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The Spacewatch Project

by Robert S. McMillan (*) - Copyright Tumbling Stone 2001


The Spacewatch Project was begun by Tom Gehrels and Robert McMillan in 1980 as a scientific exploration of the whole solar system for minor planets and comets, from the vicinity of Earth's orbit to beyond the orbit of Neptune. Spacewatch pioneered in the use of charge-coupled device CCD (dict.) electronic imagery for searching and doing astrometry of solar system objects, has been scanning the night sky with a ground-based 0.9-meter optical telescope on a regular schedule since 1984, and has been discovering Earth-approaching Asteroids (EAs) since 1989.


The Spacewatch 0.9-m telescope (left) on Kitt Peak during (Photograph by Jim Scotti)
Besides being in operation longer than any other currently active search program, Spacewatch is notable for searching fainter than most other EA surveys. Approximately 2500 square degrees, concentrated near the ecliptic and the opposition point, are searched each year by Spacewatch to a limiting visual magnitude (dict.) V of 21.5. The long reach provided by this sensitivity has made it possible for Spacewatch to discover 237 EAs, 16 Centaurs or scattered-disk objects, 17 comets, and 7 Trans-NeptunianObjects TNOs (dict.), including the large TNO (20000) Varuna (see T.S. number 2: "Varuna, goddess of Heaven and Earth"). More than 5,000 positions of EAs have been measured and reported by Spacewatch. Requests to Spacewatch by the Spaceguard Central Node for searches and recoveries of faint high priority EAs are honored regularly.
The well-documented observational efficiencies and biases of the Spacewatch survey have allowed extrapolations to the numbers and orbital distributions of the parent populations of the discovered bodies. Recent examples of such studies are Bottke et al. (2000) for EAs and Larsen et al. (2001) for populations in the outer solar system.
Spacewatch has been developing a 1.8-meter telescope with a field of view 0.8 degrees in diameter (the diagonal of the square imager). This is the widest possible field with this telescope, and is apparently as big as has ever been provided for any telescope with a paraboloid primary mirror of this diameter or greater. This telescope is also the largest in the world to be dedicated full time to discovering, recovering, and doing astrometry of asteroids and comets throughout the solar system. First light was in 2000 May and the first publishable observations (a lightcurve) were in 2000 September. Engineering trials with a 2Kx2K CCD are in progress.

A false color image of comet
Swift-Tuttle by Spacewatch (1992)

Spacewatch has also been developing a detector array nine times larger than before to be used on the 0.9-m telescope. This mosaic of CCDs will increase Spacewatch's rate of detection of EAs among objects 2-3 magnitudes fainter than are being found by the other survey groups. This is an important goal for completing the inventory of even the large EAs.
A list of the funding sources for Spacewatch, a complete publication list, photos, and other information can be found at the Spacewatch web site: http://www.lpl.arizona.edu/spacewatch/index.html.


Robert S. McMillan (*) - Lunar and Planetary Laboratory University of Arizona


References
Bottke, W. F., R. Jedicke, A. Morbidelli, J.-M. Petit, and B. Gladman 2000. Understanding the distribution of near-Earth asteroids. _Science_, 288, 2190-2194.
Larsen, J. A., A. E. Gleason, N. M. Danzl, A. S. Descour, R. S. McMillan, T. Gehrels, R. Jedicke, J. L. Montani, and J. V. Scotti 2001. The Spacewatch wide-area survey for bright Centaurs and transneptunian objects. _Astron. J._ 121, 562-579.

 


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