# Is there momemtum in space?

Discussion in 'Archived Threads 2001-2004' started by DennisHP, Jul 9, 2002.

1. ### DennisHP Second Unit

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With the somewhat recent talk about objects passing by our planet and possibly colliding with us etc.. would it be possible for a slow moving mass to be stopped by our gravitational rotation and become a part of our solar system? I am in no man's imagination any sort of scientist. It was smiply a passing thought.

2. ### BrianW Cinematographer

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Yes, there is definitely momentum in space. As to whether something could perform an orbital insertion with just the right kind of fly-by – that’s a much more difficult question to answer.

The answer is, it depends. Momentum, even in space, is conserved. Something coming from an essentially infinite distance (that is, not in orbit around Earth to begin with), unless it collides with Earth, will always have the momentum to carry itself an infinite distance (that is, out of orbit) away from Earth after its closest approach. So, no, anything not on a collision course with Earth will always have the momentum to escape our gravitational grasp, even if just barely. And an object approaching form an essentially infinite distance will always be above escape velocity on its closest approach, or it will collide with us – there’s no in between case that would allow for orbital insertion.

So the only way an approaching object can insert itself into orbit is for it to lose kinetic energy (or momentum, if you prefer – not that they’re the same thing, mind you) as it makes its closest approach to Earth. The most obvious way of doing this is for the object to momentarily come into contact with the Earth’s atmosphere. Do this just right, and you’ll have another small moon to gawk at. You might vaporize a few people in the process in the superheated trail of the object as it burns its way through the air, but that’s a small price to pay for another awe-inspiring, celestial object to spark our imaginations. Besides, you have to throw that extra energy somewhere, so into our atmosphere it goes.

Another way is for the object to collide with the Earth directly, render it asunder, and have the debris field coalesce into a small moon. The collision with Earth sufficiently lowers the momentum of the approaching object to capture it (or its molten remnants) in a stable orbit. This is what most cosmologists believe happened when the Earth acquired its one and only Moon.

By the way, this second method of capturing an object – planet-busting-collision – would kill every living thing on the planet. And since that includes me, I consider that way too high a price to pay for another moon.

3. ### Paul Jenkins Supporting Actor

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4. ### Julian Reville Screenwriter

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5. ### Bill Catherall Screenwriter

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Sorry Paul, but Brian is right. But nice picture!
Trying to get an object that is coming from a essentially infinite distance to make a stop and orbit earth is like trying to get a pendulum to stop at the bottom of its swing. It just doesn't happen without stopping its kinetic energy with some kind of resistive force. The best gravity can do is to redirect its course.

6. ### Paul Jenkins Supporting Actor

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Bill,
Distance is irrelevant in the discussion, is it not? If an object is going 1mph, it doesn't matter if it started out infinitely away from earth or not....
How about some math to back up a real answer? Any astrophysics majors out there?

7. ### Leo Hinze Stunt Coordinator

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Some of the explanations above do not quite sound right.

My qualifications are college physics courses, so I am not an expert. Here is how I understand it:

Space is an essentially frictionless environment. Momentum and energy in a closed system are conserved in space. An object traveling through space is its own closed system. Without friction, there are few influences in space to change the momentum (and energy) of an object.

However, once an object traveling through space comes close enough to be influenced by Earth, the object and the Earth become a closed system. There is some escape velocity above which an object will not be captured by the Earth's gravity. Below this speed, though, the object may either enter Earth orbit, or come crashing to the Earth. If it enters Earth's orbit, momentum of the system is conserved. The actual velocity of the object would still be the same, but in an orbit around the Earth, as opposed to whatever direction it was traveling before.

The last statement made me realize something - pretty much everything in space is in orbit around something else. The Moon orbits the Earth. The Earth orbits the Sun. The comets that we see periodically are in orbit around our Sun. Or solar system is in orbit around the theoretical center of mass of our galaxy. So any object traveling on a near-collision course with Earth is actually in orbit around something else, most likely the Sun, since it is the most influential body in the solar system.

Anyway, back to Dennis' original (sort of) question. Yes, it should be possible for an object traveling through space to be captured by Earth's gravitational field and become an orbiting satellite. However, thinking of the object as having been 'stopped' is incorrect. All the Earth's influence would do is change the object's previous orbital path into an orbital path around the Earth. The momentum of the whole system - Earth and captured object - would remain the same. For instance the Earth's rotational speed may increase or decrease, the speed around the sun may change, the objects own speed may change, etc.

I think there are a few rocket scientists from Johnson or JPL on here who can better explain this and clear up any misinformation in my rambling post.

