Could two planets survive a collision if it were a glancing blow?

Answer 1

Actually, a glancing blow isn't possible. The gravitational pull between the two planets would be so great it would basically result in a head-on collision. Any life on those planets would have long since been extinguished prior to the collision due to the gravitational force. The planets themselves would be destroyed and all that would be left would be pieces of rubble.

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I admit to having a couple problems with this answer. First, life would continue until the extinction, for if the bodies were moving towards a 'head on collision' the gravity would become such as to fragment either surface until - at worst - hours before impact. Second, there are two examples of glancing blows in our solar system that I am aware of: Mars and Earth. Most of Mars' northern surface is flat due to having become a sea of magma around four billion years ago. The object struck a glancing blow, but it was small and perhaps slow. It is believed that any ejecta remnants of the object all or mostly fell into Mars shortly after the strike. The Earth's moon was created by a similar strike, but in this case the object was much larger - although still relatively slow moving. Some of the object managed to leave the earth after striking it - along with significant portions of the Earth's atmosphere and crust. The molten ejecta eventually coalesced into the moon.

Now, I realize that in most instances either the object striking the planet (even if planet sized itself, as with the formation of the Earth's moon due to a Mars-sized body colliding with the Earth) would either meld with the planet (as with Mars' impact) or be reduced to a stream of molten slag that might later either form a moon or a ring of debris around the planet.

My question is - say, presuming the scenario in which the moon formed - if the object were moving faster and - due to orbital considerations and velocity - likely to only skim the surface of the planet - could it survive the impact. I realize that if no impact occurs, it is likely in this situation that one or both objects might be forced into a new orbit by the sling-shot situation (rather like some satellites sling shot around planets to increase their velocity and so reach the outer solar system). Could a sling shot, slowed by the friction of actual impacting / sliding along the surface - allow for the survival of both planets? I fully expect, even if survival occurs, that the surfaces of both will be reduced to magma oceans due to the cracking, crumpling, and tidal forces upon each of the crusts. But is it even theoretically possible?

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