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Given that the goal of a gravity assist is not to hit the source of the gravity well, how is using a lagrange point going to be more useful than using a planet?

Given that the goal of a gravity assist is not to hit the source of the gravity well, how is using a lagrange point going to be more useful than using a planet? The usefulness of the technique for moving a satellite around earth seems clear (for unmanned probes). The idea of using this for manned flight seems ridiculous (it's not like Arthur C. Clarke didn't know about Lagrange points). The idea of sticking a space station at Earth/Moon L1 or whatever is ridiculous. You need to decellerate into the point since it's (at best) metastable, so there's a huge practical downside. Firing your lump of ice or aluminum at a large gravity well with decent precision seems much more practical.

ColinWright 12y ago
The goal of a gravity assist is not as you describe. The goal of a gravity assist is to increase net speed, and therefore to be able to get further with less fuel. Being able to use Lagrange points - both stable and unstable - for this purpose comes as a surprise to many people, which is why I mention it. The reason it's especially useful is that we then have more places where we can perform gravity assists, giving more options. The rest of your comment leaves me confused - I don't understand what point you're trying to make. The point is that we can make use of the lumpiness of the gravitational field to do non-obvious things with less fuel than previously believed. You don't seem to be saying otherwise, so I don't understand what you are saying.
- Tloewald 12y ago
  1) The technique you're describing is going to lead to very long journeys, so this makes manned flights impractical. Or am I wrong? 2) Getting something to a Lagrange point is only useful if it's at near zero velocity (relative to the point) when it gets there -- otherwise it falls off. So, either it gets there slowly or it burns fuel to stop at the right spot.
tzs 12y ago
The main point of ITN is that it is very low energy, at a cost of being slow. You wouldn't use it for manned missions. You'd use it for unmanned missions, including cargo transport to support manned missions. For instance, suppose you have established a mining and manufacturing operation on the Moon, and want to start a long term project to establish a permanent Mars base. You can start out by putting together a bunch of supplies that Mars base will need, launching that supply package into the Earth's orbit, moving it to an appropriate Earth/Sun Lagrange point, and then nudging it into an unstable orbit that intercepts an unstable orbit around a Mars/Sun Lagrange point. When the package reaches that point, another nudge transfers it to that second unstable orbit, where it eventually reaches a Mars/Sun Lagrange point, from which it can move to Mars. The only step in that trip that uses a lot of energy is the launch to get away from the Moon. All the rest of the maneuvers (moving to/from Lagrange points, entering and exiting the unstable orbits, and switching between the unstable orbits) take very little energy. It would take a while to get there...I don't recall for sure, but I think it is something around 3 years. You make the first package large, with enough supplies to support a Mars base for 6 months or more. Then every 3 months you launch another package. When the first package arrives successfully at Mars, and the other packages in the pipeline are in good shape, you launch your manned mission to actually establish the base. Note that the manned mission only needs to carry enough food, air, and water for the trip itself. Same goes for fuel--it only needs enough fuel to get to Mars. Fuel and supplies for a return trip can be included in the supply packages coming in on the ITN. (You can do this starting from the Earth instead of the Moon, of course, but it takes a lot less energy to get from the Moon to an Earth/Sun Lagrange point than from the Earth, so it is probably worth getting mining and manufacturing set up on the Moon first and then making that the starting point for establishing bases elsewhere).
- Tloewald 12y ago
  Don't you need to decelerate into the Lagrange point? If so, why not just use conventional gravity assist to get it to Mars faster?

ColinWright 12y ago

The goal of a gravity assist is not as you describe. The goal of a gravity assist is to increase net speed, and therefore to be able to get further with less fuel. Being able to use Lagrange points - both stable and unstable - for this purpose comes as a surprise to many people, which is why I mention it. The reason it's especially useful is that we then have more places where we can perform gravity assists, giving more options. The rest of your comment leaves me confused - I don't understand what point you're trying to make. The point is that we can make use of the lumpiness of the gravitational field to do non-obvious things with less fuel than previously believed. You don't seem to be saying otherwise, so I don't understand what you are saying.

Tloewald 12y ago
1) The technique you're describing is going to lead to very long journeys, so this makes manned flights impractical. Or am I wrong? 2) Getting something to a Lagrange point is only useful if it's at near zero velocity (relative to the point) when it gets there -- otherwise it falls off. So, either it gets there slowly or it burns fuel to stop at the right spot.

tzs 12y ago

The main point of ITN is that it is very low energy, at a cost of being slow. You wouldn't use it for manned missions. You'd use it for unmanned missions, including cargo transport to support manned missions. For instance, suppose you have established a mining and manufacturing operation on the Moon, and want to start a long term project to establish a permanent Mars base. You can start out by putting together a bunch of supplies that Mars base will need, launching that supply package into the Earth's orbit, moving it to an appropriate Earth/Sun Lagrange point, and then nudging it into an unstable orbit that intercepts an unstable orbit around a Mars/Sun Lagrange point. When the package reaches that point, another nudge transfers it to that second unstable orbit, where it eventually reaches a Mars/Sun Lagrange point, from which it can move to Mars. The only step in that trip that uses a lot of energy is the launch to get away from the Moon. All the rest of the maneuvers (moving to/from Lagrange points, entering and exiting the unstable orbits, and switching between the unstable orbits) take very little energy. It would take a while to get there...I don't recall for sure, but I think it is something around 3 years. You make the first package large, with enough supplies to support a Mars base for 6 months or more. Then every 3 months you launch another package. When the first package arrives successfully at Mars, and the other packages in the pipeline are in good shape, you launch your manned mission to actually establish the base. Note that the manned mission only needs to carry enough food, air, and water for the trip itself. Same goes for fuel--it only needs enough fuel to get to Mars. Fuel and supplies for a return trip can be included in the supply packages coming in on the ITN. (You can do this starting from the Earth instead of the Moon, of course, but it takes a lot less energy to get from the Moon to an Earth/Sun Lagrange point than from the Earth, so it is probably worth getting mining and manufacturing set up on the Moon first and then making that the starting point for establishing bases elsewhere).

Tloewald 12y ago
Don't you need to decelerate into the Lagrange point? If so, why not just use conventional gravity assist to get it to Mars faster?