FAQs: Ten Frequently Asked Questions about NEO Impacts
What is an NEO?
Near-Earth-Objects (NEOs) are small bodies in the solar system (asteroids
and short-period comets) with orbits that regularly bring them close to the
Earth and which, therefore, are capable someday of striking our planet.
Sometimes the term NEO is also used loosely to include all comets (not just
short-period ones) that cross the Earth's orbit. Those NEOs with orbits
that actually intersect the Earth's orbit are called Earth-Crossing-Objects
What size NEOs are dangerous?
The Earth's atmosphere protects us from most NEOs smaller than a modest
office building (50 m diameter, or impact energy of about 5 megatons).
>From this size up to about 1 km diameter, an impacting NEO can do
tremendous damage on a local scale. Above an energy of a million megatons
(diameter about 2 km), an impact will produce severe environmental damage
on a global scale. Still larger impacts can cause mass extinctions, like
the one that ended the age of the dinosaurs 65 million years ago (15 km
diameter and about 100 million megatons).
How many NEOs exist?
There are many more small NEOs than large ones. Astronomers estimate that
there are approximately 2000 NEOs larger than 1 km in diameter, and more
than a million larger than 50 m in diameter (the threshold for penetration
through the Earth's atmosphere). The largest NEOs are less than 25 km in
Who is searching for NEOs?
Several teams of astronomers worldwide are surveying the sky with
electronic cameras to find NEOs, but the total effort involves fewer than
100 people. The most productive NEO surveys in 1997-98 are: the LINEAR
search program of the MIT Lincoln Lab, carried out in New Mexico with US
Air Force support; the NEAT search program in Hawaii, carried out jointly
by the NASA Jet Propulsion Lab and the US Air Force; and the Spacewatch
survey at the University of Arizona, funded by NASA and a variety of
private grants. Other searches in the US, France, Japan and China also
contribute to discovery of NEOs, while additional astronomers follow up the
discoveries with supporting observations.
Are any NEOs predicted to hit the Earth?
The Earth has been hit throughout its history, and certainly it will be in
the future. But none of the known NEOs is on a collision course with Earth.
All known NEOs and their predicted future positions are openly available to
everyone with access to the Internet. The problem is that astronomers have
discovered only about 10% of even the larger NEOs (diameter greater than 1
km). So 90% of them remain unknown, and we have no way of predicting the
next impact from an unknown object.
Will asteroid 1997XF11 hit the Earth in 2028 as predicted?
No! There was never a prediction that XF11 (or any other asteroid) will
hit the Earth. In March 1998 one astronomer told the press that XF11 would
come close to the Earth in 2028 and that a collision could not be ruled
out, but fortunately better calculations and additional observations
quickly revealed that there is no risk of a collision.
What is the risk of impacts?
We don't know when the next NEO impact will take place, but we can
calculate the odds. Statistically, the greatest danger is from an NEO with
about 1 million megatons energy (roughly 2 km in diameter). On average, one
of these collides with the Earth once or twice per million years, producing
a global catastrophe that would kill a substantial (but unknown) fraction
of the Earth's human population. Reduced to personal terms, this means that
you have about one chance in 20,000 of dying as a result of a collision.
Such statistics are interesting, but they don't tell you, of course, when
the next catastrophic impact will take place -- next year or a million
years from now.
How much warning will we have?
With 90% of even the larger NEOs remaining undiscovered, the most likely
warning today would be zero -- the first indication of a collision would be
the flash of light and the shaking of the ground as it hit. In contrast, if
a survey is carried out and NEO orbits are calculated, we would expect many
decades of warning. This is the purpose of the proposed Spaceguard Survey.
In almost all cases, we will either have a long lead time or none at all.
How can we protect ourselves?
NEO impacts are the only major natural hazard that we can effectively
protect ourselves against, by deflecting (or destroying) the NEO before it
hits the Earth. The first step in any program of planetary defense is to
find the NEOs; we can't protect against something we don't know exists. We
also need a long warning time, at least a decade, to send spacecraft to
intercept the object and deflect it. Many defensive schemes have been
studied in a preliminary way, but none in detail. In the absence of active
defense, warning of the time and place of an impact would at least allow us
to store food and supplies and to evacuate regions near ground zero where
damage would be the greatest.
What is the government doing about it?
The US Congress has held hearings to study the impact hazard (in 1993 and
1998), and both NASA and the US Air Force are supporting surveys to
discover NEOs. In 1997 NASA adopted the objective of finding 90% of the
NEOs larger than 1 km diameter within the next decade. In 1998 NASA created
a NEO Program Office, and it is expected that at least $3 million per year
will be spent on NASA-supported NEO searches and orbit calculations. At
current (1998) discovery rates, however, the surveys will require a
century, not a decade, to achieve 90% completeness. In addition, the US
Department of Defense is studying a space mission called Clementine 2 to
test the technology for intercepting an NEO. Other governments have
expressed concern about the NEO hazard, but none has yet funded any
extensive surveys or related defense research. A private Spaceguard
Foundation based in Europe also promotes NEO surveys on an international
David Morrison, September 1998
Department of Space Studies
Southwest Research Institute
1050 Walnut St, Suite 426
Boulder, CO 80302
Tel: (303) 546-9670
Fax: (303) 546-9687