I.   Historical Background
II.  The Beauty of Comets and Asteroids
III. How Spacewatch Evolved
IV.  Spacewatch Results
V.   Hazards Due to Comets and Asteroids
VI.  Future Work

     In 1766, Titius von Wittenburg formulated what has become known as the Titius-Bode law of planetary distances. The discovery of Uranus by Herschel in 1781 further kindled the expectation that there was a missing planet near 2.8 astronomical units (AU). In 1794 it was inferred by Chladni that meteorites have an extraterrestrial origin, and then by 1803, Olbers proposed that the meteorites come from an asteroidal exploded planet. The first asteroid, 1 Ceres, was discovered by Piazzi at Palermo in the first evening of 1801. Gauss provided the method for orbit calculation that is still used today. Other discoveries followed, and it became clear that there was not a single planet near 2.8 AU, but rather what we now call "the asteroid belt." The discovery of asteroids came slowly as it was done visually; Max Wolf introduced photography for asteroid observations in 1891. Kirkwood had pointed out in 1867 that there are resonance gaps in the asteroid belt. The first asteroid that could come relatively close to Earth was discovered by Witt in Berlin, namely Eros with a perihelion distance of 1.13 AU; von Oppolzer observed its light variations and explained them by rotation of an object of irregular shape. Reinmuth discovered Apollo in 1932, which has a perihelion distance of 0.65 AU; it was lost, but recovered in 1973.

     By the year 1900, there were centers in Berlin and Kiel to compute orbits and ephemerides for the "minor planets". After World War II this work was continued in Heidelberg and Cincinnati, and eventually this was transferred to the Minor Planet Center of the International Astronomical Union in Cambridge, Massachusetts, where Brian Marsden has been the Director since 1978. Ephemerides of Minor Planets are published by the Institute of Theoretical Astronomy in St. Petersburg. In 1944, O. J. Schmidt postulated the process of planet formation near 2.8 AU by the accumulation of particles and planetesimals, interrupted by Jupiter's perturbations such that the asteroid belt resulted instead; that work has been continued in Moscow by Safronov and others. In 1949, Kuiper initiated a series of photometric studies and the Yerkes-McDonald Survey of Asteroids, and he began to publish his studies of the origin of the solar system. Whipple proposed a "dirty snowball" model for the nuclei of comets. In 1951, Pik evaluated the effects of close approaches to planets on orbits of asteroids, and he calculated their lifetimes. By 1953 Piotrowski had published a basic study of collisions among asteroids; this work has been continued by Anders, Wetherill, and others. In 1957, Begemann and co-workers measured the first cosmic-ray exposure age of a meteorite. By 1964, Wänke had attributed large amounts of helium in certain meteorites to solar wind implantation. In 1964 also, Anders formulated a comprehensive theory of the origin of meteorites in asteroids, while J. A. Wood calculated cooling rates of iron meteorites and found that they must have originated in objects of asteroidal size, 100-500 km in diameter. In 1968, radar observations were made of Icarus.

     A first text on asteroids was edited by Gehrels (1971); there was a lively discussion of the pros and cons of asteroid missions. The first detailed proposals to conduct asteroid space missions were published by Stuhlinger et al. (1972) and by Whipple et al. (1973). Herrick was the first to propose exploitation of asteroids, namely the deliberate impact of Geographos to make another Panama Canal in the year 1994; it was considered so far out that it was not accepted for the 1971 book, but it was published posthumously in the next one (Herrick 1979), with an apology by the editor. By the middle of the 1970s extensive surveys of UBV photometry, statistics, polarimetry, radiometry, and spectrophotometry were underway. The discipline was developing! It became clear that asteroids reflect carbonaceous, silicaceous, and even metallic characteristics (spectral types classified as C, S, and M) and several others; about 50 different types have been identified by now. Their configuration became known a little later; many asteroids may be loosely reconfigured rubble piles. By this time the first systematic survey for near-Earth asteroids had come on-line at the 0.46-meter Schmidt Telescope on Palomar Mountain by Helin and Shoemaker.

     In 1980, father and son Alvarez and their associates had identified an iridium anomaly in a geological layer deposited about 65 million years ago as being due to the impact of a large asteroid or comet, thereby explaining the Cretaceous-Tertiary Extinction. In 1981 the first conference was held on the hazards due to comets and asteroids, in Colorado, but its Proceedings remained unpublished for lack of interest.

