Other
Scientific paper
Jan 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000neas.work...21i&link_type=abstract
Near-Earth Asteroid Sample Return Workshop, p. 21
Other
Cometary Collisions, Frequencies, Crossings, Meteorites, Orbital Elements
Scientific paper
The mean value of a characteristic time elapsed up to a collision of an Earth-crossing object (ECO) with the Earth equals to 100 Myr. Considering that the number of all ECOs with diameter d>1 km is equal to 750, we have about 7.5 collisions per Myr. The number of all 100-m ECOs is estimated to be about 70,000-160,000, so such objects collide the Earth on average once in 600-1400 yr. For d>70 m the frequency of collisions is greater by a factor of 2 than that for d>100 m, and so Tunguska-size objects collide the Earth once in several hundreds (-500) years. Many scientists consider that the Tunguska event was caused by the explosion of an icy body which was a debris of a comet. Most of Jupiter-family comets come from the trans-Neptunian (Edgeworth-Kuiper) belt, and long-period and Halley-type comets usually come from the Oort cloud. Duncan et al. obtained that about 10-20% of trans-Neptunian objects (TNOs) with a semimajor axis a<50 AU left the belt during last 4 Gyr and about 1/3 of Neptune-crossing objects reach Jupiter's orbit during their lifetimes. We showed that the mutual close encounters of TNOs can also play a role in their migration inside the solar system and during last 4 Gyr several percents of TNOs could change their semimajor axes by more than 1 AU due to the gravitational interactions with other TNOs. Even small variations in orbital elements of TNOs caused by their mutual gravitational influence and collisions can cause large variations in orbital elements due to the gravitational influence of planets. The results of our numerical investigations of orbital evolution of bodies under the gravitational influence of planets showed that the mean time interval, during which an object crosses Jupiter's orbit during its lifetime, is about 0.2 Myr, the portion of Jupiter crossers that reach the orbit of the Earth during their lifetimes is equal to 0.2, and the mean time, during which a Jupiter-crossing object crosses the orbit of the Earth, is about 5000 yr. It is considered that there are about 1010 TNOs with diameter d>1 km and a<50 AU. Basing on the above data, we obtained that the number of the present Jupiter-crossers with d>1 km, which came from the trans-Neptunian belt, is equal to 30,000 and there are about 170 former TNOs with d>1 km, which cross both the orbits of Earth and Jupiter (i.e., about 20% of all ECOs with d>1 km). The portion of such objects colliding with the Earth is smaller than their portion among all ECOs, because the characteristic time elapsed up to a collision with the Earth for a Jupiter-crossing body is larger by a factor of several than that for a typical Apollo object. The number of former Jupiter-crossers that move inside the orbit of Jupiter in Encke-type orbits can be of the same order (or even more) than the number of objects that cross both the orbits of Jupiter and Earth. The near-Earth objects (NEOs) that will be focused on for the understanding of potential Earth-impacting objects might have a source from the trans-Neptunian belt. The volatile-icy portion of the objects would have been ejected off due to being close to the inner solar system. The structural composition of the asteroid bodies might also be different when then the bodies formed inside of Jupiter. This might effect the type of deflection that might be used if they were a potential Earth-impacting object. The largest sample of Earth-impacting solid bodies is the Antarctic meteorites that have been recovered. It is only when robotic missions to the accessible NEOs have been done then we will get an accurate picture of sources of NEOs. Carbon based meteorites are a small sample size of the recovered antarctic and non-antarctic meteorite samples. There is a need to get more observation platforms available to search for NEOs and other potential ECOs. One solution is galvanizing amateur astronomers world-wide and also to encourage the developed world to use its telescopes that have smaller capabilities to search for ECOs.
Ipatov Sergei I.
Mardon Austin
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