[dvirsky]

What would happen if a 1g probe traveling at 10% of the speed of light hits an inhabited planet with an atmosphere? The kinetic energy is insane, but will it just burn out in the atmosphere or will it manage to hit the surface and go boom, assuming earth like atmosphere?

[kibwen]

According to the calculator at https://www.calculatorsoup.com/calculators/physics/kinetic.p... , 1 gram traveling at 29,979,245.8 m/s has 449,377,589,368.41 J, or about 449 gigajoules.

According to a quick search, the asteroid that killed the dinosaurs had about 300 zettajoules of energy. 1 zettajoule is 1,000,000,000,000 gigajoules.

So it would be on the order of a trillionth as much energy as the Chicxulub impact.

[barbegal]

The energy is something like 100 tonnes of TNT so a big bang but not catastrophic to the planet.

[cryptoz]

Sure but such a thing could easily be taken as an aggressive action or attack and traced back to us. I mean, if there is a technological civilization there, that is.

I love Breakthrough Starshot but this is an interesting risk to think about.

[cowboysauce]

Would they even notice it though? Normal bolides of similar energy happen fairly frequently (see https://catalog.data.gov/dataset/fireball-and-bolide-reports for an example). And even if they did, would they interpret it as a hostile act when similar events happen naturally without ill effect? It would mean the "attacker" posses the technology to accelerate materials to appreciable fractions of c while simultaneously not knowing that the impacts wouldn't do any damage to the target.

Should we similarly be afraid of making radio broadcasts lest our intended recipients interpret our beaming of energy as a hostile act? A somewhat similar concept is actually mentioned in the novel Blindsight. You could even go full Dark Forest and say that we're a threat simply by existing all the while brazenly advertising our presence through the oxygen in the atmosphere.

[MadnessASAP]

I would like to think that a sophisticated civilization would understand what was happening.

I also giggle at the idea of a highly advanced civilization arriving at earth to tell us humans exactly what they think of someone firing a relativistic shotgun at their planet. Even if it was only loaded with bird shot.

[eszed]

If there were a technological civilization there we'd have heard their radio broadcasts. [sad-face] - because I wish we had. It'd be nice to have neighbours.

[digging]

Interesting to think about but my gut is telling me it's not even worth planning for. It should be an infinitesimal chance of impacting a planet.

[Jabrov]

I think it's unlikely it will impact the surface because it's so small. It also depends on the material. Temperature on entry can be thousands of degrees and most materials would just vaporize

[GuB-42]

It is big but not that insane. Think about it: these probes are accelerated with a 100GW laser, they are not getting more energy. So cut the middleman and shoot that 100GW laser at the planet directly.

On Earth, we get about 1kW/m2 from the sun. So 100GW is what we get from a 100km2 patch of land. So, all the energy from the system is enough to turn night into day for an area the size of a medium sized city.

Of course, it is not a continuous beam, so it would be more like an explosion. It would be nuke-sized if it wasn't for losses, but obviously, there would be losses. But the thing is, we are not destroying planets with the energies we get from a 100GW laser.

Edit: Also, at these speeds, the probe is a bunch of high energy particles causing nuclear reactions along their way, things like chemical bonds don't make much sense. There is an XKCD What-If about relativistic object hitting earth. Not the same scale, but it may give some idea of the physics involved. https://what-if.xkcd.com/20/