     In 1981, the Spacewatch development of electronic techniques began, which had to wait for the delivery of the first 2048 x 2048 CCD before it could discover its first near-Earth asteroid in 1989. (Near-Earth asteroids are loosely defined to have elliptical orbits affected by the planets such that they can come dangerously close to the Earth at some time in their history.) It took until the early 1990s for the reality of this hazard to be more generally accepted; this was helped along by the demonstration of an impact on Jupiter by P/Shoemaker-Levy 9 in 1994.

     Comets and asteroids are dangerous; the hazard is real - it is frightening. It is the greatest danger humanity faces as it is the only one that can destroy us and our habitat in one blow. The chance of that is not small; it is about 1 in 5,000 in a lifetime. But even within that horror lies a beauty because we are on our way to solving the problem. It may even become an example for solving global problems.

     The greatest beauty lies in our research as astrophysicists. The comets and asteroids are left over from their participation in the formation of the solar system, continuing the aggregation of dust and gas from the interstellar medium. We can see the Milky Way because of its many stars, but there also are dark regions where starlight is obscured by clouds of dust. The individual particles are so small that if you had one in your hand, your eyes would not be able to resolve it because their size is near a micron, a thousandth of a millimeter. However, from such nebulae of gas and large numbers of grains, the stars are formed, and so was the solar system. The mechanisms are complex; they are described in a series of textbooks on protostars and planets (Mannings et al. 1999).

     The solar nebula had, among its dust and gas, the next step-up in size that we refer to as a planetesimal, of kilometer dimensions; later in solar-system history we call it a comet or asteroid. The Earth was formed by the accumulation of dust and gas and planetesimals; the bombarding accretion increased progressively as Earth's mass and gravity increased. Some awesomely noisy spectacle that must have been! As the impacts became more energetic, the temperature increased and, in the end, the Earth probably was mostly molten. When the material of the inner solar nebula was used up, the bombardment stopped. This occurred gradually some 4,500 million years ago. A cooling stage followed, from the outside inward, with the crust of the Earth solidifying. Yet another major wave of impacts followed about 3,800 million years ago when the outer parts of the solar system were taking final form. Comets and asteroids had made the cores of the outer planets, which then completed their accretion mostly with gas, to become the gaseous giants Jupiter, Saturn, Uranus, and Neptune. In the process the massive giants must have cleared out that part of the solar system, flinging it in all directions including those of the inner planets.

     Eventually, a steady state was reached in the solar system with the asteroids left over in the main belt, which lies mostly between 2.2 and 3.3 AU. That is still mainly in the inner part of the solar system where the Sun was close enough for the temperatures to be so high that the volatiles and gases had evaporated away. The asteroids in the inner part of the asteroid belt therefore consist mostly of sandy silicaceous materials; no gaseous activity is seen so that we call them asteroids, not comets.

     In the outer parts of the asteroid belt we see the transition to more volatile, carbonaceous types of objects. As for the comets, near the outer regions of the solar system, at distances on the order of 100,000 AU, a cold storage remained for cometary cores that we call the Oort Cloud, after Jan Oort who derived this model in a classical paper. He developed that paper in a class at Leiden University in The Netherlands, where Gehrels (1988) had the fortune to be one of the few undergraduate students attending that class. The Oort cloud is tenuous, but it is large; the number of comets might be as large as 10¹³. Because of low temperature at this great distance from the Sun, they still contain snows and ices among the dust. We call them dirty snowballs that may be dark aggregates of hundreds of kilometer-sized planetesimals tenuously held together; their composition is mostly carbonaceous.

     Back on Earth, we see in the geological layering successively the slow stages of making primitive life, until about 400 million years ago the onset of rapid evolution of more advanced species. Larger vertebrates appeared, like the dinosaurs, who roamed the Earth for some 200 million years. And then, they were gone. It has now been fairly well proven that their demise was due to impact of an asteroid or comet about 12 km in diameter. From where had it come?

