PDA

View Full Version : Can an object be thrown back to earth from space?

spifflog
04-18-2010, 09:23 AM
If an astronaut was working on the space station, removed a nut or bolt and wanted to discard it, could he just throw it back to earth?

If he tossed it toward earth, it seems that the lack of friction would allow the nut to travel toward earth, until gravity took over and then it would burn up. But I'm guessing it's not that simple. Can I get a hand?

And yes, I know he/she would never do this in any case.

beowulff
04-18-2010, 09:30 AM
Sure, he could just throw it back to Earth, but it's not going to fall back like you might expect. Both the astronaut and the nut are in orbit, so they both have a large orbital velocity. Throwing the nut downwards to the Earth is going to change the orbit slightly, but it's still going to stay in orbit, probably for many, many years. If you wanted to bring it down faster, you would want to throw it in the opposite direction of the orbit - if you could throw it as fast as the orbital velocity, it would fall "like a stone".

The Hamster King
04-18-2010, 09:36 AM
The international space state is moving about 17,000 mph. An astronaut in a spacesuit is not going to be able to throw anything faster than 100 mph. So throwing a nut or a bolt is only going to make a small difference in its orbital velocity.

As beowulff says, throwing it down toward Earth will only change its orbit. Throwing it backwards COULD work, if you could throw it fast enough. But just using muscle power, no way.

Fear Itself
04-18-2010, 09:43 AM
Wouldn't the effect be to increase the object's orbital velocity? As it moves closer to the earth, won't the velocity increase like the ice skater drawing in her arms?

Keeve
04-18-2010, 09:48 AM
Orbital mechanics is very tricky stuff. That's why "rocket scientist" is used so often as a representative example of a very smart person.

Here's what's happening. Let's say you throw the object towards the earth. A few seconds later, it is somewhat closer to the earth than before. A minute later it is significantly closer to the earth than before. Your logic tells you that it is going in a straight line, and eventually it will hit bottom.

But that's NOT what's happening. You've forgotten how fast you're traveling in orbit. While the object has indeed gone downward a few hundred feet, it has also gone eastward a few hundred miles. It's not really going straight down. It's actually going down at a very gentle slope.

What's more, you forgot that the surface of the earth is curved. As you go around the earth, the point below you keeps falling away. That's how you stay in orbit: You are falling, but so is the surface of the earth. Depending on how fast you're going, how high up you are, the angle that you're moving at, and probably a few other things, this will all determine what shape your orbit is in.

Summary: Depending on how fast you threw your object, and what direction you threw it, it might actually fall to earth - especially if you threw it westward, which slows the orbit down. But if you threw it eastward, you'd actually be pushing it into a higher orbit. If you threw it straight down, I suppose it would end up falling, but it would indeed take a long time.

AWB
04-18-2010, 10:02 AM
Wouldn't the effect be to increase the object's orbital velocity? As it moves closer to the earth, won't the velocity increase like the ice skater drawing in her arms?

That is rotational velocity, which would increase if the mass (the discarded bolt) was attached to the axis of rotation by a physical connection, not gravity.

To the OP: I believe that it would go back to Earth, burning up when atmospheric friction got to it. The astronaut couldn't aim it precisely to any particular target.

Relative to him/her, the bolt would travel straight toward Earth. But relative to the Earth, it would also be orbiting, but in a decreasing radius. Now, with the decreased radius would come a decrease in the needed linear velocity, so it's orbital path would change to a more eccentric elliptical.

The question arises: would its more eccentric orbit carry it back outside of the shuttle's orbit, or would it encounter enough friction before that to stay in a lower orbit and/or crash to Earth's lower atmosphere and burn up?

Ludovic
04-18-2010, 10:04 AM
It's true that throwing it with your hands will not enable it to even skim the edge of space, I'm wondering if a simple sling would enable you to achieve enough speed to make the object fall much faster than it normally would at shuttle distances. On Earth, a sling can achieve speeds of 200 mph easily and probably much more in space with no drag. (I'm not counting slings we'd have to develop for this purpose as this would probably be defeating the spirit of the Q.)

What's the minimum speed you'd have to achieve to be able to bounce off a significant portion of the Earth's atmosphere, such that the object will fall to earth in a couple weeks due to drag (either by throwing backward or downward.)

