Evaluation of the Apollo Asteroids as Sources of Stone Meteorites

Calculation of the secular perturbations of the earth-crossing (Apollo) asteroids shows that the node of one body, 1948 EA, seldom, if ever, crosses the earth's orbit and probably should not be included with the Apollo asteroids. Another object, 1580 Betulia, will cross the earth's orbit, although its present perihelion lies beyond the orbit of the earth. The hypothesis that chondritic meteorites are produced in earth-crossing orbits as collision fragments of these Apollo asteroids has the attractive feature that the short (∼107 years) cosmic-ray ages, characteristic of such an initial orbit, are in agreement with measured ages. A quantitative study of this hypothesis indicates difficulties that must be overcome before this hypothesis may be considered plausible. (1) If the Apollos are ordinary asteroids, gravitationally perturbed into earth-crossing orbits, the steady-state ratio of falls from fragments of Apollo asteroids to falls from fragments of other asteroids will be to a good approximation R = TA/TC ∼ 0.01 to 0.1, where TA is the lifetime of Apollo asteroids (∼107 years) and Tc is the collision lifetime (108 – 109 years) of meteorites in the asteroid belt. In the steady state, long exposure ages should predominate. (2) The expected mass yield of meteorites from the known Apollo asteroids is too small by a factor of about 500. Although there are certainly many undiscovered Apollo asteroids, it is unlikely that they are sufficiently numerous or sufficiently massive to produce the observed meteorite flux of ∼109 g/yr. Effects of the ∼104-year periodic variations in eccentricity and inclination do not change this conclusion. These difficulties are in part avoidable if, as proposed by Opik, the Apollo asteroids are cometary cores, especially if there also exist numerous fragmented cores within cometary nuclei.