The committee notes that objects smaller than 140 meters in diameter are also capable of causing significant damage to Earth. The best-known case from recent history is the 1908 impact of an object at Tunguska in the Siberian wilderness that devastated more than 2,000 square kilometers of forest. It has been estimated that the size of this object was on the order of approximately 70 meters in diameter, but recent research indicates that it could have been substantially smaller (30 to 50 meters in diameter), with much of the damage that it caused being due to shock waves from the explosion of the object in Earth’s atmosphere. (See, e.g., Chyba et al., 1993; Boslough and Crawford, 1997, 2008.) The committee strongly stresses that this new conclusion is preliminary and must be independently validated. Since smaller objects are more numerous than larger ones, however, this new result, if correct, implies an increase in the frequency of such events to approximately once in three centuries.
All told, the committee was struck by the many uncertainties that suffuse the subject of NEOs, including one other related example: Do airbursts from impactors in this size range over an ocean cause tsunamis that can severely damage a coastline? This uncertainty and others have led the committee to the following recommendation:
Recommendation: Because recent studies of meteor airbursts have suggested that near-Earth objects as small as 30 to 50 meters in diameter could be highly destructive, surveys should attempt to detect as many 30- to 50-meter-diameter objects as possible. This search for smaller-diameter objects should not be allowed to interfere with the survey for objects 140 meters in diameter or greater.
In all cases, the data-reduction and data-analysis resources necessary to achieve the congressional mandate would be covered by the survey projects themselves and by a continuation of the current funding of the Smithsonian Astrophysical Observatory’s Minor Planet Center, as discussed in the report.
Obtaining the orbits and the physical properties of NEOs is known as characterization and is primarily needed to inform planning for any active defense of Earth. Such defense would be carried out through a suitable attack on any object predicted with near certainty to otherwise collide with Earth and cause significant damage. The apparently huge variation in the physical properties of NEOs seems to render infeasible the development of a comprehensive inventory through in situ investigations by suitably instrumented spacecraft: the costs would be truly astronomical. A spacecraft reconnaissance mission might make good sense to conduct on an object that, without human intervention, would hit Earth with near certainty. Such a mission would be feasible provided there was sufficient warning time for the results to suitably inform the development of an attack mission to cause the object to miss colliding with Earth.
In addition to spacecraft reconnaissance missions as needed, the committee concluded that vigorous, ground-based characterization at modest cost is important for the NEO task. Modest funding could support optical observations of already-known and newly discovered asteroids and comets to obtain some types of information on this broad range of objects, such as their reflectivity as a function of color, to help infer their surface properties and mineralogy, and their rotation properties. In addition, the complementary radar systems at the Arecibo Observatory in Puerto Rico and the Goldstone Solar System Radar in California are powerful facilities for characterization within their reach in the solar system, a maximum of about one-tenth of the Earth-Sun distance. Arecibo—which has a maximum sensitivity about 20-fold higher than Goldstone’s but does not have nearly as good sky coverage as Goldstone—can, for example, model the three-dimensional shapes of (generally very odd-shaped) asteroids and estimate their surface characteristics, as well as determine whether an asteroid has a (smaller) satellite or satellites around it, all important to know for planning active defense. Also, from a few relatively closely spaced (in time) observations, radar can accurately determine the orbits of NEOs, which has the advantage of being able to calm public fears quickly (or possibly, in some cases, to show that they are warranted).
Finding: The Arecibo and Goldstone radar systems play a unique role in the characterization of NEOs, providing unmatched accuracy in orbit determination and offering insight into size, shape, surface structure, and other properties for objects within their latitude coverage and detection range.