Sunday, March 10, 2013

One Reason a Walk in Space Would Not Be a Drag

I have not blogged in quite a while. Some of the air went out of my Manned Orbiting Laboratory (MOL) balloon with my blockbuster exposé of the little-known and under-appreciated fact that the Gemini-B capsule was to be depressurized and off-limits to its pilots during for the duration of their 30-day MOL reconnaissance missions. (I am being facetious: no blocks were busted and my exposé has not stimulated any follow-up whatsoever.) Never fear: I plan to return to the biomedical aspects of MOL, especially since no one is doing it, or has done it in several decades.

However, this blog entry is on another favorite topic, or topics, actually. One topic is the earliest history of extravehicular activity (EVA), or space walking (or as the Russian more accurately put it, space swimming). My research interest is especially in the parallel development of underwater neutral buoyancy for EVA training by several different groups simultaneously with little interaction, and how one group seems to have prevailed in an almost Darwinian way. (For a spoiler on who won, go here .) 

The other favorite topic is orbital mechanics, in which I have had no professional training whatsoever, and lack competence in any but the simplest mathematical skills required, but for which I have a fascination bordering on reverence, and so have memorized a few convenient facts.

During on-line research into EVA history, I discovered an interesting newspaper column from April 1963 (ref. 1) containing some of the earliest public discussions about EVA during NASA’s upcoming Gemini missions. The columnists, Robert S. Allen and Paul Scott, reported that NASA had calculated that the astronaut floating outside of his Gemini capsule would be traveling at the same speed as his capsule, but that this “untested theory” was challenged by unnamed astronauts who had actually flown in space. The astronauts reportedly believed that the spacewalker would travel at a slower speed than the capsule, just like the “firefly” particles knocked off their spaceships during flight, and thus would be pulled along by the 20-foot tether connecting him to his Gemini. They did not say why he would lag so.

This is funny because it is true, but probably not in the way the authors meant. In fact, I cannot say how the authors meant it: they offered no explanation beyond the observation that it was an untested theory—as if the deductions of Newton, Kepler, LaGrange, Einstein and others had not been confirmed by four centuries of measurements on natural and man-made satellites.

I assume that Allen and Scott were afraid that wind resistance would hold the extra-vehicular astronaut back, just as a skydiver falls behind his airplane when he jumps. After all, even in the 21st century many people still do not understand that wind resistance is minimal at orbital altitude because the atmosphere is vanishingly thin. This misapprehension seems to underlie the oft-repeated wonderment that rendezvousing spacecraft can approach each other even at the blistering speed of 17,500 miles per hour—as if there is some force or perturbation inherent in that speed that would cause a hazard—like, say, wind turbulence, maybe.

In the grossest sense, we now know from almost five decades of observations that EVA astronauts do not lag behind their host space vehicles. Interestingly enough, in a pure vacuum a passive and completely immobile astronaut really would lag behind for reasons having nothing to do with wind resistance. Orbital mechanics is the reason. But the effect would be negligible. The speed of an object in orbit around a planet depends on, among other things, the sum of the mass of the primary object (in this case, the Earth) and the mass of the secondary object (the astronaut or the capsule). An amateur orbital mechanicist, on whose calculations lives and treasure do not depend, may comfortably ignore the contribution of the mass of any man-made satellite to that sum since the mass of the Earth is literally billions of billions of times larger.

But a small, lightweight satellite such as an astronaut really will be travelling infinitesimally slower than another more massive satellite such as a Gemini capsule at the same orbital altitude. This is because the sum of the masses of the Earth plus the astronaut in the Earth+astronaut system is very slightly less than that of the Earth plus the capsule in the Earth+capsule system. The difference is so small that it would probably take years for the astronaut to lag behind the capsule by the length of his 20-foot tether. During that time, many other factors would overwhelm the drift-away: the very tiny “wind resistance” from the microscopically thin atmosphere at that altitude actually would do more to cause the lighter astronaut to lag behind the heavier, denser capsule, but even that effect would be overwhelmed by other factors such as the astronaut’s maneuvering in performing his assigned tasks—after all, nobody goes EVA without some purpose requiring movement, and certainly not to float passively for a few years—and the usual small air leaks from his spacesuit, among other things.

