THE HAZARD PRESENTED BY SMALL ASTEROIDS AND STRATEGIES FOR
DETECTING THEM
Programs are currently in place to discover potentially hazardous Earth-crossing asteroids
(PHAs) larger than one kilometer in diameter. The census should reach its goal of being
approximately 90 percent complete by 2008.
Two surveys of scientific priorities for the next decade conducted by the National Research
Council (Astronomy and Astrophysics in the New Millennium and New Frontiers in the Solar
System) have recommended that discovery be extended to asteroids of smaller size.
The following statement summarizes the reasons for this extended survey and a potential strategy
for carrying it out.
HAZARD PRESENTED BY ASTEROIDS LESS THAN ONE KILOMETER IN DIAMETER
Asteroids with sizes smaller than one kilometer in diameter present a significant hazard. While
larger asteroids cause more damage per event, they also occur much less frequently. Somewhere
in a size range above 1 km (0.6 miles), impacts cause global environmental effects that can put
the entire population of the Earth at risk, even those that are a hemisphere away from the impact
site. Such events may occur only once in a million years. In contrast, a "Tunguska-sized"
impact, like the one that occurred over Siberia in 1908, occurs perhaps once per 1,000 years.
The quantitative assessment of the hazard presented by smaller asteroids is currently in progress.
The actual damage that will occur depends on a number of factors, including not only the size of
the asteroid and the frequency of occurrence but also on modeling of such effects as the blast
damage, earthquakes, fire, and tsunamis. The extent of the damage also depends on the
composition of the asteroid and the angle at which it impacts the atmosphere.
The large damage that would be caused by the impact of an asteroid larger than 1 km in diameter
is the concern of the present "Spaceguard Survey." With the technology at hand a decade ago,
when Congress first called for a study of the impact hazard, the cost to conduct a survey of these
most hazardous large NEOs were found to be worth the projected savings in terms of the risk
posed by such large impacts. That task is now well underway, and much improved technology
has become available. Accordingly, it is appropriate to ask whether it is worth implementing a
next generation survey to find smaller and less hazardous, but more frequent impactors.
The smallest asteroid that can penetrate the atmosphere to or near enough to the ground to cause
damage is about 50 meters (150 ft) in diameter. Smaller objects generally explode high in the
atmosphere, at most dropping a few small fragments (meteorites) to the ground nearly without
harm. There is no recommendation from studies by the National Research Council at the present
time to search for asteroids smaller than 200-300 meters because of the limited potential for
damage and because most of the time such small asteroids are too far away and therefore too
faint to be detected with current techniques. Systematic searches for larger objects will continue
to discover asteroids in this small size range when they venture close to the Earth.
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Accordingly, the appropriate size range for a new survey for potentially hazardous asteroids is
200-300 meters to one kilometer
STRATEGY FOR DISCOVERING
METERS TO 1 KILOMETER
AND
CATALOGUING ASTEROIDS
IN THE
SIZE RANGE 200-300
In order to extend the "Spaceguard Goal" (90% completion in about 10 years of surveying) to a
smaller size of ~300 meters it is necessary to meet three requirements. First, it must be possible
to reach a limiting visual magnitude of 24 according to the NRC report. Second, this limiting
magnitude must be reached with a short exposure time (ideally less than 20 seconds). For fixed
objects, it is possible to lengthen the exposure time in order to reach fainter limiting magnitudes;
moving objects, such as PHAs, however, will move significantly during long exposures and will
be more difficult to detect when their images are elongated. Third, it is necessary to survey a
large area of the sky (several thousand square degrees) six times during each lunation in order to
link the observations of any single asteroid and to derive a preliminary orbit.
Unlike the present Spaceguard survey to about magnitude 19.5, this more ambitious survey
cannot rely on amateur astronomers or other smaller observatories to follow up detections. Not
only are such faint objects beyond the range of most other telescopes, but the sheer number of
objects to be tracked becomes so great that every field imaged contains multiple objects
requiring follow-up. Thus the only practical scheme is for the survey telescope to cover the
entire sky multiple times per month, thereby providing the data both for discovery and for
tracking and orbit determination. The National Optical Astronomy Observatory has funded a
study to model the cadence required, optimum exposure times, and methods of prompt data
processing.
The Large Synoptic Survey Telescope (LSST), recommended by name by the two NRC reports,
is being designed to achieve these goals. It has a large aperture (8.3 m) and will reach V = 24 in
20 seconds; it has a wide field of view and can survey the entire visible sky in a few days; and it
will be able to slew and settle quickly in order to maximize throughput. The community of
interested scientists is developing a costed conceptual design for the LSST that will meet the
goal of creating, over about a decade of observing, a catalog of PHAs that is about 90 percent
complete down to diameters of 200-300 meters, with the exact details of completeness and
limiting size to be determined after more detailed modeling of the predicted population of PHAs
and of LSST performance.
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