Space Security Index

Space Security 2011
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Trend 1.4: Increased recognition of the threat from NEO
collisions and progress toward possible solutions
Near Earth Objects are asteroids and comets whose orbits bring them in close proximity
to the Earth or intersect the Earth’s orbit. NEOs are subdivided into Near Earth Asteroids
(NEAs) and Near Earth Comets (NECs). Within both groupings are Potentially Hazardous
Objects (PHOs), those NEOs whose orbits intersect that of Earth and have a relatively
high potential of impacting the Earth itself. As comets represent a very small portion of the
overall collision threat in terms of probability, most NEO researchers commonly focus on
Potentially Hazardous Asteroids (PHA) instead. A PHA is de ned as an asteroid whose orbit
comes within 0.05 astronomical units of the Earth’s orbit and has a brightness magnitude
greater than 22 (approximately 150 m in diameter). 112
Initial e orts to nd threatening NEOs focused on the so-called “civilization-killer” class,
which are NEOs 1 km in diameter or larger. In 1998, NASA agreed to undertake a survey
to discover 90 per cent of these objects by 2008. Of the estimated 1,100 objects in this
class, NASA tracks approximately 80 per cent. 113 In 2003, a NASA Science De nition
Team published a report that recommended the search be extended to include all NEOs
down to 140 m in diameter. 114 Impacts of this class of objects would have the potential to
wipe out regions of the Earth’s surface. Discovery of these objects, along with those over 1
km in diameter, would identify around 90 per cent of the risk the Earth faces from NEO
collisions. 115
Figure 1.9: Number of large* NEAs discovered by year (2001-2010) 116
* 1 kilometer in diameter or larger
ere is now a growing consensus that the greatest threat is not from asteroids that can
destroy the entire Earth, but those that have the potential to destroy large areas such as
cities. ese are objects approximately 45 m in diameter, one of which caused the Tunguska
explosion in Siberia in 1908. Researchers estimate that there are over 700,000 NEOs of
this size, of which approximately three per cent are estimated to pose some sort of threat of
impact. 117 Although objects of that size cause much less damage, they impact the Earth at a
much higher frequency than kilometer-sized objects.
Technical research is ongoing into ways of mitigating a NEO collision with the Earth. is
is proving to be a di cult challenge due to the extreme mass, velocity, and distance of any
impacting NEO. Mitigation methods are divided into two categories depending on how
much warning time there is for a potential impact event. If the warning times are in the