One question often asked at this point is, "If a vacuum is such a good insulator, then how do you cool a spacecraft?" Heat builds up in a spacecraft from its electronics, its fuel cells, its rocket engines and incoming solar radiation, among other things. All of this heat needs to go somewhere or the spacecraft will overheat. However, the spacecraft is floating in the world's biggest thermos -- the vacuum of outer space. So how does a spacecraft dump its excess heat?
It turns out that heat dissipation is a fairly significant part of the spacecraft design process. For example, if you look at this page you will see that Skylab had a gold coating to reject infrared radiation coming from the sun, and a large radiator to dissipate heat that built up. A space radiator can use nothing but infrared heat radiation to dissipate heat, so it must be much larger than a similar radiator on Earth, where convection plays a big part in the cooling process (almost all radiators on Earth use fans to improve the effects of convection). Similarly, the inside of the space shuttle's cargo bay doors are lined with radiators. Once the shuttle is in orbit, one of the first things the crew does is open these doors so that heat can radiate away, as this page explains.
So if space is a giant vacuum and a vacuum is an insulator, why do astronauts get cold fingers on space walks? The cold-finger problem is actually quite interesting. This article discusses some of the reasons.
For more information on thermoses and related topics, check out the links on the next page.