METEOROIDS CAPTURED INTO EARTH ORBIT BY GRAZING ATMOSPHERIC ENCOUNTERS

Some meteoroids, such as the one that produced the daytime fireball of August 10, 1972 that passed over the western United States and the Eu ropean fireball of October 13, 1990, graze the atmosphere of Earth bef ore returning to space (at reduced speed). Other grazing meteoroids, s uch 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 mu st ultimately plunge to ground after further orbital dissipation in su bsequent atmospheric passages unless the gravitational pull of the Moo n and Sun or other intervention raise its perigee above the atmosphere . A spherical atmospheric model is used to integrate the passage of me teoroids in grazing atmospheric encounters. It is found that the corri dor for capture narrows with increasing values of V-infinity, the appr oach velocity of the meteoroid prior to gravitational acceleration by Earth. As an example, if V-infinity = 5 km s(-1), stony meteoroids wit h 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-infinity = 15 km s(-1) and h = 40 km, they are only captured if R is between 1.5 and 2 m. Irons with V-infinity = 5 km s(-1) and h = 40 km, are captured if R is between 1 and 3.5 m, while if V-infinity = 15 km s(-1), they are c aptured if R is between 0.6 and 0.9 m. The cross section for orbital c apture of iron meteoroids and small stony meteoroids is about 0.001 th at for directly hitting Earth. Large stones are never captured except at very low impact velocities because of the large increase in drag re sulting from fragmentation. Published by Elsevier Science Ltd.