9.1 Overview

Concern over the cosmic impact hazard motivated the U.S. Congress to request that NASA conduct a workshop to study ways to achieve a substantial acceleration in the discovery rate for near-Earth asteroids. This report outlines an international survey network of ground-based telescopes that could increase the monthly discovery rate of such asteroids from a few to as many as a thousand. Such a program would reduce the time-scale required for a nearly complete census of large Earth-crossing asteroids (ECAs) from several centuries (at the current discovery rate) to about t 25 years. We call this proposed survey program the Spaceguard Survey (borrowing the name from the similar project suggested by science-fiction author Arthur C. Clarke nearly 20 years ago in his novel Rendezvous with Rama).

In addition, this workshop has considered the impact hazards associated with comets (both short-period and long-period) and with small asteroidal or cometary objects in the tens of meters to hundreds of meters size range. The object is not elimination of risk, which is impossible for natural hazards such as impacts, but reduction of risk. Emphasis, therefore, is placed upon the greater hazards, in an effort to define a cost-effective risk-reduction program. Below we summarize our conclusions with respect to these three groups of objects: ECAs, comets, and small (Tunguska-class) objects.

1) Large ECAs (diameter greater than 1 km, impact energy greater than a million megatons). These objects constitute the greatest hazard, with their potential for global environmental damage and mass mortality. About two thousand such objects are believed to exist in near-Earth space, of which fewer than 10 percent are now known. Between a quarter and a half of them will eventually impact the Earth, but the average interval between such impacts is long -- more than 100,000 years. While some of these objects may break up during entry, most will reach the surface, forming craters if they strike on the land. On average, one ECA in this size range passes between the Earth and the Moon every few decades.

The proposed Spaceguard Survey deals effectively with this class of objects. Telescopes of 2-3 m aperture can detect them out to a distance of 200 million kilometers. Since their orbits bring them frequently within this distance of the Earth, a comprehensive survey will discover most of them within a decade and can achieve near completeness within 25 years. Specifically, the survey modeled here, covering 6000 square degrees of sky per month to magnitude V = 22, is calculated to achieve 91 percent completeness for potentially hazardous ECAs in 25 years. The most probable outcome of this survey will be to find that none of these objects will impact the Earth within the next century, although a few will need to be followed carefully to ensure that their orbits do not evolve into Earth-impact trajectories. In the unlikely case (chances less than 1 percent) that one of these ECAs poses a danger to the Earth over the next century or two, there will be a warning of at least several decades to take corrective action to deflect the object or otherwise mitigate the danger.

2. Comets. Comets with short periods (less than 20 years) will be discovered and dealt with in the same manner as the ECAs described above; they constitute only about 1 percent of the ECA hazard in any case. However, comets with long periods (more than 20 years), many of which are entering the inner solar system for the first time, constitute the second most important impact hazard. While their numbers amount to only a few percent of the ECA impacts, they approach the Earth with greater speeds and hence higher energy in proportion to their mass. It is estimated that as many as 25 percent of the objects reaching the Earth with energies in excess of 100,000 megatons are long period comets. On average, one such comet passes between the Earth and Moon per century, and one strikes the Earth every million years.

Since long-period comets do not (by definition) pass frequently near the Earth, it is not possible to obtain a census of such objects. Each must be detected on its initial approach to the inner solar system. Fortunately, comets are much brighter than asteroids of the same size, as a consequence of outgassing stimulated by solar heating. Comets in the size range of interest will generally be visible to the Spaceguard Survey telescopes by the time they reach the asteroid belt (500 million km distant), providing several months of warning before they approach the Earth. However, the short time-span available for observation will result in less well-determined orbits, and hence greater uncertainty as to whether a hit is likely; there is a greater potential for "false alarms" with comets than asteroids. Simulations carried out for this report indicate that only 35 percent of Earth-crossing long-period comets greater than 1 km in diameter will be detected with at least 3 months warning in a survey of 6000 sq degrees per month. By increasing the area of the survey to include the entire dark sky, as many as 77 percent could be detected.. Increasing telescope aperture to reach fainter magnitudes (V = 24) improves the discovery rate still further. Because of the continuing hazard from comets, which may appear at any time, the cometary component of the Spaceguard Survey should be continued even when the census of large ECAs is essentially complete.

