# Effects of a Super High Velocity Cannon on a Planet

In the 23rd century, humans are at war with a technologically advanced alien species. Humans discovered a method of faster than light travel during early stages of this war when an alien capital ship was destroyed and their FTL drive reverse engineered.

Humanity, now facing the loss of the solar system and Earth if they can't turn the tide of the war, propose a new solution: blitz and raze. Basically, a few massive FTL-capable ships will be built with a single massive cannon designed to launch a projectile at a small percentage of the speed of light, which will then storm and destroy alien planets. The eventual goal is to hold an alien capital world hostage to force a peace.

Thus, what would the effect of a projectile, say a telephone-pole-sized tungsten or osmium rod, fired at a few percent of the speed of light(between 1% and 5%) at a planet be?

Would it completely destroy the planet, or merely cause an extinction level event? The damage needs to be enough to take out nuclear-hardened bases with protected production facilities for essentials such as food and ammo. An extinction-event level asteroid blotting out the sun isn't enough, ripping the planet in half would be. Would a cannon like this this work?

• It seems to me that a better route would be to weaponize the FTL drive. Maybe there is a bunch of stuff built in to prevent that. Jan 12, 2021 at 21:18
• Randall Munroe wrote a post exploring the effect of a large projectile hitting a rocky planet at various relativistic speeds: what-if.xkcd.com/20. TL;DR: You'll need much more than 0.01-0.05c to get a strategically serious splash, and the effect isn't any different than a nuclear explosion, chemical explosion, or other event which delivers an equivalent amount of energy. Jan 13, 2021 at 2:05
• The FTL drive I had in mind was technically a slip-space drive similar to that in Halo. Basically the ship is in another dimension and can't interact with anything in the physical space we inhabit until it leaves. Jan 13, 2021 at 2:58
• This is outside the frame of the question, but you can use just about any serious weapon if you can somehow use the FTL drive to fire on the planet with impunity. Instead of a few massive ships with heavy weapons, build a swarm of lighter vessels. - Think like firebombing Dresden and Tokyo, instead of vaporizing downtown Hiroshima and Nagasaki. Jan 13, 2021 at 5:21
• Why not just put the FTL drives on some asteroids, and make the ships themselves the projectiles? If you have a nice 1km long asteroid, that's much more effective than any cannon. Jan 13, 2021 at 6:47

# Not much

A telephone-pole sized rod of tungsten would mass about 4.5 tonnes

4500kg, moving at 5% lightspeed, has kinetic energy of 5e17 J
That's the energy of about 120 Megaton Nuclear bomb.
A very respectable KaBoom indeed, but nowhere near Earth-Shattering. It's barely city-flattening, really.

at 1% lightspeed, the energy is less than 1/25th as much.
Relativity does not get a big mention at 4% c, and is virtually undetectable at 1% c

If you want to

damage needs to be enough to take out nuclear-hardened bases with protected production facilities for essentials such as food and ammo. An extinction-event level asteroid blotting out the sun isn't enough, ripping the planet in half would be.
then you need to seriously up the speed and/or mass of the projectile.

You want something stronger than a extinction-level asteroid impact? Thus something more that about 1.0e26 J energy.(that's the energy of the infamous dinosaur-killing Chicxulub impactor)

At 5% lightspeed, your projectile would need to mass 200 million times as much.. About 900 million tonnes. Say as heavy as 150 Great Pyramids of Giza.

Not so practical. Not for a gun, in any case. Maybe for a pushed meteor.

Let's try upping the speed. Same telephone-pole sized projectile.
To achieve that energy, you need to up the speed to 0.999999999993 c
That is 800 times the energy of the protons in the LHC particle accelerator.

I don't think your projectile gun is a practical idea.

• You beat me to it. But yeah, you need a lot more mass or go much, much faster. Jan 12, 2021 at 21:17
• I wouldn't describe 120MT as "barely city-flattening". The fireball alone would be about 6km across. Jan 12, 2021 at 23:06
• I imagine using the MIRV concept would greatly increase your damage rather than just focusing all the energy in your impact into one area. Jan 12, 2021 at 23:46
• Re: the fireball size. The effect, I suspect, would be roughly comparable to a low depth underground nuclear explosion. That is, significant portion of energy will be absorbed by the ground. There would still be a fireball, but not as devastating as with a low-altitude atmospheric explosion. Jan 13, 2021 at 0:13
• I'm going to keep this open until tomorrow, but this pretty much answers it. Thanks! Jan 13, 2021 at 2:48