I just read a Sci-Fi novel that featured a space elevator: i.e., a cable or stalk running from the earth to geosynchronous orbit.
In the story, they stopped at the elevation of low earth orbit (same as the ISS). The story said that the passengers still felt a significant amount of gravity at this height. Me: “wait, but the astronauts in the ISS don’t feel gravity. Why would the characters in the story?”
They made a distinction between gravity and freefall. The ISS is in freefall around the earth, hence no apparent gravity in the ISS, but the space elevator is not in freefall so they do feel gravity.
So my questions:
Is this correct?
If so, how much gravity would a passenger feel at the height of the ISS? (Call it 250 miles high.)
Gravity varies as inverse square. So about 0.88 G.
If you step off the elevator at 250 mile altitude, you’ll fall straight down. In order not to fall straight down, you need to move sideways at about 7 km/s, so that you are falling in a gentle arc that matches the curvature of the earth. Which is what the ISS is doing. And which is why it’s so much more work to launch a satellite into orbit than to just go straight up to the same altitude.
Gravity at 250 miles is still about 90% of the value at the surface.
Free fall is when only gravity is acting on a body. The entire ISS is in free fall (an orbit is free fall). But the people in the space elevator are supported by a stationary floor that stops them falling, so they feel the floor resisting the force of gravity and holding them up.
ETA: Think of an orbit (like the ISS) as moving horizontally very fast, and having gravity pull you down with just the right amount of force that the fall pulls your horizontal trajectory downward with the same curvature as the surface of the earth. So you are continuously falling “around” the earth.
The space elevator is (more or less) horizontally stationary (acutally it’s moving with the rotation of the earth, but that’s a low speed by comparison).
The space elevator top station is positioned above the geosynchronous level, so that it is moving faster than an orbital body would be at that level. Thus extra force is needed to keep it from wizzing off, which is where the tension in the cable comes from.
So if you get off the elevator at the geosynchronous level, you will seemingly hover there.
If you get off lower, you will fall toward earth.
If you get off higher, you will enter an elliptical orbit with your present altitude as the perihelion. If the space elevator top station were really high, you might be able to step off high enough to have escape velocity, in which case you would zoom off in a hyperbolic arc.
To calculate the answer to your second question, use the fact that the force of gravity is inversely proportional to the square of the distance from the centers of mass. On the surface of the earth, you’re 3959 miles from the center. At an altitude of 250 miles, you’re 4209 miles from the center.
Also note that when you’re in free fall, you’re weightless regardless of what your altitude is. If you’re in an elevator in a two story building and the elevator starts falling, you’re (briefly) weightless until you hit the ground.
If we ever get to the point of building space elevators, we’ll probably do this, at least for some of them. It’s a great way to launch to locations beyond the Earth.
And NASA actually has drop towers for zero-G research. You put your experiment in a capsule and drop it from the top of the tower. The experiment experiences free fall for 5 seconds before it’s stopped by a braking system.
Passengers in the Vomit Comet enjoy weightlessness for ~30 seconds at a time as the plane flies a ballistic trajectory. Once the plane has nosed over into a significant descent, the pilot pulls out of the dive, climbs steeply, and then the cycle begins again. A typical flight, AIUI, includes maybe a dozen of these 30-second periods of weightlessness.
The OP has us anchored at geosynchronous orbit, so the top floor would be orbital velocity … we’d need a top floor further than geosynchronous orbit, and rockets to maintain the new position, if we wanted to be at escape velocity there at the top floor … weather permitting …
That’s correct. I was the Facility Manager for the ZGF during the 1990s. It is a very controlled stop, the drop vehicle stops in 15 feet or so from 112 mph. It does peak at 50 Gs, but very briefly. It is not rated for dropping live subjects. The G level during the drop is 10 -6 Gs, lower then on orbit.
