Objects (NEOs) are comets and asteroids that have been
nudged by the gravitational attraction of nearby planets into
orbits that allow them to enter the Earth's neighborhood. Composed
mostly of water ice with embedded dust particles, comets
originally formed in the cold outer planetary system while most of
the rocky asteroids formed in the warmer inner solar system
between the orbits of Mars and Jupiter. The scientific interest in
comets and asteroids is due largely to their status as the
relatively unchanged remnant debris from the solar system
formation process some 4.6 billion years ago. The giant outer
planets (Jupiter, Saturn, Uranus, and Neptune) formed from an
agglomeration of billions of comets and the left over bits and
pieces from this formation process are the comets we see today.
Likewise, today's asteroids are the bits and pieces left over from
the initial agglomeration of the inner planets that include
Mercury, Venus, Earth, and Mars.
As the primitive, leftover
building blocks of the solar system formation process, comets and
asteroids offer clues to the chemical mixture from which the
planets formed some 4.6 billion years ago. If we wish to know the
composition of the primordial mixture from which the planets
formed, then we must determine the chemical constituents of the
leftover debris from this formation process - the comets and
In terms of orbital
elements, NEOs are asteroids and comets with perihelion distance q
less than 1.3 AU. Near-Earth Comets (NECs) are further restricted
to include only short-period comets (i.e orbital period P less
than 200 years). The vast majority of NEOs are asteroids, referred
to as Near-Earth Asteroids (NEAs). NEAs are divided into groups (Aten,
Apollo, Amor) according to their perihelion distance (q), aphelion
distance (Q) and their semi-major axes (a).
||q<1.3 AU, P<200 years
||Earth-crossing NEAs with semi-major axes
smaller than Earth's (named after asteroid 2062 Aten).
||a<1.0 AU, Q>0.983
||Earth-crossing NEAs with semi-major axes
larger than Earth's (named after asteroid 1862 Apollo).
||a>1.0 AU, q<1.017
||Earth-approaching NEAs with orbits
exterior to Earth's but interior to Mars' (named after
asteroid 1221 Amor).
||a>1.0 AU, 1.017<q<1.3
||Potentially Hazardous Asteriods:
NEAs whose Minimum Orbit Intersection Distance (MOID)
with the Earth is 0.05 AU or less and whose
absolute magnitude (H) is 22.0 or brighter.
||MOID<=0.05 AU, H<=22.0
(NEOs) are small objects 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 (ECOs).
atmosphere protects us from most NEOs smaller than a modest office
building (40 m diameter, or impact energy of about 3 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. The probable consequence
would be an "impact winter" with loss of crops worldwide
and subsequent starvation and disease. 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
There are many more
small NEOs than large ones. Astronomers estimate that there are
approximately 1100 Near Earth Asteroids (NEAs) larger than 1 km in
diameter, and more than a million larger than 40 m in diameter
(the approximate threshold for penetration through the Earth's
atmosphere). The largest NEAs are less than 25 km in diameter.
There are probably many more comets than NEAs, but they spend
almost all of their lifetimes at great distances from the Sun and
Earth, so that they contribute only about 10% to the census of
larger objects that strike the Earth, and probably less than 1% of
NEOs less than 1 km in diameter.
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 survey is the LINEAR search
program of the MIT Lincoln Lab, carried out in New Mexico with US
Air Force and NASA support. The LINEAR team, which operates two
search telescopes with one-meter aperture, discovers more
asteroids than all the other searches combined. Other active
survey groups include the NEAT search program in Hawaii, carried
out jointly by the NASA Jet Propulsion Lab and the US Air Force;
the Spacewatch survey at the University of Arizona, funded by NASA
and a variety of private grants, the LONEOS survey at Lowell
Observatory in Flagstaff Arizona, supported by NASA grants, and
the Catalina Sky Survey in Tucson Arizona, also supported by NASA.