     The orbits of the cometary nuclei in the Oort Cloud are stable, but they may be perturbed by the gravitational attraction of stars or massive interstellar clouds passing by the solar system. Or, the perturbation might be a periodic effect due to the solar system's motion up above and down below the plane of the galaxy. The latter is the so-called "Shiva effect," a marked periodicity of 29 million years in the major extinctions on Earth [Smoluchowski et al. 1986, Rampino and Haggerty (1996); it has recently been reviewed in Gehrels (1999)]. In any case, the effects of such distant encounters have been modeled to show that the cometary orbits may be perturbed to be either tossed out of the solar system, or to be brought into elliptical orbits toward the inner regions of the solar system. In the latter case we see them as active comets: as they come closer to the heat of the Sun, melting of the snows and ices occurs, and then the evaporation of the contained gasses and volatiles brings forth the coma and tails. As the material heats up, the gas comes out of the nucleus in huge jets as from a volcano. Together with the gases, dust particles come pouring out so that a glorious dust tail may be seen as well as a tail of ionized gases, in addition to the coma closer in.

     Regarding the asteroid belt, its members generally have stable orbits that are nearly circular, but there are so many of them that collisions occur. If such collisions take place where there are rhythmic resonance encounters with Jupiter - and the gravity of other planets plays a role too - complex interactions take place. The collisional fragments may be either tossed out of the solar system or brought into elliptical orbits including toward the inner regions of the solar system. Here is a steady state, with a balance of supply and demand. The supply is due to the collisions and perturbations, while the demand is caused by gravitational sweeping up by the planets and the Moon. The lifetime of asteroids that approach the Earth-Moon system is typically ten million years. Eventually they will all collide with either Mars, the Earth and Moon, Venus, Mercury, or even with the Sun.

     One can summarize the beauty of comets and asteroids as follows. The primary one is that they participated in the original formations, in our very own origins and well-being. Without them, we would not have had this Earth, we would not have been here ourselves. These roles are best studied with their statistics, their magnitude - frequency relations, as a basis for a dynamical and often collisional history.

     The subsequent beauty is that the Darwinian gradual evolution of life was impacted. Major extinctions are seen in the geological fossil record. The voracious dinosaurs were eliminated so that our type of mammals had a better chance to evolve, and here we are.

     A further beauty lies in the opportunity for sending space missions to comets and asteroids, eventually landing on them and sampling their surfaces for detailed study. There is an increasing interest in missions to comets and asteroids, in Europe and Japan as well as in the United States, with other countries participating. There also appears to be a beginning in commercial spaceflight and exploration of asteroidal material (Lewis et al. 1993). Groundbased observations are needed to find suitable candidates for space programs; the techniques may be used on the missions themselves (Gehrels 1986). There have been fly-by encounters of main-belt asteroids Gaspra and Ida (for which the first asteroid satellite "Dactyl" was found) by the Galileo spacecraft; the NEAR spacecraft flew by Mathilde and by Eros. An overview of the asteroids and their utilization has been made by Kowal (1996).

     An astronomer's beauty is that it is fun to hunt them (Gehrels 1988). It is a challenge to develop new techniques for finding the elusive bodies. The discovery of new objects, especially comets and fast-moving nearby objects brings unforgettable moments for the observer, and tension too because they must be pursued before they might be again.

     In 1969 the idea sprouted to build a telescope dedicated to the small objects in the solar system, the comets, asteroids, and satellites. A most encouraging colleague is Aden Meinel, who was then Director of the Optical Sciences Center of the University of Arizona. He acquired for that plan of a dedicated telescope a 1.8-meter mirror blank from a military program. It is a precious piece of optics, of light-weight construction from low-expansion material, fused silica. It was sagged to f/2.7 but that was as far as the dream came to be realized at that time. The idea of a dedicated Asteroid Telescope was too far ahead of its time; too few people were active in these fields at that time. When it became clear that there was not going to be an Asteroid Telescope soon, Meinel borrowed the mirror for the Multi-Mirror Telescope (MMT). It was used for a decade in the MMT, biding its time to work on the more exciting studies of asteroids. It was returned in 1993, when the 1.8-meter Spacewatch Telescope was finally going to be built (Perry et al. 1998).

    The telescope was designed and built in-house because there was never enough funding to go shopping outside. It took years to build, but the Spacewatch crew stayed the course. The advantage of the arduous procedure comes now, during the operation, because the crew learned every screw and symbol and transistor, as they had done for the 36-inch Spacewatch Telescope. So now they know how to keep them both in operation, continually improving with new electronics and computer routines.

