Why does an orbit decay as in the "MIR" orbit

Factual Questions
1

I have always wondered?
Why does an orbit around a planet not stay there and how long does it take for an orbitting space station to stop orbitting a planet?
And why does it decay anyways?
The Mir was up there for 15 years I think.
I trust your reponces.

2

A Low Earth Orbit (LEO) such as Mir’s is close enough to be influenced by atmospheric, gravitational and other effects. As a satellite encounters these forces, its velocity is reduced and its orbit changes shape. Each change “delta-v” brings the satellite in contact with more and more atmosphere (usually at the bulge around the equator) and the orbit ultimately ends in a ballistic trajectory towards Earth.

Mir was up for 15 years, but not without help. The orbit was frequently ‘boosted’ to maintain its height.

Once you get well beyond the effects of low Earth orbit, which is very useful for a lot of applications, it is possible to put a satellite in an orbit that shouldn’t decay. Vanguard 1, for example, has been up there without reboost since 1958.

3

An extremely good website for this and other space questions is http://www.badastronomy.com . The Bad Astronomer himself is a frequent contributor to this Board.

4

I’ll translate Stephen’s reply into simple English for y’all:

Outer space is not totally empty of debris, dust, air molecules, etc. And there’s a lot more of this stuff close to the earth than there is farther away.

When an orbiting object hits these things, it slows the orbiter down a bit. Now that it is moving slower, it can’t stay in such a high orbit. Then, being lower and closer to earth, there is even more dust and air, which slows it down even more.

Eventually, it is going so slow that it cannot stay in orbit at all, and that’s when it crashes.

5

There’s another factor worth mentioning. The extent of a planet’s atmosphere is not static. This year, increased solar activity has caused Earth’s atmosphere to expand. This expanison increases the amount of friction on objects in orbit, reducing their velocity below the necessary escape v.

6

Those Russians are probably in for seven years of bad luck, now that they broke the Mir.

–Nott

7

Thank you, AskNott. That was a truly awful pun. Now I feel it is safe to say:
Mir today, gone tomorrow. :smiley: