Over the past several decades, research has clearly demonstrated that major impact events have occurred throughout Earth’s history, often with catastrophic consequences. The Chicxulub impact apparently caused a mass extinction of species, possibly resulting from a global firestorm due to debris from the impact raining down around the planet. It may also have caused dramatic cooling for a year or more and global climatic effects that may have lasted a long time (e.g., O’Keefe and Ahrens, 1989). Many species became extinct at this time (including perhaps 30 percent of marine animal genera), but many survived and ultimately thrived in the post-dinosaur world. It may be that impacts throughout the history of this planet have strongly helped shape the development and evolution of life forms.
Several recent events and new analyses have highlighted the impact threat to Earth:
As Comet Shoemaker-Levy 9 came close to Jupiter in 1992, tidal forces caused it to separate into many smaller fragments that then may have regrouped by means of self-gravity into at least 21 distinct pieces (e.g., Asphaug and Benz, 1994). These pieces impacted Jupiter in July 1994, creating a sequence of visible impacts into the gaseous Jovian atmosphere. The resultant scars in Jupiter’s atmosphere could be readily seen through Earth-based telescopes for several months. In July 2009, a second object, though much smaller than Shoemaker-Levy 9, impacted Jupiter, also causing a visible dark scar in the Jovian atmosphere. Such clear evidence of major collisions in the contemporary solar system does raise concern about the risk to humanity.
In December 2004, astronomers determined that there was a non-negligible probability that near-Earth asteroid Apophis (see Chapter 4 for more details) would strike Earth in 2029. As Apophis is an almost 300-meter-diameter object, a collision anywhere on Earth would have serious regional consequences and possibly produce transient global climate effects. Subsequent observations of Apophis ruled out an impact in 2029 and also determined that it is quite unlikely that this object could strike during its next close approach to Earth in 2036. However, there likely remain many Apophis-sized NEOs that have yet to be detected. The threat from Apophis was discovered only in 2004, raising concerns about whether the threat of such an object could be mitigated should a collision with Earth be determined to have a high probability of occurrence in the relatively near future.
In June 1908, a powerful explosion blew down trees over an area spanning at least 2,000 square kilometers of forest near the Podkamennaya Tunguska River in Central Siberia. As no crater associated with this explosion