How big a meteorite to sterilise the Earth?

Simply put, how massive would a meteorite have to be to sterilise our planet without destroying it? I’m aware that there will be some variation depending upon the speed of impact. And how long would it take for the surface to become habitable? Could mankind speed up this latter process?

Why? Well, suppose we discover a habitable planet (with no detectable sentient life if that bothers you) which we wish to colonise. Sure we could risk exposure to alien germs; but why bother when you can sterilise the place, then colonise without such a risk? Your colonisation craft are going to take a very long time to get there anyway, so time is not of the essence.

Meteor Impact Calculator
Assuming that boiling the oceans would be enough to sterilize the planet, a 325 km diameter rock moving at 20 km/sec would do the trick.
To actually kill everything you’d probably have to melt the top couple miles of surface rock too. Perhaps a collision with Ceres could do that.

I do not know how big one would have to be to sterilize a planet down to bacteria in the deep ocean but I suspect if it did that you are approaching if not surpassing destroying the planet outright (or at least something on the order of what struck the earth to form our moon). The planet may ultimately recover but it’d be like starting from scratch like a new planet formation and take tens or hundreds of thousands of years (heck…maybe millions).

FWIW the meteor that likely did in the dinosaurs was ~6.5 miles wide and travelling at 15 miles per second (54,000 mph) and that only destroyed about 70% of the life on the planet so your planet killer will need to be bigger. Moving an Ark Ship would be a lot easier than trying to accelerate something that big to respectable speeds much less guide it with the precision to hit a planet in another solar system.

Ah, I would expect that we’d find a suitable asteroid in the target system. Give the thing a series of gentle nudges at the right times and let gravity do the rest.

That would make the most sense but since you said the colonization craft would take a long time to get there I assumed the sterilization would happen long before someone arrived in the destination system to strap rockets to a local meteor.

If you’re going to hit a habitable planet with a rock big enough to melt its crust, it’s going to take millions of years before the crust becomes solid again. And what happens to the water and air and dirt and stuff, presumably the reason you wanted to colonize the planet in the first place?

If you want a lifeless sterile ball of rock there’s millions of them out there in the universe. There’s no way to destroy all life on a planet and still have a planet capable of supporting life.

Send an automated craft ahead of the colonisation fleet.

Do we need to melt the whole crust? Simply raising it above 100[sup]o[/sup]C for a little while should do the trick, and that’s well below the melting point of most rocks.

But how many of them are capable of supporting life?

And how long for the crust to reform after the impact and melting? It would be really annoying if we got there and discovered that we would have to wait another ten million years until everything had cooled down enough to have nice watery oceans, let alone actual pleasant temperatures and beaches.

Thing is, an average temperature of 100C isn’t enough, because that’s just an average. Even if the seas are boiling in parts of the planet, they’re still only temperate in other places.

So you have to get the temperature waaaaay over 100C, especially if you’re only using one rock. Any rock big enough to wipe out all life on the planet is going to be big enough to render the planet uninhabitable for a million years of so.

And after all life on the planet is extinct, after you’ve waited a million years…or 100,000 years…you still don’t have a habitable planet, because there’s no atmospheric oxygen left. You do know that oxygen comes from plants, right? Without plants making oxygen the atmosphere is going to be nitrogen, CO2, methane, ammonia, and other such things.

And you can’t just seed the place with plants, because plants need oxygen too. You’d have to dump anaerobic bacteria down there, wait for hundreds of thousands of years for them to colonize the planet, then try to establish cyanbacteria to start the photosynthetic cycle again. How many centuries do you want to wait?

There’s no such thing as a lifeless habitable planet. A lifeless planet requires that the humans there live in pressurized artificial environments. You might as well live on the Moon. Heck, the Moon would be easier, since you don’t have to worry about erosion and corrosion from the toxic atmosphere on your habitats.

To emphasize my main point.

There’s no way to sterilize a planet capable of supporting life unless you change it into a planet that’s incapable of supporting life. Then you’ve got a lifeless hunk of rock. Maybe you can terraform that lifeless hunk of rock somehow…but you could do that with an already existing lifeless hunk of rock. If your colonists want to spend the next hundred generations inside domes while waiting for the terraforming to complete, why couldn’t they have just stayed in space?