Researchers Examine Options For Detecting And Countering
Threats From Space
This is not the plot of a movie: a
new report from the National Research Council lays out real options
NASA could follow to detect more near-Earth objects (NEOs) –
asteroids and comets that could pose a hazard if they cross Earth's
orbit. The report says the $4 million the US spends annually
to search for NEOs is insufficient to meet a congressionally
mandated requirement to detect objects that could threaten
Congress mandated in 2005 that NASA discover 90 percent of NEOs
whose diameter is 140 meters or greater by 2020, and asked the
National Research Council in 2008 to form a committee to determine
the optimum approach to doing so. In an interim report
released last year, the committee concluded that it was impossible
for NASA to meet that goal, since Congress has not appropriated new
funds for the survey nor has the administration asked for them.
In its final report, the committee lays out two approaches that
would allow NASA to complete its goal soon after the 2020 deadline;
the approach chosen would depend on the priority policymakers
attach to spotting NEOs. If finishing NASA's survey as close
as possible to the original 2020 deadline is considered most
important, a mission using a space-based telescope conducted in
concert with observations from a suitable ground-based telescope is
the best approach, the report says. If conserving costs is
deemed most important, the use of a ground-based telescope only is
The report also recommends that NASA monitor for smaller objects
– those down to 30 to 50 meters in diameter -- which recent
research suggests can be highly destructive. However, the
report stresses that searching for smaller objects should not
interfere with first fulfilling the mandate from Congress.
Beyond completion of that mandate, the report notes the need for
constant vigilance in monitoring the skies, so as to detect all
dangerous NEOs. In addition, the nation should undertake a
peer-reviewed research program to better investigate the many
unknown aspects connected with detecting NEOs and countering those
that could be a threat. The U.S. should also take the lead in
organizing an international entity to develop a detailed plan for
dealing with hazards from these objects.
In addition, the report recommends that immediate action be
taken to ensure the continued operation of the Arecibo Observatory
in Puerto Rico. NASA and NSF should support a vigorous
program of NEO observations at Arecibo, and NASA should also
support such a program at the Goldstone Deep Space Communications
Complex. Although these facilities cannot discover NEOs, they
play an important role in accurately determining the orbits and
characterizing the properties of NEOs within radar range.
THE SCOPE OF THE HAZARD
Near-Earth objects are asteroids and
comets that orbit the sun and approach or cross Earth's
orbit. An asteroid or comet about 10 kilometers in diameter
struck the Yucatan peninsula 65 million years ago and caused global
devastation, probably wiping out large numbers of plant and animal
species including the dinosaurs. Objects as large as this one
strike Earth only about once every 100 million years on average,
the report notes. NASA has been highly successful at
detecting and tracking objects 1 kilometer in diameter or larger,
and continues to search for these large objects. Objects down to
sizes of about 140 meters in diameter -- which NASA has been
mandated to survey for -- would cause regional damage; such impacts
happen on average every 30,000 years, the report says.
While impacts by large NEOs are rare, a
single impact could inflict extreme damage, raising the classic
problem of how to confront a possibility that is both very rare and
very important. Far more likely are those impacts that cause
only moderate damage and few fatalities. Conducting surveys
for NEOs and detailed studies of ways to mitigate collisions is
best viewed as a form of insurance, the report says. How much
to spend on these insurance premiums is a decision that must be
made by the nation's policymakers.
The report also examines what is known about methods to defend
against NEOs. These methods are new and still immature.
No single approach is effective for the full range of near-Earth
objects, the committee concluded. But with sufficient
warning, a suite of four types of mitigation is adequate to meet
the threat from all NEOs, except the most energetic ones.
- Civil defense (evacuation, sheltering in place, providing
emergency infrastructure) is a cost-effective mitigation measure
for saving lives from the smallest NEO impact events and is a
necessary part of mitigation for larger events.
- "Slow push" or "slow pull" methods use a spacecraft to exert
force on the target object to gradually change its orbit to avoid
collision with the Earth. This technique is practical only
for small NEOs (tens of meters to roughly 100 meters in diameter)
or possibly for medium-sized objects (hundreds of meters), but
would likely require decades of warning. Of the slow
push/pull techniques, the gravity tractor appears to be by far the
closest to technological readiness.
- Kinetic methods, which fly a spacecraft into the NEO to change
its orbit, could defend against moderately sized objects (many
hundreds of meters to 1 kilometer in diameter), but also may
require decades of warning time.
- Nuclear explosions are the only current, practical means for
dealing with large NEOs (diameters greater than 1 kilometer) or as
a backup for smaller ones if other methods were to
Although all of these methods are conceptually valid, none is
now ready to implement on short notice, the report says.
Civil defense and kinetic impactors are probably the closest to
readiness, but even these require additional study prior to
reliance on them.
Given the significant unknowns about many aspects of the threat
and its mitigation, the report recommends that the US start a
peer-reviewed, targeted research program on the hazards posed by
NEOs, and how to deal with them. Because this is a
policy-driven, applied research program, it should not be in
competition with basic scientific research programs or be funded
from them, the report adds.
The study was sponsored by NASA at the request of
Congress. The National Academy of Sciences, National Academy
of Engineering, Institute of Medicine, and National Research
Council make up the National Academies. They are private,
nonprofit institutions that provide science, technology, and health
policy advice under a congressional charter. The Research
Council is the principal operating agency of the National Academy
of Sciences and the National Academy of Engineering.