Originally Posted By: Arney
I know that the US tracks thousands and thousands of bits of orbital debris. How could the collision of two satellites be a surprise to them?

The orbits drift slightly due to the predictable effects of the Earth, Sun and moon and the random effect of traces of atmosphere.
There was a close approach prediction but you have to make a business decision for each approach warning. If you use up propellant to move out of the way then you reduce the life of the satellite and reduce service if you needed to have a craft in that particular orbit. Whether they should have moved it is something their lawyers will be arguing with the insurers lawyers.
Of course there is the also the "it's never happened before so don't worry" effect.

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But we've gotten to the point where there is enough junk out there that they occasionally collide with other junk, thereby creating a lot MORE debris

That's a worry as more satellites are launched, especially at the mid orbits of Iridium and GPS. Where any debris takes a long time to decay.

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I'm not sure how long it would take for gravity to pull these very small bits of debris out of orbit, if at all.

It's not gravity as such that pulls them down - it's the braking effects of the atmosphere. The height of the atmosphere mostly depends on the sun's activity and the last solar cycle was a lot less active then usual which meant the atmosphere was lower and there was less drag in low earth orbit. This has been great for the ISS and Hubble but means that junk hasn't been cleaned out as quickly.