penetration of metallic foils or plates. Recovered metallic surfaces in which crater and perforation dimensions can Data from satellite impact experiments and the scanning of be characterized and in situ detectors where a threshold recovered spacecraft surfaces and solar cells offer an extended time base for examining, using a consistent methodology, micro-ballistic limit (F max , the maximum thickness of material particle fluxes at low Earth orbital (LEO) altitudes. Revised that would be just perforated by the impacting particle) is estimates of the ratio of natural micrometeorites to space debris exceeded are considered.
at micrometer dimensions at ศ500 km altitude show that the Because of the small dimensions of the candidate imdebris population is not as dominant as previously believed. pactors, namely unbound meteoroids and orbital material New data show that, despite a predicted growth in the debris (space debris and captured interplanetary particles), the population, the particle flux has not changed appreciably in effect of the Earth's atmosphere can be significant at some this size regime over the period 1980-1994. Of penetrations of altitudes of LEO deployment. Using appropriate orbital spacecraft surfaces of a 4 to 5 m thickness 18 ุ9 / ุ6 % are due parameters, exposure conditions and atmospheric density to interplanetary meteoroids, whereas above ศ30 m thickness profiles for the relevant epochs, the types of orbit which the interplanetary component dominates. Results of studies of could access the detectors have been examined. atmospheric drag depletion of orbital components, confirmed by flux data at differing altitudes, show that at altitudes of AVAILABLE SATELLITE DATA ศ240 km the meteoroid population dominates even at micrometer sizes due to the extremely short lifetimes of orbital particles. The Timeband Capture Cell Experiment (TiCCE) on Exposure of detectors in such low orbits represents an opportu-ESA's European Retrievable Carrier (Eureca) provided nity to sample the interplanetary meteoroid population without the most recent opportunity for postflight study of impactor contamination from space debris.