Bad Astronomy: Misconceptions and Misuses Revealed, from ...

254 BEAM ME UP
9. . . . Our Hero gets a lock on them and fires! A huge ball of
expanding light erupts past us, accompanied by an even faster expanding
ring of material as the Dreaded Enemy’s engines explode.
Explosions in space are tricky. Stuck here as we are on the Earth,
we expect to see a mushroom cloud caused by the superheated air
in the explosion rising rapidly, accompanied by an expanding circle
of compressed air formed by the shock wave as it moves along
the ground.
The lack of air in space strikes once again. In the vacuum of
space there is nothing to get compressed. The expanding shell of
light that is the trademark of most science-fiction explosions is just
another way to make viewers feel more at home. The debris itself
expands more slowly; pieces fly out in all directions. Since there is
no up or down in space, the explosion will tend to expand in a
sphere. The debris will no doubt be very hot, so we might actually
see what looks like sparks exploding outward, but that’s about it.
Of course, it’s a lot more dramatic to have nifty things happen
during an explosion. The quickly expanding shell of light looks
really cool, if implausible. Sometimes, though, it makes some sense.
In the movie, 2010: the Year We Make Contact, Jupiter is compressed
by advanced alien machinery until it is dense enough to
sustain nuclear fusion in its core. The core ignites, sending a huge
shock wave through the outer atmosphere. This would get blown
off and be seen as an expanding shell of light. That was relatively
accurate and fun to watch, besides.
A special effect tacked on in recent movies is the expanding
ring of material seen in explosions. This started with Star Trek VI:
The Undiscovered Country, when Praxis, the Klingon moon, exploded.
The expanding ring that results is for my money the most
dramatic effect ever filmed. I also have to give this scene the benefit
of a doubt. The expanding ring we see during a large explosion
on Earth is shaped by the ground itself. You can think of it as part
of the explosion trying to move straight down but being deflected
sideways by the ground. In space, you wouldn’t get this ring, you’d
get a sphere. But the explosion in Star Trek VI was not a simple
one; it’s possible the expansion was distorted by the shape of the
moon. A flat ring is unlikely but not impossible.