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Old 2004-05-25, 16:02   #1
ewmayer
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Default Off-Topic: "Mission to Mars"

This flick (starring Tim robbins, Gary Sinise and some other nice folks) was on cable a couple of weeks ago, which was the first time I'd seen it.

At one point about midway through the film, expedition leader Tim Robbins winds up free-floating in his spacesuit in an orbit a couple hundred feet lower than his crew and their vehicle, with no pssst left in his suit's maneuvering jets, just a few agonizing feet out of reach of the end of a cable-on-a-reel thingie one of his crew (played by Connie Nielsen) has shot toward him from where she is floating, in an orbit roughly halfway between Timmay and their spacecraft. The crewmember needs to get closer to him to reach him, but as the movie loudly reminds us, her jetpacks are exactly at 50% used up, i.e. if she goes even a few feet further, she won't be able to get back herself. She doesn;t want to let Tim die like that, so there's all this wailing and then Tim does this great noble gesture and lets the air our of his suit so that he can die a quick death rather than by way of slow agonizing asphyxiation, and his crew can save themselves. Really touching, blah, blah.

Except that Tim's death was completely unnecessary, which kinda takes the emotional impact out of the scene for nitpickers like me. ;)

So, can anyone else who's seen the movie tell me why the Crew-babe could have easily rescued Timmay in that scenario? :D
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Old 2004-05-25, 16:49   #2
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Quote:
Originally Posted by ewmayer
So, can anyone else who's seen the movie tell me why the Crew-babe could have easily rescued Timmay in that scenario? :D
Throwing back the empty jetpacks and acquiring momentum, maybe?

I noticed the scene and made the same reflections... All I could say was that maybe the ship was moving in the other way, and the shortage of jetpack propulsion was calculated bearing the movement of the ship in mind.

It obviously did not convince me.

Luigi
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Old 2004-05-25, 17:02   #3
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Were orbital mechanics taken into account? The "to speed up, slow down" effect is noticable at around 200' - 150m range.

Also, if each vents a tad bit of air, they can zoom zoom.

It would take twice the power to send both back to the spacecraft as one.

Last fiddled with by Uncwilly on 2004-05-25 at 17:03
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Old 2004-05-25, 17:16   #4
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You're getting warm - the whole reason they need more than a tiny puff from the thrusters just to get them started in the first place is that they're at different elevations. While there's no air resistance, basic physics is still the same, i.e. to go to a higher elevation you need to do work, in this case via thrusters. In fact you also need to fire your thrusters to go to a LOWER elevation in orbit, because you need to slow yourself down, and the only way to brake in space is via retrorockets (the same kind they fire on the shuttle when they need to start their descent).

Now we can't expect the movie to pay attention to all the details of orbital mechanics, but just assuming that to move M amount of mass up or down in orbit a distance H, you need fuel proportional to M*H, she could've still easily rescued Timmay with the tools/fuel she had. Think about it....
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Old 2004-05-25, 17:49   #5
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Could she have let the cable out due to gravitational pull? If I understand your hints, the reel at elevation x is at speed (or is it velocity?) s. As the cable is let out, the elevation (of the end of the cable) becomes x-l but still at the same s. To avoid the effects of gravity, the end of the cable needs to be at a speed faster then the reel. Since it maintains the same v at both ends, gravity will pull on it towards the helpless hero. He can grab it and then . . . But wouldn't that make his speed faster, oh, he is falling to earth so his speed is the same as her speed and she can still be in a safe stable orbit . . .
I don't know. This is making my head hurt, yet I find it a fascinating problem. Am I even close?
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Old 2004-05-25, 18:39   #6
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OK, there are two ways of answering this. First, let's pick our parameters. Based on the scene, let's say Timmay is 100 meters below the orbiting craft he needs to get back to, and that CrewBabe has just enough fuel in her suit thrusters to go up or down in orbit 100 meters total. I.e. she can go down 100 meters and reach Timmay but leave herself zero propellant, or she can go down 50 meters and just have enough propellant to go back up 50 meters to her craft. Let's say her grappling-hook-on-a-reel thingie has 45 meters of cable, leaving her a few meters short of being able to reach Timmay with the end of it from the 50-meter midway point between him and the orbiter. According to the movie Timmay has no propellant left in his jet packs, but has something on the order of an hours' worth of air left in his breathing tanks, just long enough for an extended, heart-wrenching agonizingly-slow-death-by-asphyxiation scene. ;)

EASY ANSWER (given that Hollywood is going to dumb things down somewhat): In the scene, they clearly neglect the effect of the person's mass on fuel consumption, because her fuel-left thing is at 50% when she stops. So just going with that level of dumbness, if her reel were 50 meters long, Timmay could grab the end of it, she could reel him in and jet them back to the orbiter. But her reel is a little too short. Still no problem - jet the extra few meters, leaving yourself (say) 45% of the fuel you need to get back. Using that all up takes you 45 meters back up, leaving you 10 meters shy of the orbiter. At that point just shoot your reel thingie A SECOND TIME, back toward the orbiter, and have the guy manning that catch it, then reel yourself up. Problem solved.

HARD ANSWER: In real life even if CrewBabe were able to reach Timmay with the reel thingie at precisely the midway point, the action of reeling him in would pull him up but pull her down, so (assuming they have roughly equal mass to keep the math simple) they'd both wind up at 75 meters from the orbiter. In our scenario, things are even worse - she has to jet to 55 meters below the orbiter to reach Timmay with the reel, and after reeling him in (again assuming equal mass for simplicity) they both wind up 77.5 meters from the orbiter. Because with twice the weight they burn twice the fuel for every meter in elevation, she burns the rest of her fuel (45 meters' worth for one person, i.e. 22.5 meters' worth for 2), and they wind up a whopping 55 meters below the orbiter, which is too far even if they use the shoot-the-reel-a-second-time-back-toward-the-orbiter trick of the easy answer above. But Timmay *and* CrewBabe have decent amounts of air left in their breathing tanks, so especially with 2 pairs of hands, they could do a controlled venting and shoot themselves the rest of the way. OK, that's way too easy, so let's assume that all the air in their tanks isn't enough to accomplish that. So have the THIRD GUY, the one still at the orbiter, tether himself to it (I assume they have something they can use - tie together a couple of bedsheets or something), then use his suit's propellant to get down to where he can reach the end of their reel thingie, a mere 10 meters below him. And they all lived happily ever after...

I guess the lesson of the story is: never send an actor to do an astronaut's job. ;)
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Old 2004-05-25, 18:44   #7
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Quote:
Originally Posted by nomadicus
Am I even close?
Both of them were caught in Martian orbit, and centrypet (sp?) force would have driven both towards the center of the planet... The question is: had our hero enough air to survive?

Luigi
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Old 2004-05-25, 18:48   #8
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Quote:
Originally Posted by ET_
Both of them were caught in Martian orbit, and centrypet (sp?) force would have driven both towards the center of the planet... The question is: had our hero enough air to survive?

Luigi
Too late
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