3. Smaller Asteroids, Comets, and Meteoroids (diameters from about 100 m to 1 km; energies from 20 to a million megatons). These impacts are below the energy threshold for global environmental damage, and they therefore constitute a smaller hazard in spite of their more frequent occurrence. Unlike the large objects, they do not pose a danger to civilization. The nature of the damage they cause depends on the size, impact speed, and physical nature of the impacting object; only a fraction of the projectiles in this size range will reach the surface to produce a crater. However, detonation either at the surface or in the lower atmosphere is capable of severe local damage, generally on a greater scale than might be associated with a large nuclear weapon. Both the Tunguska (1908) and Meteor Crater impacts are small examples of this class. The average interval between such impacts for the whole Earth is a few centuries; between impacts in the inhabited parts of the planet is a few millennia; and between impacts in densely populated or urban areas is of the order of 100,000 years. More than one million Earth-crossing objects probably exist in this size range, with several passing between Earth and Moon each year.

The Spaceguard Survey will discover as many hundreds of objects in this size range every month. By the end of the initial 25-year survey, it will be possible to track the orbits of as many as 100,000, or about 10 percent of the total population. If the survey continues for a century, the total will rise to about 40 percent. Since the interval between such impacts is greater than 100 years, it is moderately likely that the survey will detect the "next Tunguska" event with ample warning for corrective action. However, in contrast to the ECAs and even the long-period comets, this survey will not achieve a near-complete survey of Earth-crossing objects in the 100-m size range in less than a several centuries with current technology. If there is a societal interest in protecting against impacts of this size, presumably advanced technologies will be developed to deal with them.

9.2 Survey Network: Cost and Schedule

The proposed Spaceguard Survey network consists of six telescopes of 2-3 meter aperture together with a central clearinghouse for coordination of the observing programs and computation of orbits. It also requires access to observing time on existing planetary radars and optical telescopes for follow-up. For purposes of this discussion, we assume that the Spaceguard Survey will be international in operations and funding, with the United States taking a leadership role through the Solar System Exploration Division of NASA.

The Spaceguard Survey Telescopes

The six survey telescopes required for the Spaceguard Survey are new instruments optimized for the discovery of faint asteroids and comets. While it is possible that one or more existing telescopes could be retrofit for this purpose, we expect that the most cost-effective approach is to design and construct telescopes specifically for this project. For purposes of this Report, we consider a nominal telescope design of 2.5 m aperture and 5.2 m focal length with a refractive prime-focus corrector providing a field-of-view of at least 2 degrees. The telescope will have altitude-azimuth mounting and be capable of pointing to an accuracy of a few arcsec and tracking to a precision of a fraction of an arcsec at rates up to 20 times sidereal. We assume that each telescope will be located at an existing observatory site of proven quality, so that no site surveys or new infrastructure development (roads, power, etc.) is required. The nominal aperture of 2.5 m is optimized for the ECA survey, but we note that larger telescope aperture (3 m or even more) would permit long-period comets to be detected at greater distances and thereby provide both greater completeness and months of additional warning.

An instrument of very similar design has recently been proposed by Princeton University for a wide-angle supernova survey. Cost estimates for this telescope are summarized in Table 9.1, adapted from their current (1991) proposal to the National Science Foundation. We believe that the SPACEGUARD Survey Telescopes could similarly be built for about $6 million each, including observatory building, but not including the focal plane of several mosaiked CCD detectors or the supporting data processing and computation capability. For each telescope, we allocate $1 million for the focal plane and $1 million for computer hardware and software, for a total cost per installation of $8 million. If these six telescopes were purchased together, the capital costs would thus be about $48 million.

For an estimate of operating costs, we assume that each telescope will require the following staffing: 2 astronomers, 2 administrative support personnel, 3 telescope operators, 1 each senior electronic and software engineers, and 2 maintenance and support technicians, for a total of 11 persons. Additional funds will be needed for transportation, power, sleeping accommodations for observers, and other routine costs associated with the operation of an observatory; the exact nature of these expenses depends on the location and management of the pre-existing site where the telescope is located. The total operations for each site should therefore run between $1.5 million and $2.0 million per year. In making this estimate we assume that each survey telescope is dedicated to the Spaceguard effort, and that it will be in use for about three weeks (100-150 hours) of actual observing per month. If it is intended that the telescope be used for other unrelated purposes when the Moon is bright, we assume that the other users will pay their prorata share of operation costs.

The Spaceguard Survey Operations Center should provide overall coordination of the international observing effort, including rapid communications among the survey telescopes and those involved in follow-up observations. The Spaceguard Survey Operations Center will also compute orbit ephemerides and provide an ongoing evaluation of the hazard posed by any object discovered by the Survey. Similar functions are performed today for the much smaller number of known asteroids by the Minor Planet Center in Cambridge, Massachusetts. Scaling from that operation, we estimate an initial cost of $2 million for computers and related equipment, and an annual operating cost of $2 million.

