There are multiple scenarios for asteroid disasters-even small objects can cause massive firestorms and large strikes risk mass extinctions and climate change
IRWIN I. SHAPIRO et al in 10,( Harvard-Smithsonian Center for Astrophysics, Chair FAITH VILAS, MMT Observatory at Mt. Hopkins, Arizona, Vice Chair MICHAEL A’HEARN, University of Maryland, College Park, Vice Chair ANDREW F. CHENG, Johns Hopkins University Applied Physics Laboratory FRANK CULBERTSON, JR., Orbital Sciences Corporation DAVID C. JEWITT, University of California, Los Angeles STEPHEN MACKWELL, Lunar and Planetary Institute H. JAY MELOSH, Purdue University JOSEPH H. ROTHENBERG, Universal Space Network, Committee to Review Near-Earth Object Surveys and Hazard Mitigation Strategies Space Studies Board Aeronautics and Space Engineering Board Division on Engineering and Physical Sciences, THE NATIONAL ACADEMIES PRESS, http://www.fas.harvard.edu/~planets/sstewart/reprints/other/4_NEOReportDefending%20Planet%20Earth%20Prepub%202010.pdf)
Over the last several decades, research hasclearly 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 across 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). While many species became extinct at this time (including perhaps 30 percent of marine animal genera), 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: 1. 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 via 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. 2. 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 a near-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. Also we became aware of the threat from Apophis only in 2004, raising concerns about whether we would be able to mitigate the threat of such an object, should Earth collision be determined to have a high probability of occurrence in the relatively near future. 3. 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 was located, scientists initially argued against an asteroid or comet origin. However, subsequent analysis and more recent modeling (see, e.g., Chyba et al., 1992; Boslough and Crawford, 1997; 2008) have indicated that modest-sized (the Tunguska object may have been only 30 to 50 meters in diameter) objects 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 similarly sized objects. 4. 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. 5. On October 6, 2008, asteroid 2008 TC3 was observed by the Catalina Sky Survey (Chapter 3) on an Earth-collision course. Although the object was deemed too small to pose much of a threat, Spaceguard and the Minor Planet Center (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 kg (in 280 fragments) of material from the meteorite. These recent events, as well as our 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 our ability to respond to the threats that they might pose to humanity.