A diagram showing how the asteroid-billiards deflection strategy would work.
Credit: Natan Eismont (Space Research Institute of the Russian Academy of Science)

This concept, which Dunham laid out in his FISO talk, is basically a scaled-up version of the kinetic-impactor method. 

It involves launching a robotic spacecraft out to a small NEA — one 33 feet (10 m) wide or so. The probe would land on (and anchor itself to) the asteroid, then fire up its thrusters to set up a "gravity assist" flyby of Earth. (Alternatively, the probe could pluck a boulder off a larger asteroid and then fly off with that rock, Dunham said.) 

This speed-boosting, trajectory-altering flyby would steer the spacecraft-asteroid combo toward the hazardous object. As it neared its target, the rock-riding probe would refine its course using onboard ranging instruments, as well as reflectors and transponders placed on the big and dangerous rock, Dunham said.

The collision, when it came, would be much more powerful and effective than a smashup generated by a naked spacecraft serving as the kinetic impactor, he said.

There would be a lot of additional mass and momentum involved, after all. Consider, for example, that the Mbozi meteorite in Tanzania, which scientists first spotted in 1930, is just 10 feet (3 m) long but weighs 18 tons (16 metric tons).

There are enough small NEAs flying around in Earth's neighborhood to make this strategy a real possibility, Dunham said. Indeed, calculations that he, Natan Eismont of the Space Research Institute of the Russian Academy of Science, and their colleagues performed suggest that humanity could feasibly snag a dozen or so such nearby asteroids and steer them into holding-pattern orbits; these cosmic projectiles could then be "activated" as needed.

"So, you'd have a whole bunch of these things ready to throw at any asteroid coming towards you," Dunham said.

This asteroid arsenal could be especially useful for dealing with long-period comets, which spend most of their lives in the dark depths of the outer solar system and are therefore very difficult to find and track, he added.

The asteroid-snagging idea has applications beyond planetary defense, Dunham said: Pre-positioned space rocks could serve as inviting targets for crewed exploration efforts. Indeed, NASA had been developing just such a plan until last year, when the Trump administration canceled the agency's Asteroid Redirect Mission.

Dunham stressed that the asteroid-billiards concept needs to be studied in much more detail before it can be fully implemented. He'd like to see the idea demonstrated first on a benign target rock, citing NASA's proposed Double Asteroid Redirection Test (DART) mission as an illustrative example.

If all goes according to plan, DART would slam a spacecraft into the 500-foot-wide (150 m) moon of the asteroid (65803) Didymos in October 2022, in a test of the textbook kinetic-impactor deflection strategy.