How’d They Do That Tuesday: Lagrange Points

Putting together yesterday’s post about planet hunting and intergalactic civilizations got me thinking about some of the ideas for colonizing space right here in our own solar system. One of the more outlandish ideas out there is to fly space stations to colonize the Lagrange Points around Earth.

In other words: Home, Home on Lagrange.

Sorry I couldn’t resist. But in all seriousness, what are these points all about?

Lagrange points are wonderful features of orbital mechanics. They’re five points around the orbit between the Earth and the Sun where a satellite can park itself and remain the same relative spot. No thrusters needed, no orbital corrections, a spaceship can just relax and let gravity do all the work. These spots (labeled L1-L5) move along with Earth around its orbit, so though they don’t say fixed in space, they stay fixed along Earth’s orbit.

The first point, L1, is easy enough to understand. It’s the point exactly between the Earth and sun where the pull of gravity is exactly equal. The pull of Earth’s gravity and the Sun exactly cancel each other out, and a satellite can hang out in that space indefinitely. L2 is on the far side of the Earth (as seen from the Sun) where the masses of the two massive bodies are exactly in line with each other and form a single gravitational pull. For the L2 point, it would be like orbiting around a single object with the masses of the Earth and Sun combined. The more mass an object has, the stronger the pull of gravity, so the farther from the center of gravity the satellite would have to orbit to stay in sync with the rest of the system. The L3 point combines the two masses the same way, only it’s on the line on the far side of the Sun.

L4 and L5 only work if one body is at least 24.96 times more massive than the other. Our sun is thousands of times greater than the Earth, so no problems there. In order to stay balanced between the Earth and the Sun, L4 and L5 are the same distances away from both bodies. To find these points, all one has to do is draw an equilateral triangle out from the Sun and Earth and L4 and L5 are where the triangle’s third point intersects Earth’s orbit. There, the pull of gravity is equal between the two bodies so the satellite won’t fall towards either the Sun or Earth. Also, because it’s on the same orbit as Earth, its speed around the sun will be the same, locking it in place around the Earth.

Of course satellites aren’t the only things that can take advantage of the Lagrange points. Scientists discovered two big clusters of gas following Earth at its L4 and L5 spots. The Earth and Sun isnt the only system with these points either. Any pair of massive bodies orbiting each other have them. At the L4 and L5 spots between Jupiter and the Sun are entire asteroid fields.

We’ve already started using the first two Lagrange points for observatories. The Wilkinson Microwave Anisotropy Probe (WMAP), which studies the cosmic microwave background radiation, has been parked at L2 since 2001. The Hubble’s replacement, the James Webb Space Telescope, will be moving in next door once it’s launched in 2013. Wilder proposals have been suggested for the other points as well. The points have long been a popular spot for science fiction writers to put space colonies and the like. My favorites are all the stories about parallel Earths located at L3 that we might never know were there because they’re always eclipsed by the Sun. Spooky right?

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