What would a planetary collision be like?

Hi everyone, this is my first post here.

I have been wondering what a collision of Earth and another planet would look like to us, and what effects there would be before the impact. Here in my scenario:

A rogue planet, roughly earth-sized, is moving slowly towards us. To make the approach as slow as possible, perhaps it is coming in just behind the Earth's orbit, moving slightly faster, so it is catching up for a slow impact rather than slamming into us at mind-boggling speed.

My questions are:

1 How close would it need to be for us to see it with the naked eye?
2 What effects would it have before the impact - e.g. its gravity on our oceans and atmosphere?
3 Would we be able to see the details of its surface, as we can with the moon, or the colour of it?
4 Just before the impact, would we actually see it break through the clouds in the sky, or would there be no sky at that point? Would it all happen too quickly anyway because the acceleration from the gravity?
5 Would there be any other interesting effects (apart from the end of the world!)


Thanks!

1 It will depend on the planets albedo and size, but recall that we can see Mars at well over 60 million kilometers distance
2 The tidal effects would be rather unpleasant. The entire world would resemble the Passamaquoddy.
3 Yes. We may not spend much time looking, though.
4 I suspect that well before it gets within the Roche Limit, let alone the atmosphere, nobody would be looking anywhere.
5 Too many to count, many of them sociological.

Thats not a idea I ever want to test... Hmmm... and welcome,.
1, Way out there near Neptune distance ... we would have been aware of it for months... or years...
2, At two lunar distances or more the gravity would be creating havoc.
3, Oh yes we would see detail. But would be a little distracted by more pressing issues...
4, No. We would have transported to a safe distant earth like environment before it got that close...
5, If we did not leave then yes. Mass hysteria. Absolute pointless mayhem and anarchy...
6, The actual collision would be a 'Earth Shattering' event... pun intended. out of the debris field would be born a new asteroid field...

PONDERING humanities fate is why I eat 'chocolate'.

What would a planetary impact be like? Nasty! ;-)

TomK wrote:

Thats not a idea I ever want to test... Hmmm... and welcome,.
1, Way out there near Neptune distance ... we would have been aware of it for months... or years...
2, At two lunar distances or more the gravity would be creating havoc.
3, Oh yes we would see detail. But would be a little distracted by more pressing issues...
4, No. We would have transported to a safe distant earth like environment before it got that close...
5, If we did not leave then yes. Mass hysteria. Absolute pointless mayhem and anarchy...
6, The actual collision would be a 'Earth Shattering' event... pun intended. out of the debris field would be born a new asteroid field...

PONDERING humanities fate is why I eat 'chocolate'.

With the conditions involved, it would actually coalesce into a new planet, not a new asteroid belt. The bits and pieces would still be gravitationally bound.
/nitpick

Others may correct me, but I'm not sure there is a scenario where a planet would creep up from behind the earth at roughly the same speed. It would come into the solar system at higher than escape velocity, so I think it would take a fantastic set of coincidences, like a bunch of gravity assists, to slow it down.

Go rent "When World's Collide" (1951)

As with everything Hollywood, their remaking it now to be released in 2012,
but like all remakes it'll probably stink. Go rent the original.

The two planets would attract each other gravitationally, so even if they do creep up on each other at first (as seems to have happened during the Big Splash collision) by the time they actually collide they will be going pretty fast with respect to each other.

Quite a lot of the energy of impact would be emitted as light, so expect a big flash.

Clive_D wrote:

TomK wrote:

Thats not a idea I ever want to test... Hmmm... and welcome,.
1, Way out there near Neptune distance ... we would have been aware of it for months... or years...
2, At two lunar distances or more the gravity would be creating havoc.
3, Oh yes we would see detail. But would be a little distracted by more pressing issues...
4, No. We would have transported to a safe distant earth like environment before it got that close...
5, If we did not leave then yes. Mass hysteria. Absolute pointless mayhem and anarchy...
6, The actual collision would be a 'Earth Shattering' event... pun intended. out of the debris field would be born a new asteroid field...

PONDERING humanities fate is why I eat 'chocolate'.

With the conditions involved, it would actually coalesce into a new planet, not a new asteroid belt. The bits and pieces would still be gravitationally bound.
/nitpick

One should never let the facts spoil a good story...
I was simply stating that (New asteroid belt) That the earth as we know it would be completely destroyed.
The environment we live on and in would be gone.
Yes from the two Earth sized masses would be born a new planet or planet and moons... It could get complicated.
As a related point to this, I have a question; Part of the definition of a planet makes the statement that to be a planet it must have cleared its orbit of debris... ( or something like it ). Could I argue that the presence of the moon defies that. It is after all the coalesced debris of a collision.
I to can not imagine a gentle collision... The gravity of the masses at a intersecting trajectory would be violent.
No gentle bump. Utter and complete destruction is assured. A awfully messy outcome for the inhabitance living on the surface of either body...

Astrotones wrote:

Others may correct me, but I'm not sure there is a scenario where a planet would creep up from behind the earth at roughly the same speed. It would come into the solar system at higher than escape velocity, so I think it would take a fantastic set of coincidences, like a bunch of gravity assists, to slow it down.

you are quite right. The best we could hope for is something at 20km/s probably closer to 40km/s

Excellent topic question!

I wonder what would happen if the rogue planet approached the earth so slowly, like 1mph, so that instead of a big collision they carried on attached to each other like an enormous double scoop of icecream. There'd be some strange sunsets then. You could look up from the valley and see land going into the sky. And be able to drive up it.

dannyadamsuk wrote:

Excellent topic question!

I wonder what would happen if the rogue planet approached the earth so slowly, like 1mph, so that instead of a big collision they carried on attached to each other like an enormous double scoop of icecream. There'd be some strange sunsets then. You could look up from the valley and see land going into the sky. And be able to drive up it.

lol we have our winning answer