was located, scientists initially argued against an asteroid or comet origin. However, subsequent analysis and more recent modeling (see, e.g., Chyba, 1993; Boslough and Crawford, 1997, 2008) have indicated that modest-sized objects (the Tunguska object may have been only 30 to 50 meters in diameter) moving at high supersonic speeds through the atmosphere can disintegrate spontaneously, creating an airburst that causes substantial damage without cratering. Such airbursts are potentially more destructive than are ground impacts of similar-size objects.
A stony meteorite 1 to 2 meters in diameter traveling at high supersonic speeds created an impact crater in Peru in September 2007. According to current models with standard assumptions, such a small object should not have impacted the surface at such a high velocity. This case demonstrates that specific instances can vary widely from the norm and is a reminder that small NEOs can also be dangerous.
On October 6, 2008, asteroid 2008 TC3 was observed by the Catalina Sky Survey (see Chapter 3) on a collision course with Earth. Although the object was deemed too small to pose much of a threat, the Spaceguard Survey1 and the Minor Planet Center (see Chapter 3) acted rapidly to coordinate an observation campaign over the following 19 hours, with both professionals and amateurs to observe the object and determine its trajectory. The 2- to 5-meter-diameter object entered the atmosphere on October 7, 2008, and the consequent fireball was observed over northern Sudan (Figure 2.2) (Jenniskens et al., 2009). Subsequent ground searches in the Nubian Desert in Sudan located 3.9 kilograms (in 280 fragments) of material from the meteorite.
These recent events, as well as the current understanding of impact processes and the population of small bodies across the solar system but especially in the near-Earth environment, raise significant concerns about the current state of knowledge of potentially hazardous objects and the ability to respond to the threats that they might pose to humanity.