Missions to the Moon and Beyond
The concept of a momentum exchange tether was first envisioned by the late Dr. Robert Forward some twenty years ago. The way it works is that the tether would be electrically charged, using solar panels. The electric change would cause the tether to “push” against the Earth’s magnetic field, causing it to rotate. The tether would have a counterweight on one end and a “catch mechanism” on the other. The tether would be placed in an elliptical orbit of about three hundred to five thousand kilometers around the Earth.
A space craft destined for the Moon would be launched into low Earth orbit. At the proper moment, the space craft would “dock” with the catch mechanism of the tether. The tether would fling the space craft out of Low Earth orbit and send it on a path to, say, the Moon without expending any rocket fuel.
Because the momentum of the space craft leaving low Earth orbit is transferred to the tether, lowering its orbit, the tether would have to spend several weeks “reboosting” itself. It could do this, either using chemical rockets, electrically powered ion rockets, or a technique called “electrodynamic tether reboost.” The latter would use electrical energy flowing through the tether to add orbital energy to it, thus increasing its orbit. NASA researchers think that in this way, a momentum exchange tether could be used for a lunar mission once a month.
Recently, NASA researchers at the Marshal Space Flight Center in Huntsville, Alabama, in partnership with faculty and students at the Tennessee Technological University in Cookeville Tennessee, tested the momentum exchange tether concept. They specifically developed the most challenging part of the concept, how the tether could catch a space craft so that it could fling it out of low Earth orbit.
The technique these researchers came up with consisted of an extendable boom on the space craft. The catch mechanism on the tether would consist of a box. The box would close around the boom and latches on with hooks on the boom. When it is time to release the space craft, the hooks release and the space craft is sent on its way.
Other testing at the Marshal Space Flight Center demonstrated that the orbit of the tether could be determined with enough precision to make the catch. Also, the tether materials could be built that would be light, strong, and resistant to space debris damage.
NASA researchers believe that a tether could be build, launched, and deployed so that after about six to ten missions to the Moon would pay for itself in saved fuel and weight costs. Other missions that similar tether could be designed for is helping boost space probes to other destinations around the solar system and satellites, such as communications satellites, to geo synchronous orbit.
If and when such a tether would be deployed, it would be a visible object in the night sky. Robert Heinlein once famously said that when one gets to low Earth orbit, one has made it half way to anywhere. Such a momentum exchange tether could become a bridge to anywhere.
