Defending Planet Earth Near-Earth-Object Surveys and Hazard Mitigation Strategies (2010) / Chapter Skim
Currently Skimming:
2 Risk Analysis
2 Risk Analysis
Pages 12-28
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 12... ...
More rarely, larger objects impact Earth. It is now widely believed that the impact of an approximately 10-kilometer-diameter object formed the Chicxulub Crater near the Yucatan Peninsula about 65 million years ago, very likely resulting in the extinction of the dinosaurs.
|
From page 13... ...
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)
|
From page 14... ...
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.
|
From page 15... ...
To determine what fraction of the entire NEO population has been detected, it is necessary to compute the total expected number of objects from knowledge of the properties of known NEOs and how objects are expected to get brighter and fainter as they and Earth move around their orbits. Using computer models one can determine the fraction of all NEOs of different sizes that will be detected for a particular survey strategy.
|
From page 16... ...
will also be published in the upcoming European Space Agency conference proceedings of the April 27-30, 2009, 1st International Academy of Astronautics Planetary Defense Conference: Protecting Earth from Asteroids. 3Spacewatch was one of the first NEO discovery systems, established in 1981 and run by the University of Arizona.
|
From page 17... ...
Furthermore, as scientists pass 90 percent survey completion, we are approaching the tails of the distribution of orbits where the model is far less robust. The Size Distribution of NEOs and Potentially Hazardous NEOs Most NEOs with diameters under half a kilometer remain undiscovered, although many of the larger objects in this size range have been identified in past surveys.
|
From page 18... ...
. Scientists do not know the specific orbits of undiscovered NEOs, but can use what is known about their population and size distribution to perform a probabilistic "risk assessment" for this fraction.
|
From page 19... ...
the mass distribution of potentially hazardous NEOs striking Earth; this component is obtained by calculating the masses of the objects on the assumption that they have densities of 2.5 g cm–3. Table 2.1 is based on such information to provide an approximate indication of the average impact interval and impact energy for objects of various sizes.
|
From page 20... ...
is deposited along the entry path, with significant downward momentum transferred to the ground. Accordingly, these researchers suggest that smaller explosions, with net yields of 3 to 5 MT, may be sufficient to produce Tunguska-like impact events.
|
From page 21... ...
Impacts Capable of Producing Global Effects The motivation for the original Spaceguard Survey was to find all of the NEOs larger than 1 kilometer in diameter capable of striking Earth. According to Toon et al.
|
From page 22... ...
. For example: • The NEO and potentially hazardous NEO size distributions may not follow the simple law as shown by the dashed line in Figure 2.4 but instead may have a dip, as illustrated by the open circles.
|
From page 23... ...
for the NEO population as it was known before the Spaceguard Survey. The plot shows estimated average fatalities per year and clearly indicates that most of the threat comes from the larger objects that exceed the global catastrophe threshold, even though the probability of an impact by these objects is very low relative to that for smaller objects.
|
From page 24... ...
This limiting value, according to survey simulation of potentially hazardous NEOs, could remove a significant proportion of the remaining statistical hazard that still exists after the conclusion of the Spaceguard Survey. The completion of this survey does not change the probability of Earth impact for any undetected NEO.
|
From page 25... ...
. Additional information could change the relative statistical hazard associated with the various size ranges of NEOs as the following data are obtained: • Orbital distributions and collision probabilities for subkilometer-sized impactors; • More reliable estimates of the effects of Tunguska-like and larger impacts, including tsunami damage; and • Maps that more realistically account for human population distribution and growth.
|
From page 26... ...
All the other estimates in Table 2.2 are based on the attribution of causes of actual fatalities from ongoing threats that may change in the future. In contrast to other known natural hazards, there has been no significant loss of human life to impacts in historical times, due to the low frequency of major impacts and the higher probability of impact in unpopulated TABLE 2.2 Expected Fatalities per Year, Worldwide, from a Variety of Causes Cause Expected Deaths per Year Shark attacksa 3-7 Asteroidsb 91 Earthquakesc 36,000 Malariad 1,000,000 Traffic accidentse 1,200,000 Air pollutionf 2,000,000 HIV/AIDSg 2,100,000 Tobaccoh 5,000,000 NOTE: The entries in this table are of various types.
|
From page 27... ...
However, options for effective mitigation become much more limited when threatening objects are identified with only months to years, rather than decades or centuries, before impact. Thus, one of the greatest elements of risk associated with NEOs is the publicís expec tation that governments will provide protection against any threat from NEOs, even as governments and agencies have been unwilling so far to expend public funds in a concerted effort to identify, catalog, and characterize as many potentially dangerous NEOs as possible, as far in advance of a damaging impact event as feasible.
|
From page 28... ...
Presentation to the Survey/Detection Panel of the NRC Committee to Review Near-Earth Object Surveys and Hazard Mitigation Strategies, January 28-30, 2009. Hills, J.G., and M.P.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.