Soft entry of micrometeorites at grazing incidence or by aerocapture

A study is made of the conditions under which micrometeorites can enter the Earth's atmosphere without being melted or without having their nuclear tracks annealed. The two mechanisms studied are: (1) aerocapture followed by entry from a bound orbit; and (2) direct entry at large zenith angles. Mechanism (2) is generally more efficient if the maximum temperature is limited to 1600 K; some grains as large as 400 [[micro]meter] across can survive unmelted. Both mechanisms are competitive for maximum temperatures of 800 K. The estimated probabilities of entry (compared with the total flux to the Earth) are a few times [10.sup.-3] for unannealed particles and several percent for unmelted ones.