QotW: Would a moon explosion be more dangerous?

This is a great question. Sci fi films love to show a good explosion on the moon, but what would actually happen if this was to occur? Well, our great friend and University of Cambridge’s public astronomer Matt Bothwell, is here to make sense of this outer-space occurrence.

When explosions happen on Earth, there are two things that can cause damage: the shrapnel, and the blast wave. Shrapnel are the pieces of metal from the actual thing that got exploded, which get pushed outwards at high speed by the explosion. As you can imagine, getting hit by a piece of shrapnel is pretty bad for you. On the Moon, because there is no air the shrapnel won’t be slowed by air resistance. Plus Lunar gravity is weaker than Earth gravity, so the shrapnel will fly much further and much faster before hitting the ground.

Any shrapnel from a Lunar experiment will be much more dangerous, over a much longer range. This is the same reasoning of why space junk is incredibly dangerous to satellites in our orbit.

The other part of the explosion, the ‘blast wave’, is a pulse of high-pressure air which travels supersonically outwards from the explosion. Blast waves can be very damaging — they can knock buildings down. The good news for us is that there is no air on the Moon, which means no blast wave. The same goes for meteorite collisions. When meteors hit the Earth, one of the most dangerous things is the air burst, when the meteor slams into the Earth’s atmosphere and causes a huge explosion. But on the moon, no atmosphere means no air-burst.

The blast wave we experience on Earth is the air molecules being pushed together and propelled outwards from the explosion. This is usually more deadly than the fireball an explosion would produce. But if there’s no air to compress, the blast wave can’t occur and so the explosion is less dangerous. When we posted this question to our forum user evan_au agreed with Matt that in the vacuum of the moon, shrapnel would travel further and be more dangerous.

So Neil. Our Lunar explosion will have more dangerous shrapnel, but no blast wave at all. What that actually means in terms of how powerful or dangerous our explosion is, is hard to say without an experiment. We just need to convince NASA!