> Not having a helmet on, though, would be very bad for your health.>

NASA accidentally tested it in 1966: http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970603.h...

The speed in your definition would refer to the temperature of the particle, and not the temperature of the vacuum. This paper has both the vacuum and the particle not in thermal equilibrium, so the temperature of the vacuum is not due to the speed of the particle moving within it.

You can also have temperature from a photon flux.

> Common misconception... a vacuum is actually an incredibly efficient insulator

Yes, it is even a very real problem for EVAs and spacecraft. One of the first things the Shuttle does is open up its radiators to help get rid of its internal heat. The astronauts doing EVA have a problem with heat too, they have to wear an internal cooling suit with fluid piped around their bodies.

http://physics.about.com/od/glossary/g/temperature.htm http://en.wikipedia.org/wiki/Temperature "n the classical thermodynamic approach to temperature, temperature of an object varies with the speed of the particles it contains, raised to the second power. Therefore, temperature is tied directly to the mean kinetic energy of particles moving relative to the center of mass coordinates for that object. Temperature is an intensive variable because it is independent of the bulk amount of elementary entities contained inside, be they atoms, molecules, or electrons. In order for the temperature of a system to be defined, the system must be in thermodynamic equilibrium. Temperature may be considered to vary with position only if, for every point, there is a small neighborhood around that point can be treated as a thermodynamic system in equilibrium (i.e. local thermodynamic equilibrium). In the statistical thermodynamic approach, degrees of freedom are used instead of particles."