Overview video (different from the one NASA published at the time) and with a clip of catch mechanism testing - provided to NASASpaceFlight.com at the time of testing in 2006.

NASA researchers were working on an alternative form of space travel to the Moon, involving a ride on a giant slingshot that utilizes the technology of momentum-exchange tethers.

NSF article in 2006:

"These giant structures – roughly 55 miles in length – would rotate end-over-end in space, catch a spacecraft, before “throwing” it on a path to take it to the Moon.

The concept shows that passengers on such a ride would wait in orbit for the right moment when the tether’s ‘catch mechanism’ would snare their spacecraft and accelerate them for a few minutes at twice the force of gravity. Once thrown free from the tether, the astronauts would coast to the Moon over a four day transit period – before conducting their missions as they would on any other lunar mission.

Using a tether to go to the Moon could make lunar travel much more affordable, by cutting in half the amount of rocket propellant needed to make the mission work. Less rocket propellant means smaller rockets and lower costs for future space travelers. The tether could be re-used, over and over again, like a bridge across a river.

The tether would have to recharge itself to be ready for action again, by using a technique common to every electrical motor on Earth. The tether would run electricity through wires in the tether, and that electric current would ‘push’ against the Earth’s magnetic field just like the winding of a motor pushes against the magnets of its armature.

By pushing against the Earth itself, the tether could recharge its orbit over a few weeks and be ready for another payload. Best of all, since the electricity comes from solar panels, there is no need for any rocket propellant, saving lots of mass and cost that would otherwise have to be launched from the ground.

Development of the tether moved along In spring 2005, engineers from NASA’s Marshall Space Flight Center (MSFC) and faculty and students from Tennessee Technological University (TTU) in Cookeville, Tennessee, tested one of the most important parts of the tether-the ‘catch mechanism’.

Unlike a typical space rendezvous, a tether catch must take place instantaneously, so the team had to go back to the drawing board and come up with a totally different approach to linking two spacecraft, and it had to work in a split-second.

They came up with a technique that uses an extensible boom on the spacecraft and a box-like structure on the end of the tether. When the boom goes through the opening of the box, it closes around the boom and latches onto hooks on the end of the boom. When it’s time to get off the tether and go to the Moon, the spacecraft reverses the hooks on the boom and falls away onto its new trajectory.

Testing of the catch mechanism at TTU demonstrated that it could catch a simulated payload – about the size of a coffee can – that was shot up to the mechanism to simulate the free-fall of space flight. Other testing at NASA showed that the orbit of the tether could be determined with enough precision to make the catch, and that tether materials could be built that was light, strong and resistant to space debris damage.

The tether system, with its solar panels, mechanisms, and power systems, can be seen in a video generated by NASA. With a mass some six to ten times that of its payload, it will ‘pay’ itself off after 6-10 uses since a typical Moon mission has to carry an equal amount of fuel to the payload it wants to send to the Moon. Engineers anticipate that the tether system will be able to recharge quickly enough to support monthly trips to the Moon.

Interestingly enough, the engineers also think that the tether will be a very visible object in the night sky, like a line rotating end over end in space. With a little luck and a lot of hard work, that line in the sky might be pointing us straight to the Moon."

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