Houngan
4753
I really liked it, but yeah, it didn’t seem super accurate to have debris circling at different speeds at the same orbital height. Or maybe I missed the explanation.
Menzo
4754
LOL you didn’t miss anything. They definitely gloss over the physics of it, to the benefit of the movie.
Romalar
4755
Yeah, generally things orbiting at the same height are going around the same speed relative to the Earth. However, they can be going at any angle and be able to hit each other, making the relative speed of any collision as much as double that speed, and this is what really matters here.
Houngan
4756
I see my time reading The Integral Trees wasn’t wasted!
CraigM
4757
I think people* have a hard time conceptualizing the speeds at play here. Something in Low Earth Orbit is moving, relative to earth’s surface, at 20x the speed of a high speed rifle round.
At those speeds it doesn’t take much deviation in orbital angle to create huge speed differences. Even 10 degrees of orbital separation of two objects moving at the same orbital velocity can create a speed vector of 1360 m/s, which is still about 4x that of a high speed rifle round.
In Gravity the objects have far less of a velocity vector, as they are visible to the naked eye and take several seconds to cross a relatively narrow field of view. They appear to be something closer to 2-300 mph/ 89-178m/s. Which would be a relatively small orbital angle/ inclination change.
Basically space is fast, and things can go pear shaped in a hurry.
*definitely not calling you out. Even as someone who understands the physics of orbital mechanics it is hard for me to visualize because it falls so far outside of human scale we lack a frame of reference. We evolutionarily do not think at that scale, so it is an entirely abstract concept for everyone.
JoshL
4758
Houngan
4759
To that point, I do or did kinda get it, my main problem was a thing at the same relative speed at the same orbital level blows up, and then comes around again and has a much different speed. Again, maybe I missed the explanation, but I could see something at a higher or lower orbital level throwing off debris both faster and slower, and the fast stuff goes higher and the slow stuff goes lower, but if it’s at the same level the only thing you should encounter would be the stuff at your level that was the center without delta-V relative that should just hang around and not rip stuff to shreds. If there was stuff at different levels that was going through orbital decay, then I get the relative speed difference, but again I don’t recall that being a thing.
CraigM
4760
See that’s the thing. My 10 degree of inclination is for objects at the same orbital height, and therefore same orbital velocity.
(also pointing out that objects in higher orbit actually move slower. The lower your orbit the faster they are moving)
So my example was two objects each moving at the same 7.8km/s orbital speed. Their orbits are 10 degrees off from eachother. Basically think one has an orbital angle that has a maximum latitude of Cape Canaveral (in the 28.4 latitude band), and another object with a maximum latitude on par with Washington DC (38.7). These two objects would have the same orbital speed, just with different orbital inclinations. Essentially their orbits would ‘cross’ each other twice per orbit. These two objects, moving at the same speed relative to earths surface would travel across each other at a speed in excess of 1300 m/s. That is, the geometry of their orbits means they are moving relative to each other at high speed.
Think of it like two cars. If two cars are on the same road going the same direction at 50mph, their relative speed to each other is, effectively, 0. If those two cars are going in opposite directions it is basically 100mph in a head on collision. They follow a geometric progression as the angle changes, so two cars traveling perpendicular (90 degrees) have a relative speed of 50* square root 2 relative speed.
You know triangle geometry and all that :)
So, yes, there is no requirement of different orbits, orbital decay, or anything. Simply two objects on the same orbital height traveling the same speed, but with different angles of inclination would produce huge velocity vectors to each other.
Houngan
4761
Hahaha, no offense, but I’m not engaging on that level! Higher orbit stuff blows up, some stuff goes faster and reaches higher orbit assuming it blew up conveniently on the exact orbital plane, some stuff goes slower and reaches lower orbit, even though relative speed might be the opposite, okeydoke. On a circle stuff at a higher orbit with the same period has to go faster, yes? Real world it all becomes a bunch of bullshit ellipses that degrade immediately, but come on.