Even in the absence of these confounding effects, orbital mechanics itself (themselves?) would cause another effect much more quickly. If the slower astronaut were to find himself lagging behind the capsule, being dragged along by his tether, he would actually be boosted by the tether to the same speed as the capsule. But this would be faster than the speed required—and allowed—at that distance between the two bodies in the “Earth+astronaut” system. Thus he would find himself propelled into a slightly higher orbit, in effect flying above his capsule as if he were a kite in a stiff breeze, but without the breeze.

The column from 1963 also said something else that appealed to another of my interests, namely early spacesuits for astronauts. It said that on the upcoming daylong Mercury flight of astronaut Gordon Cooper, he would remove parts of his spacesuit for comfort. In fact, no Mercury astronaut removed so much as his gloves or helmet in flight, and Cooper was not planning to do so. At that time, the B.F. Goodrich was working on a prototype spacesuit for the Gemini program, due to start in just over a year, that would have permitted the astronauts to unzip their spacesuit sleeves and “pants legs” for comfort. It appears that Allen and Scott simply inferred that Cooper would be wearing such a suit on his Mercury flight. However, the Goodrich suit was not adopted for Gemini and was not developed further.

All of the foregoing makes me wonder if the columnists were extrapolating from incomplete information or from their incomplete understanding of some real information. They got maximum value out of the “quotation mark” key on their typewriter, using it to apply emphasis to too many words in their column: “spectacular”, “flight plan”, “heat, cold and meteoroid puncture”, “firefly”, “environmental control system”, “life line” (twice) and “walk in space”, “walking” and “walk”. This suggests that they were unfamiliar with the vocabulary of spaceflight, no sin during those early days of the space age.

Or maybe they were being strung along by real space professionals, perhaps the astronauts themselves. For example, they reported the orbital velocity in question as being 15,000 mph—nearly 15% lower than the actual speed of about 17,500 mph—and called the Gemini hatch a “trap door” (without the quotation marks, surprisingly) as if they were being spoon-fed a series of simplified factoids.

I hope space professionals then and now would rather correct misunderstandings than perpetuate them. However, these columnists did not usually write about spaceflight topics, so they were already out on a limb. One of them, Robert Allen, a Washington D.C. correspondent and Washington bureau chief for The Christian Science Monitor, had actually published Soviet propaganda early in his career, albeit during World War II to make the Soviet Union attractive as an ally of the United States (ref. 2).

  1. Allen, Robert S. and Paul Scott, “Astronauts Will Try Daring Experiments,” Washington Report, Oakland Tribune, April 9, 1963, p. 17, (accessed Mar. 10, 2013).
  2. “Robert S. Allen,” (accessed Mar. 7, 2013).

1 comment:

  1. Also, each particle in a spacecraft is in a slightly different orbit unless it is precisely at the center of mass. If you are higher, your orbit should be a tad slower. If lower, you should be faster. To the side or ahead or behind will have similar effects. But because you are attached to the vehicle, you get dragged or pushed along, micro-accelerated one way or the other. From this comes the term microgravity.

    With this in mind, NASA carefully designed Spacelab missions doing microgravity research so the most sensitive experiments were as close as possible to the center of mass. And it designed ISS so the U.S. lab module would occupy the same "sweet spot." There's even an acceleration contour map/

    It also means that an astronaut outside his spacecraft would be in a slight different orbit. So theoretically, EVA astronauts could have wound up "dangling" behind the spacecraft on the tether, or dragging the spacecraft behind them, if they ever stayed still long enough for the effects to build.

    Isaac Newton, call your answering service!

    Dave Dooling, Alamogordo, NM


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