But there's an enormous difference between cosmic dust, which generally doesn't pose any threat to satellites or spacecraft, and the man-made "junk" that are essentially destructive whizzing bullets, cannonballs, and worse. The total mass is essentially irrelevant, what matters is the mass of individual objects.
> Or, at least they are in orbits where danger from natural debris is vastly greater than from space junk.
Are you sure? A quick online search reveals only two satellites appear to have been destroyed -- an Iridium satellite was destroyed in 2009 when it was hit by a junked Russian satellite, while in 1993 the European satellite Olympus was destroyed by a meteor.
So it would seem that so far, natural debris and space junk are tied 1-1. Why do you think natural debris is that much more of a risk?
There are plenty of sats that have just stopped working, even broken apart, for unknown reasons. Telling the difference between a defect and a meteor impact is mostly impossible. Lots of spacecraft have just stopped working for unknown reasons. Until we start inspecting every failed satellite, as we do aircraft, we won't really know.
And note where the 2009 collision happened: over the arctic. This was one polar orbit colliding with a near-polar orbit, right in the danger zone I mentioned above.
a rock the size of pebble or as small as a grain of sand will cause a very bad day when you realize that some of the objects have incredible amounts of kinetic energy due to the speeds involved (the speed of the rock or the speed of the thing to be damaged).
For many satellites the biggest cross sectional area are solar panels. So the most likely part of a satellite impacts by debris will be there.
In terms of probability with debris impacts on satellites the common effect is a slight drop in power output from the solar panels [0].
Keep in mind stuff in orbit is pummeled by micrometeoroids pretty regularly. These are literally dust size grains of material in orbit. They face pretty constant abrasion yet remain fully functional.
But that's the entire point -- fragments of space junk can be the sizes of pebbles you're talking about.
Cosmic dust, on the other hand, is just dust. It can be abrasive but satellites are designed to handle it, as is the ISS. It may have high velocity but the fact that its mass is miniscule means it can be shielded against.
> Or, at least they are in orbits where danger from natural debris is vastly greater than from space junk.
Are you sure? A quick online search reveals only two satellites appear to have been destroyed -- an Iridium satellite was destroyed in 2009 when it was hit by a junked Russian satellite, while in 1993 the European satellite Olympus was destroyed by a meteor.
So it would seem that so far, natural debris and space junk are tied 1-1. Why do you think natural debris is that much more of a risk?