Shock Waves in Space
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?
It is a perceptive question. Acoustic waves (sound) are produced when the
atoms/molecules are displaced from their equilibrium rest position by a
cyclic oscillating force. The restoring force for small displacements is
given by Hooke's Law (the displacement of the atoms/molecules is
proportional to the applied force). The acoustic wave can be a shear wave
in which the displacement is perpendicular (up and down) to the direction
of propagation of the wave. It can be longitudinal (compression) in which
the displacement is in the direction of the direction of propagation (back
and forth). Or the wave may be torsional, that is, the material twists back
and forth around the direction of propagation. Mathematically, a torsional
wave can be "decomposed" into a combination of compressional and shear
Now even in the vacuum of space both material particles and light can be
generated by a cyclic process. This produces wave-like behavior in that the
motion of the particles and/or light is periodic. But since there is no
"restoring" force this type of periodic motion is not considered to be
acoustical. Analogously, a machine gun sends out a periodic stream of
projectiles, but one does not categorize the stream of bullets as an
acoustic wave because THERE IS NO RESTORING FORCE. The key characteristic of
an acoustic wave is a restoring force operating.
You cannot have a shock wave in empty space, but it is possible for
the effects of an explosion to be felt away from the source. Let us
just look at what actually occurs and see what effects are possible.
For purposes here, an explosion is a rapidly expanding cloud of gas,
mixed with debris from whatever structure used to contain the gas.
In an atmosphere, if the gas and debris are moving faster than the
speed of sound, they will produce a shock wave, and this will transfer
energy from the gas and debris to the surrounding air, slowing the gas
and debris down and limiting the distance they will travel. In space,
with no air to absorb the energy of the explosion, the gas and debris
will just continue to travel at constant speed until they hit something.
When the gas and debris collide with an atmosphere, they could produce
a shock wave, but only if they arrive in sufficient concentration to
force the air molecules they hit to move together. If the explosion
occurred very far away, the gas molecules would be very spread out by
the time they hit the atmosphere, and they would behave more as a bunch
of separate, rapidly moving particles than as a gas. The air molecules
they hit would not be forced to move as a unit in the direction the gas
was moving; air molecules would be allowed to move aside as well, and no
shock wave would result.
Any astronauts in the vicinity would get hit by some fraction of the
gas/debris, and would absorb essentially all of that fraction's energy.
If they were close, they would absorb a significant fraction of the
total energy of the explosion. If they were far away, they would
absorb only a small fraction. (If you divide the area of the
astronaut's silhouette by the area of a sphere whose radius is the
distance from the explosion to the astronaut, you will have the
fraction of the explosion's energy absorbed by the astronaut.)
The "vacuum" of space is a slight exaggeration. It is not completely
any atoms, but the molecules that are present are spread so widely as to be
easily enough ignored. The difficulty in establishing any kind of wave
pattern means that neither sound nor explosions could be felt or heard.
Now since most explosions as we know them on Earth involve relatively small
amounts of solids or liquids (on occasion gasses, such as fumes from fuel)
becoming very hot gas very quickly, Then I suppose their shock wave could be
felt at a small distance in space. Actually, all you would likely feel is
the rapidly expanding gas of the explosion itself.
As we think of a shock wave on earth, no. We usually feel the intense
compression wave from an explosion. It is a sound wave that is strong
enough that not only does our ear drum sense it, but we can feel it with our
skin. In space there is nothing - no matter - to compress so a traditional
"shock wave" would not travel.
Yet, there are things that could be created by an explosion that travel in
space. Light does not require matter to travel and, although the force is
usually much smaller - light can transmit force. Your class might find it
interesting to explore the idea of a solar sail. The solar wind can be seen
in the effect on the tail of a comet. Someday we might "sail" through space
in the same way that Columbus "sailed the ocean blue" but using light
instead of air to fill the sail. More information can be found at
Explosions in space (such as explosions on the surface of the sun) also send
out particles that could be considered a shock wave. Although our
earth-orbiting satellites sometimes suffer from these explosions, we on the
earth are largely protected by our magnetic field.
NASA launched an experiment investigating the exposure to space that your
class might find interesting. it was called LDEF - Long Duration Exposure
Facility. You can read more at
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
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...
Sound, of course, requires matter to be transmitted and thus does not
propagate in space.
However, an explosion implies the presence of matter. Typically it would be
accompanied by generation of large quantities of gases (even if the
explosion occurs in space). This matter (gas and particles of solids) would
expand rapidly outward, striking any space capsules. I would expect that
would be heard by the occupants of the space capsule.
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Update: June 2012