John DiFool
04-18-2010, 10:12 AM
Note that the film Mission to Mars had some issues with the concepts discussed here.

Stranger On A Train
04-18-2010, 10:43 AM
That is rotational velocity, which would increase if the mass (the discarded bolt) was attached to the axis of rotation by a physical connection, not gravity.

To the OP: I believe that it would go back to Earth, burning up when atmospheric friction got to it. The astronaut couldn't aim it precisely to any particular target.

Relative to him/her, the bolt would travel straight toward Earth. But relative to the Earth, it would also be orbiting, but in a decreasing radius. Now, with the decreased radius would come a decrease in the needed linear velocity, so it's orbital path would change to a more eccentric elliptical.

The question arises: would its more eccentric orbit carry it back outside of the shuttle's orbit, or would it encounter enough friction before that to stay in a lower orbit and/or crash to Earth's lower atmosphere and burn up?Fear Itself is correct; the velocity of the bolt will increase as it is thrown toward Earth. Because the change in momentum is so small, it would go into a more elliptical orbit (assuming that the astronaut is in a circular orbit when it is thrown) and it would be unlikely to return to Earth in any short timeframe. Remember that momentum has to be conserved, so if the bolt is thrown with enough impulse to toss is back to Earth, the astronaut's velocity (and thus, trajectory) will also change as a ratio of the bolt's mass to the astronaut's mass, which may not seem like much as the bolt is probably a fraction of a percent of the mass of the astronaut, but it is probably enough that he won't be able to return to his spacecraft. Not a good idea.

I have a nitpick with your use of the term "friction" here; in the context of your last question, you mean aerodynamic form drag, not friction. Skin friction (which is a somewhat inexact term that describes a phenomenon that is distinct from tribological friction) is due to the cohesive losses in the boundary layer that forms around an object moving through a comparatively viscous fluid; that is, the boundary layer attempts to attach to the moving body but the fluid can't accelerate fast enough, and so even though the external pressure is high enough that voids don't appear there is an internal shearing action in the fluid that results in heating, which is transmitted (mostly by direct conduction) to the body. However, in the thermosphere and even the mesosphere, the air density is so low that it essentially has no viscosity and can't be treated as a fluid continuum, and such "drag friction" effects don't apply. Instead, the drag that occurs is due to impulsive momentum transfer between the body and individual air molecules, and what heating occurs is almost exclusively due to ram pressure forming a compressive shock wave which radiates heat onto the body. Often the body shape can form a series of shock waves at different points that will interact (shock-shock interaction) to provide amplified heating in unexpected locations. For simulating body-air interactions at those regimes computational fluid dynamics fails, and methods that actually simulate individual atmospheric particles (called Direct Simulation of Monte Carlo Dynamics) are used.

Stranger

Koxinga
04-18-2010, 11:07 AM
What's more, you forgot that the surface of the earth is curved. As you go around the earth, the point below you keeps falling away. That's how you stay in orbit: You are falling, but so is the surface of the earth.

"The knack of flying is learning how to throw yourself at the ground and miss." - Douglas Adams

ZenBeam
04-18-2010, 12:36 PM
Starting from an initially circular orbit, if the astronaut throws the bolt straight towards (or away from) the Earth, the bolt will be in an elliptical orbit that crosses the astronaut's original circular orbit. It will spend part of its orbit closer to Earth, and part of its orbit nearer to Earth.

If the astronaut throws the bolt opposite his orbital direction, the bolt will again be in an elliptical orbit, but this orbit will touch the original circular orbit at its largest distance from Earth (apogee), and will be closer to Earth than the circular orbit the rest of the time. This perigee will be closer than the perigee of the elliptical orbit from the first paragraph. Since the atmosphere increases closer to Earth, this case will have more drag over the course of an orbit than both the original circular orbit and the bolt's elliptical orbit from the first paragraph, and the bolt will fall from orbit more quickly.

If the astronaut throws the bolt in the same direction as his orbital direction, the bolt will be in an elliptical orbit always farther from Earth than the circular orbit (except when it grazes the circular orbit). This orbit will have the least drag per orbit, and will take the longest to decay due to drag.

