I saw the opening of *Gravity* and it was pretty terrifying, but to quote Billy Bob Thornton in *Armageddon*, it’s a big-ass sky.
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Orbital debris cannot just “drift off into space”. An object’s kinetic energy defines its orbit. Without a means of propulsion, an object’s kinetic energy cannot increase (unless during a collision, another object transfer’s some of its kinetic energy).
Over time, friction with molecular gases (the remnants of Earth’s atmosphere that extend many thousands of kilometers above the planet) and collissions with dust and other debris gradually robs an object of its kinetic energy, reducing its orbital height until it reaches thicker parts of the atmosphere and its rate of deceleration increases sharply, initiating re-entry.
Between 100 km and 500 km in altitude, an unboosted satellite will re-enter in anywhere between a matter of hours to a couple of years, depending on the satellite’s size and density. Beyond 1000km in altitude, a satellite will remain in orbit for many thousands of years.
Should a Kessler event produce a cloud of debris above 1000 km in altitude, safe space flight through that region becomes impossible.
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