Science Workshop Reveals Evolving Perspective on Asteroid Threat
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 online,
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 on Scientific Requirements for Mitigation of Hazardous
Comets and Asteroids 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.
CONTACTS:
Douglas Isbell
ublic Information Officer
National Optical Astronomy Observatory
Phone: 520/318-8214
E-mail: disbell@noao.edu
Prof. Erik Asphaug
University of California at Santa Cruz
Phone: 831/459-2260
E-mail: asphaug@es.ucsc.edu
