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A common point of drama in science fiction is the loss of a tether during a spacewalk threatening to (or actually causing) send an unlucky astronaut hurtling into space.

In reality, is such a thing actually an unrecoverable situation?

If an astronaut is tethered to a spacecraft and the line is severed, the astronaut becomes a new orbiting body. The energy released from a snapping tether can't be too considerable given the forces usually used to move spacecraft around. If some kind of explosive decompression (assuming our hero is properly suited) provides a considerable ejection force, I can see recovery being a little more difficult, but probably still not impossible. This of course assumes that a severed tether is not his only form of life support.

From the astronaut's point of view, the craft may drift slowly away (and vice versa) but both are still cycling around the planet below. Assuming the astronaut could keep calm and keep his wits about him and isn't too impacted by atmospheric drag, wouldn't it be possible to do some small pulses of thrust on either a small propulsion unit with the astronaut, or a thruster on the craft such that they can rendezvous on the next or a future orbit and let him climb back aboard?

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    $\begingroup$ The energy released from a snapping tether can't be too considerable No, but the energy that caused the tether to snap can. Imagine a force working on the astronaut that stretches the tether to breaking point. $\endgroup$ – Jan Doggen Nov 30 '16 at 10:03
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    $\begingroup$ @JanDoggen: Comparing to energies of a rocket engine, that's still next to nothing. Although the delta-V to reentry from LEO is very small, and if the astronaut unluckily "broke free" into retrograde direction, entering the atmosphere is a very real risk. $\endgroup$ – SF. Nov 30 '16 at 10:58
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    $\begingroup$ Not to drift too far away from the thread topic (see what I did there?) but... a free-floating astronaut should be able to correct for a tumble without any external force (such as throwing an object). They can pinwheel their arms, until the rotation is canceled. $\endgroup$ – DaveC426913 Nov 30 '16 at 14:41
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    $\begingroup$ @DaveC426913: An astronaut who was rotating slowly might be able to keep his torso orientation roughly constant by waving his arms, but he would need to keep waving his arms constantly to maintain his torso orientation. The rotational momentum of the astronaut as a whole could not be changed without ejecting mass, but as long as he didn't have too much rotational momentum, he could concentrate it all in his arms. Doing anything else useful while keeping his arms in motion, however, might be difficult. $\endgroup$ – supercat Nov 30 '16 at 16:47
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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – called2voyage Dec 2 '16 at 14:37
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This is a great question. I wanted to provide an answer which cited some specific, real-world situations. Currently the only people in space are those aboard the International Space Station. If anyone could potentially get into a scenario as you describe in your question, it would be them.

Currently, on spacewalks, a huge number of safety procedures are used to ensure the safety of the astronaut. One primary component is the use of tethers to keep the astronaut tied to the actual space station. If this tether were to break, there are secondary means of keeping the astronaut from drifting into space.

Once the astronaut comes untethered, his primary concern is to remove any velocity he may have which is taking him away from the space station. The cause of this velocity can depend on a variety of factors. Often in movies, its because something exploded and the astronaut is flying away from his station/shuttle quite rapidly. In a scenario like this, the astronaut can use his SAFER unit. This is a pack worn by astronauts during extra-vehicular activities (EVA) which is designed for just such an emergency. It has thrusters on it capable of slowing down the astronaut and sending him back towards the station/shuttle. These thrusters are of course limited and can only handle changing the astronaut's velocity by about 3 m/s. If they're moving faster than that, they need an additional way to slow themselves down.

If, for whatever reason they can't use the thrusters, or it isn't enough, they're likely going to have to rely on Newton's Third Law — for every action, there is an equal and opposite reaction. What this means is that they can slow themselves down by throwing any objects they have on hand. If they happened to have a wrench in their hand when the unfortunate event occurred, they could throw the wrench in the direction they were traveling. The act of ejecting the wrench in a particular direction would cause them to slow their motion in that direction. At this point, the SAFER unit might still be useful, even if it is out of propellant. They can always throw that! If they need to, they can try throwing anything they can safely dispose of which can help slow their speed and bring them back to the space station.

There is one final saving grace that I can think of. The ISS in particular has robotic arms on the outside. These arms are meant for doing space work by those inside the ISS and include things like performing repairs and helping ships to dock. It is quite possible that an arm could be used to grab onto a free-floating astronaut, or else provide the astronaut with something to grab onto. This of course relies on the fact that 1) the astronaut is close enough, and 2) the arm has not been destroyed during the accident that sent the astronaut into space.

So ultimately, a free-floating astronaut who has been untethered is not in an ideal situation, but certainly not unrecoverable. There are several redundant mechanisms which the astronaut can use to save themselves in emergency situations. Redundancy is the key to safe space travel!

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    $\begingroup$ Also, correct me if I'm wrong, but I think the Zvezda only has fuel for a couple m/s delta V (and a fresh Progress or ATV not a ton more), so it may be less useful than you think. The ISS is too big for "gracefully swooping in". $\endgroup$ – hobbs Nov 29 '16 at 21:31
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    $\begingroup$ Throwing something is useless. The astronaut could never apply the force precisely along a line through their own center of gravity. Throwing mass would mostly impart angular momentum, I.e. just make them spin, compounding the problem. $\endgroup$ – Jim Garrison Nov 30 '16 at 2:50
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    $\begingroup$ @JimGarrison How about with feet, i.e. jumping upwards? On earth, people manage to jump without spinning and get about 3m/s delta-V. $\endgroup$ – jpa Nov 30 '16 at 6:04
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    $\begingroup$ @JimGarrison the best possible way to "throw" something would be to put it "below" your feet and then "jump" away from it. That way you get maximum force out of it with minimal angular momentum. The problem would be aligning yourself right, so that you actually propel yourself in the correct direction to ease your recovery. Even then it is a huge risk because you could still drift completely out of control as you have to guess the right momentum visually... but instead of drifting away unrecoverably it would still be worth a try. $\endgroup$ – Adwaenyth Nov 30 '16 at 7:14
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    $\begingroup$ It seems very unlikely that the robot arm would be useful. If we're in a situation where SAFER was insufficient, the astronaut was initially travelling at more than 3m/s. If they did that for even three seconds, they're already 10m away and, since SAFER didn't stop them, they're still moving away. The arm itself is only 17m long at maximum extension and I assume it moves pretty slowly. $\endgroup$ – David Richerby Nov 30 '16 at 11:08
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Once the astronaut & space vehicle part ways, they're on two separate orbits. If the force that separated them is impulsive (instantaneous force in a single direction - as in pushing off the spacecraft and forgetting your tether) those orbits intersect at the point of departure.

