On a daily basis, about one hundred tons of interplanetary material drifts down to the
Earth's surface. Most of the smallest interplanetary particles that reach the Earth's
surface are the tiny dust particles that are released by comets as their ices vaporize in the
solar neighborhood. The vast majority of the larger interplanetary material that reaches
the Earth's surface originates as the collision fragments of asteroids that have run into one
another some eons ago.
With an average interval of about 10,000 years, rocky or iron
asteroids larger than about 100 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 several 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 mitigation 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.