Viable, organic material has been discovered at altitudes of around 40 km above the Earth (Wainwright et al. (2004). Int J. Astrobiol. 3(1), 13–15). Assuming an extraterrestrial origin, this raises the question of how the material survived air breaking in the atmosphere from hypervelocity speeds. The Earth is under constant bombardment from interplanetary and interstellar dust particles, with a daily influx of over 60 tonnes of material incident upon the upper atmosphere. The majority of this material is in the form of micro-meteoroids with typical radii ranging from 0.01 μm to a peak of around 200 μm (Ceplecha et al. (1998). Space Sci. Rev. 84, 327–471).

Classical work on meteoroid ablation suggests that these particles should be annihilated by atmospheric deceleration. Recent work suggests that molecular sputtering of surface molecules may provide an alternative way to decelerate, without such intense heating. In general, the mathematics of atmospheric entry are complex; here we review some of the main parameters (particle size, initial velocity, entry angle and composition) that contribute to the heating of meteoroids during their descent. Comparing the heating profiles for meteoroids descending from inside and outside of the Earth's shadow, it is found that the maximum temperatures reached by 10 μm meteoroids can be 10–15% lower if the meteoroid descends from within the shadow of the Earth, compared to those decelerating during daylight. The possibility that micrometre-sized particles can decelerate subject to maximum temperatures ~300 K offers a mechanism for the survivability of the recovered organic material.