Inspired by the recent movie Melancholia, depicting the destruction of the Earth by a colliding rogue planet. I was wondering just what would really happen in such a scenerio. For starters, tidal effects are usually ignored. The planets would start pulling each other to pieces well before the actual collision. Even if the planets didn’t actually collide, I doubt human life would survive having all the water in the Pacific basin dragged around the planet in a three-mile high wave. Or if not that, at some point before the actually disintegration of the Earth, the Earth’s crust would be pulverized by Richter scale 20 earthquakes.
Well, since a collision with a 10-mile-wide asteroid would cause an extinction-level event, I don’t think that much of anything will survive a collision with a body the size of the Earth.
So, what are you asking? Hit, or near miss?
As you mentioned earlier about the tidal effects, the Roche limit is seemly ignored in virtually every Hollywood planet collision. After all, we’re talking about the destruction of the earth! Being stretched and pulled apart–what couldn’t Hollywood do with that?
ETA: user_hostile wasn’t there when I loaded the thread.
I think he’s asking “What would the actual process be as another Earth-sized object approached. When would we see tidal effects, when would we see earthquakes, how large, etc.”
I don’t have any answers, but I could image that if the speed of approach was slow enough, we might well have highly destructive stuff happening due to tidal effects for days or weeks before the moment of closest approach / impact.
Conversely, if the speed of apprach was fast enough, we might well pass through the roche limit and on to the actual impact in mere seconds. In which case the Earth wouldn’t realy have time to covert itself into a ring formation and observers would see something not too far from two spheres colliding.
The moon is now understood to have been formed by the impact of a Mars-sized planet. The Roche limit probably is not an important factor for two bodies of about the same size. They will smash into each other largely intact, melt because of the heat, and either reform as one larger body or disintegrate into many smaller ones depending on initial conditions. Nothing survives that.
Late add … For two twin Earths colliding, the Roche limit is on the order of 10,000 miles. For the kinds of relative speeds we usually see with meteors & such, that distance would be crossed in a few seconds, tops.
Could that be because – as explained in the article you cite – the Roche Limit doesn’t apply to solid bodies, but only to one which is “held together only by its own gravity”?
If a gas sphere or liquid sphere or even a sphere of dust or boulders would be heading toward Earth, the Roche limit would indeed be relevant. But Hollywood likes to deal with solid bodies, and given that starting point, they are actually correct to be ignoring the Roche Limit. (or so it seems to me, based on that Wiki article.)
I also think the time scale of the collision is just too small for tidal effects to be all that much.
Comet Halley (as per Wikipedia) was said to have a relative speed of 157,838 mph to the Earth at some point during it’s passing. Presumably this was during the part closest to Earth as when it was close to the Sun, the relative direction of orbits would have meant a lower relative (but higher absolute) speed.
Any rogue planet would be coming in far faster than that. Perhaps ~10 minutes from crossing the Moon’s orbit to blammo.
Regular tides lag a bit from the Moon’s zenith. (In fact, the peak neap tides lag a couple days behind Moon-Sun alignment.) I don’t see large masses of water moving across the globe in the short window of time as it nears. And solid ground, even less noticeable motion. Not sure if the winds start going crazy a couple minutes before impact. The first real damage is probably going to be when it hits the atmosphere. Then after that … who cares what kills you.
Relative to their size, planets are spheres of dust and boulders- their mechanical strength is negligable.
The linked Wiki article did mention that density is a factor: if one of the bodies is more than twice as dense as the other, the Roche limit of the less dense body will be less than it’s radius. So that’s one out. And if the planets are colliding at typical speeds (20km per sec) or more, there’d be a few hours or less for tidal effects to be felt. So maybe it isn’t as significant as I thought.