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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 understanding 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. JUPITERâFAMILY AND LONG-PERIOD COMETS It is possible for comets from the outer solar system to cross Earthâs orbit. ï· Jupiter-family comets (JFCs). Referred to as short-period comets with orbital periods less than 20 yearsâthey are a source region for bodies that evolve to become NEOs. ï· Long-period comets (LPCs). Take more than 200 years to orbit the Sun. They originate from the Kuiper belt. LPCs can be NEOs. ï· Isotropic comets, which encompass LPCs and Halley-type comets (HTCs), generally have periods greater than 20 years and have an arbitrary boundary in orbital period at 200 years. All of these comets originate in the Oort Cloud. 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. FIGURE 7.1 The main near Earth asteroid source regions are two asteroid groups (Hungaria and Phocaea), four main-belt escape 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 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_fi nal_e-version.pdf, p. 11. PREPUBLICATION COPY â SUBJECT TO FURTHER EDITORIAL CORRECTION 7-1
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 140 meters in diameter (see Figure 7.2). 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. ï· The probability of an impact by an LPC is only 1 percent that of a NEO impact.1 ï· 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. FIGURE 7.2 Fireballs reported by U.S. government sensors between April 15, 1988, and March 15, 2019. The 2013 Chelyabinsk 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. 1 G.H. Stokes, 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. PREPUBLICATION COPY â SUBJECT TO FURTHER EDITORIAL CORRECTION 7-2