The program is currently called Spacewatch and is run out of the Lunar and Planetary Lab at the University of Arizona. They use a 0.9 meter telescope at the Steward Observatory on Kitt Peak, and on every clear night during lunar New Moon, they take hundreds of images of the sky to search for asteroids brighter than magnitude 21 that are moving at large angular speeds from night to night. The above photo shows the Near-Earth Asteroid 1994XM1 discovered in 1994. The asteroid was about 550,000 km away at the time of this image, on its way to a record close approach of some 105,000 km, just 12 hours later!
The apparent rates of asteroids that will impact the Earth within about 100 days has been studied by astronomers Hills and Leonard ( see Astronomical Journal, v.109 p. 401 for more details), and 10 days before impact the maximum speed of these objects in the sky is about 300 arcseconds per day. Spacewatch's current method of detection can see objects moving as slow as 150 arcseconds per day. As of May 28, 1995, Spacewatch has discovered 97 Near Earth Asteroids of which half are larger than 500 meters. These were culled from 120,000 asteroids actually seen in their survey. 240 of these asteroids were followed in order to make positive identifications of the 97 'NEAs'.
A larger aperture, 1.8 meter Spacewatch Telescope is now under construction under grants from NASA and the Air Force Office of Space Research. You can always write your Congressperson to impress upon them that just one of these 500 meter babies would substantially alter Earth's ecosystems, kill millions of people, and make any major earthquake of hurricane look like a minor nuisance!
According to a discussion of their progress to date at Spacewatch: The scientific conclusions are the most important, of course, rather than merely discovering new objects and following them up for orbit improvement. Soon after Spacewatch began the automatic detection of near-Earth asteroids in 1990, it found smaller ones than 100 meters in diameter surprisingly more often than had been expected. For the larger asteroids, the discoveries occurred as expected from previous data sets, and linear extrapolation to fainter magnitudes had brought a prediction for smaller objects. With respect to that linear extrapolation, the frequency seems to be increasing for objects smaller than 100 meters; at the 10-meter size, the frequency is 40 times the number expected from the prediction (Rabinowitz 1993, 1997; also see Scotti et al. 1991). The excess was found to agree with observations made by military reconnaissance satellites (Tagliaferi et al. 1994); objects of 10-meter size hit the Earth's atmosphere as often as a few times per year.
The interpretation of the excess took some time and a few false starts, until Galileo on its way to Jupiter passed by main-belt asteroids Gaspra and Ida; crater counts on their surfaces also show that excess of small objects (Chapman et al. 1996). One interpretation is that it is due to debris from cometary activity, that the smaller objects come from the surfaces and interiors of comets when they are active (Ceplecha 1997).
A Spacewatch paper has been written on the statistics of asteroids in the main belt (Jedicke and Metcalfe 1998, see also Durda et al. 1998), and additional studies of magnitude - frequency relations are on the way.
Another major result of the Spacewatch program is the discovery of three objects, (5145) Pholus, 1993 HA2 and 1995 GO, with semi-major axes near 22 AU, eccentricity about 0.6 and inclination near 20 degrees. Additional discoveries keep coming in of objects orbiting the Sun at such distances where a chance encounter with Uranus, for instance, can greatly affect the orbit. A surprising finding was made by various spectro-photometrists for Pholus and HA2, namely that they are by far the most reddish objects in the solar system, which is interpreted as being due to organic material on their surface. They differ from (2060) Chiron, which is not so reddish and has a more circular orbit, near those of Saturn and Uranus. With number 2060, Chiron obtained the usual asteroid identification, but cometary activity was eventually observed so that Chiron is now considered to be a large comet.
Statistical studies made by Jedicke and Herron (1997) showed that the population of those "Centaurs" may be as large as that in the asteroid belt. The Centaurs may be intermediate objects transferring from the ones farther out yet, to comets and asteroids in the inner parts of the solar system (Bailey et al. 1992, also see Marsden and Steel 1994, and Valtonen et al. 1995). David Jewitt and other astronomers in Hawaii and later also elsewhere have indeed discovered objects still farther out. To date about 100 of these trans-Neptunian objects (TNOs) are known, and one speaks of the "Kuiper Belt" beyond the orbit of Neptune. During the past 5 years our knowledge of the number of solar system objects has vastly increased!
The first step towards mitigation of the hazard lies in astronomy: the calamitous asteroids have to be found first. Next, there must be enough follow-up observations to allow the computation of precise orbits. We must know where they are, and if and when they might collide with Earth. It is curious to note that these practical problems in saving humanity has to be led by astronomers, the scientists who have lived so long in ivory towers satisfied with the remoteness of their subject. Astronomers now have to find the 1,700 "near-Earth objects" of diameters of 1 kilometer and larger that could cause global demise.
The Spacewatch Telescope, with its CCD scanning, came to the forefront of trying new techniques because it had been originated in 1980 already, not per se for the hazards, but for statistical studies of all comets and asteroids anywhere in the solar system. It may be possible to find the 1,700 with a fair degree of completion within the next few decades because Spacewatch is no longer the only professional electronic discovery program. Amateur astronomers have been finding near-Earth comets and asteroids for years, with their own CCD equipment. The Lincoln Laboratories presently has the most successful program by far; it uses a telescope in New Mexico with a new CCD that was developed by them particularly for fast reading out. Spacewatch is being redesigned with a larger array of new detectors, and their 1.8-m reflector is coming on line so that it should then not be far behind anymore. Ted Bowell manages the Lowell Observatory Near-Earth Object Survey, LONEOS, which is successful in finding 1-km and larger potentially hazardous asteroids. The Near-Earth Asteroid Tracking (NEAT) program is run by Eleanor Helin at the Jet Propulsion Laboratory with its CCD system on a telescope in Hawaii. A program that is successful away from the ecliptic is run by Steve Larson with a Schmidt Telescope of the University of Arizona. The program at the Observatoire de la Côte d'Azur is run by Alain Maury. The most recent group to join the hunt is at an observatory near Beijing, China. The Anglo-Australian Observatory (AAO) had an effective program running, which was discontinued by the Australian government, but it may be revived with NASA support by Steve Larson and Rob McNaught (AAO). The National Observatory of Japan has announced that it will join the hunt with two new telescopes.
Copyright 1997 Dr. Sten Odenwald
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