The Spaceguard Survey

Executive Summary

Impacts by Earth-approaching asteroids and comets pose a significant hazard to life and property.  Although the annual probability of the Earth being struck by a large asteroid or comet is extremely small, the consequences of such a collision are so catastrophic that it is prudent to assess the nature of the threat and prepare to deal with it.
The first step in any program for the prevention or mitigation of impact catastrophes must involve a comprehensive search for Earth-crossing asteroids and comets and a detailed analysis of their orbits. At the request of the U.S. Congress, NASA has carried out a preliminary study to define a program for dramatically increasing the detection rate of Earth-crossing objects, as documented in this Workshop Report.

The greatest risk from cosmic impacts is associated with objects large enough to perturb the Earth's climate on a global scale by injecting large quantities of dust into the stratosphere.
Such an event could depress temperatures around the globe, leading to massive loss of food crops and possible breakdown of society. Such global catastrophes are qualitatively different from other more common hazards that we face (excepting nuclear war), because of their potential effect on the entire planet and its population.
Various studies have suggested that the minimum mass impacting body to produce such global consequences is several tens of billions of tons, resulting in a groundburst explosion with energy in the vicinity of a million megatons of TNT.
The corresponding threshold diameter for Earth-crossing asteroids or comets is between 1 and 2 km . Smaller objects (down to tens of meters diameter) can cause severe local damage but pose no global threat.

Current technology permits us to discover and track nearly all asteroids or short-period comets larger than 1 km diameter that are potential Earth-impactors. These objects are readily detected with moderate-size ground-based telescopes. Most of what we now know about the population of Earth-crossing asteroids (ECAs) has been derived over the past two decades from studies carried out by a few dedicated observing teams using small ground-based telescopes.
Currently several new ECAs are discovered each month. At this rate, however, it will require more than a century to approach a complete survey, even for the larger objects. What is required to assess the population of ECAs and identify any large objects that could impact the Earth is a systematic survey that effectively monitors a large volume of space around our planet and detects these objects as their orbits repeatedly carry them through this volume of space. In addition, the survey should deal with the long-period comets, which are thought to constitute about 10 percent of the flux of Earth impacts. Long-period comets do not regularly enter near-Earth space; however, nearly all Earth-impacting long-period comets could be detected with advance warning on the order of a year before impact with the same telescopes used for the ECA survey. Finally, it is desirable to discover as many of the smaller potential impactors as possible.

No object now known has an orbit that will lead to a collision with our planet during the next century, and the vast majority of the newly discovered asteroids and comets will also be found to pose no near-term danger.
Even if an ECA has an orbit that might lead to an impact, it will typically make hundreds of moderately near passes before there is any danger, providing ample time for response. However, the lead time will be much less for a new comet approaching the Earth on a long-period orbit, as noted above.

The survey outlined in this report involves a coordinated international network of specialized ground-based telescopes for discovery, confirmation, and follow-up observations. Observations are required from both the northern and southern hemispheres, monitoring about 6000 square degrees of sky per month.
In order to provide reliable detection of objects as small as 1 km diameter over a suitably large volume of space, the telescopes should reach astronomical magnitude 22.  The telescopes that are suitable to this survey have apertures of 2-3 meters, moderately wide fields of view (2-3 degrees), focal-plane arrays of large-format CCD detectors, and automated signal processing and detection systems that recognize the asteroids and comets from their motion against the background of stars. The technology for such automated survey telescopes has been demonstrated by the 0.9-m Spacewatch telescope of the University of Arizona.
For purposes of this study, we focus on a Spaceguard Survey network of six 2.5-m aperture, f/2 prime focus reflecting telescopes each with four 2048x2048 CCD chips in the focal plane.

In addition to the discovery and verification of new Earth-approaching asteroids and comets, the Spaceguard Survey program will require follow-up observations to refine orbits, determine the sizes of newly-discovered objects, and establish the physical properties of the asteroid and comet population.
Observations with large planetary radars are an especially effective tool for the rapid determination of accurate orbits, but are not useful as a primary search method because of their limited range. Potentially hazardous objects will require radar data in order to ensure that they will miss the Earth or, if this is not the case, to determine the exact time and location of the impact. Desirable for this program would be increased access to currently operational planetary radars in California and Puerto Rico, and provision of a suitable southern-hemisphere radar in the future.
We anticipate that much of the optical follow-up work can be accomplished with the survey telescopes themselves if they are suitably instrumented, although one or more dedicated follow-up telescopes would greatly improve our ability to study faint and distance asteroids and comets.
The survey program also requires rapid international electronic communications and a central organization for coordination of observing programs and maintenance of a database of discovered objects and their orbits.

Numerical modeling of the operation of the Spaceguard Survey network indicates that as many as a thousand ECAs will be discovered per month. Over a period of two decades we will identify more than 90 percent of potentially threatening ECAs larger than 1 km in diameter, as well as detecting most incoming comets about a year before they approach the Earth.
At the same time, tens of thousands of smaller asteroids (down to a few meters in diameter) will also be discovered, although the completeness of the survey declines markedly for objects smaller than about 500 m.
The advantage of this survey approach is that it achieves the greatest level of completeness for the largest and most dangerous objects; however, if continued for a long period of time, it will provide the foundation for assessing the risk posed by smaller impacts as well. Continued monitoring of the sky will also be needed to provide an alert for potentially hazardous long-period comets.

The survey can begin with current programs in the United States and other countries, which are providing an initial characterization of the ECA population and can serve as a test bed for the technologies proposed for the new and larger survey telescopes. A modest injection of new funds into current programs could also increase current discovery rates by a factor of two or more, as well as provide training for personnel that will be needed to operate the new survey network.
For the new telescopes, we assume the use of modern technology that has, over the past decade, substantially reduced the construction costs of telescopes of this aperture. The initial cost to build six 2.5-m telescopes and to establish a center for program coordination is estimated to be about $50M (FY93 dollars), with additional operating expenses for the network of about $10M per year. If construction were begun in FY93, the survey could be in operation by about 1997. Over the first decade of operation (to 2007), the survey would require appropriations approaching $100M, perhaps half of which could be provided by the United States and half by international partners.

The international survey program described in this report can be thought of as a modest investment to insure our planet against the ultimate catastrophe. The probability of a major impact during the next century is very small, but the consequences of such an impact, especially if the object is larger than about 1 km diameter, are sufficiently terrible to warrant serious consideration.
The Spaceguard Survey is an essential step toward a program of risk reduction that can reduce the risk from cosmic impacts by up to 75 percent over the next 25 years.