A third component of the Spaceguard Survey Program is follow-up, including radar and optical observations. As noted previously in this Report, it would be desirable to have one or more dedicated planetary radars and large-aperture optical telescopes (4-m class). However, we anticipate that a great deal of useful work could be done initially using existing planetary radars and optical facilities. Therefore, for purposes of this Report, we simply allocate a sum of $2 million per year for the support of radar and optical observing on these instruments.

Spaceguard Management and Cost-Sharing

The total estimated capital costs for the Spaceguard Survey are $50 million, with operating costs of $8-$10 million per year. We anticipate that these costs would be shared among several nations with advanced technical capability, with the maximum expenditure for the U.S. (or any other nation) of less than half the total amount. For purposes of U.S. budgeting, we assume that NASA will pay the cost of two telescopes ($16 million) and the Operations Center ($2 million), and will support operating costs of $5 million per year.

Management of the U.S. component of the Spaceguard Survey could be accomplished by NASA in one of two ways. (1) The telescopes could be constructed and operated by universities or other organizations with funding from NASA Headquarters through grants or contracts, as is done today with the NASA IRTF telescope on Mauna Kea (owned by NASA but managed by the University of Hawaii under a five-year contract) or the 0.9-m Spacewatch Telescope on Kitt Peak (owned and operated by the University of Arizona with partial grant support from NASA). (2) NASA could construct and operate the telescopes itself through one of its Centers (JPL or Ames, for example); the Centers might contract with universities or industry for operations but would retain a more direct management control. Similarly, the Spaceguard Survey Operations Center could be located at a NASA Center or could be supported by grants or contracts at a university or similar location, such as the present Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics. In any case, international cooperation and coordination is essential, and an international focus is required from the beginning in planning and supporting this program.

Initial Steps

The construction of the new Spaceguard Survey telescopes will require approximately four years from the time funding is available. In the meantime, several steps are essential to ensure a smooth transition from the present small surveys to the new program. (1) An international coordination effort should be initiated by NASA, independent of but coordinated with the International Astronomical Union Working Group on Near Earth Objects, in order to plan for the orderly development of the Spaceguard Survey network. (2) The small cadre of current asteroid observers should be strengthened. Additional expenditures of about $1 million per year on existing teams would allow for expansion of personnel, purchase of badly needed new equipment, and greater sky coverage. Consequently, the discovery rate of ECAs should double to quadruple, thereby also increasing our confidence in modeling the population of such objects and planning the requirements for operation of the full-up survey. (3) In order to gain additional experience with the kind of automated CCD scanning techniques proposed for the Spaceguard Survey, efforts should be made as soon as possible to place in operation a telescope that utilizes these techniques; one such option is the proposed 1.8-m Spacewatch telescope at the University of Arizona. Efforts are also required in studying the use of CCD arrays and in developing appropriate software to support CCD scanning. (4) Continuing support should be provided for research on near-Earth asteroids and comets, including their dynamics and their physical properties. For purposes of this study, we assume an increase of $2 million/year beyond current NASA expenditures for these programs, to be maintained during the transition period.

Proposed Schedule for NASA Funding

On the assumption that the Spaceguard Program can begin in a modest way in FY 93 and will reach full funding about FY95, we suggest the following possible schedule for new NASA support of this effort

9.3 Conclusions

The Spaceguard Survey has been optimized for the discovery and tracking the larger ECAs, which constitute the greater part of the cosmic impact hazard. If any large ECAs threaten impact with the Earth, they could be discovered with ample lead-time to take corrective action. The Spaceguard system also will discover most incoming long-period comets, but the warning time may be only a few months. Finally, the great majority of the new objects discovered by the Spaceguard Survey will have diameters of less than 1 km; these should be picked up at a rate of about a thousand per month. It is therefore reasonably likely that even the "next Tunguska" projectile (20 megatons energy) will be found by the Spaceguard Survey if it is continued for a century or more.

The Spaceguard Survey should be supported and operated on an international basis, with contributions from many nations. The total costs for this system are of the order of $50 million in capital equipment, primarily for the six survey telescopes, and $10-15 million per year in continuing operating support. However, these estimates will vary depending on the aperture and detailed design of each telescope, the nature of the international distribution of effort, and the management of the survey. In particular, larger telescopes would be appropriate if greater emphasis is to be given to the search for long period comets. Whatever the exact cost, however, the proposed system can provide, within one decade of its initial operation, a reduction in the risk of an unexpected large impact of about 50 percent at a relatively modest cost. Of course, additional and much greater expenditure would be required to deflect an incoming object if one should be discovered on an impact trajectory with the Earth, but in that unlikely event the cost and effort would surely be worth it. The first and essential step is that addressed by the Spaceguard Survey: to carry out a comprehensive survey of near-Earth space in order to assess the population of near-Earth asteroids and comets and to identify any potentially hazardous objects.

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