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Long‐period Comets and the Oort Cloud
Author(s) -
WEISSMAN PAUL R.
Publication year - 1997
Publication title -
annals of the new york academy of sciences
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.712
H-Index - 248
eISSN - 1749-6632
pISSN - 0077-8923
DOI - 10.1111/j.1749-6632.1997.tb48335.x
Subject(s) - comet , asteroid , impact crater , near earth object , physics , astrobiology , period (music) , astronomy , orbit (dynamics) , astrophysics , acoustics , engineering , aerospace engineering
The long‐period comets pose a unique problem for the impact hazard problem. Because of their very long orbital periods and generally large distances from the Sun, they cannot be surveyed and catalogued in the same manner as the near‐Earth asteroids and short‐period comets. They appear at random, uniformly distributed on the celestial sphere. Current technologies can detect long‐period comets at distances of ∼5 AU, giving somewhat less than a one year warning time for potential Earth impactors. The mean impact probability for a long‐period comet crossing the Earth's orbit is 2.2 to 2.5 × 10 −9 per perihelion passage. The mean impact velocity is ∼52 km sec −1 , but the most probable impact energy is characterized by a velocity of 56 to 58 km sec −1 . The estimated current impact rate for cometary nuclei large enough to create 10 km diameter (or larger) craters on the Earth is between 5 × 10 −7 and 2.8 × 10 −6 per year, with a best estimated value of 1.0 × 10 −6 per year. Nuclei large enough to initiate global climatic disturbances strike the Earth, on average, every 16 Myr. The impact frequency may be increased substantially for brief periods of time during cometary showers, initiated by major perturbations of the Oort cloud. Improved technologies are needed to detect approaching long‐period comets at large heliocentric distances so as to increase the warning time for potential impactors.