Other astronomers (many of them amateurs) follow up the
discoveries with supporting observations.
As of the end of
2004, astronomers had discovered more than two thirds of the
larger Near Earth Asteroids (diameter greater than 1 km). None of
the known asteroids is a threat, but we have no way of predicting
the next impact from an unknown object.
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 40,000 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 placeónext
year or a million years from now. The purpose of the Spaceguard
Survey is not to improve these statistical estimates, but to find
any individual rock that may be on a collision course.
With so many 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 the current surveys actually discover a NEO
on a collision course, we would expect many decades of warning.
Any NEO that is going to hit the Earth will swing near our planet
many times before it hits, and it should be discovered by
comprehensive sky searches like Spaceguard. In almost all cases,
we will either have a long lead time or none at all.
Some press reports
express concern that an asteroid could hit the Earth coming out of
a "blind spot", such as the daylight sky or high
southern latitudes where no Spaceguard telescopes are looking.
Some worry that if an asteroid is found after its closest approach
to Earth, this is an indication that the system is not working.
These concerns seem to be based on the misconception that we are
trying to detect asteroids as they approach the Earth on their
final plunge toward impact. In fact, any such last-minute warning
system is impractical as well as unproductive. In this survey, it
makes no difference if a NEA is discovered on approach or
departure from the vicinity of the Earth. The important thing is
that it is discovered and its orbit determined. The only effect of
blind spots, whether they be due to sunlight or moonlight or bad
weather or lack of a southern hemisphere survey telescope, is to
slow down the completion of the NEA catalog. Objects in blind
spots will be picked up later, usually within a few years, in a
more favorable geometry.
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.
The US Congress
held hearings to investigate the impact hazard in 1993, 1998, and
2002, and both NASA and the US Air Force are supporting surveys to
discover NEOs. In 1998 NASA formally initiated the Spaceguard
Survey by adopting the objective of finding 90% of the NEAs larger
than 1 km diameter within the next decade (that is, before the end
of 2008). In 1998 NASA also created a NEO Program Office, and
today $3-4 million per year is being spent on NASA-supported NEO
searches and orbit calculations. 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 promotes NEO surveys
internationally, and further interest on an international basis is
provided by the International Astronomical Union, the
International Council of Scientific Unions, and the United
Survey and most associated search and tracking programs are
concentrating on NEAs larger than 1 km in diameter -- large enough
to risk a global ecological catastrophe if one of them hit the
Earth. But there are many more smaller undiscovered NEAs, and we
are likely to be hit somewhere on Earth by one of these, with an
energy equivalent to a large nuclear bomb, sometime in the next
couple of centuries. The last such impact was in 1908 in Tunguska
( Siberia ) with an estimated explosive energy of 15 megatons. The
actual risk to each of us from Tunguska-like impacts is very small
-- much less than the risk of larger impacts, and indeed much less
than the risk from many common natural hazards, such as
earthquakes and severe storms. Nevertheless, there are people who
are interested in this problem. In 2003 NASA completed a study of
these sub-km impacts and concluded that it was both technically
feasible and cost-effective to to mount an expanded Spaceguard
Survey, with much larger telescopes, to search for these smaller
Hazardous Asteroids (PHAs) are space rocks larger than
approximately 100m that can come closer to Earth than 0.05 AU.
None of the known PHAs is on a collision course with our planet,
although astronomers are finding new ones all the time.
Propulsion Laboratory is introducing a new Web site that will
provide a centralized resource for information on near-Earth
objects - those asteroids and comets that can approach Earth.
on the logo below to visit the site-
The B612 is a non-profit organization founded by a
group of astronauts and scientists whose goal is to predict and
prevent catastrophic asteroid impacts on Earth. B612 aims to find
potentially threatening asteroids, track their trajectories, and
to demonstrate the technology to alter the orbit of an asteroid in
a controlled manner.
Credit: NASA, B612 Foundation