The International Space Station (ISS) is orbiting at nearly 7.66 km/s. At such high speeds, how do astronauts perform tasks outside the ISS? Or is it all relative like astronauts are having same speed (w.r.t. Earth) inside and outside of ISS which is equal to the orbiting speed of ISS?
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28$\begingroup$ How can you stay on a skateboard? How can you stay on Earth's surface, for that matter, which moves with hundreds of miles per hour around the Earth's axis in most inhabited places? $\endgroup$– Peter - Reinstate MonicaFeb 10, 2020 at 2:41
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23$\begingroup$ If it stops, it drops. $\endgroup$– Dawood ibn KareemFeb 10, 2020 at 5:21
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6$\begingroup$ The space station isn't weightless, ever. It is experiencing nearly as much pull of gravity towards the Earth as people on the surface feel. -- Astronauts are weightless for the same reason that people parachuting don't drift away from each other until they open their parachutes: They're all going the same speed, unless/until acted on by an external force. As there's no wind in space, there's no external forces. (Obligatory XCKD: what-if.xkcd.com/58 ) $\endgroup$– GhedipunkFeb 10, 2020 at 6:14
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7$\begingroup$ Don't be afraid, but every one of us is orbiting around the sun at ~30km/s and around the center of our galaxy at ~230km/s. $\endgroup$– Eric DuminilFeb 10, 2020 at 10:12
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5$\begingroup$ @Peter-ReinstateMonica A person walking from the back of a train to the front of the train doesn't experience any significant gravitational attraction from the train. But they still move along with the train, and can change their relative velocity at will. We can experience relative velocity in everyday life. On a plane, on a ship, on a train, on a skateboard. I don't think its conceptually too different from moving around the Sun, or the center of the galaxy, or wherever. $\endgroup$– PolygnomeFeb 10, 2020 at 12:18
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2 Answers
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Let's look at Newton's first law:
Law I: Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.
In modern mathematical speech, this can be stated more precise.
In an inertial frame of reference, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.
For an EVA, atmospheric drag is negligible. When an astronaut leaves the ISS, they do not experience any slowdown due to drag. They just keep their velocity. Since before leaving they were orbiting the earth along with the ISS, they'll orbit along with the ISS after leaving. By pushing against the handles on the outside of the ISS, they can gain momentum and move around on the surface of the station.
So no, the ISS does not slow down or become stationary w.r.t. Earth. But the ISS is more or less stationary w.r.t. the astronaut.
And of course, there is the obligatory XKCD (What-If? Orbital Speed) you should definitely read!
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$\begingroup$ Let's just hope they don't schedule space walks during orbital maneuvers. Oops! $\endgroup$– gerritFeb 10, 2020 at 15:09
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3$\begingroup$ @gerrit As long as the astronauts are tethered or are holding onto a handle, that isn't a problem. Acceleration due to reboosts is small enough that you can hold on, and astronauts are typically also tethered. Still, for obvious reasons reboosts are not scheduled during spacewalks. $\endgroup$ Feb 10, 2020 at 16:02
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$\begingroup$ Newton's first law doesn't really apply here because the ISS travels not in a straight line but in circles, being constantly acted upon by the force of gravity. However, the same force applies to the astronaut, so their trajectory remains the same as of the spaceship. But this follows mostly from 2nd Newton's law and the law of gravity. $\endgroup$– IMilFeb 11, 2020 at 5:49
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$\begingroup$ @IMil respectfully, i disagree. Newtons first law is exactly why the astronaut ends up in (almost) the exact orbit as the ISS. Yes sure, how that orbit looks like follows from the other laws, but still. Since there is nothing acting on the astronaut, it ends up all the same. $\endgroup$ Feb 11, 2020 at 7:26
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$\begingroup$ @Polygnome what do you mean by "there is nothing acting on the astronaut"? ISS and astronaut are a mere 400 km above the Earth surface. The force of gravity acting on them is about 90% of that acting on you and me, therefore they constantly experience acceleration of around 8.8 m/sec^2. That's pretty non-negligible, and ISS can't really be called an inertial frame of reference by any standard. $\endgroup$– IMilFeb 11, 2020 at 11:46
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Not necessary!
Astronauts are in orbit around the Earth, traveling at the same speed as their space ships.
This is true whether they are inside or outside of the space ship.
So if they go outside, they travel along side it without any need to slow down. Of course since they are in circular orbits around the center of the Earth, if they wait 20 minutes they will gently return to the ship because the orbits of the astronaut and the ship will intersect in two places. (for more on that 20 minutes, see How to estimate which astronaut ends up furthest from the ISS after one orbit?)
That's because each orbit is in a different plane that passes through the center of the Earth.
Here are some photos from What is the farthest that a “human satellite” has been from their spacecraft? and some Space Exploration SE favorite videos to help illustrate this
NASA video of McCandless: Astronaut Bruce McCandless II Floats Free in Space, video and much more: NASA Remembers Astronaut Bruce McCandless II.
above: "This Feb. 7, 1984 photo made available by NASA shows astronaut Bruce McCandless II participating in a spacewalk a few meters away from the cabin of the Earth-orbiting space shuttle Challenger, using a nitrogen-propelled Manned Maneuvering Unit." Photo: AP. From here
below: "On Feb. 12, 1984, Bruce McCandless ventured away unrestrained from the safety of his spaceship, which no previous astronaut had done. He could do it because of a brand-new, jet-powered backpack." Photo: NASA. Cropped from here.
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$\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$– called2voyage ♦Feb 11, 2020 at 13:49