# What exactly will happen if a small planet collides with Earth?

This is a video of a simulation of Ceres (a dwarf planet) hitting Earth at 2000km/s.

What strikes me the most, is how fast the shockwave is moving. It takes about 6 minutes for the shockwave to pass through Europe.

So if I saw the collision on the news, I would be able to go on a cliff and watch the shockwave travelling towards me, knowing that within seconds I will be dead and minutes later all life on Earth will be dead.

Earth in the end has a surface temperature of more than 4,000oC.

(This simulation was created with a game-simulator; I know many other elements of this "game" are realistic, but can't tell how accurate this particular simulation is.)

Question:

Assuming same sized object, moving at a more "reasonable" speed (e.g. 50km/s):

• What will the actual effects of such a collision be (shockwave, temperature, oceans vaporizing, dust, etc)?
• Would the shockwave be able to kill me if I were in Europe and the object hit US?
• How fast would the shockwave travel and how much time would it take to engulf a country like Iceland?
• For how long will I be able to watch the shockwave from a 500m high cliff before it reaches me (assuming it will reach me)?
• Some of what you want to know is answered in XKCD what if 20 – RedGrittyBrick Nov 15 '15 at 11:48
• That video is sheer nonsense. 2000 km/s? Sorry. There's no shockwave. There's just utter obliteration. A Ceres-sized object colliding with the Earth at that speed would impart about ten times the gravitational binding energy to the Earth. – David Hammen Nov 15 '15 at 20:48
• @DavidHammen Oh, he indeed changed the velocity from 10 to 2000. I didn't notice it. If we ignore completely the video, is it answerable? Assuming 10km/s? – Fermi paradox Nov 15 '15 at 20:59

Very generally speaking, a crater is about 10 times the diameter of the meteor, with a direct hit.

so at some 950 km in diameter, we can guesstimate a crater roughly covering 9,500 km, which is 1/4 the way around the Earth. If we give an impact speed of slightly greater than escape velocity of 12-13 km/s, it would take over a minute to complete it's impact into Earth, but more realistically, in less time than that, the Earth would begin to rebound and push the impacting Ceres back outwards as it's still moving into the earth and that rebound creates enormous stress and crazy heat and to anyone watching it, I would think a blinding flash.

Ceres is large enough to do some crazy-gonzo damage, essentially boiling the oceans and killing virtually all life on earth. It's something in the ballpark of a million times the mass of the dinosaur killing meteor.

I realize I've not answered your question on pressure waves, which is a great question despite the arguably bad simulation video.

shockwave speed: just a remark that the speed involved in collision already gives you the order of magnitude: for Europe size (~ 3.000 km ) / 10 km/s = 300 s = 5 mn.

now for the effect of the collision, since energy is proportional to the mass and to the square of the collision speed, it is very different depending of the collider caracteristics (M + v).

also, a remark that there is also a light flash (comprising IR), that travel faster (as for thunderstorm), so you have to be far enough to be killed by the shockwave.

There is also a huge destructive seismic shockwave travelling through the floor, possibly faster than the atmospheric shockwave (2 to 13 km/s) depending of the speed of collision. So you might better use a balloon than go to the cliff...

• I don't know how reliable the simulator is, so we should avoid drawing conclusions from it. – Fermi paradox Nov 16 '15 at 20:42