Airbursts are also detected by the arrays of microbarographic sensors deployed by the DOD and the Comprehensive Test Ban Treaty (CTBT) Organization. This international network, called the International Monitoring System, consists of seismic, infrasound, radionuclide, and hydroacoustic stations. The data are not publicly available; the scientific community would benefit from unfiltered access to the data produced by these arrays.

One of the least understood aspects of the airburst phenomenon is whether and how these events play a role in the formation of tsunamis. There has been significant debate on the effects of ocean impacts, both by direct impact into and by airbursts above the water. Some investigators suspect that an airburst over an ocean may be much more devastating than a similar-sized impact event directly into the water. The modeling of direct oceanic impacts suggests that the impact splash is significant and will be detrimental to those nearby, but that the wavelength of the resultant waves generated is not of sufficient length to cause a tsunami. Other studies suggest on the contrary that even this type of impact may be enough to generate a tsunami-like phenomenon depending on the terrain that such impact-generated waves may encounter. Still others have found that, based on numerical simulations and on data from nuclear oceanic tests, tsunamis are not generated by impact events.

More recent work on airburst events over the ocean suggests that this too is an area of uncertainty. Previous investigations have treated these types of airbursts in a fashion similar to nuclear explosions that deliver their energy from a single point. If this treatment were correct, then the resultant blast waves would not produce a tsunami-type of event. However, a recent study suggests that NEOs entering the upper atmosphere and exploding there act more like a linear series of nearly simultaneous explosions (Boslough and Crawford, 2008). These blast effects are not as localized as those from the single source models, in which the momentum of the object is carried downward into the atmosphere and produces a shock wave. If the shock wave were sufficiently strong to depress a wide area of the ocean’s surface, the resultant rebound effect of the ocean would create a classic tsunami. Hence the threat from small NEO airbursts over the ocean might present their most significant hazard to humanity given that most of the world’s population is concentrated on or near oceanic coastlines.


Finding: U.S. Department of Defense satellites have detected and continue to detect high-altitude airburst events from NEOs entering Earth’s atmosphere. Such data are valuable to the NEO community for assessing NEO hazards.


Recommendation: Data from NEO airburst events observed by the U.S. Department of Defense satellites should be made available to the scientific community to allow it to improve understanding of the NEO hazards to Earth.


Finding: Preliminary theoretical studies on low-altitude atmospheric Tunguska-like airbursts from asteroids as small as 30 meters in diameter suggest that significant risk exists from these NEOs.


Finding: Current models for the generation of tsunamis by impacts into or airbursts above the ocean are not yet sufficiently reliable to establish threat levels to coastal communities.


Recommendation: Additional observations and modeling should be performed to establish the risk associated with airbursts and with potential tsunami generation.

IN SITU CHARACTERIZATION RELEVANT FOR MITIGATION

Detailed knowledge of the physical characteristics of several representative NEOs would improve understanding of the overall NEO population and help the design and implementation of the mitigation techniques that may be employed should an NEO threaten Earth (but that understanding may well not improve the knowledge of a specific object on an impact trajectory). Although the physical characteristics of an individual NEO that might strike Earth cannot be accurately predicted in advance, the knowledge of the range of possible characteristics will greatly aid in advance planning and might be essential if there is no opportunity to perform detailed characterization studies of the incoming NEO. Dedicated space missions such as Near Earth Asteroid Rendezvous (NEAR)



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