A comet or asteroid hitting the Earth would be interesting. Enough so that this scenario has been played out by computer animators for years, with highly variable results in terms of accuracy and quality. Vastly less likely than getting whacked by a comet or asteroid would be getting whacked by a *planet,* for reasons which I hope I don’t need to go into. As a result, this scenario has been simulated far less often, and usually fairly badly. However, there are two such simulations I’ve seen that seem to be both reasonably accurate and visually “appealing” (well, as much so as the depiction of an event which would wipe out all life on the planet, including the extremophile bacteria kilometers below the surface can be).
This one comes fromt he Canadian/Japanese documentary series “Miracle Planet.” The show discussed the heavy bombardment phase of the formation of the Earth, some four or so billion years ago. At the time the inner solar system was lousy not only with comets and asteroids, but planetoids and minor worlds in vast numbers. On occasion, such worldlets would hit the inner planets, such as Earth. To illustrate this, the documentarians simulated a 500-km-wide asteroid hitting the modern Earth in the Pacific off Japan:
[youtube zc4HL_-VT2Y]
The second one is a typical Youtube video… cropped from a longer video, the audio replaced with music, and then re-posted without proper attribution or context. Feh. Anyway, it appears to show the impact of the hypothesized Mars-size planet Thea with the *very* early, somewhat smaller Earth. The result of this impact was the splashing out into space the material that would become the Moon. While this event has been animated a number of times, this one seems to be unique in that it shows the Earth reacting to Thea *before* impact. And this is as it should be: on the scale of worlds, the hardest stone has less apparent structural strength than water; materials properties play no meaningful role in determining the form of the stone… just gravity and pressure. So when a world the size of Mars gets within a few thousand miles of the surface of Earth, the surface will respond. The gravity of Thea will pull *upwards* as seen from the point of view of an unfortunate observer on the surface of Earth directly below Thea. Of course, at no point is the upward pull of Theaa match for the gravity of Earth. If the surface gravity of Thea is, say, .35 g’s, while the surface gravity of Earth is 1.0 g’s, then right at the moment of impact, the gravity felt be someone on the surface of Earth would be .65 g’s.
However, it’s not *exactly* just that simple. As Thea approaches and the surface gravity decreases, the shape of the planet will deform. Gravity on the Thea-facing side will be a little lower than normal; gravity on the anti-Thea side will be a little higher than normal. The world will seek a gravitational balance, resulting in an egg-shape, pointy end pointing at Thea. Since this will be process that occurs over a span of only hours or even minutes, the way this egg-shaping will be done is by the magma flowing as a vast tide, and the solid surface simply cracking along a million fissures. The magma won’t be sucked out of the ground by Thea’s gravity, but if you make cracks in the surface hundreds of miles long and dozens of miles deep, the magma will come out of it’s own accord due to the subsurface pressure. From space, this will have the approximate appearance shown here.
[youtube zZD1R54r3Tc]
If the impact velocity is 25,000 miles per hour, and Thea is 4000 miles in diameter, then the time it takes from the moment it first touches Earth to the moment the last little bit of the far side of Thea passes that impact point – and is swallowed up by the ruined form of Earth – is 0.16 hours… a bit short of ten minutes. The impact alone, never mind the conversion of Earth into an incandescent blob that does not even remotely resemble a sphere, would be a long-drawn-out horror movie, far longer than the few seconds Hollywood might grant the event.