Robittybob1

1. Nothing says it couldn't 2. I have already given you a reference to 2 above. http://www.scienceforums.net/topic/88421-when-two-planets-collide-what-is-their-terminal-velocity/page-2#entry861986 3. I will do 3 when I get back to the office where I have a sheet set up with the calculations. Initial speed in these cases approximates zero (just off balance and that is all). I am very surprised that that is not just standard physics to you. So far you have just linked to papers that don't even mention Lagrangian Points so what are you talking about.

Show me a reference that says anything about the L3 lagrangian point please. All those other references seem so far off the topic. http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/lagpt.html Still doesn't say it is impossible. Obviously a spacecraft parked at L3 becomes very difficult to communicate with having the Sun in the way. Some references of the instability on the L3 position hint it is due to the influence of the Moon on the Earth -Moon system. So what about when the Moon wasn't there. Was that a period of increased stability? Also in the planet building scenarios I work with the terrestrial planets formed prior to the Sun going main sequence hence the physical size of the protosun would be considerably larger for a while. I'd say that increased the stability too.

Giving him clues as to what to argue. That is allowed isn't it?

I sense a bit of guessing here. I can't see a planet forming at L4 or L5. I know objects get lodged there but do they form there? There is more chance of a planet forming at L3 even if it is ultimately unstable. 100,000 years maybe enough time. I'd like to see some figures. Even so if a planet did form at L4 or L5 and became unstable and drawn to the Earth it would not impact at a speed similar to an object coming in from infinity. I read about that yesterday that it was easier to send a spacecraft to a trojan at the L4 or L5 position, so presumably to reverse applies if an object comes in from the same location. Earth has a trojan at the L4 Lagrangian Point. http://en.wikipedia.org/wiki/2010_TK7 OK let's put some figures on our hunches shall we? http://en.wikipedia.org/wiki/Nebular_hypothesis This is the bit that made me feel anti -

So where do you get that time limit from? I might have read them had you not been so condescending. How long do you think it would take to pull a planet around from the Earth's L3 position?

Doesn't that prove my point for we are saying soon after forming at the L3 it became unstable and later impacted the Earth. Once I do the calculation you will see (and believe) that a planet coming from L3 will impact slower than from infinity.

David - That would depend on the qualities of the atmosphere surely. Early atmosphere had methane and CO2 at high levels and both are greenhouse gases so it could even be balanced if the blanket of thick dense atmosphere absorbed as much heat as it allowed out. Taking that to mean the surface was 1000 degrees. Venus has got a situation approaching this even today.

Well you agreed it was a trojan planet, so there are limited places it could have come from if that was the case, and none of them have anything to do with "infinity". The L3 is a postion co-orbiting with the Earth on opposites sides of the Sun (300 million km). OK it is a long way around the orbit but if it was perturbed by Venus and if it got out of the balanced situation it could begin a journey being pulled toward the Earth, slowly at first but after sometime it would impact. I don't believe the impact speed is the same for all starting positions especial from a Lagrangian point. http://en.wikipedia.org/wiki/Lagrangian_point

To get a Giant Impact collision of course. I had already calculated the speed an object hit earth would get from infinity (hope I did it right) but how did that speed compare to a planet that just coming in from the L3 position (furthest point)?

So does it get enough velocity?

Venus maybe.

So where would it get that extra velocity from? Could the GIT be made to work if the only extra speed Theia had was due to the gravitational attraction of the Earth? If it is already co-orbiting with the Earth it average distance to the Sun is the Earth radius so it shouldn't be falling toward the sun as comets and Near Earth Objects do. If it caught up with the Earth from behind, as the Earth is orbiting the Sun, and the Earth pulled it in faster it could hit the Earth off center and flick matter during impact in the right direction of rotation to form the Moon.

So was Theia a trojan planet before hand or not IYO?

If is going faster than escape velocity it is most likely falling toward the Sun as well.

I had a way of calculating impact velocity on another computer so I'll check it out whether a smaller planet will fall faster. But on the surface all things should fall at the same rate. But when they say from infinity that means it is not falling just toward the Earth but toward the Sun as well (Generally) as could happen in chaotic interaction of 3 or more planetary bodies. I've seen simulations of three body problems and not all of them have a stable pattern. But if a planet came around the back of Jupiter and was slingshot down toward the Sun in a very eccentric orbit it could get very high velocities as it approached the Earth and the Sun combined. To have these low velocities it must have started in an orbit close to the Earth (the word was Trojan planet in one video) so it never really starts from infinity and is relatively close to begin with. But if the Trojan planet is formed at the same distance to the Earth it could have a similar composition in any case, that is my argument. http://en.wikipedia.org/wiki/Trojan_%28astronomy%29 This gels with me particularly at the L3 point. http://en.wikipedia.org/wiki/Near-Earth_object As seen from the speed quoted if objects start from further out they can get to be much faster.

You might get to tell me what they mean by that?

I found the NASA article very interesting. Thanks for that.

So what can we put into Impact Earth site to mimic the Giant Impact? We want the impactor to glance off the surface. It has to exchange material and leave 7/8 mass behind and have enough energy to climb out to at least 50,000 km to reform the Moon. Can the trajectory be made to curve around the Earth so the remnants have enough angular momentum to stay up there? It is not just a matter of lifting it to 50,000 km but it has to be able to orbit as well. Each piece of molten ejecta will be cooling in space before regrouping. Will the regrouping material still have enough energy to stratify the Moon into a mantle outer core and inner core?

Doesn't that suggest to you there is a problem with the maths? Well maybe not. He did the research not me.

Ultimately we want the situation that forms the Moon. Now with the Moon orbiting the Earth in the same direction as the Earth spins, that impact scenario did not happen. It would need to hit in a direction that would tend to speed up the rotation of the Earth wouldn't it?

Well I heard with my own ears someone say to me they have seen a miracle. So if anyone was to answer your question with "yes" you have then to consider whether you'd believe them. Was it DivineFool who said "As defined, delusion is a "belief" but a miracle is an "event". If you believe in a miracle, then you may be deluded." So you are no better off knowing the answer.

So what that means is that Kelvin calculated that the very hot temperatures and hence high radiation early on had little effect on the time it took for a metal sphere to cool. So any starting temperature above 3.16 times the final temp measured in Kelvin had less than a 1% effect. Initially that is hard to believe or accept but it is clear the time taken to cool once the surface nears the final temperature is so much longer than the initial rapid cooling, I came to understand this. It is only considering the effect on the surface temperatures.

Does that mean it could just about stop the Earth spinning a day being longer than a week long after impact.

What would happen if it was a Mars sized planet whacking into a nearly Earth sized planet?

So was that rain forecasted?