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Name: Lois
Status: educator
Grade: 6-8
Location: TN
Country: N/A
Date: 4/1/2005

One of my students asked a great question we would all like the answer to. They understand there would be no sound in space, but would shock waves from an explosion in space be felt either on Earth or especially if astronauts were on a mission in space?

There is no air or water to carry waves, so some explosions send out much light and little shock. Quiet flashes, almost. But gasses and vaporized solids thrown out by the explosion do carry impulse, and there is no air to slow them down, so they coast outwards a long ways, getting thinner and weaker as they go. If astronauts were close enough or the explosion heavy enough, yes, it could hit hard. Down on Earth, we would not feel the blast unless the explosion was so big that it affected a large fraction of the depth of the atmosphere.

Suppose a lump of material in space gradually got hotter and hotter until it finally vaporized very fast. By gradual I might mean 1 second, or 0.01 second. The gasses leaving the lump first would be barely hot enough to evaporate, so they would be slow. The gasses leaving last would be hottest, so their molecules would be moving faster through space. The faster ones might catch up with the slower ones and, if still dense enough, make a clumped-up sheet of expanding gas, which would hit surrounding objects in a shorter time than the gradual explosion took from start to finish. This kind of impulse-compression is one of the core concepts of a "shock wave", in air or wherever.

On the other hand, if the slow explosion was too small or too slowly heated, the "catching up" would not occur until so far away from the explosion that the gasses were so spread-out that they would pass through each other transparently. Then the explosion might merely feel like a breath of hot wind.

There are a few other things to be said about explosions in space, too. There is no speed-of-sound to help estimate the speed of the shock wave. Hot, thin explosions can send out shock waves that travel considerably faster than sound in air. If the explosion emits light or X-rays onto a solid object, and if they are intense enough to make a bit of the surface evaporate abruptly (have to be pretty close), then the vapor instantly expanding away from the object will push on the object, applying some "blast".

In a way, the vacuum of space allows more different styles for an explosion than the atmosphere.

I am curious what source of explosion source your students were imagining...

Jim Swenson

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