Explosions in Outer Space

Explosions in outer space on the big screen are magnificent to behold. Brilliant colors of stuff shooting outward in all directions, decreasing their density as the inverse square of the distance. The sound of the explosion rocks the spaceship with a thundering roar just as the light flash is first seen. Finally, bits of debris scream past. What do you think about this space physics?

Answer

The colors of the explosion are probably okay, and perhaps one can appreciate the beautiful outward-going streamers isotropically distributed. However, very seldom does a real explosion, even in a vacuum, distribute the stuff isotropically. One also would expect bits and chunks of vastly different sizes, with a large chunk or two left over near the explosion origin.

As a dramatic example, in 1987 Supernova 1987A in the Large Magellanic Cloud, bound to our Milky Way galaxy, blew apart, dumping practically all its energy into neutrinos, but there was still enough energy to produce a lot of photons for a bright flash of visible light that was first seen by amateur astronomers in Japan. Today that light flash continues to expand with decreasing intensity, and the gas cloud of particles also continues to stream outward, impacting molecules and other stuff in various directions to help “paint” the region in beautiful colors. Data seem to indicate that the remains at the origin consist of two small, massive objects orbiting a common barycenter.

The real problem with hearing an explosion in space is that there is no medium to carry the sound waves, so no sound from the original explosion should be heard by the spaceship crew or by the audience in the theater. The light travels at nearly 3 × 10⁸ m/s, and the sound would transit at the snail’s pace of 3 × 10⁴ m/s or slower. The light flash comes before the sound for any safe distance. Of course, any debris from the explosion hitting the spacecraft would make an impact sound carried by the walls of the ship and by the internal artificial atmosphere. Then there is the noise of the other spaceship going past, but that is another story.