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Comment by elorant | Hacker News Reader

And what happens if the probe crashes on the planet with that much kinetic energy?

Total annihilation of the tiny, light probe and its contents happens. It will either burn up in the atmosphere before impact, or if none is there this event be negligible compared to all other space objects impacting said planet.

Also figuring 10g of mass, an online calculator suggests it would have about 4 megatons of tnt worth of kinetic energy, or 8% of the Tsar Bomba. Not negligible but not planet destroying either.

I get 1.85 * 10^13 joules for 10 grams at 0.2 c from https://physicscalc.com/physics/relativistic-kinetic-energy-..., or about 4.4 kilotons of TNT equivalent. I think that you may have slipped a factor of 1000 somewhere (calculating for a 10 kilogram probe, maybe?)

I also checked it, but you were faster to publish. My result was also 4.4 kilotons TNT equivalent, not megatons.

That's almost suspiciously similar to the Tunguska event, [1] a 3-30 megaton event from 1908, which flattened a large area of Siberian forest. Airburst meteorites hit fairly frequently in our solar system.

I don't even know if you could detect a difference between an extrasolar high-velocity (60,000 km/s) low-mass (10^-2 kg) airburst and an interplanetary 'low'-velocity (27 km/s) high-mass (10^+9 kg) airburst.

Importantly, these events are purely thermal, not nuclear, so there would be no radioactive fallout.

[1] https://en.wikipedia.org/wiki/Tunguska_event

for comparison, the 2013 Chelyabinsk meteor was "estimated ... to be equivalent to the blast yield of 400–500 kilotons of TNT"

And it caused broken windows, other damage, over 1000 injuries.

This would be 10x larger?

To be fair, if that meteor had hit over a big city, it would have caused much more damage. More probable if it had hit over open ocean, it would have done very little.

https://en.wikipedia.org/wiki/Chelyabinsk_meteor

Space is massive and planets with life on them are (at best) extremely scarce.

This probe crashing in to a planet that any life form cares about would be extremely unlikely.

If it's a few grams of mass and a few m2 of area, it will evaporate if it hits any atmosphere.

It will be the size of a compact disc though. I'm not sure it will encounter enough atmosphere to absorb that much kinetic energy before it hits the ground. Meteors enter the atmosphere at 37 miles per second; this will be going a thousand times as fast. It will probably look like an instantaneous vertical spike of incredible brightness followed by a fairly large impact explosion. I imagine that at .2c the atoms of atmosphere would be essentially stationary, so the trail through the atmosphere would be a cylinder of total vacume surrounded by a rapidly expanding shell of exotic high energy particles. The air would be ionized and conduct a lot of electric current too, so this would be followed by a humongous lightning bolt. All that momentum is focused in a single direction, straight down- so I imagine the probe would penetrate pretty deep into the planet's crust, straight through to the mantle, maybe, which might cause a volcanic eruption. Would be quite a sight to see, I imagine. Like god's railgun.

> All that momentum is focused in a single direction, straight down

Well, no. Straight down is possible, but not necessary or even necessarily likely.

I think there's relatively little empirical evidence with respect to relativistic collisions at macroscopic scales, but isn't there a simpler conservation of energy argument to be made? Absent fancy orbital mechanics maneuvers, isn't the upper bound of the 'planet killing' energy of impact the total energy imparted by the accelerating lasers?

Or am I missing something (could be physics was 30+ years ago.)

> isn't the upper bound of the energy

The energy of a 10g object at 0.2c is not just bounded, it is calculatable, and not really that large. The earth has had worse encounters, multiple times, in the last century. The above wildly overstates even a "best case straight down direct hit" scenario.

https://news.ycombinator.com/item?id=33177030

Sure, but the energy of a bomb explodes outwards in all directions, such that at a distance of R the amount of energy per square meter decreases according to 1/R^2. while this projectile's energy is concentrated in a tiny area and all in the same direction, so it won't dissapate the same way. A cherry bomb and a bullet with the same amount of total kinetic energy are going to act in very different ways. And this thing is going at .2c. The time it would take to go from very top of the atmosphere to ground is 1.6 milliseconds (on Earth, assuming 60 miles thick atmosphere). That's not a meteor. And certainly not a bomb. That's a railgun.

I did misspeak I said "straight down", that's not necessarily true. Could be along a diagonal at any angle.

> Sure, but the energy of a bomb explodes outwards in all directions. while this projectile's energy is concentrated in a tiny area and all in the same direction ... That's not a meteor.

With respect, I think that _is exactly_ how a meteor works.

So the extreme velocity times smaller mass gives us a middling kinetic energy. The difference, the extreme velocity, means that the energy is burned up as the object disintegrates into atoms over milliseconds not seconds as a regular meteor with the same energy would. Atoms and small fragments won't sustain their velocity.

There's a fast bright flash of superheated air, a clap of thunder as the trail collapses, done.

Yes, you know I did some more research, and it's too late to change what I wrote earlier- but you're probably right, or at least I'm not sure anymore. I still think my version would be more fun! But, I'm the kind of person who is willing to reevaluate in the face of evidence and I think I was probably not correct.

Respect to you as well!

Then the loincloth-wearing natives will see it and worship it as their new God, thus violating the Prime Directive

This is the greatest thing I've read all week

It's also way overestimating things. More like 10x the 2013 Chelyabinsk meteor

https://news.ycombinator.com/item?id=33177030

"Straight down" is odds against. Over a place that's not empty land or water is also odds against.

This NEEDS to be a scene in a disaster pr0n movie. Where's JJ's Bad Robot when you need it?

At least the c. elegans don't survive to fuck up the biosphere, as per the root comment.

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