A Second Line of Defense
The desire of the technical community is to identify a threatening asteroid or comet decades prior to impact and then develop and employ systems to either destroy the object or alter the object’s trajectory to avoid a collision with Earth. The goal of the Spaceguard program is to identify the most destructive asteroids and comets, those greater than 1 kilometer in diameter that threaten the Earth. This is the best outcome, the best approach and the best plan. But inherent weaknesses exist in any plan. As a result, we should ensure that a second line of defense, the ability to evacuate a threatened impact area, is in place.
As the size of the asteroids decreases, the quantity of Earth threatening asteroids increases. As a result, there are a large number of asteroids that may never be picked up and tracked under the Spaceguard program. Therefore it is quite possible that an asteroid (< 1km) may impact Earth with very little advanced warning.
I thought it might be worthwhile to describe a comet/asteroid impact scenario. The best way I can do this is by comparing it to a hurricane.
Predicting hurricanes is a tricky business. There are many variables involved in making predictions. We might initially spot the hurricane forming from satellite photographs. We may send specialized aircraft into a forming hurricane to extract information that will aid in the assessment. Finally as the hurricane approaches the coastline, the government will issue general warnings covering a large area. During the next few hours, the data resolution becomes sufficiently accurate to make a projection of where the hurricane will make land and a specific warning is issued which defines a narrow band of coastline as the target. The affected area is evacuated.
I expect an impact scenario may play out in a similar manner. Asteroids and comets are discovered using optical telescopes. Over time, additional optical sightings made during the discovery apparition or later apparitions allow us to make crude predictions about orbital path and impact probability. At some point in time as further sightings from optical telescopes are added into the data set, the asteroid trajectory is assessed to have a substantial possibility of impact. At this point, another tool is brought into service. Delay-Doppler radar is used to reduce post-discovery uncertainty and refine the asteroid trajectory. Radar provides several orders of magnitude finer resolution than obtainable from optical telescopes and also provides information on the asteroid density, shape and internal structure. Because the level of uncertainty is reduced dramatically, the probability-of-impact equations begin to produce a very high probability of impact. The threat is recognized and mitigation efforts to deflect or destroy the incoming comet/asteroid are undertaken. If the mitigation effort fails, the next option is to fall back on a second line of defense, evacuation. Delay-Doppler radar is a key and critical element for evacuation planning. Only radar can reduce trajectory uncertainty to the level required to make point-of-impact predictions. Radar data will allow a general warning to be issued describing the potential impact site. As the asteroid approaches Earth, the resolution of the radar data improves and point-of-impact hazard area narrows in size. A detailed warning is issued and the area is evacuated.
It is my belief that the major threat presented by an impact of a small-to-medium asteroid (< 1km) or a small comet fragment is the shock wave, thermal radiation, debris and electromagnetic effects. If individuals can be evacuated outside the 1-psi overburst pressure zone, there is a high probability that their lives can be saved from the impact. In order for this to occur, the point of impact must be determined with a fair degree of accuracy and there must be sufficient warning time to allow evacuation.
Knowing the exact point-of-impact for an ocean impact is also critical. This data would allow analysis of the propagation of the ocean shock wave on the surrounding continents in order to project which coastlines would be affected by a resulting tsunami. From this process, reliable coastal evacuation orders could be given.
I have a major concern: The U.S. has two asteroid/comet radar telescopes: Arecibo and Goldstone. These instruments' radar capabilities and the NEA radar investigations that use them are supported by NASA, but this support has been dramatically reduced in early 2002. The decision to reduce support of this capability undermines the nation's ability to obtain the information needed to predict and mitigate a collision by an asteroid or comet. Radar reconnaissance is as critical to a successful mitigation program as it is to trajectory predictions. It is the only reliable ground-based technique for determining critical properties: size, shape, density, internal structure and whether the object is a binary system. And should mitigation efforts fail, this capability is critical to a second line of defense. The loss of these sites will severely limit the ability to define the point-of-impact with sufficient resolution to issue specific warnings and to implement an evacuation plan.