Sir - Mr Alan Mason says that, "mass times velocity says it all". Actually, it doesn't. He seems to be mistaking momentum (MV) for kinetic energy (half MV2) - more than a slight difference when one is talking of speeds of 50,000km/h, or 8.5 miles/s.
A given mass requires a velocity of about 12,000km/h to achieve a kinetic energy equal to the explosive energy of the same mass of high explosive. Consequently, a 0.5kg-weight object travelling at 50,000km/h would pack the punch of 9kg of dynamite. One can readily see how a fleck of paint might kill a space walking astronaut.
That the French satellite survived a collision with a "briefcase-sized" fragment suggests that the closing speed must have been quite small, as would be the case if their orbits were roughly similar. It is crossing orbits and opposed orbits which produce the worst situations."
DAVID MORRELL
Chelsea, London, UK
[This letter was published in Flight International, 16-22 October, but, during editing, some of the figures were wrongly interpreted. Apologies to reader Morell, Ed.]
Source: Flight International
