Magnetout, while I can’t give you a speed, I do seem to recall that if you want to hit the Earth from orbit with a thrown rock, you actually throw out the back of your spacecraft, behind you along the orbit you just travelled. This maximizes the reduction of orbital velocity, and so the rock drops to earth faster. I could be wrong, especially because I can’t think where I’m dredging this up from, but it sounds right to me.
To update Jack Handey:
“If you ever drop some bolts into the void of outer space, let 'em go, because man, they’re gone.”
I know; I mentioned that. I want to know how fast it would have to go if we wanted to do it by propelling the bolt directly at the Earth.
Well, I’ll be pretty peeved if I got hit by that bolt from the blue.
Well, I’m going to ask the serious question! I took an orbital mechanics class in college. If an Astro/Cosmonaut has five bolts for a required four-bolt job, what is the hurt if he gently glides that fifth bolt towards the earth (kind of like tossing out some extra trash)? If the bolt reenters the atmosphere and burns up, no loss to anyone, right? But if he just tosses it over his shoulder, then it becomes a major problem.
My question is this: we all know that certain things burn up in the atmosphere. A single bolt will burn up. A major spacecraft will not. Can’t we just draw a line in the sand and say, “Hey, any debris less than 5 tons can be jettisoned for burnup?” and save everyone a helluva lot of trouble?
Tripler
I’m thinking about the debris situation here. . .
Okay. I see what you mean now.
Wow, that’s deep!
As per posts (and my question) above - I’m pretty sure that tossing it gently toward Earth will not actually get it there; it’s not floating motionless in space, it’s in orbit, the same as the object it fell off of.
If we could be certain the objects would leave orbit and burn up, then yes, it would be fine. Mass wouldn’t be the only consideration; a 5 ton sphere of solid steel would not burn up as easily as a 5-ton framework of thin steel braces, for example.
I Googled “space junk,” and yikes! I had no idea there was so much crap up there!
According to one of the sites I visited, satellites in geosynchronous orbit might stay up there for centuries before falling out of orbit. But what about objects in lower orbits? Does anyone have any information on what the typical “lifespan” of a piece of junk in low-Earth orbit is? Years? Decades? I’m thinking mainly of the orbits that the space shuttle and non-geosynchronous satellites use.
Yeah, it’s a big issue. Something to consider when you want to put up that Space Station or Space Elevator. (Arthur C. Clarke talked about the tashk of cleaning all this stuff out of orbit before building his space tower in “The Fountains of Paradise”) There have been whole reports written about ways of cleaning out this space junk. While I’m sure there stuff up there before we started, I think the bulk of space debris is from human launches over the past forty-odd years.
I have no idea what the lifetime of orbit is vs. height. You know that it has to increase as you get higher simply because the atmosphere thins out. I think it’s a pretty small effect when you get as high as geosynch orbit. Satellites that high stay up for decades. But Low Earth Orbit takes its toll. They have to keep sending up spy satellites into LEO all the time. LEO is grat for high photo resolution, but it takes its toll in atmospheric drag.
I don’t know the answer to Mangetout’s question, but I’ll bet you have to propel a bolt toward the earth a lot faster than you can throw it. I think it’s a deceptive situation in that we see the Earth as “down”, so once you derail something from orbit, it ought to fall, right? But orbit itself is a form of falling. It’s just that the earth keeps curving away from you faster than you can fall into it.
It helps if you visualize the gravitational potential – the “gravity well” – with a centrigugal potential added to it. That’s the fictitious potential that maintains you in orbit, visualized as a potential field. It’s like an “anti-well” sticking up out of the center. Now you can see your stable circular orbit as the lowest point in the surface of those combined potentials. The model actually works if you turn it out of wood or plastic or something and put a ball in it.
Now perturb the ball, moving it inward or outward, and you see that it rocks back and forth, inward and outward (that’s the oscillation" I talked of above). Whether you throw it inward or outward, the result is to add that oscillationm that turns your circular orbit into a slightly elliptical orbit. You can try throwing your bolt down to eaerth, but that cebntrifugal potential rises awfully fast, and goes to infinity at the center. It’s likely not going to get to the surface. The best you can hope for is to send the bolt into an elliptical orbit that dips into the atmosphere enough to slow it down.
Otherwise, your best strategy is to reduce the size of that centrifugal potential by a significant amount, so that the size of your orbit decreases. The best way to do this is to throw the bolt backwards, against the orbit direction, really hard.
Actually, you really want to slow down the speed and push yourself into an elliptical orbit that propels you earthward, which is what you do to come back to earth if you’;re in a space ship. But it’s more than you can muster with a good arm.