Buzz Advocates Lagrange Point Spaceport

thrasymachus writes "Buzz Aldrin has an editorial in the New York Times (free reg req) advocating a spaceport at a Lagrange point between the Earth and the moon over simply more moon missions. He emphasizes the cost and practicality of such a station, as well its potential as a 'bridge to the heavens.'"

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[erinacht]

It's actually quite a good read but not enough to make me want to register...

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Lagrange Points

[DarkDust]

A really interesting article about Lagrange Points can be found here. What I found really fascinating is the fact that it seems like that the earth pulls/pushes dust around space on the earth-moon Lagrange Points L4 and L5.

Re:The Moon or Lagrange? I still choose Mars.

[jdh-22]

I never knew what a Lagrange point was. I found that Wikipedia gave the best explanation.

Lagrange Point

For those who dont know what Lagrange Points are..

[Manhigh]

AKA Libration Points...

For any heavenly body with a satellite in a relatively circular orbit, there are 5 points where gravitational forces and centripetal accelerations cancel each other out. Three fall on a line that connects the two bodies, and the other form a pair of equilateral triangles with the heavenly bodies.

L1 between the two bodies
L2 on the far side of the smaller body
L3 on the far side of the larger body
L4 is the "leading" equilateral point
L5 is the "trailing" equilateral point

L4 and L5 are relatively stable. Putting a station at L1-3 would require more propellant to keep it there, though not an unreasonable amount.

Personally, I'd rather go for a base on the Moon that at a libration point. Sure, it requires more propellant to get to and from there, but its also a permanent fixture, rather than something that would need to be disposed of eventually.

Re:The Moon or Lagrange? I still choose Mars.

[Keebler71]

Well direct injection and aerocapture (not aerobraking which still required capture propellant) have their own drawbacks. In the case of direct transfer injection, you are much more constrained by launch windows than if you use a parking orbit. As for aerocapture (my thesis by the way), while there are significant propellant cost savings you now have to have a robust thermal protection system (TPS). Aerocapture has not yet been used on any mission as it is relatively high risk (due in large part to insufficient understanding of the Mars atmosphere and what type of heat loads would be encountered). The closest to aerocaputre that has been accomplished were the direct entries of the Apollo missions. Moreover, aerocapture guidance is still in its infancy (again, point designs have been done but we need a guidance algo that is robust enough to account for huge errors in Mars' atmospheric model). Further complicating matters is the fact that for large (manned spaceflight sized) missions, the heat loads and instantaneous heaing rates really push the limit of what current ablative technologies can sustain.

Don't get me wrong, I think that eventually we will have the technology to stage missions from Mars. However, I think that by the time we overcome some of the human related issues (long zero-g exposure, radiation, psycological, etc...) we will likely have made other advances that mitigate the benefit of launching from Mars over simply staging from Earth (i.e. cheaper access to space here at home).

I am not saying that Zubrin has misled you, and I never finished reading my copies of his books. He does an excellent job of pointing out the current deficiencies with our current technologies and policies, but I think he is overly optimistic in the alternatives that he suggests.

Why yes, I am ALSO a rocket scientist! :)

Re:Where would this point be?

[starfarer42]

Your imagery is wrong. There are no dimples, except for the gravitational effect of whatever you put at the Lagrange point.

For one thing, you have to include the centripetal forces exerted on the satellite as it's orbiting. The Lagrange points are places where the centripetal forces exactly cancel the gravitational forces.

The L4 and L5 points are stable. If the satellite drifts out of position then the gravitational and centripetal forces acting on it will nudge it back into position.

But the L1, L2 and L3 points are unstable. If the satellite drifts even slightly then the gravitational and centripetal forces will not be cancelled and they will actually pull it further out of position. Even a very small force, like that exerted by the solar wind, would push it out of position given enough time. So a station at L1 would always need some kind of propulsion system to keep it positioned correctly.

See this link for more info.

Re:Pick a point, any point

[barawn]

Yes. And while all the Lagrange points are stable, the L4 and L5 points are even more stable (more massive objects can sit in them and catch the ride, as it were).

No. While all the Lagrange points are *balanced* - that is, there's no net acceleration towards either of the two objects, only L4 and L5 are stable. If you nudge something at L1,L2, or L3, they fall away.

L1 is between the two objects. This is obvious why it works: because one object pulls one way, and one object pulls the other way. Where the two pulls are equal, there's no net force.

L2 is on the other side of the (smaller) mass. Since it's farther away from the (larger) body, it should orbit slower than the (smaller) mass, but the added gravity makes it orbit at the same speed as the (smaller) mass, making it stationary.

L3 is on the other side of the (larger) mass. Same reasoning, just substitute "faster" for "slower".

All of these three are unstable: if you push something at L1, it goes towards the body you pushed it towards, ditto with L2,L3.

They talk about L1/L2/L3 because of the positional convenience of them. Yes, you have to active stationkeep, but this isn't impossible, and the drift rate would still be slow for reasonable timespans.

Regarding L4 and L5, L5 is more convenient than L4 because of dynamics of the Earth-Sun-Moon system, rather than just the Earth-Moon system.