A single impact can't put something into a higher circular orbit-- it can create an elliptical orbit with a higher apogee (max distance), but the correspondingly nearer perigee (closest approach) means that it experiences more atmospheric drag and falls out of orbit faster.
I thought so too, but apparently this is not universally true. Some of the debris can be put in an orbit with a longer lifetime than the original satellite. I got into a long twitter argument with experts in this field, and I was proven false: some of the debris can have longer orbital lifetime and go to higher orbit.
> That seems unlikely, as that requires like 11km/s which is more than the sum of the two velocities[...]
Isn't the first problem here that you're assuming a spherical cow in a vacuum physics problem? Two satellites colliding aren't two indivisible pebbles colliding.
They're going to pulverize on impact, and some of the now-expanding debris cloud might even contain combustibles the satellites were carrying.
The debris also won't only collide once, there'll be a series of rapidly occurring re-collisions. Some of those might impart extra velocity on some of the expanding debris.
Granted, I don't know the math and this is intuition from Kerbal Space Program, but I'm not sure that's right. Wouldn't two objects colliding at 0° and 90° orbits result in a higher apoapsis and the same periapsis? Wouldn't it be equivalent to a normal impulse? Some pieces of one of the satellites would have more energy than they did before the impact.
I don't think the perigee needs to be lower? Consider a collision fragment that breaks off tangentially to the orbital path and moves faster than the speed needed to maintain a circular orbit: its new perigee should be the radius of the circular orbit, but the apogee will be higher.
