What You See
In a movie, an explosion in space will look pretty much just the same as an explosion here on earth - or, at least, the hollywood version of an explosion here on earth. For most audiences this is an Acceptable Break From Reality, or possibly not even noticed.
In otherwords, explosions in space are big fireballs or pillars of flame. Glowing debris and a Planar Shockwave are optional, but likely. It's almost mandatory to combine them with sound effects so massive your seat vibrates. And most everyone is just fine with that, because of the Rule of Cool.
Is Not What You Get
In reality, however, explosions in outer space are nothing like that.
There's definitely no Planar Shockwave or Mushroom Cloud, as a similar effect only happens on earth because of interactions between the explosive, the atmosphere, the plane of the ground, and gravity. Several of those four factors are missing in space, even if your explosion is happening on a large pressurized ship with artificial gravity. You still have no atmospheric resistance outside the vessel, because space is a vacuum. The shockwave of an explosion is contained within the vessel where the explosion happened. It doesn't spread into space because, a normal shockwave happens because of displaced air, and there is no air in space.
As a result, the correct shape of the explosion is nearly spherical. Force is applied evenly in all directions, sending debris nearly evenly in nearly every direction. There's also no air resistance to impede the movement of debris, so the explosion should expand almost too quickly for the human eye to process. There's no oxygen to sustain the fire and reactions. There's also a great big void for the heat to radiate out into, as opposed to atmosphere that would retain heat in a terrestrial explosion. The result would be a rapid burnout of the fireball. The big glowing sphere appears and vanishes in microseconds, leaving behind an ever-expanding spherical halo of debris.
Again, there's no atmospheric resistance to slow it, so the debris will just keep going at the same speed until it impacts something or gets caught in a gravity well. Luckily, space is big and empty, and there's really no need for ships to travel anywhere near each other. If not for that fact, one explosion could wipe out a whole armada. Unless the Admiral is an idiot, the space fleet will be barely within (or possibly beyond) visual contact of one another. At that range, the debris is going to be so spread out it will probably miss you. Keep your fingers crossed.
Game and Story Use
- Familiar Hollywood style explosions are, well, familiar, and thus easy for the players to visualize.
- Realistic space explosions are trippier, and possibly creepier, because they don't match our expectations and imaginings. Describing them accurately will make your game memorable, and can establish the "otherworldness" of outer space.
- If the PCs are supposed to be scientists or veteran astronauts, you'll need to take care to explain that this is what space explosions are supposed to look like. You don't want the PCs assuming there's some clue hidden in the oddity of the explosions, and going chasing after a red herring that their characters should have known was perfectly normal.