Long‐period Comets and the Oort Cloud

Abstract

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.

Impact induced melting and the development of large igneous provinces

“…Recent calculations imply formation of s 450 terrestrial craters of D s 100 km since the late heavy bombardment, and cratering rate estimates suggest that a large 200 km crater may occur every 150 Ma and a 500 km crater every 450 Ma [40,41]. Similar impact rates are implied independently by studies of comets and for the combined probabilities of comets and asteroids ; Weissman [42] indicates that the impact probability of long-period comets large enough to produce craters s 10 km is about 1 Myr 31 and estimates an interval of 1.7U10 7 yr between potentially catastrophic long-period comet impacts. Both comets and asteroids cause impacts, but comets can have much higher velocities [43].…”

Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis

“…However, comets with dimensions of tens of meters will disintegrate at high altitude. Weissman (59) estimated that a comet would have to be >350 m in diameter to penetrate Earth's atmosphere and form a crater, depending upon angle, velocity, etc. Such an event would be at least 500 × more energetic than the Tunguska event.…”

Large Aerial Bursts: An Important Class of Terrestrial Accretionary Events

“…Weissman (1997) calculates that 27% of long-period comets are lost to "random disruption." The reason these comets fall apart is not clear; Weissman (1997) suggested that is associated with the heating of the nucleus during perihelion passage. That the probability of disruption is greatest on the first perihelion passage is attributed to a selection effect that removes those objects that are weakest first.…”

“…Weissman (1997) noted that the mean infall velocity of long-period comets is 51.8 km s 21 and that the energyweighted mean velocity is 57.7 km s 21 .…”

“…Both Shoemaker et al (1990) and Weissman (1997) reckon that even comets with nuclear diameters and energies sufficient to excavate a 10-km-diameter crater probably release almost their entire energy in the atmosphere. Shoemaker et al (1990) noted that the size distribution of Earthcrossing comets and asteroids differ, and that at masses greater than ,10 15 g the impact rate from comets exceeds that from asteroids.…”