Re: Shockwaves in Space

Technically speaking, a shockwave will occur when the propagation
velocity of an object or material exceeds the speed of sound
*in the medium within which it is travelling*.  So your suspicion
about shocks in space is justified, in some sense.

Of course, space isn't entirely empty, of course, and in the
neighborhood of an object like a supermassive star, the matter
density may be rather high compared to the average density of
the interstellar medium.  Thus, when a star undergoes a supernova,
the explosion expels material at velocities higher than the
speed of sound in the surround medium, creating a shock.  The
exploding satellite is a less clear situation - it would depend
on the violence of the explosion, the altitude of the satellite,
etc.

Anyway, the point is that you do need some ambient medium to generate
a shock.  A supernova shock typically travels at a few hundred km/s;
compare this to the initial velocity of material expelled during the
explosion itself, which is of the order 10,000 km/s.  The shock
may last on the order of a million years or so, before dissipating.
However, the supernova explosion generates a lot of *light*, so much
that they are observable even in very distant galaxies.  Thus for the
example you quote, which was estimated at 12 billion light years,
we are detecting the light from the explosion, and not the shock
(which doesn't travel all that far on an astronomical scale).

For more info on supernovae, see
http://lheawww.gsfc.nasa.gov/users/jonathan/snr_group/