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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?

Cornelis
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learner
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    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? – Peter - Reinstate Monica Feb 10 '20 at 02:41
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    If it stops, it drops. – Dawood ibn Kareem Feb 10 '20 at 05:21
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    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: https://what-if.xkcd.com/58/ ) – Ghedipunk Feb 10 '20 at 06:14
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    If nothing else, consider the inconvenience involved in slowing down to a stop from many kilometres per second, and having to boost back up again when you were done. – Starfish Prime Feb 10 '20 at 07:58
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    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. – Eric Duminil Feb 10 '20 at 10:12
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    @EricDuminil I wanted to makes the same comment at first but refrained because there is a crucial difference: We are not only orbiting the sun with the rest of the Earth because we are in the same orbit (we are a percent or so off because of Earth's rotation, and satellites are off much more); we come along because we are bound to Earth's gravity. The space station, by contrast, does not exert significant gravity on the space walkers -- they are indeed in very similar orbits. – Peter - Reinstate Monica Feb 10 '20 at 10:50
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    @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. – Polygnome Feb 10 '20 at 12:18
  • @Polygnome You are right, the concept of relative speeds (or rather, relative rest) is the same in all cases. But the reason we don't move apart over time is different. – Peter - Reinstate Monica Feb 10 '20 at 12:29
  • If you want to get a decent understanding of how this works, you should play Kerbal Space Program – llama Feb 10 '20 at 17:36
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    @Ghedipunk The space station isn't weightless, ever. This isn't really true beyond the technicalities of it being a microgravity environment and not strictly a perfectly zero-g frame. A stable orbit is a true spacetime geodesic, so anything following that trajectory will be weightless. I think you're trying to point out the misconception that the ISS is in a gravity free environment, which it is not - despite it, and the astronauts, being effectively weightless. – J... Feb 10 '20 at 23:02

2 Answers2

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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!

Polygnome
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  • Let's just hope they don't schedule space walks during orbital maneuvers. Oops! – gerrit Feb 10 '20 at 15:09
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    @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. – Polygnome Feb 10 '20 at 16:02
  • 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. – IMil Feb 11 '20 at 05:49
  • @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. – Polygnome Feb 11 '20 at 07:26
  • @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. – IMil Feb 11 '20 at 11:46
  • @IMil You are knowing very well what I mean. The forces acting on the ISS and the astronaut are equal, except very, very small differences in gravity & drag. There is nothing else acting on the astronaut, so the astronaut behaves exactly like the ISS unless acted upon. And since we have tethers and handles, any miniscule differences in forces during an EVA is cancelled anyways. I think my answer very intuitively describes why the astronaut doesn't float away or the ISS doesn't need to stop. If you are unhappy with it, please provide your own answer. – Polygnome Feb 11 '20 at 11:53
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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.

astronaut Bruce McCandless II participating in a spacewalk

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.

Bruce McCandless ventured away unrestrained from the safety of his spaceship

uhoh
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