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Asteroid Deflection by Tether

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.

Comments on this entry are closed.

  • James M. Essig April 23, 2009, 20:26

    Hi Paul;

    I like the idea of attaching a tether to the asteriod that holds a long train of membranous reflectors wherein the reflectors are perhaps supported by very low pressure inflatable toriodal rings.

    If the system could be attached to the asteriod several years out, then the asteriod should be nudguable out of harms way.

    My brother John an I came up with some very simple ways to make inflatable toriods out of high modulus, low strain membranous materials, that are made of low cost metalized Mylar, Nylon, and the like materials. In the vacuum of space, the absolute pressure within such toriods can be very small and the pressure required to inflate them into a rigidizable structure goes down with increased major and minor diameters or the toroids.

    I sometimes wonder if such very low cost simple inflatable structures could be of benefit in deflecting an asteriod.

  • Darnell Clayton April 27, 2009, 22:09

    But tethers just sound too boring for politicians (saving the world via rockets sounds much better on TV). ;-)

    Joking aside, there does not seem to be much interest in asteroid annihilation outside of the UN (which has suffered serious credibility over the years) and non-profits (who politicians love getting photo ops with, but not sitting down and addressing the issues).

    I seriously think that the only way Earthen nations will DO something about this is when they actually discover an asteroid that will strike our planet within their lifetimes (or election terms).

  • ljk May 6, 2009, 13:05

    Why it may be fruitless to attempt nuking an NEO:

    A Giant Crater on 90 Antiope?

    Authors: P. Descamps, F. Marchis, T. Michalowski, J. Berthier, J. Pollock, P.Wiggins, M. Birlan, F. Colas, F. Vachier, S. Fauvaud, M. Fauvaud, J.-P. Sareyan, F. Pilcher, D.A. Klinglesmith

    (Submitted on 5 May 2009)

    Abstract: Mutual event observations between the two components of 90 Antiope were carried out in 2007-2008. The pole position was refined to lambda0 = 199.5+/-0.5 eg and beta0 = 39.8+/-5 deg in J2000 ecliptic coordinates, leaving intact the physical solution for the components, assimilated to two perfect Roche ellipsoids, and derived after the 2005 mutual event season (Descamps et al., 2007).

    Furthermore, a large-scale geological depression, located on one of the components, was introduced to better match the observed lightcurves. This vast geological feature of about 68 km in diameter, which could be postulated as a bowl-shaped impact crater, is indeed responsible of the photometric asymmetries seen on the “shoulders” of the lightcurves. The bulk density was then recomputed to 1.28+/-0.04 gcm-3 to take into account this large-scale non-convexity.

    This giant crater could be the aftermath of a tremendous collision of a 100-km sized proto-Antiope with another Themis family member. This statement is supported by the fact that Antiope is sufficiently porous (~50%) to survive such an impact without being wholly destroyed. This violent shock would have then imparted enough angular momentum for fissioning of proto-Antiope into two equisized bodies.

    We calculated that the impactor must have a diameter greater than ~17 km, for an impact velocity ranging between 1 and 4 km/s. With such a projectile, this event has a substantial 50% probability to have occurred over the age of the Themis family.

    Comments: 30 pages, 3 Tables, 8 Figures. Accepted for publication in Icarus

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0905.0631v1 [astro-ph.EP]

    Submission history

    From: Franck Marchis [view email]

    [v1] Tue, 5 May 2009 15:48:26 GMT (576kb)


  • ljk June 11, 2009, 14:08

    Military Hush-Up: Incoming Space Rocks Now Classified

    By Leonard David

    SPACE.com’s Space Insider Columnist

    posted: 10 June 2009 05:35 pm ET

    For 15 years, scientists have benefited from data gleaned by U.S. classified satellites of natural fireball events in Earth’s atmosphere – but no longer.

    A recent U.S. military policy decision now explicitly states that observations by hush-hush government spacecraft of incoming bolides and fireballs are classified secret and are not to be released, SPACE.com has learned.

    The satellites’ main objectives include detecting nuclear bomb tests, and their characterizations of asteroids and lesser meteoroids as they crash through the atmosphere has been a byproduct data bonanza for scientists.

    The upshot: Space rocks that explode in the atmosphere are now classified.

    “It’s baffling to us why this would suddenly change,” said one scientist familiar with the work. “It’s unfortunate because there was this great synergy…a very good cooperative arrangement. Systems were put into dual-use mode where a lot of science was getting done that couldn’t be done any other way. It’s a regrettable change in policy.”

    Scientists say not only will research into the threat from space be hampered, but public understanding of sometimes dramatic sky explosions will be diminished, perhaps leading to hype and fear of the unknown.


    Most “shooting stars” are caused by natural space debris no larger than peas. But routinely, rocks as big as basketballs and even small cars crash into the atmosphere. Most vaporize or explode on the way in, but some reach the surface or explode above the surface. Understandably, scientists want to know about these events so they can better predict the risk here on Earth.

    Yet because the world is two-thirds ocean, most incoming objects aren’t visible to observers on the ground. Many other incoming space rocks go unnoticed because daylight drowns them out.

    Over the last decade or so, hundreds of these events have been spotted by the classified satellites. Priceless observational information derived from the spacecraft were made quickly available, giving researchers such insights as time, a location, height above the surface, as well as light-curves to help pin down the amount of energy churned out from the fireballs.

    And in the shaky world we now live, it’s nice to know that a sky-high detonation is natural versus a nuclear weapon blast.