No. That’s not how orbit works. If they are in Orbit of Mars, then they are traveling at precisely the right velocity fall towards Mars and miss by going sideways.
The imparting of motion to astronaut R sends him heading, for the sake of discussion, towards Mars. What actually happens is that affects his orbital path slightly, so instead of a circular orbit, his orbit is now somewhat eliptical, and ends up lopsided to the far side of Mars.
Astronaut B goes after Astronaut R. She fires her thruster to move in the same direction and thus catch up with Astronaut R. She could apply a low thrust and take a long time to get there, or fire a long thrust and get there faster and therefore be closer for the return trip.
Either way, when she reaches Astronaut R, she has to grab on, then apply thrust to slow both with respect to the spacecraft. Then she has to apply more thrust to get on a return trajectory.
Here is where it is tricky. Depending upon how long it takes to catch up to Astronaut R, how fast the orbit, how long the orbital path, that velocity that they acquired will have shifted their orbit somewhat, such that they no longer stay close the the spacecraft. As they travel the orbit around Mars, the two orbits are different, and thus diverge. As they continue the orbits, those paths will have slightly different travel times, and will travel closer and farther from matching, but will likely diverge so the close points are farther and farther apart.
Orbital mechanics is a tricky bitch, so I can definitely see how the writers might not have understood, or even if somehow they did, realized the vast majority of the audience would not understand. So they “simplified” to what could be understood.
I think you nailed it. The somewhat misleading dialogue may be due to scriptwriting economy, but the situation is reasonably accurate. I disagree that most people get that, as discussions of this scene have popped up before where people (like the OP) posit that “there is no friction in space,” therefore once they start moving they need no fuel. This is incorrect. There is no turning without fuel, no adjustments, no maneuvers, no stopping. If you run out half way, you keep going straight on whatever vector you had last. When trying to reach that (moving) ship from hundreds of yards away, they would have to be dead accurate–an error of a foot may as well be a furlong. That would be impossible without some way to maneuver.
More evidence that it’s a mistake is what happens when she stops: she decides to shoot her tether gun at him so he can grab the end. Nobody in the movie sees the problem with this. If she didn’t have enough fuel to reverse both her and his motion, then a tether wouldn’t have helped (of course, a tether would have helped greatly if it was long enough to attach to the spacecraft).
From beginning to end they treat fuel as something that needs to be spent in order to move. When she goes after him, she resets the fuel gage and they show it counting down from 100%. There is a nearly constant spray of exhaust out the back of her suit. When it reaches 50% she stops.
If she has enough fuel to travel halfway there, stop and come back, she could have travelled the whole way, stopped and come back.
You beat me to it. Once two objects in orbit are far enough apart, the divergence in their orbits will make it appear that the two objects are “drifting apart.” (And, in their own orbiting frame of reference…they are.) The divergence can be represented (in that frame of reference) as an acceleration away from each other. One could run out of fuel partway through a traverse.
I believe relative orbital velocity was observed in the Gemini missions, when they first attempted rendezvous between two orbiting objects. (A Gemini capsule and an unmanned orbiting probe.)
No, she could not. I will grant you the whole fuel gauge thing is somewhat silly and just a crutch for the audience–particularly the flashing “Point of No Return” on her gauge. The spray of the jets you see is just the slow acceleration, the trimming, and the deceleration.
There are three problems with the “if she could go halfway, she could go all the way” scenario:
First, if she reaches her husband and brings him back, it will more than double her mass. Double the mass and you double the force needed to move it (yes, even in space). If it took 50% of fuel for her to go halfway with just her mass, there is no way she would have fuel to move the combined mass back the whole way.
Second, the ship and her husband are on vectors away from each other. With each second that passes, they get further apart. By the time she reaches her husband, they may be 50% further away then he was when she was at 50% fuel. Now that have to travel 150% of the distance to get back to the ship. They would have to spend fuel to travel much faster in order to intercept it. She would not be able to do that even if it were not for the additional mass.
Third, as I noted previously, only part of the fuel is needed achieve a general velocity, the bulk of fuel to ensure that you are going in the right direction. So, even if she did not have the added mass and even if she did not have to travel the added distance, she still would not have enough fuel because that fuel is needed to maneuver. Unless by blind luck she happened to be on an exact intercept vector with the ship when she is halfway back, she will just end up tumbling in space.
That means to say she can’t take it slow?.. which seems correct to me…
Theoretically, she could just go really slow and use little fuel.
But he was drifting away… You need fuel to correct your path and the longer the path, the more correction to do !.. So going really slow can just cost more in fuel required for path correction.
Yes, if she had infinite time and omniscient accuracy she could have accomplished what she wanted with the available fuel; she had neither of those things, however.
I thought the film did a good job at showing how vast space was, and how difficult it was to locate anything. Any rescue had to happen relatively quickly otherwise she would soon be unable to identify where the ship was when trying to return.
Close. The first US orbital rendezvous attempt was with a Gemini spacecraft and the expended booster that took it into orbit. It failed because the planners of that attempt didn’t consult with the orbital mechanic guys who would have told them you have to speed up to slow down and slow down to speed up when you’re in orbit. You don’t just eyeball the target, point the nose at it, and goose the rocket with a burst.
And I believe some of the space suit rescue stuff was covered in Heinlein’s “Space Cadet”. Not all, but some.
Yes, they would be able move, but they would not be able to move fast enough to catch the ship that is moving away from them. They have a limited amount of time.
I did watch Gravity last night, and (going spoilery here obviously):
George Clooney makes a very analogous sacrifice, claiming that he will continue to drag her out away from the ISS with him, even though she is wrapped up in a parachute cord around her leg which is also wrapped around the ISS. I couldn’t for the life of me grasp why she couldn’t simply reach down, grab the line wrapped around her leg, and drag the two of them back in towards the station. The Bad Astronomer agrees with me.
Cool! I hadn’t known of that one. In SF, Larry Niven notes the (apparent) contradiction between where you try to go and where you actually go. Forward equals up; up equals back; back equals down; down equals forward.
I remember reading that the Gemini astronauts would get caught in awkward “spiraling” approaches to rendezvous, which they called Whifferdills. Much of this is because one’s frame of reference when in orbit is, itself, an accelerating frame of reference, and thus conventional Newtonian laws don’t apply in the expected fashion.
So exactly how long was that tether? Apparently it was long enough for the first astronaut to travel all the way to the vehicle and latch on. That looked a considerable distance - 500 yards? Then after reaching the spacecraft, the second astronaut sets off after him, stops some distance away, and fires the tether. It is just a few feet too short to reach the first astronaut.
So why didn’t she move closer, refire the tether, then thrust back, and when a few feet from the vehicle, her buddies reach out and grab her? Or use the fact that she slowed down their departure and then follow up with the stated plan B to start up the vehicle and come chase them down?
There were solutions.
I was going to suggest recoiling the tether and then using it to get back, but wouldn’t recoiling the tether to pull them together increase the tension and thereby pull on her? Or would that pull be constant, already the force of pulling the guy along? Anyway, if you pull them togehter and travel most of the way back, then use the tether to shoot back to the first two astronauts. Bingo.
Those are all valid points, and those are clearly things that would need to be considered in that situation, but the characters in the scene don’t discuss any of those things. They very clearly subscribe to the idea that if you use more than 50% of your fuel to go out, you absolutely won’t have enough fuel to get back. They treat the “point of no return” as an absolute fact.