     At the time of the dedication of the 1.8-meter and its building, Gehrels made a wager with representatives of the Tohono O'odham Nation that within 20 years there will be ae. So now he goes around the Nation, which is a big one, giving peptalks to high school students. The immediate goal is to get some of them into the astronomy summer camps run by Don McCarthy of the Steward Observatory.

     The general goal of the Spacewatch programs is the discovery of small objects in the solar system for the study of their statistics and dynamical histories. It had been done before with photographic plates (Kuiper et al. 1958; van Houten et al. 1970, 1989, 1991); one of the finest interpretations of the magnitude - frequency relations still is the one made by Anders (1965). Spacewatch is now to make such studies from observations made with much faster detectors, the charge-coupled devices (CCDs), and computer processing which were coming on the horizon in the early 1970s particularly for Soviet submarines.

     To develop scanning techniques with CCDs, a venerable 0.9-meter Newtonian telescope had been made available in 1982. It was the original Steward Observatory Telescope erected in 1921 on the campus in Tucson and moved to Kitt Peak in 1962. It is now called "The Spacewatch Telescope. "On Kitt Peak, 80 km west of Tucson, there are some 18 telescope systems, mostly under the control of the Kitt Peak National Observatory. The University of Arizona has a site of its own for a few telescopes such as the 0.9-m Spacewatch Telescope and also now the 1.8-m Spacewatch Telescope. The crew for the Spacewatch work is dedicated to not only keeping the 0.9-m in operation 18-20 nights per month and analyzing and publishing the data, but also to developing new techniques and bringing the 1.8-meter on line.

     An early design was for a straight prime-focus 1.8-m f/2.7 reflector, but there always were shortages in funding, so the telescope was made more compact to save in the size of the dome, keeping it down to the range in which competitive bids could be obtained. The result is a "folded prime focus" with a flat secondary mirror, which has a diameter of 76 cm. This causes about 6% more loss of light than what would have occurred due to the blocking of the light by the instrumentation at prime focus; this is the price that will always have to be paid for an affordable telescope.

     Regarding the work at the 0.9-m Spacewatch Telescope, which is a Newtonian, the south port has a dewar permanently installed, which contains a Tektronix 2048 x 2048 CCD with a pixel size of 24 microns; the system has about 70% quantum efficiency. It began in the 1980s with a 320 x 520 CCD surveying for gamma-ray bursters, debris in geosynchronous regions, satellites of asteroids, brown dwarfs, the tenth planet, Lou Frank's cometesimals (in vain), and for astrometry of comets and asteroids (Gehrels 1991, 1995). The computer programming for automatic detection has required about eight man-years of work, mostly by Scotti (1993) and by Rabinowitz (1991) actually later. The charge-coupling property of the CCD is used to transfer the charges from row to row when the telescope drive is turned off, such that the sky is being scanned (McMillan et al. 1986). The width of the CCD is crossed by the stars in 146.53/cos(d) seconds, where d is the declination. The principle is that of a "bucket brigade" in fire fighting: the light is converted photo-electrically into charges, which are transferred from pixel to pixel until the end. From the last readout row, which is called the end register, the data are read into a Solbourne Sun-Station computer system. Three consecutive scans are made for each region on the sky, of about half a degree width and about 7 degrees length, for half-an-hour duration of the scan. In intricate bookkeeping, the computer lists the pixel coordinates and a brightness parameter of as many as 50,000 stars and asteroids that may occur in the scan. By comparing the listings obtained during the three scans, the computer finds the moving objects.

     The distance of the asteroid is learned from its rates, using a simple principle, which is best illustrated by an airplane flying overhead where it has great angular velocity. As it flies away into the distance, it slows down in apparent angular rate, even when its own linear velocity remains the same. Similarly, when an asteroid is at great distance from us, its angular rate is low; this method works especially well, with the rate being inversely proportional to the distance, near the direction opposite that of the Sun, "near opposition" (Rabinowitz 1991, Jedicke 1996).

     There also is a routine for computerized "streak detection" of objects that move so fast that they will make a trail during the 146.53 secs exposure. In addition, the human eye is efficient at discovering faint trails, and the observer therefore watches intently as the scan goes by. While the computer is set to discover objects down to a level of 3.35-sigma, long trails are reliably recognized by eye and the brain even at the 1-sigma level. The scans are watched carefully also in a search for faint comets; it is a fun job because of all the galaxies, meteors, spacecraft, and hydrogen clouds going by! The third scan is especially exciting because the new discoveries by the motion-detection routine are then reported. During inspection the following day it is found that about one-third of the new discoveries is not real. The fakes and errors can be minimized by setting the level of detection to a signal level higher than that of 3.35 sigma, but that would be at the cost of losing faint discoveries.

