Our story about the discovery of bubbles around Earth that are 18,000,000 Fahrenheit spurred several readers to ask how spacecraft flew through and discovered them without melting.

The short answer: Temperature is a measure of the average energy of each particle in a gas or other substance. Heat is the total of the energy of all the particles within something. Since the gas in space is very, very thin, the high temperature does not translate to lots of heat.

For a longer answer, I turned to George Parks, a researcher at the University of California, Berkeley who led the team that discovered the space bubbles: “While hot gases in space are not any different from hot gases anywhere, in space the density is very low, on the order of a few particles per cubic centimeter. The low density means that space gases behave as individual particles. The thermal conduction on individual particles is very small and negligible even though the particles have [high] energies. What this means is that there is hardly any heat transfer to the spacecraft.”

Peter Edmonds, Chandra Press Scientist at the Harvard-Smithsonian Center for Astrophysics, pointed me to this web site, which goes into the issue much more deeply (scroll down to “What is the difference between Heat and Temperature?”).