8. ### Bill Catherall Screenwriter

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9. ### CharlesD Screenwriter

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10. ### Danny R Supporting Actor

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You folks are forgetting gravity braking. Objects CAN be captured with just gravity under the right set of circumstances. This is because objects in space are not stationary but are themselves moving. The difference in vectors can be transfered through the gravitational interaction.

If for instance a meteor approaches earth along its own orbit (opposite direction), and then slingshots around, it will lose speed relative to the earth. This is because the time of influence on the incoming path is not the same as the time of influence on the outgoing path. At just the right speed, capture is possible.

The exact opposite effect has been used to send our Voyager space probes out of the solar system. By sending them so they approach first the inner planets from the same direction they are orbiting, they are pulled faster and faster yet the planet is also moving away from them. When they eventually slingshot away, they do so at a different vector. Thus the probe has gained some of the planets speed. (and very very slightly slowed the planet an equal amount of energy)

Also other bodies can help in capture. Its quite feasible to use the moon to alter an incoming asteroid's vector just enough to allow the Earth to capture it in a highly eliptical orbit. Of course the original objects speed has to be exact. However the math proves that this could be used to capture materials hurled our way from future miners in the asteroid belts without the use of retro-rockets to slow them.

it is going too slow the object will enter Earth's gravitational field and eventually fall to the surface

But the point is that it will NEVER be going too slow. As it approaches earth, it will gain the velocity it needs to escape, because earth's own gravity will provide it. Only if some force works to slow the object somehow (aero or gravity braking) will it be captured.

11. ### BrianW Cinematographer

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Paul, that picture is absolutely breathtaking. Thanks!

12. ### Jack Briggs Executive Producer

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Objects traversing their own trajectories are captured by larger objects' gravitational pull all the time. So, yes, it can happen and does happen. Phobos and Deimos did not start out as moons of Mars, but were stray asteroids captured by the planet's gravity well.

Again, all the time.

But that's not why I am posting here.

Paul: I used to be an amateur astronomer. I am very familiar with the astrophotography produced by amateurs. Very familiar. And, really, the image you posted possesses resolution that exceeds just about any ground-based telescope with which I am familiar.

With all due respect, may I ask if you have inadvertently given us an image of the "Orion Nebula" produced by, say, an orbiting device? By, say, the HST?

If I am incorrect, I will buy you dinner and drinks next time you're in Los Angeles.

(Really, amateur telescopes cannot resolve that kind of detail. In fact, no ground-based telescope can.)

13. ### CharlesD Screenwriter

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14. ### CharlesD Screenwriter

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Jack,
Paul never said that his "geek" brother was an amateur or described his telescope.

15. ### Alex Spindler Producer

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I think orbital insertion can happen pretty easily (assuming it's deliberate - observe the penguin:
http://home.tampabay.rr.com/ferneaux/penguin/
*Edit : Meant to say for level 4 and afterwards that there are several times when you can get the penguin into a stable orbit. It's typically elliptical but occasionally gets to be almost completely circular.

16. ### Bill Catherall Screenwriter

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17. ### Paul Jenkins Supporting Actor

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18. ### Paul Jenkins Supporting Actor

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Jack,
btw, I like steak for dinner
But rarely get to LA, how about a DVD instead?
Paul

19. ### BrianW Cinematographer

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This question reminds me of a ripoff county fair game that regularly took my friends’ money when I was a kid. The game consisted of a bowling ball on two steel rails that curved up and down like a roller coaster. The object of the game was to push the bowling ball over the hump, and then get it to stay in the valley on the other side without it coming back over the hump back to you. The trouble was that there was almost no friction, so if the ball had enough energy to clear the hump and enter the valley, it almost certainly had enough energy to exit the valley back over the hump. It really is like letting go of a pendulum and asking it not to return to its starting point.

If the celestial object in question is moving 1 MPH when it enters Earth’s gravitational influence, then it will be moving at escape velocity plus 1 MPH at its closest approach (ideally, but not feasibly, through the Earth's center of gravity to just keep the math simple), assuming it’s not on a collision course. And it will (definitely) be moving at 1 MPH when it leaves Earth’s gravitational influence. (Keep in mind, this is all relative to the Earth's reference frame.) In order for orbital insertion to take place, the object must be traveling at exactly 0 MPH when it enters Earth’s gravitational influence, which is impossible, since entering Earth’s influence would necessarily mean that it’s moving toward us at some finite speed. It is simply not possible for an object to be captured in orbit without some kind of inelastic collision taking place to diminish the kinetic energy of one or both of the objects. I believe this is what's happening all the time.

On the other hand:

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