     The work schedule of the Spacewatch observer can be demanding. On a winter night he gets into the dome by about 6 p.m. and is stopped by twilight at about 6:30 a.m. It is a vigilant operation without time off. At 7 a.m. the computer begins the analysis of the discoveries, by calling up snapshots for each one of the discovered objects. This is a large number, as many as 3,000 snapshots for a good winter night. The observer then needs several hours in the daytime to sort out the real discoveries, which becomes a thrill when a special object is recognized. That can be a comet or a near-Earth object or a distant one such as the so-called Centaurs that move in the space of Saturn and Uranus.

     As many as 700 real asteroids may then remain from the inspection of 2,100 snapshots; these are mostly in the main belt and nearly all of them are new, previously unknown. With a limiting magnitude near 21.9, the surveying is far beyond the present completion limit in the asteroid belt, which lies near the 17th magnitude. The positions are reported on e-mail to the Minor Planet Center in Cambridge, Massachusetts. Since 1995 Spacewatch has been repeating scanned regions twice during each lunation, that is with about five days in between. The five-day interval allows the determination of better orbits for statistical studies than the one-hour interval in the single nights of the original scans. As yet, Spacewatch is the only survey program obtaining such better orbits in routine surveying. It is presently done for some 25,000 asteroids per year; that number may soon become much higher yet through modifications of the telescope and the use of multiple and faster detectors.

     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.

     Concerning the dangers, it is an unpleasant topic that people adjusted to slowly, even though the hazards have always been with us and been known for decades. Meteorites have come falling down such as at Dhajala near Ahmedabad in India on January 28, 1976. The schoolmaster declared a holiday after the noisy break-up and crash of the object so that the children could search the area. Even though it was a stony meteorite the children knew the desert so well that they found nearly a dozen of the fragments.

     As we move on to larger impacts, there is the crater in Northern Arizona near Flagstaff. It was made by a 35-meter metallic object some 40,000 years ago. The kinetic energy was about 300 "Hiroshimas", 300 times the energy used at Hiroshima in August 1945. At a similar scale of impacts is the one at Tunguska, Siberia, in 1908 with an air burst in the upper atmosphere that was heard as far away as London. The energy was also near 300 Hiroshimas, but as far as we know no people were killed. This would have been different if it had been over the great city of Moscow, for example, which would have been obliterated by the blast.

     A larger impact site is the Ries Crater, 2 hours on the autobahn southeast of the Frankfurt Airport, which has an exquisite visitors' center in. From its church steeple one can see the surrounding rim of a 20-km crater. A ratio of 20 has been derived for the crater/projectile diameters so that the size of this object must have been near 1 km. The age of impact craters is derived from dating the stressed rocks near the rims; this one appears to be 15 million years old. There is a smaller crater some 40 km to the west was made at the same time, apparently due to a satellite.

     The largest near-Earth asteroids are limited to about 10 km in diameter. That is interesting especially because the ones in the main belt are as large as 1,000 km; if one of those hit the Earth, the Earth would be severely damaged and life eliminated entirely. The asteroids in the main belt are generally in stable orbits, but there are so many of them that collisions do occur. These are violent events at a typical velocity of 5 km/sec, such that the debris can reach great distances and the gravitational effects of Jupiter will then either thrown them out of the solar system, or bring them inward. The observation that the largest are near 10 kilometers therefore seems to confirm that we are dealing with fragmentation in the asteroid belt. For each factor of 10 smaller in size, the number increases by a factor of about 100. This is seen in the magnitude-frequency law in the asteroid belt (page 355 of van Houten et al. 1970), again, this is understood as showing a fragmentation law.

     Modeling of the impact disasters by various groups of scientists indicates that the asteroids larger in diameter than about 1 kilometer do cause holocaust on a global scale; the smaller ones like the ones of Arizona and Tunguska cause "only" regional devastation. Of the objects larger than 1 kilometer there are about 900 (Bottke et al. 2000) They impact the Earth on average once in about 330,000 years. However, one should not dismiss the danger because it happens only once in 330,000 years. It is merely a statistical expression. The chance of it happening tomorrow is just as great or just as small as some tomorrow 330,000 years from now.

