B Why don't humans orbit the Earth?

[Prog47]

Let's say a single human exits the atmosphere? Why doesn't he orbit the Earth like the moon does? Why does he drift in space instead?

 

[Orodruin]

Your premise is incorrect because you are not specifying the initial conditions properly. What happens will depend on the speed an direction of how the person moves.

 

[CWatters]

+1

What makes you think he will "drift in space"? What does that even mean?

 

[fresh_42]

Let's say a single human exits the atmosphere? Why doesn't he orbit the Earth like the moon does? Why does he drift in space instead?

He doesn't drift in space. He falls back on earth. If he leaves a spaceship, he has all the orbital velocity the spaceship has, so he stays near the spaceship. It's the same thing as if you jump up and down in a train. You land on the place where you jumped off. If he leaves Earth with the right velocity and angle, he could orbit earth.

 

[Prog47]

He doesn't drift in space. He falls back on earth. If he leaves a spaceship, he has all the orbital velocity the spaceship has, so he stays near the spaceship. It's the same thing as if you jump up and down in a train. You land on the place where you jumped off. If he leaves Earth with the right velocity and angle, he could orbit earth.

Okay I was just thinking about this in the wrong way. I thought why does the moon orbit the earth? Why can't a human? I guess it depends on his velocity

 

[CWatters]

A human can. Think about what happens on a space walk outside the ISS. The man and ISS are both in orbit around the Earth (hopefully extremely similar orbits!).

 

[OscarCP]

If an astronaut leaves the spacecraft while not attached with a tether or other restraint to it then, of necessity must leave with at some speed relative to it. That puts him or her in a slightly different orbit. The faster the astronaut leaves, the greater the difference between the astronaut's orbit and that of the spacecraft .

Two things happen:
(1) the spacecraft and its departing passenger, who leaves by pushing outwards with normal muscular strength, will move at slightly different speeds, along slightly different orbits, with slightly different orbit periods: they would turn around the Earth at slightly different rates. The result is that the astronaut drifts away from the spacecraft with a slow long-term drift equal to their average velocity difference. Why "average?"
Because their velocities are not quite constant: each velocity consisting mainly of a very slowly varying drift plus a once per revolution oscillation: almost like a sine wave plus a constant in each of the three dimensions of space.
(2) Having slightly different periods and due to those oscillations, the two orbits also turn around each other forming a Lissajous' curve that is the composite of their relative oscillations in three dimensions. The time it would take to complete each turn is approximately the same as the mean of the two orbit periods, and pretty close to each of them. As they are in a low Earth orbit, every some ninety minutes, give or take.

Putting (1) ad (2) together: the astronaut, as seen from the spacecraft shall:
(a) Drift gradually away, while staying in orbit (if the push to get out is not so extraordinarily strong to be enough to cancel much of the orbital velocity (of some 7 - 8 km/s), something so strong that most likely will kill the astronaut).
(b) Be seen to turn round and round the spacecraft along a helix that is the combination of (1) and (2).
If the drift is slow enough, the astronaut shall keep circling close the spacecraft for a very long time, until due to air drag is slowed down to the point that stops orbiting and falls burning on reentry into the thicker atmosphere. That will take quite a while.

Do you remember "Gravity"? The 2013 movie where two astronauts, played by Sandra Bullock and George Clooney, until then linked together are separated when the man let's go his end of the band keeping them together, so the woman can jump away and reach an entrance to the refuge offered by an abandoned space station? In this movie, he not only drifts away, but also falls to Earth, to inevitably burn in reentry much before he can die of thirst, hunger or run out of the breathable air in his spacesuit tanks.

Well: no, that can't happen like that and is a major flaw in the movie that, after all, is about "gravity."
Instead of falling down after letting go of her, he would quite slowly drift away while turning round and round the space station, in plain sight from the place where she finds refuge thanks to his sacrifice. And she could be close to him long enough for him to die in her proximity while his agony is broadcast by his suit radio, and then for his corpse to continue to cycle her, relentlessly, for a long time (with enough bad luck).

Much more dramatic and also gruesome this way, don't you think?

 

[DaveC426913]

Well: no, that can't happen like that and is a major flaw in the movie that, after all, is about "gravity."
Instead of falling down after letting go of her, he would quite slowly drift away while turning round and round the space station, in plain sight from the place where she finds refuge thanks to his sacrifice.

Doesn't that kind of depend on which way he was pointing - and how fast they were spinning when he let go?

If he let go when he was moving "backwards", his new more elliptical orbit could approach atmo at perigee - only 45 minutes away.

I guess he probably couldn't cancel sufficient orbital v, but that's really more of a quantitative (i.e. implausible) error, not a qualitative (i.e. impossible) error.

 

[OscarCP]

Not, really, because the direction he moves is irrelevant in the context of your comment, assuming that he pushes off using normal muscle strength (or some low-power astronaut propulsion system, such as a small rocket pack). He cannot change his speed this way, until then the same as his spacecraft of some 7 - 8 km/s (much faster than a rifle bullet) nearly enough to fall out of orbit in any time less than after making many revolutions around the Earth and only because of air drag slowing him down. Something that on a small and rather compact object, like a suited astronaut, would be very weak force taking a long time to have enough effect to deorbit the astronaut. Probably longer to what that would take for the spacecraft , maybe a few years. That is why small satellites without propulsion means to counteract drag are expected to stay in orbit for more than one year, at the very least. Otherwise it would not make a lot of sense to go through the trouble and the expense of building and launching them if they are going to last too short a time, like some sort of space mayflies, before drag brings them down too soon to do much that is useful.

 

[PeroK]

Do you remember "Gravity"? The 2013 movie where two astronauts, played by Sandra Bullock and George Clooney

I'm sceptical that you could learn physics from a Sandra Bullock movie. That said, I don't think I've actually seen one.

 

[OscarCP]

Bullock: She's OK, I have seen several pretty decent movies with her as lead actress and, in my opinion, she is a good actress.

But why this movie about "gravity" (among quite a few other science fiction movies and shows) is so deeply flawed, in this case, when it comes to how gravity shapes the orbits of things in space -- the physics' specialty known as "orbit dynamics", also a main component of "astrodynamics", the physics and basic engineering of space travel -- is something I would put the blame squarely on the movie makers and their science adviser(s), or on how little the former care about the advice of the latter -- even while their names are proudly listed in the movie's closing titles. The thing is that, while some good science fiction movies and shows play fast and loose with the physics involved, that is in incidental "rubber science" and also "far out, but maybe one day they might be shown to be, somehow, if not quite as depicted, possible" things (e.g. faster than light travel, telepathy, telekinesis), not in those that shape the main story, as in this case does "gravity.". Because this movie was presented as being "realistic", and "Gravity" is just not so, when it comes to ... gravity. (The cascading of dangerous space junk, unfortunately, is not science fiction, but an increasing possibility, with the deployment of so many satellites at much the same altitudes, particularly these days.)

 

[Melbourne Guy]

...is something I would put the blame squarely on the movie makers and their science adviser(s), or on how little the former care about the advice of the latter...

I know it's off topic to the OP, but this brief interview with Gravity's science adviser, Kevin Grazier, suggests accurate orbital mechanics was not a motivator for the plot:

https://www.spaceanswers.com/space-exploration/gravity-the-science-behind-the-movie/

 

[berkeman]

A necropost has resurrected this old thread. The Mentors are looking into forking off the new thread. Wait one please...

 

[berkeman]

After review, this old thread will remain closed. Please start a new thread if there is a valuable discussion to be had. Thanks.