Abstract

Some meteoroids, such as the one that produced the daytime fireball of August 10, 1972 that passed over the western United States and the European fireball of October 13, 1990, graze the atmosphere of Earth before returning to space (at reduced speed). Other grazing meteoroids, such as Peekskill, penetrate deeper into the atmosphere and lose enough energy to plunge to ground. It is evident that if a grazing meteoroid is within some critical range of closest approach distance and speed, it is captured into a gravitationally bound orbit around Earth. It must ultimately plunge to ground after further orbital dissipation in subsequent atmospheric passages unless the gravitational pull of the Moon and Sun or other intervention raise its perigee above the atmosphere. A spherical atmospheric model is used to integrate the passage of meteoroids in grazing atmospheric encounters. It is found that the corridor for capture narrows with increasing values of V, the approach velocity of the meteoroid prior to gravitational acceleration by Earth. As an example, if V= 5 km s^-1, stony meteoroids with closest-approach distances of h = 40 km above the Earth are captured if their radii, R, are between 3 and 9 m while if V = 15 km s^-1 and h = 40 km, they are only captured if R is between 1.5 and 2m. Irons with V = 5 km s^-1 and h = 40 km, are captured if R is between 1 and 3.5 m, while if V = 15 km s^-1, they are captured if R is between 0.6 and 0.9 m. The cross section for orbital capture of iron meteoroids and small stony meteoroids is about 0.001 that for directly hitting Earth. Large stones are never captured except at very low impact velocities because of the large increase in drag resulting from fragmentation.

Corresponding author. Correspondence to: J. G. Hills