But yeah, I get the point of equivalent orbits on different inclinations, I just didn’t think that was really involved in the movie. No way they were trying to get to other orbital bodies on different inclinations for the very reason you said, did they actually lay out that it was a different inclination that created the debris?
Houngan
4763
Okay, NOW I get it! ;)
I see you replying, and what about tidal locking Frodo? Huh, Huh? You got nothing for tidal locks. (I know that would take centuries, but still, neener.)
antlers
4764
If you blow up stuff, the stuff will go out from the explosion in a variety of different orbits. The explosion will be unlikely to change the velocity of the stuff to take it out of orbit entirely (unless it is a ridiculously powerful explosion). When you perturb an orbit, the new orbit will take it through the point of perturbation again; the period and the apogee/perigee will change. So what’s going to happen is you have thousands of fragments with somewhat different orbits, but their orbits will tend to pass through a donut that kind of surrounds the orbit of the original object, with the width of the donut proportional to the energy of the initial explosion. Over months and years as orbits decay, the inner edge of this donut will descend toward the earth and the fragments in it will get sparser. These fragments could hit anything that passes through the doughnut, so eventually anything that’s in a lower orbit. The relative velocity of the fragments when they encounter each other again will be comparable to the speed at which there were ejected from the explosion, so probably hundreds of meters per second. But if they encounter satellites that are in orbits with different inclinations (like polar orbits), the relative velocity may be thousands of meters per second.
Testing out an anti-satellite weapon on a target in this high an orbit was unbelievably irresponsible.
CraigM
4765
Was a phone finger drawing of concept, typing on computer now because fuuuuuccckkkk typing that on a phone.
Heh, So space is counter intuitive, of course. The faster you accelerate the higher orbit you reach, but the slower orbital speed you have. To go faster you actually slow down to a lower, and faster, orbit.
There is no stable orbit for two objects at the same elevation to have different speeds. Either one object is going to go to a lower eliptcal orbit, because its speed is too slow to maintain the same elevation, or one object will expand to a higher eliptical orbit, because it is moving too fast and will travel further from earth’s surface.
I don’t think the movie ever really expresses why, or explains it, because it would be very technical, of interest to only a small subset of crazy people like me, and doesn’t meaningfully change the drama of the situation.
Object go fast, hit space station and orbiter. They break, now what? That’s about what they need for the premise, and about what the movie delivers. It doesn’t particularly bother me because I know how fast these things move, and so it is plausible for two objects to collide with huge velocity vectors even if they have the same relative orbital speed and orbital height.
Houngan
4766
Yeah, you had nothing for tidal locking. Game, set, and match!
Just kidding of course, thanks for getting way down in the weeds, I’m certainly smarter for it. Both you and @CraigM . I can see where the movie could be more reasonable by the idea of a few nasty doughnuts of debris orbits before everything went elliptical and decayed or wandered into the universe.
Related question, how quickly would an elliptic orbit circularize, or an altitude adjustment due to speed, happen to objects of this scale? I’m guessing “quickly”, but I’m curious how quickly.
Matt_W
4768
In the absence of air resistance, never.
CraigM
4769
I mean there is nothing that forces it to do so. Elliptical orbits can be perfectly stable. You would need some force acting on the object at apogee or perigee to circularize it. In fact in LEO it is more likely an object would ‘circularize’ by brushing the atmosphere, lowering apogee. But it also means the perigee lowers, as it loses energy, leading ultimately to reentry.
Basically everything orbits at some level of elliptical. Perfectly circular orbits basically don’t exist in the real world. The earth is slightly elliptical to the sun, the moon slightly elliptical to earth, most satelites have some variability in orbit, and are probably the closest to ‘true’ circular. But even so you would be defeated by the lumpy and uneven surface of the earth.
Houngan
4770
You misspelled “satellite” so your points are all wrong. Again, :)
Yeah, that was a dumb question. I wondered what happened to speeds/orbit if something goes “up” (or some other angle) with an impulse… but I’ve watched Kerbal, I do already kind of know and didn’t need to ask.
Space, man.
Matt_W
4772
If you mean burn away from the surface of the planet, it kind of pivots the orbit around the point of the burn.
