# Is there an upper limit for the internal size of space stations?

I was reading about the various TransHab proposals (at http://www.astronautix.com/craft/traodule.htm, among others), and I started to think about pushing the idea to its limits. Assuming you had a large enough payload area on a rocket (or space shuttle - once upon a time), you could launch some pretty big modules. Most of the plans called for multi-"story" modules, with different levels. But a much bigger, simpler version would, of course, simply have no levels, and be just one giant cavity. Reminiscent of Star Wars landing bays, right?

But there's a catch. Most (okay, all) of the modules for space stations over the years are tight, cramped, and not for the overly claustrophobic. One upside is that you can pull yourself around using bars on the "walls". In a large, cavernous module, that would be hard (unless you plan on using an MMU all the time!). Artificial gravity could make this even worse, by trapping any astronauts on a single side!

So my question is this: Is there an upper limit to the interior of space stations because it becomes impossible to move around in a controlled fashion? Are space station modules doomed to be thin and cramped forever?

Note: I don't think that this is a duplicate of Are there any theoretical size limits of man-made space stations/structures?, simply because I am interested in the interior of such a space station - my hypothetical module could simply be the size of a gymnasium and still be of a problematic scale, as opposed to the miles-long structures discussed in the other question.

• I imagine there are many pressures to build as small as possible: less structural mass to launch into space, less volume to maintain a comfortable environment in, less cross-section to get hit by micrometeroids and debris, and so on. – Russell Borogove Aug 26 '14 at 20:53

Have you seen tours of the space station? The modules are not particularly cramped. Some parts are, especially where they connect with other modules, but most seem quite spacious actually.

Kibo, the Japanese module and also the largest module on the station, has seemed somewhat too spacious for the astronauts, as they could get stuck in the middle. [1]

From wikipedia, these are Kibo's dimensions, and thus a version of an answer to your question:

Length: 11.19 m (36.7 ft)
Diameter: 4.39 m (14.4 ft)


Since they could to get stuck in Kibo, I'd bet that it would be tough to build a module much larger without "levels" or some kind of infrastructure to get around.

• Of course, if that infrastructure is a personal "jet-pack" you put on when you get up in the morning... that would be awesome. – Nickolai Aug 26 '14 at 20:40
• Do remember, that the inside volume is very different from the outside volume. Remove thickness of hull, which is not insignificant, since it has space to absorb micrometeorite impacts and decelerate them. Probably lose a foot at least. Then the racks, convert the round space to a square internal space, probably less than 9 feet across. – geoffc Aug 29 '14 at 13:30
• Skylab was even bigger, like 20ft in diameter – Innovine Oct 17 '18 at 11:33
• @Nickolai The article you cited didn't say astronauts were getting stuck, simply that they could get stuck, "Space Station Astronauts COULD Get Stranded in Kibo." (emphasis added) – Bob516 Jan 18 at 22:28

With respect to "getting stuck" in a large open space in an orbiting space station, I believe the dangers have been exaggerated.

First: A "stuck" astronaut would need to work pretty hard to get stuck. If he/she is moving from the wall of the compartment to the middle, it's tricky to stop in mid-air. There could be a line run from one side to the other, then removed. Or fellow astronauts could do it to a unsuspecting victim. Sort of the orbital equivalent to being stuffed in a high-school locker...

OK, say you are stuck. This means that you are in your own orbit around the earth, unaffected by anything the space station does. The center of mass of the space station is following its orbit, and all the parts of the station (except you) are pulled along in the same orbit by the structural integrity of the station.

If the center of mass of the space station is located inside the large space, and that's where you're stuck, well, then you are stuck (except see below). As a bonus, you'll still be stuck if someone changes the station's orientation.

Picture the line traced by the center of mass of the station as it orbits and how this line passes through the structure of the space station. You can only be stuck, motionless, if you are located on this line. You can only be stuck, with oscillations, if you are located close to this line. The oscillations get bigger, around the line, as you get farther from the line. If the oscillations are too big, you'll get close enough to something to grab it.

Finally, if you are stuck, you are no longer affected by any decay in the space station orbit. The ISS changes speed, and thus altitude, because of the slight amount of air friction, and must be periodically boosted back up. Eventually the space station will come to meet you. The time scale for rescue would depend on the state of the earth's atmosphere and the shape and orientation of the station.

Then, you could always sneeze, or ...But I digress...

• you cannot get "stuck" in the center, unless you collide with something. If the chamber is empty, when you push against a wall you'll continue to travel right across the chamber to the other side, then bounce and traverse all the way across again. You'd need to collide with something of just the right mass and velocity to cancel out your relative motions, and this is very unlikely. – Innovine Oct 17 '18 at 11:36
• @Innovine I could see it being a practical joke though, a right of initiation. Also, the information provided for the "stuck" comments in the highest-voted answer are appreciated, I wanted to read more about that. I didn't think "danger" when I first read it, honestly my first thought was "Ha... Hahahaha..." and I imagined a PHD astrophysicist astronaut (a person of extreme prestige and caliber) just like "well, dangit... not again!" Thanks for the expansion on the stuck part. – Magic Octopus Urn Oct 17 '18 at 13:51

As Nickolai suggested, this is more of a technical problem, and so there are several imaginable technical solutions. Reading the article he linked, it seems to be a temporary annoyance in Kibo, rather than a serious problem.

"Swimming" might work for relatively small distances. If modules get bigger, you would of course need a more sophisticated solution, such as a small hand-held cold gas thruster. Although it would take practice to operate in a way that doesn't send the astronaut spinning wildly. (This solution appears in the anime "Planetes", which is excellent, btw)

A simpler solution would be to create an air draft in the station that pushes the astronauts towards one side. This approach is of course limited in how large the module can be, because the required energy to create such a draft would become unreasonable.

Obviously some draft always has to exist in a space station, or the CO2 that the Astronauts produce wouldn't get transported away from them. So you could rescue yourself by sending the air conditioning into overdrive.

• "or the CO2 that the Astronauts produce wouldn't get transported away from them" - Isn't the movement of the lungs enough to take care of that? – LocalFluff Aug 27 '14 at 13:16
• To take it out of the body, yes, but once it's outside the body, in zero-G it just stays in front of your face. So without ventilation, when astronauts breath they are basically create a cloud of CO2 around their heads. – Nickolai Aug 27 '14 at 14:00
• A simpler, cheaper solution might be as suggested in the referenced article: a few lengths of cord stretched between strategically chosen anchor points. – Anthony X Aug 29 '14 at 2:36
• In case of emergency, an astronaut can take their t-shirt off and flap it. Whatever produces an small air current when done in Earth produces an small thrust when done without gravity. – Pere Jul 3 '19 at 13:53