     It is essential to take the impact energy into account, which is computed with the expression for kinetic energy, 1/2 mv², assuming a density typical for meteorites of 3 g/cm³. The relative velocity of the asteroid with respect to Earth is typically 20 km/sec. The kinetic energy of impact of an asteroid with a diameter of 1 km is then derived to be near a million times the energy used at Hiroshima in 1945 (5 x 10²⁰ ergs, or equivalent to the explosion of 13,000 tons of TNT).

     What do we mean by a global hazard? What could happen? The extreme case occurred 65 million years ago, clearly marked by a discontinuity in the deposit layering between the Cretaceous and the Tertiary geological periods. It is referred to by the German initials as the K-T event. The K-T crater was identified, after years of searching and neglect of previous literature, under mud and soil so that it is not immediately recognized as a crater. It is called the Chicxulub Crater after the nearby village of Chicxulub in the Yucatan Peninsula of Mexico; the primary diameter is 180 km, but it has rings out to a diameter of 400 km. Impact modeling shows that the object must have come in while throwing off fiery blobs of its own material causing fires over large areas of the Earth. The impact explosion of the 12-km object threw enormous quantities of dust from the ground and from itself high up into the Earth's stratosphere. First, there was total blockage of sunlight for perhaps half a year, during which most of that material settled in a layer a few centimeters thick between the Cretaceous and Tertiary deposits. Below it, the evidence for dinosaurs is found. Above it, three-fourths of previously living species are missing.

     It was a large object at an exceptionally bad place for an impact, namely within only about the 2% of the Earth's surface having a deep layer of limestone. This occurs in equatorial regions where 200-400 million years ago there had been tropical rain forests; a similar area would be the Barrier Reef of Australia. By the impact explosion, the carbon and nitrogen of the limestone were freed into the Earth's atmosphere where they combined with the oxygen into CO₂ etc., the gases that cause acid rain and greenhouse effects. The global warming may have been to as much as 10 degrees Centigrade, lasting some five-thousand years, and that is what killed the dinosaurs (Smit 1994).

     The extinction effects as well as the detection and mitigation of dangerous objects was studied in a variety of workshops, beginning with one in Colorado in 1981. Based on these conferences and related studies and writings there followed the book edited by Gehrels (1994). It is part of the Space Science Series of the University of Arizona Press, designed to produce texts for graduate students and source books for scientists and engineers. The book procedures include the combination of authors from various backgrounds to write a chapter together, and this is followed by helpful refereeing and editing. The 120 participating authors combined their work in 46 chapters, bringing a wide experience in the detection of these objects, their statistics and characteristics, modeling of impacts, and a variety of techniques that could be used to avoid a disaster.

     However, the material of that book is dated from the beginning of the 1990s, while the science and engineering are developing fast. A next volume has been published by Remo (1997). Continued observations and studies are always needed and forthcoming.

     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 900 "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 900 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 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 was revived with NASA support by Steve Larson and Rob McNaught (AAO). The National Observatory of Japan has joined the hunt.

     What if we find a dangerous asteroid? International rocketry experts and planetary scientists have held various conferences to answer this question, one of them in Tucson in 1993, resulting in the textbook mentioned before (Gehrels 1994). The rocketry and celestial mechanics for missile encounters with a dangerous object appear well established. They could be executed with short forewarning. When the warning time is short, and the masses are always large, a chemical explosion may not be sufficient. The tremendous energies needed to push the thing aside may require more than chemical weapons such that we have to resort to the greatest forces that can be unleashed from nuclear weapons. We therefore asked, already in 1981, the people familiar with nuclear engineering to take an interest in these problems. There seems to be sufficient expertise in various nations such that in an emergency a mitigation system could be put together and launched within several months. A nuclear warhead would be exploded at a short distance from the object, a "stand-off explosion," in order to ignite surface material so that the spallation would yield an impulse away from the source of action.

     Such application of nuclear engineering would bring the intense satisfaction that mankind had developed the dreadful weaponry for its survival rather than for its demise.

Acknowledgment: Funding for Spacewatch is summarized elsewhere on this web site.