Reddit reviews The Right Stuff

The speed of sound in a medium is equal to the square root of the bulk modulus K divided by the density ϱ. Since the temperature of space is a constant 2.73 Kelvin, we can use the isothermal bulk modulus relation, which is just equal to the pressure P. Since atoms in space are vastly outnumbered by CMB photons, we can just use the radiation pressure of isotropic radiation which is P=u/3 where u is the energy density. The effective mass density ϱ (again ignoring the negligible number of atoms) can be derived from the energy density u with a rearrangement of E=mc^2 giving u/c^2 . Plugging back into the speed of sound equation we get: [ (u/3) /( u/c^2 ) ]^1/2 = [ 1/3 c^2 u/u ]^1/2 = [ 1/3 c^2 ]^1/2 = [1/3]^1/2 c. So the speed of sound in space is 107,550 miles/s.

The fastest spacecraft ever were the Helios probes which reached 43.63 miles/s while falling towards the Sun, or about Mach 1/2465. So: no, not quite.

We will break it someday, though! The governments of Earth will gather a group of hot young space-fighter jocks, True Brothers who are willing to go up in a hurtling piece of machinery and put their hides on the line and then have the moxie, the reflexes, the experience, the coolness, to pull it back in the las yawning moment! (Starring Dennis Quaid and Ed Harris, based on the novel by Tom Wolfe.)