Assessing the Asteroid Threat
6 Sep 2002
(Source: National Optical Astronomy Observatory)
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National Optical Astronomy Observatory
Prof. Erik Asphaug
University of California at Santa Cruz
Direct measurements of the surface properties and interior structures of asteroids and comets should be fundamental elements of future spacecraft missions to these primitive solar system bodies, according to participants in a scientific workshop held in Arlington, VA, from September 3-6.
Such information is vitally important for preparing a variety of approaches for the diversion of Near-Earth Objects which may someday threaten Earth. Evidence presented at the workshop suggests that gentle thrusts applied for decades, rather than traditional explosives, are likely to be needed to change their orbital paths. This will require early detection together with knowledge of their geologic properties.
Sponsored by NASA, the workshop was designed to find common ground among researchers on the reconnaissance and exploration of Near-Earth Objects. "Unlike volcanoes or earthquakes, the NEO hazard was only recently identified, and we have just begun to understand its implications," said meeting organizer Erik Asphaug of the University of California at Santa Cruz. "This is the only major natural hazard which can, in principle, be made predictable and even eliminated if we find the dangerous ones and learn how to modify their orbits over time."
Astronomers have determined precise orbits and estimated the sizes of approximately 1,500 Near-Earth Objects (NEOs), according to conference presentations. More than 600 of the estimated 1,000 asteroids larger than one kilometer in diameter (a size that could cause widespread calamity on Earth) have been detected so far. This represents good progress toward the goal mandated by Congress for NASA to discover 90% of these objects by 2008. While no known asteroid is on collision course with Earth, ongoing detection should alert us to serious threats.
Significant topics of discussion at the workshop included large uncertainties in the state of scientific knowledge of asteroid surfaces, despite great advances in recent years. There is increasing evidence that most asteroids larger than a few hundred meters have complex interiors and may be loosely bound conglomerates which might resist explosive diversion. To almost everyone's surprise, about a sixth of NEOs are now observed to have moons, which would complicate any effort to change their orbits.
While scientific goals of researching the early history of the solar system and mitigation goals of protecting the Earth are very different, the kinds of asteroid studies needed to address both goals are largely identical, several participants noted. "Learning more about them is the first step," Asphaug said. Gathering a wide variety of measurements is critical for fully understanding the history and properties of NEOs, given their great diversity and their many observed dissimilarities from presumed analogues like the surface of the Moon.
Because we know so little, physical characterization was seen by researchers as going hand-in-hand with potentially useful technological developments. For example, a large, lightweight solar concentrator was discussed that could vaporize a small surface area for measurements of composition; thrust from the escaping material could be measured to test concepts for solar-powered asteroid deflection.
Because close-calls are far more likely than actual impacts, attendees also discussed the deployment of radio transponders for precision tracking of dangerous objects. Many researchers expressed the need for high-performance propulsion systems that could power a spacecraft to a rapid rendezvous with an NEO.
Ground-based observatories such as the proposed 8.4-meter Large Synoptic Survey Telescope (a high priority in the most recent Decadal Survey of astronomy by the National Academy of Sciences) can be effective tools to detect 80-90% of the NEO population down to a diameter of 300 meters within about a decade of full-time operations. A spacecraft orbiting close to the Sun and looking outward in tandem with such a telescope might reduce this time to five years. NEOs in this size range can cause widespread regional damage on Earth, although the workshop scientists agreed that the detailed effects of impacts of any size remain poorly understood.
Ground-based radar observations of close-approaching NEOs will also remain a uniquely important and flexible method to study a variety of objects, attendees agreed. Radar is capable of imaging and accurately tracking the closest Earth-approachers.
Few countries outside of the United States are spending significant resources on the NEO hazard, and this international imbalance must be remedied if the threat is to be fully understood within the next few decades, according to several speakers. For example, there are currently no active ground-based NEO searches in the Southern Hemisphere. Despite the spectacular success of NASA's recently concluded Near Earth Asteroid Rendezvous mission, and excitement surrounding Japan's upcoming MUSES-C mission (the first-ever sample return from an asteroid, to be launched in December), researchers agreed that more substantial investigations are required if we are to learn how to change an asteroid's orbit.
Scientists must take better advantage of opportunities to explain new detections and their related risks to the media and the public, attendees agreed. With advanced search systems coming on line, asteroids will be discovered at an increasing rate, with orbits which may initially appear dangerous. Only detailed follow-up on a case-by-case basis can prove each new discovery to be non-threatening. This process must be communicated more carefully, scientists agreed, in the manner that hurricanes are tracked by the weather service until the "all-clear" is announced.
The workshop was attended by 70 scientists from the United States, Australia, Europe and Japan. It was co-sponsored by Ball Aerospace, Science Applications International Corp., Lockheed Martin Corp., the National Optical Astronomy Observatory and the University of Maryland. A formal report on the workshop will be submitted to NASA by the end of 2002.