Micrometeoroid

Whipple, in collaboration with Fletcher Watson of the Harvard Observatory, led an effort to build an observatory to directly measure the velocity of the meteors that could be seen. At the time the source of the micro-meteorites was not known. Direct measurements at the new observatory were used to locate the source of the meteors, demonstrating that the bulk of material was left over from comet tails, and that none of it could be shown to have an extra-solar origin.[6] Today it is understood that meteoroids of all sorts are leftover material from the formation of the Solar System, consisting of particles from the interplanetary dust cloud or other objects made up from this material, like comets.[7]

Lunar sample 61195 from Apollo 16 textured with "zap pits" from micrometeorite impacts

The early studies were based exclusively on optical measurements. In 1957, Hans Pettersson conducted one of the first direct measurements of the fall of space dust on Earth, estimating it to be 14,300,000 tons per year.[8] This suggested that the meteoroid flux in space was much higher than the number based on telescope observations. Such a high flux presented a very serious risk to the high-orbiting Apollo capsules and for missions to the Moon. To determine whether the direct measurement was accurate, a number of additional studies followed, including the Pegasus satellite program, Lunar Orbiter 1, Luna 3, Mars 1 and Pioneer 5. These showed that the rate of meteors passing into the atmosphere, or flux, was in line with the optical measurements, at around 10,000 to 20,000 tons per year.[9] The Surveyor Program determined that the surface of the Moon is relatively rocky.[10] Most lunar samples returned during the Apollo Program have micrometeorite impacts marks, typically called "zap pits", on their upper surfaces.[11]

The "energy flash" of a hypervelocity impact during a simulation of what happens when a piece of orbital debris hits a spacecraft in orbit

Whipple's work pre-dated the Space Race and it proved useful when space exploration started only a few years later. His studies had demonstrated that the chance of being hit by a meteoroid large enough to destroy a spacecraft was extremely remote. However, a spacecraft would be almost constantly struck by micrometeorites, about the size of dust grains.[6]

Whipple had already developed a solution to this problem in 1946. Originally known as a "meteor bumper" and now termed the Whipple shield, this consists of a thin foil film held a short distance away from the spacecraft's body. When a micrometeoroid strikes the foil, it vaporizes into a plasma that quickly spreads. By the time this plasma crosses the gap between the shield and the spacecraft, it is so diffused that it is unable to penetrate the structural material below.[15] The shield allows a spacecraft body to be built to just the thickness needed for structural integrity, while the foil adds little additional weight. Such a spacecraft is lighter than one with panels designed to stop the meteoroids directly.

For spacecraft that spend the majority of their time in orbit, some variety of the Whipple shield has been almost universal for decades.[16][17] Later research showed that ceramic fibre woven shields offer better protection to hypervelocity (~7 km/s) particles than aluminium shields of equal weight.[18] Another modern design uses multi-layer flexible fabric, as in NASA's design for its never-flown TransHab expandable space habitation module,[19] and the Bigelow Expandable Activity Module, which was launched in April 2016 and attached to the ISS for two years of orbital testing.[20][21]