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Buttonstc's avatar

Why is this satellite going to fall from the sky? And how can they predict the day so accurately ?

Asked by Buttonstc (23679 points ) September 23rd, 2011 from iPhone

Maybe I don’t have a good enough understanding of how the Earth’s gravitational field works.

In my understanding, that’s what holds them up there in orbit without them floating off into outer space, right?

So, if all these satellites are sitting up there orbiting around the Earth day after day and year after year, what changes?

Why/how does one suddenly “decide” it’s had enough of this?

Yes, I realize that inanimate objects really don’t decide anything, but I just couldn’t think of a better way to phrase it. Indulge me, please :)

Presumably gravity stays the same and it didn’t go on a diet to change it’s weight, so?

So, how come? And why now instead of 2 weeks ago or 6 months from now?

Obviously I wasn’t a Science major but I really am seriously wondering about this and feeling a bit dense about it.

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7 Answers

King_Pariah's avatar

The force it exerts while spinning around the earth that counters gravity is what kept it up. The satellite was actually more or less decommisioned back in 2004(?) and its inertia has finally worn off to the point where gravity is now reeling it in for the kill so to speak.

CWOTUS's avatar

Every satellite in orbit around the Earth exists (right now) in a more or less circular path at a precisely known distance from the surface of the planet. But because entropy is inevitable, and some energy is lost with each revolution – no matter how little – the astronomers and rocket scientists at NASA can predict with pretty good accuracy when the orbit will decay to the point where the thing will start to encounter enough of the planet’s atmosphere that the orbit will decay quickly. It’s as if you were driving on a freeway and no longer accelerating, but not holding the wheel, either. You’d start to drift to the side of the road and maintain pretty good speed for awhile… until your car found that guardrail. That would bring you to a screeching halt.

Earth’s atmosphere is like a guardrail for objects in near-Earth orbit. They hit it, and the friction builds up (exactly as it would with your car sliding along a guardrail), which causes heat to destroy the satellite even as its orbit decays faster and faster and it fully enters the atmosphere and then falls. It doesn’t fall in a straight line, either. It’ll streak across the sky like a shooting star of fairly long duration. (I haven’t checked to see how long that is expected to last, but it should take awhile, I’d suppose.)

Every satellite in near-Earth orbit will eventually fall to Earth, it’s just a matter of time and decreasing energy / increasing entropy. (On the other hand, I think the Moon is eventually going to break free from Earth orbit and go wandering into space someday. Not in our lifetimes.)

Nullo's avatar

Inertia would propel the satellite in a straight line. Earth’s gravity compels it instead to stay nearby, and the satellite falls into an orbit. The pull of gravity will eventually overcome the satellite’s inertia and drags it into the atmosphere where it effectively starts crashing into the myriad air molecules that are unable to get out of the way in time. These collisions generate friction which creates heat and before long the whole thing is a fiery mess (railgun test fire for an example of a similar phenomenon). And still gravity continues its relentless pull.

Figuring out where the satellite will hit is a fairly simple matter of ballistics.

Buttonstc's avatar

I knew I could count on some of the resident Fluther tech genuises. I actually (sort of) understood most of that. Thanks.

But, now I know why they make so many allusions to Rocket Science. Good thing they’re not counting on the likes of someone like me to accurately calculate that kind of stuff :)

KateTheGreat's avatar

On another note @CWOTUS should write textbooks. Goodness, your guardrail example was brilliant.

Buttonstc's avatar

@Kate

I agree. Great observation.

I think he would also make a great Science teacher (if he were willing put up with all the teens who think they know everything already)

:D

CWOTUS's avatar

Aw shucks, thanks.

On the other hand, can you tell that I have taught my kids to drive?

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