LawMonkey
04-18-2010, 12:43 PM
You know, this thread makes the whole concept of 'nuking it from orbit' much more complicated than I ever really thought about. Supposing you've got your orbital launch platform up there, you can't just lob your whatever down at the earth--you've got to figure out how to counteract its orbital velocity in such a way as to get it wherever it is you want blown up, right?

ZenBeam
04-18-2010, 12:52 PM
Well, it's not like it's... Oh wait. It is.

Chronos
04-18-2010, 02:28 PM

chrisk
04-18-2010, 02:42 PM
You know, this thread makes the whole concept of 'nuking it from orbit' much more complicated than I ever really thought about. Supposing you've got your orbital launch platform up there, you can't just lob your whatever down at the earth--you've got to figure out how to counteract its orbital velocity in such a way as to get it wherever it is you want blown up, right?

This is true. On the other hand, when you've got nuclear warheads, it generally isn't too hard to come up with rocket-propelled missiles to put them on and a little computer guidance. Or so I've heard.

Cheshire Human
04-18-2010, 03:10 PM

It's not that we don't read what you write, it's that we don't care what you wrote. What, you think our lives revolve around you?... :D

ETA: Que "grandma's grating voice" here... :D

billfish678
04-18-2010, 03:33 PM
Not so long ago a shuttle astronaut accidently just LET go of a tool bag on a space walk. It didnt stay in orbit too long.

You THROW something in the right direction (and with orbital mechanics, that isnt as obvious as you would think) you could lessen its orbital lifespan significantly I think. But, you aint just going to send it the the ground...

It would also help greatly if said thing was more nerf football like than tiny heavy bolt like.

Keeve
04-18-2010, 04:19 PM
For those who are trying to follow this thread, but want something dumbed down to the point where it doesn't contain the word "elliptical", try this:

Things which orbit close to the earth go pretty fast. For example, most of our satellites, including manned missions, go around the earth every hour an a half or so. Things which orbit higher up need a lot more time, like the moon, which takes 29 1/2 days.

Now, let's say you take an object in orbit, and push it straight down towards the earth, not east and not west, but straight down. All of a sudden, this object is going faster than it needs to go to stay in orbit! So while you thought you were pushing it down to the earth, you've actually acheived the exact opposite - because it is going so fast, it will end up climbing back up!

Eventually, as a result of climbing up, it will start to lose some speed, and head back down again. But it is so far from earth, and moving at just the right angle, that it won't actually hit the ground. It will gain so much speed that it will just pass by the earth and head uphill again. In short, it is still in orbit, but a very ellipt whacky-shaped one,

Malacandra
04-18-2010, 04:41 PM
Starting from an initially circular orbit, if the astronaut throws the bolt straight towards (or away from) the Earth, the bolt will be in an elliptical orbit that crosses the astronaut's original circular orbit. It will spend part of its orbit closer to Earth, and part of its orbit nearer to Earth.

And these differ in what way?

04-18-2010, 04:54 PM
No, you can't hit the Earth from orbit with a baseball pitch. It's not big deal rocket science. You just miss the Earth. If you were half as good as you needed to be, you wouldn't be an astronaut, you'd be in the Majors.

Tris

04-18-2010, 04:54 PM
And these differ in what way?

Ah, but in the other part of its orbit, it'll be in closer proximity to earth.

ZenBeam
04-18-2010, 05:02 PM
And these differ in what way? Sorry, half the orbit will be nearer the Earth than the circular orbit, and the other half will be farther.

Ludovic
04-18-2010, 05:29 PM
Ah, but in the other part of its orbit, it'll be in closer proximity to earth.That doesn't reveal the whole picture. For at least part of its orbit, the object's location will approach the earth's moreso than the thrower's does.

Chronos
04-18-2010, 06:08 PM
But only for 2045 minutes.

Super Kapowzler
04-18-2010, 06:32 PM
Not so long ago a shuttle astronaut accidently just LET go of a tool bag on a space walk. It didnt stay in orbit too long.

.

I think that was one of our Best and Brightest from Minnesota!!!
Tool-Bag, You ROCK!

Just sayin' and shit.

tim314
04-18-2010, 07:17 PM