Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
7 Impact Hazards Not Explicitly Considered by the George E. Brown, Jr. Act Near Earth objects (NEOs) are not the only objects in space that can potentially impact Earth. As understand- ing of the solar system has advanced and more telescopic observations have been made, scientists have identified other objects that could pose an impact hazard and are also of scientific interest. These are summarized in this chapter for the sake of completeness. JUPITER-FAMILY AND LONG-PERIOD COMETS It is possible for comets from the outer solar system to cross Earthâs orbit. â¢ Short-period comets (also referred to as Jupiter-family comets, JFCs) with orbital periods of typically less than 20 years; â¢ Long-period comets (LPCs), including isotropic comets and Halley-type comets (HTCs), with longer orbital periods. The contribution of multiple NEO source regions is shown in Figure 7.1. Most NEOs come from the inner and central main belts; few come from the outer main belt or JFCs. For LPC and HTC populations, the goal should be to know the number-flux density of such comets (i.e., the number of comets per unit time per size bin) through near-Earth space, because the absolute number is huge and it is extremely difficult to identify them all individually, because the vast majority are too distant. OBJECTS WITH DIAMETERS LESS THAN 140 METERS The George E. Brown, Jr. Near-Earth Object Survey Act specified 140 meters as the lower limit for the NEO survey requirements in the 2005 NASA Authorization Act. Objects smaller than 140 meters are being found and catalogued in existing visible surveys. Although the completeness level of these surveys is low, it is important to discover and catalogue these objects. The 2013 Chelyabinsk fireball and the December 2018 fireball that exploded over the western Pacific Ocean had energies of 440 and nearly 200 kilotons, respectively. The Chelyabinsky fireball resulted in major damage to buildings. Both meteoroids were estimated to be significantly smaller than 140Â meters in diameter (see Figure 7.2). 44
After an analysis of all of the integration results, Granvik et al. identified six primary main belt source regions that can supply NEOs: two highâinclination asteroid groups (i.e., the Hungarias and Phocaeas) and four primary escape routes (i.e., the ïµ6 secular resonance and the 3:1, 5:2 and 2:1 meanâmotion resonances with Jupiter) (Figure 2â1). The inclusion of JFCs, which come from the outer solar system, yield a seventh source. GEORGE E. BROWN, JR. ACT IMPACT HAZARDS NOT EXPLICITLY CONSIDERED BY THE 45 FIGURE 7.1â The main near Earth asteroid source regions are two asteroid groups (Hungaria and Phocaea), four main-belt Â scape e Figure 2-1. Main NEO source regions: two asteroid groups (Hungaria and routes (via the Î½6 secular resonance; 3:1, 5:2, and 2:1 mean-motion resonances with Jupiter), and the Jupiter-family comets (JFCs). SOURCE: G.H. Stokes, B.W. Barbee, W.F. Bottke, Jr., routes (ïµ6S.R. Chesley, P.W. Chodas, J.B. Evans, et al., 2017, Report Phocaea), 4 main belt escape M.W. Buie, secular resonance; 3:1, 5:2, and of the Near-Earth2:1 mean-motion resonances with to Determine thethe Jupiter family comets Object Science Definition Team: Update Jupiter), and Feasibility of Enhancing the Search and Charac- (JFCs). Science Mission Directorate, https://cneos.jpl.nasa.gov/doc/2017_neo_sdt_final_e-version.pdf, p. 11. terization of NEOs, NASA The orbital pathways followed by asteroids from a given source traversing NEO space was characterized by computing how much time the bodies spent within a series of semimajor axis, eccentricity, and inclination (a, e, i) bins. The resultant residence time probability distribution, defined by the variable Rs (a, e, i), where the s subscript is the label for the source, describes the nature of the steady state orbital distributions of NEOs coming from a source. Each Rs function was then multiplied by a source specific absolute magnitude H distribution, Ns (H), whose properties were defined in Granvik et al. (2016). Finally, these functions were added together with weighting functions such that the sum was 1. The result was a NEO model of the form N (a, e, i, H). If all seven of the Rs and Ns (H) functions were 100% accurate and were added together in the right proportion to one another, N (a, e, i, H) would represent a complete debiased orbital and absoluteâmagnitude distribution of the NEO population. To calibrate the variables and thereby determine which sources provide most NEOs, the model had 2.2 Population Debiasing to be compared in some way with known NEOs. The problem is that known NEOs are biased by observational selection effects that favor the discovery of bodies that spend long periods of time in a surveyâs search volume above the detection threshold. This led Granvik et al. (2016) to compute the observational selection effects associated with Catalina Sky Survey (CSS), which had a large and FIGURE 7.2â Fireballs reported by U.S. government sensors between April 15, 1988, and March 15, 2019. The 2013 Chely- abinsk fireball is visible at upper center right, and the December 2018 Bering Sea event is at upper right. These objects were all well below 140 meters in diameter. SOURCE: NASA Jet Propulsion Laboratory, Center for Near Earth Object Studies, âFireballs Reported by U.S. Government Sensors,â https://cneos.jpl.nasa.gov/fireballs, accessed March 15, 2019; courtesy NASA/JPL-Caltech. 2017 Report of the NEO Science Definition Team | 11
46 FINDING HAZARDOUS ASTEROIDS USING INFRARED AND VISIBLE WAVELENGTH TELESCOPES INTERSTELLAR OBJECTS Interstellar objects, of which only one has been discovered in the history of astronomy, can be treated as part of the LPC population and are a miniscule fraction thereof. â¢ The probability of an impact by an LPC is only 1 percent that of a NEO impact.1 â¢ The energy of an Earth impact would be high, because velocity at Earth orbit would be high, and energy is proportional to square of velocity and is calculated at ~30 percent larger than a typical NEO impact. Definitions of these types of NEOs are included in this report for the sake of completeness and to explain why they should be considered within the context of the George E. Brown Act requirement, although they are not a driver in meeting the requirement. 1 G.H. Stokes, B.W. Barbee, W.F. Bottke, Jr., M.W. Buie, S.R. Chesley, P.W. Chodas, J.B. Evans, et al., 2017, Report of the Near-Earth Object Science Definition Team: Update to Determine the Feasibility of Enhancing the Search and Characterization of NEOs, NASA Science Mission Directorate, https://cneos.jpl.nasa.gov/doc/2017_neo_sdt_final_e-version.pdf.