Diverting incoming asteroids is a high priority item, and so is a mission to a nearby asteroid for a close-up study of its composition and a shakeout of operating technologies. Think about the movie Deep Impact. Nukes are used to break up an incoming object, in this case a comet, but the resultant deadly chunks are still headed toward Earth. The planet suffers one disastrous collision, but it turns out to be survivable due to quick thinking and the willingness of a spacecraft crew to sacrifice themselves by blowing up the remaining impactor.

Get past the Hollywood cliffhanger elements and Deep Impact had its moments (in any case, I’ll sign off on any movie with Robert Duvall in it). The use of nuclear weapons in the movie does raise a legitimate question — do we know enough about what might hit us to predict what would happen if we did try to destroy it this way? That’s one reason we need early missions to study Earth-crossing asteroids, and it’s also a reminder that keeping our deflection options open means looking at entirely new solutions.

Tethers for Deflection

Enter David French, an aerospace engineering doctoral student at North Carolina State University. French has come up with a technique for deflecting an Earth-crosser that is, in its scale, a reminder of the magnitude of the danger. It involves attaching a long tether and ballast to the incoming object. “You change the object’s center of mass,” says French, “effectively changing the object’s orbit and allowing it to pass by the Earth, rather than impacting it.”

We’re talking about a tether between 1,000 and 100,000 kilometers in length, the latter being long enough to wrap around the earth two and a half times. An extreme solution? Perhaps, but considering the political obstacles we face in deploying any sort of nuclear technology, maybe it’s best to keep even unlikely options open. In any case, tethers (especially electrodynamic ones) have long interested NASA and include uses that could morph advanced tethers into payload delivery systems as the necessary background work is accomplished.

Electrodynamic Tethers for Propulsion

Electrodynamic tethers can provide propulsion because of the force a magnetic field exerts on the wire when an electrical current is passing through it. Do this with a tether in Earth orbit and the Earth’s magnetic field can do the accelerating, launching a payload connected to the wire without the need for fuel. Thus the MXER (Momentum Exchange Electrodynamic Reboost) tether, which throws the payload and then replaces lost kinetic energy by providing power to a conducting section of tether that allows the MXER station to regain altitude, leaving it recharged for another spacecraft launch.


Image: The Momentum-Exchange/Electrodynamic-Reboost Tether Concept. Credit: Tethers Unlimited.

The point being, tethers of various kinds have an interesting future (imagine what we might do with a tether system adapted for the magnetic fields in Jupiter space), and one that may adapt to this new use. The best space work doubles up on resources and extends existing ideas into new directions, so we’ll see what may come of the asteroid tether concept. French will present it at the AIAA SPACE 2009 Conference this fall.

In the meantime, the larger picture is that even those who disparage the need for space exploration can see the necessity of protecting the planet from danger, a fact that may ultimately be our best insurance for overcoming short-sighted opposition and developing a robust deep space infrastructure. “The prospect of hanging,” said Samuel Johnson, “concentrates the mind wonderfully.” So too will the prospect of future impacts as we continue to discover and catalog near-Earth objects.

The Tethers Unlimited site has descriptions of numerous types of tethers (this is Robert Forward’s old company). For MXER, one interesting take is Sorensen, K.F., “Conceptual Design and Analysis of an MXER Tether Boost Station”, AIAA Paper 2001-3915, available here.