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Asteroids
Image courtesy of
NASA
Near-Earth Objects (NEOs)
are comets and asteroids that have been nudged into orbits
that allow them to enter the Earth's neighborhood.
With an average interval of
about 100 years, rocky or iron asteroids larger than about
50 meters would be expected to reach the Earth's surface
and cause local disasters or produce the tidal waves that
can inundate low lying coastal areas. On an average of
every few hundred thousand years or so, asteroids larger
than a kilometer could cause global disasters. In this
case, the impact debris would spread throughout the
Earth's atmosphere so that plant life would suffer from
acid rain, partial blocking of sunlight, and from the
firestorms resulting from heated impact debris raining
back down upon the Earth's surface. Since their orbital
paths often cross that of the Earth, collisions with
near-Earth objects have occurred in the past and we should
remain alert to the possibility of future close Earth
approaches. It seems prudent to mount efforts to discover
and study these objects, to characterize their sizes,
compositions and structures and to keep an eye upon their
future trajectories.
Because of the ongoing search efforts to find nearly all
the large NEOs, objects will occasionally be found to be
on very close Earth approaching trajectories. Great care
must then be taken to verify any Earth collision
predictions that are made. Given the extremely unlikely
nature of such a collision, almost all of these
predictions will turn out to be false alarms. However, if
an object is verified to be on an Earth colliding
trajectory, it seems likely that this collision
possibility will be known several years prior to the
actual event. Given several years warning time, existing
technology could be used to deflect the threatening object
away from Earth.
The key point in this process
is to find the threatening object years ahead of time so
that an orderly international campaign can be mounted to
send spacecraft to the threatening object. One of the
techniques suggested for deflecting an asteroid includes
nuclear fusion weapons set off above the surface to
slightly change the asteroid's velocity without fracturing
it. High speed neutrons from the explosion would irradiate
a shell of material on the surface of the asteroid facing
the explosion. The material in this surface shell would
then expand and blow off, thus producing a recoil upon the
asteroid itself. A very modest velocity change in the
asteroid's motion (only a few millimeters per second),
acting over several years, can cause the asteroid to miss
the Earth entirely. However, the trick is to gently nudge
the asteroid out of harm's way and not to blow it up. This
latter option, though popular in the movies, only creates
a bigger problem when all the pieces encounter the Earth.
Another option that has been discussed includes the
establishment of large solar sails on a small threatening
object so that the pressure of sunlight could eventually
redirect the object away from its predicted Earth
collision.
No one should be overly concerned about an Earth impact of
an asteroid or comet. The threat to any one person from
auto accidents, disease, other natural disasters and a
variety of other problems is much higher than the threat
from NEOs. Over long periods of time, however, the chances
of the Earth being impacted are not negligible so that
some form of NEO insurance is warranted. At the moment,
our best insurance rests with the NEO scientists and their
efforts to first find these objects and then track their
motions into the future. We need to first find them, then
keep an eye on them.
Click here for a list of
near earth objects being tracked at NASA and their
probability of an impact with earth.
Links
The Sky Is Falling
Closer encounter: Nasa plans landing on 40m-wide asteroid
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