A problem with using a traditional kinetic impactor in one piece, like the one used by Deep Impact, to deflect a hazardous NEA, is that its effects depend on the subsurface composition of the asteroid.

But what if the impactor consisted of gravel which were released into a stream by a spacecraft in approaching trajectory? The same kinetic energy would be spread out over a larger surface on the asteroid and during a longer time. Shouldn't this prevent fragmentation? And as a bonus also eliminate the risk of hitting a steep topological feature obliquely so that the ejecta flies out sideways.

  • $\begingroup$ Fracturing a rubble pile isn't that hazardous so long as it's done well in advance. (Note: the real-world of planetary protection assumes it's done well in advance. The movie version assumes it's done two days in advance. Too late.) With years between the kinetic impactor's impact on the rubble pile and the rubble pile's impact on Earth, the rubble pile simply pulls itself back together after being hit and ruptured apart a bit. $\endgroup$ – David Hammen Apr 23 '15 at 21:30

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