If you can wait 1 rev (about 90 minutes in LEO) you will cross the spacecraft orbit at this same point with the same relative velocity that you left it. In fact, if you departed perpendicular to the orbit plane (cross-track), such that you created a plane change, you'll come back in just half a rev.

However, as David points out, if your departure maneuver had an in-track component such that your orbital period changed relative to the spacecraft, you will cross its path ahead of or behind the vehicle. In this case, the spacecraft will need to perform a phasing maneuver to sync up. The best place to do this is likely at the intersection of the orbit planes, so you might have to wait two revs or more depending on how much delta v your crew is willing to spend to get you back.

This is assuming unperturbed 2 body orbits - if there's a large drag difference between the space vehicle and the astronaut you could end up with a bigger difference when you come back around. Similarly, J2 will push things around a bit if the the eccentricities or inclinations of the orbits are significantly different.

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  • $\begingroup$ +1 for pointing out nonlinear relative motion. You may add a link to this paper. $\endgroup$ – Andreas Nov 29 '16 at 21:44
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    $\begingroup$ What is "J2"? .. $\endgroup$ – Steve Nov 29 '16 at 21:50
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    $\begingroup$ Steve - the earth isn't round. It's kind of a lumpy oblate spheroid. To account for this, you need a geopotential model (en.wikipedia.org/wiki/Geopotential_model) to accurately model the earth's gravitational field. The J2 zonal term of the spherical harmonic gravity model is the dominant term and corresponds to the earth's equatorial bulge (equatorial radius of the earth is 6378 km vs 6357 at the poles) This causes the right ascension of ascending node and argument of perigee to precess at rates that are a function of apogee, perigee and inclination. Search on "J2 perturbation" $\endgroup$ – CoAstroGeek Nov 29 '16 at 23:22
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    $\begingroup$ Andreas - I've only had time to take a very brief look at your link. Are you familiar with the Clohessy-Wiltshire equations? en.m.wikipedia.org/wiki/Clohessy-Wiltshire_equations $\endgroup$ – CoAstroGeek Nov 30 '16 at 14:05
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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – called2voyage Dec 2 '16 at 14:33
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You're right, losing a spacewalk tether by itself isn't an unrecoverable situation. In fact, on a normal spacewalk, anytime the tether isn't taut, it's effectively the same as not having it. With a bit of maneuvering, it's certainly possible to recover an extra-vehicular object, without having to reel it in by tether.

The problem, at least in movies, is that when the tether breaks, there's usually something else that's gone very wrong - an explosion, impact by space junk, etc. Now there's a serious situation that demands the attention of the crew on board, and may affect maneuverability or other spacecraft systems. If the person left outside isn't brought on board quickly, they could drift away, have their orbit decay, lose life support, or otherwise become much more difficult to recover.

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  • $\begingroup$ Trouble is (theoretically) that if the fellow on EVA has no thrusters, even a tiny drift or tumble is unrecoverable, so he better hope someone on board has a rope or other retrieval device that can be tossed his way. (Or do like Matt Damon and puncture the pressure suit and hope the thrust gets him home before he passes out :-) ) $\endgroup$ – Carl Witthoft Nov 29 '16 at 15:59
  • $\begingroup$ Would a drift or tumble be unrecoverable? The drift would be caused by the force exerted on both astronaut and spaceship, minutely affecting their orbits. A tumble would just be a change in the astronaut's attitude; he should not be able to affect his own orbit without using a thrust or force of some kind (even something like throwing a hammer would give him a slight impulse) but if he is just waving his arms and legs, he might start to spin a bit (or a lot) but he shouldn't cause himself to change orbits. He should also be able to have some control on stopping that spin with more movement. $\endgroup$ – Omaha Nov 29 '16 at 16:30
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    $\begingroup$ Or perhaps you are saying the "drift or tumble" is what causing him to leave the tether to begin with? $\endgroup$ – Omaha Nov 29 '16 at 16:31
  • $\begingroup$ @Omaha: Rotational momentum will be conserved unless the astronaut ejects something. If the astronaut were to point his forearms roughly parallel to his torso and start rotate them clockwise, that would cause him to rotate counterclockwise about his torso as long as his arms were rotating (the clockwise rotational momentum of his arms would need to be balanced by counterclockwise rotation somewhere else) but if he had any rotational momentum the only way to keep his torso orientation constant would be to be continuously rotating some other part of his body. $\endgroup$ – supercat Nov 30 '16 at 17:29
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If your loose astronaut has no thruster capability of his own, all may not be lost if he was operating from a capable spacecraft with a crew member aboard. Any of Gemini, Apollo, or the US space shuttle were able to make small translational maneuvers under pilot control. At tethered distances the spacewalker and pilot should be able to see each other, and absent major damage to the craft, recovery should be possible if nothing else goes wrong.

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