3
$\begingroup$

My gut impression is that an orbit would negate the feeling of gravity, and that if gravity were to be felt, it would indicate the orbit is decaying; an orbit, in effect, is the speed (or is it velocity?) at which the pull of gravity is effectively 0. So a passive orbit would be out.

I would imagine that for a person within a spacecraft that's in a decaying orbit to feel ~1g, the spacecraft would need to be actively exerting force away from the gravitational pull to maintain that 1g.

Examples would be:

  1. Spacecraft hovering above Earth's surface (not orbiting), with the hovering actively maintained, results in 1g being felt by the passenger.
  2. Spacecraft "hovering" at a certain point away from the sun, probably with some kind of decaying(?) orbit actively maintained.

In my mind, the 1st example makes sense.

Something like the 2nd example is what I'm wondering is possible or not.


For answers: I'm mostly looking for a yes/no + expansion|correction.

Bonus points:

  • A) I'm assuming the force required to actively maintain such a "hover" with 1g force would be, basically, 1g? (~9.8 m/s?) (and where would mass enter the equation, I wonder..?)
  • B) Is there existing verbiage for "an actively maintained decaying orbit"?
$\endgroup$
  • $\begingroup$ @NathanTuggy is it really artificial gravity, though? :P $\endgroup$ – wTyeRogers Apr 7 '18 at 15:36
  • $\begingroup$ If you're using thrust continuously to create acceleration that simulates gravity? Yes. (The tag is used for linear and rotational gravity simulators like centrifuges, mostly the latter; it doesn't rely on hypothetical graviton generators or anything like that.) $\endgroup$ – Nathan Tuggy Apr 7 '18 at 16:05
  • $\begingroup$ However, the continuous thrust wouldn't be to create acceleration, but to negate unwanted acceleration from (natural) gravity. This is in order to not slip further into a gravity well, but to maintain a position in the well where 1g would be felt by the passenger. #artificial-ground? :3 $\endgroup$ – wTyeRogers Apr 7 '18 at 16:11
  • $\begingroup$ I'm having trouble figuring out what you are asking in your part 2. "Hovering" in a "decaying orbit"? Can you clarify / think through what you are actually wanting an answer to? $\endgroup$ – Organic Marble Apr 7 '18 at 17:06
  • $\begingroup$ When you sit in a chair at home, the chair produces a 1g force, creating an acceleration which balances the Earth's pull. If it did not (and neither did the floor, ground, etc.), you would be in free-fall toward the Earth's center. Ok, semantically this is still wrong; technically the Earth does not "pull". $\endgroup$ – bitchaser Apr 8 '18 at 19:51
4
$\begingroup$

As long as you are in free fall, you will experience no sensation of gravity. If you are in an elevator plummeting towards the ground in a giant vacuum chamber, then you and the elevator are both falling together, so you aren't pressed against the elevator floor. You are certainly under the force of gravity, but it just doesn't feel that way because the elevator is falling too. You can illustrate this by putting some paperclips in a plastic bottle and tossing it up in the air. The paperclips will float freely inside the bottle.

In the same way, the astronauts in the ISS feel like there is no gravity because they themselves are in orbit around the earth, just like the station. They are actually falling, but they are moving sideways so fast that the direction they are falling keeps changing and they never hit the earth.

You could build a rocket that would fly up to the height of the space station and just hover there for a few minutes until it ran out of fuel. While it was hovering the astronauts would feel the force of the earth's gravity at that height, which is something like 80% of the force at sea level. I think this is what you mean, and it would totally work, except that you have to keep the engines running, and you can never do that for very long.

But, if you are going to do that, there is no reason to put yourself near an actual gravity field. You can use that rocket power to actually accelerate towards your destination (or slow yourself down as you approach your destination) and you will still have as much of an experience of gravity as if you hovered near Jupiter or something.

Unfortunately, as I said, no current rocket technology can maintain 1 g of acceleration for very long. This is why space travel today is mostly a matter of quick bursts of acceleration followed by coasting, with the exception being continuous but very gentle acceleration by ion drives.

$\endgroup$
  • $\begingroup$ "While it was hovering the astronauts would feel the force of the earth's gravity at that height, which is something like 80% of the force at sea level. I think this is what you mean, and it would totally work, except that you have to keep the engines running, and you can never do that for very long." Perfect! If it were near Jupiter (like you warned against) at a height to attain ~100% of Earth gravity, my remaining curiosity is: how much maintenance energy would that require? (P.S. A previous version of my question included something along the lines of "btw I'm a physics noob.") $\endgroup$ – wTyeRogers Apr 8 '18 at 22:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.