Proposed Next-Generation Space Station

WallytheWalrus writes "This NewScientist.com article discusses the proposed next generation of telescopes and space stations. The concept presented with little fanfare by the NASA Exploration Team (NEXT) consists of placing a space station about 5/6ths of the way to the moon at one of a handful of local Lagrangian Points. This station would act as a springboard for constructing new telescopic mirrors, maintaining the telescopes that use them, and as a haven for future manned exploration missions. If only NEXT's budget was more than $4 million a year...."

Comments

lagrangian points

[agurkan]

These are points where the gravitational pull of two bodies, such as the Earth and the Moon, cancel each other out, providing a stable location to position spacecraft.

I am very surprised The New Scientist makes such a mistake. These points are stable mainly because of rotation. In a nonrotating system, there is only one equilibrium point, and that is unstable.

Re:lagrangian points

[GileadGreene]

I am very surprised The New Scientist makes such a mistake. These points are stable mainly because of rotation. In a nonrotating system, there is only one equilibrium point, and that is unstable.

You are correct about the contribution of rotation to teh formation of the libration points. However, these points are not all stable. L4 and L5 (the triangular points) are stable (at least in a linear sense). L1, L2, and L3 are unstable. That said, you can establish periodic orbits around the unstable points, so they aren't completely useless :-)

Re:lagrangian points

[kst]

The point where the gravitational pulls of the Earth and the Moon cancel each other out is somewhere inside the Earth crust.

The center of gravity of the Earth-Moon system is about 1000 miles below the Earth's surface.

The point where the Earth and the Moon would exert an equal gravitational force on a third body is in space, much closer to the Moon than to the Earth. I think the actual balance point, the L1 point, is somewhat closer to Earth because of the contribution of rotation -- "centrifugal force", which of course isn't really a "force" unless you use a rotating frame of reference ... blah blah physics blah blah vague handwaving blah blah.

Deja vu all over again

[GileadGreene]

Didn't we just have this story a few days ago? Oh well - guess we can talk about it again:

While the concept of placing a space station at a libration (or Lagrange) point seems nice on the surface, it's a very tough proposition in reality.

The problem is that the myth of a libration point as simply some kind of nifty stable point in space where gravity balances has been propagated for a while now. I've seen this mistake turn up in countless places, including some otherwise reputable textbooks. The reality is far more complex, and difficult to analyze.

For starters, the L1, L2, and L3 are unstable. That means that anything put there will tend to drift away over time. Not only that, but the L points don't even exist in reality - they are an artifact of a simplified gravitiational model (three bodies only). Once you incorporate the eccentricity of the primaries, and the effects of the other planets, you find that the L points are not so much points as variable regions of space with rather messy dynamical properties that we still don't fully understand. Oh, sure, you can mess around with numerical explorations and experiments, and there are a couple of series approximations that give reasonable first guesses at some particular solutions, but we are still a long way from being able to characterize and predict the full dynamics in one of these regions.

So, placing some thing actually at a libration point is out. But, as it turns out, you can establish periodic or near-periodic orbits around the approximate region of the libration "point" (so-called halo or lissajous orbits). We still don't really undertsand these orbits that well either, but we know enough to be able to have successfully put some unmanned probes out at the Sun-Earth L1 point (e.g. ISEE-3, SOHO, and most recently Genesis). Note that these are all Sun-Earth L1 missions, not Earth-Moon which would add another layer of complexity due to the influence of the Sun's gravity of the Earth-Moon system.

At present, the process of designing a new trajectory for a libration point mission consists of a fair amount of trial and error, and iteration. Techniques have improved some in the last decade (check out the work by Martin Lo at JPL and Kathleen Howell at Purdue on using dynamical systems theory to find transfers to/from halos), but it's still a lot of work to generate a finished trajectory that meets all of the necessary constraints. Trying to do this kind of thing with a manned, maneuvering spacecraft is going to be extremely difficult. In particular, any kind of rendezvous between two or more spacecraft will be difficult, since it's tough to predict where your spacecraft is going to go (very non-linear dynamics). Planning L point trajectories in real time really isn't that feasible until techniques improve a lot more.

This is a very active field of research, but there's still a long way to go before we're likely to be really ready for manned missions that do anything other than hang around on their own at L1 for a while.