SOFT ENTRY OF MICROMETEORITES AT GRAZING-INCIDENCE OR BY AEROCAPTURE

A study is made of the conditions under which micrometeorites can ente r the Earth's atmosphere without being melted or without having their nuclear tracks annealed. The two mechanisms studied are: (1) aerocaptu re followed by entry from a bound orbit; and (2) direct entry at large zenith angles. Mechanism (2) is generally more efficient if the maxim um temperature is limited to 1600 K; some grains as large as 400 mu m across can survive unmelted. Both mechanisms are competitive for maxim um temperatures of 800 K. The estimated probabilities of entry (compar ed with the total flux to the Earth) are a few times 10(-3) for unanne aled particles and several percent for unmelted ones. (C) 1997 Academi c Press.