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Published online by Cambridge University Press: 14 August 2015
Recent investigations of the terrestrial influx of small meteoroids (sizes l00µm–1 cm) are reviewed, and it is shown that (i) Previous radar measurements have underestimated the influx of these bodies by at least an order of magnitude; (ii) The true height distribution of meteors in the atmosphere indicates a very low bulk density, in general of order 0.01-0.10 gm cm-3; (iii) Several Apollo asteroids have associated meteoroid streams, implying a genetic relationship and hence the possibility that these asteroids are in fact extinct or dormant cometary nuclei; and (iv) Along with the other three retrograde intermediate-period comets, P/Halley most likely originated in the Kuiper Cloud of comets just beyond the planetary region, and not in the distant Oort Cloud as is usually assumed.