Push/pull methods fail-they take decades and are not useful for larger objects
IRWIN I. SHAPIRO et al in 10,( Harvard-Smithsonian Center for Astrophysics, Chair FAITH VILAS, MMT Observatory at Mt. Hopkins, Arizona, Vice Chair MICHAEL A’HEARN, University of Maryland, College Park, Vice Chair ANDREW F. CHENG, Johns Hopkins University Applied Physics Laboratory FRANK CULBERTSON, JR., Orbital Sciences Corporation DAVID C. JEWITT, University of California, Los Angeles STEPHEN MACKWELL, Lunar and Planetary Institute H. JAY MELOSH, Purdue University JOSEPH H. ROTHENBERG, Universal Space Network, Committee to Review Near-Earth Object Surveys and Hazard Mitigation StrategiesSpace Studies BoardAeronautics and Space Engineering BoardDivision on Engineering and Physical Sciences, THE NATIONAL ACADEMIES PRESS, http://www.fas.harvard.edu/~planets/sstewart/reprints/other/4_NEOReportDefending%20Planet%20Earth%20Prepub%202010.pdf)
“Slow push” or “slow pull” methods. For these options the orbit of the target object would be changed so that it avoided collision with Earth. The most effective way to change the orbit, given a constraint on the energy that would be available, is to change the velocity of the object, either in or opposite to the direction in which it is moving (direct deflection—moving the object “sideways”—is much less efficient). These options take considerable time to be effective, of the order of decades, and even then would be useful only for objects whose diameters are no larger than 100 meters or so.
Asteroids are giant balls of rubble, no deflection methods would be able to do anything but break them into more parts.
Oberg 98 [James, Author on Russian and US space programs, US Space Command, “Planetary Defense, Asteroid Deflection & The Future of Human Intervention In The Earth’s Biosphere,” July 23, 1998, SM, accessed: 7/11/11, http://abob.libs.uga.edu/bobk/oberg.html]
Not long ago, astronomers thought of asteroids as rocks, perhaps rubble covered, but still mainly single bodies. But evidence has accumulated that asteroids are rubble piles all the way through, loosely bound together by what is generously called "gravity" (escape velocity is 11,000 meters per second on Earth but less than 1 meter per second on a typical small asteroid). The Shoemaker-Levy object was torn apart by a close brush with Jupiter in 1992 so when it fell back onto Jupiter two years later it was a string of smaller objects. Crater chains on the moons of Jupiter, on Earth's moon, and on Earth itself also point to the gravity-induced disintegration of many asteroids prior to impact. Asteroids which rotate fast enough to fling pieces clear are extremely rare -- only two are known -- which suggests that these are the rare single-rock objects. What this means is that big impulses -- say, from another asteroid collision or from a nuclear detonation -- would more likely disperse the material than deflect it.Pushing an asteroid has been likened to clearing a landslide off a road, rather than rolling a rock. So other techniques -- gentle pushes over long periods -- may prove to be required. There are plenty of such ideas. None of them will prove workable, I predict, but the second and third generation ideas will turn out to be quite feasible. And information from a reborn Clementine-2 may powerfully augment new astronomical discoveries.