not contain any ice. Based on our observations, we conclude that (596) Scheila was most likely impacted by another main belt asteroid less than 100 m in diameter.

HUBBLE SPACE TELESCOPE OBSERVATIONS OF MAIN-BELT COMET (596) SCHEILA

David Jewitt1,2,3, Harold Weaver4, Max Mutchler5, Stephen Larson6 and Jessica Agarwal7
jewitt@ucla.edu
1 Department of Earth and Space Sciences, UCLA, 595 Charles Young Drive East, Box 951567, Los Angeles, CA 90095-1567, USA
2 Institute for Geophysics and Planetary Physics, UCLA, 3845 Slichter Hall, 603 Charles Young Drive East, Los Angeles, CA 90065-1567, USA
3 Department of Physics and Astronomy, UCLA, 430 Portola Plaza, Los Angeles, CA 90095-1547, USA
4 The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
5 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
6 Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721-0092, USA
7 Institute for Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany

The Astrophysical Journal Letters Volume 733 Number 1
David Jewitt et al. 2011 ApJ 733 L4 doi: 10.1088/2041-8205/733/1/L4

Abstract:
We present Hubble Space Telescope Observations of (596) Scheila during its recent dust outburst. The nucleus remained point-like with absolute magnitude HV = 8.85 ± 0.02 in our data, equal to the pre-outburst value, with no secondary fragments of diameter ≥100 m (for assumed albedos 0.04). We find a coma having a peak scattering cross section ~2.2×104 km2, corresponding to a mass in micron-sized particles of ~4×107 kg. The particles are deflected by solar radiation pressure on projected spatial scales ~2×104 km, in the sunward direction, and swept from the vicinity of the nucleus on timescales of weeks. The coma fades by ~30% between observations on UT 2010 December 27 and 2011 January 4. The observed mass loss is inconsistent with an origin either by rotational instability of the nucleus or by electrostatic ejection of regolith charged by sunlight. Dust ejection could be caused by the sudden but unexplained exposure of buried ice. However, the data are most simply explained by the impact, at ~5 km s–1, of a previously unknown asteroid ~35 m in diameter.

Link: Paper (Collisional Excavation of Asteroid (596) Scheila)

Link: Paper (Hubble Space Telescope Observations of Main-belt Comet (596) Scheila)

Link: News Article

Link: YouTube Video (Scheila Asteroid Crash)