When planning to defend Earth against asteroids on collision courses, the most cinematic option is to blow the thing to pieces. But that would probably be the worst thing to do, thanks to the discovery that common rubble-pile asteroids are held together by little more than gravity and cohesion.
The study, published today in the journal Nature, focused on asteroid (29075) 1950 DA, which is famous for being one of the closest asteroids to Earth and one of the most likely to impact the planet in the next several centuries.
1950 DA also is also something of a riddle to planetary scientists. They noticed that the body is spinning slightly faster than expected given its rubble and dust composition.
Alexandra Witze, reporting for Nature News:
Researchers have suspected that undetected cohesive forces help to hold some asteroids together — especially ‘rubble-pile’ asteroids, which are agglomerations of dirt and rock. Some of these rotate slowly enough for the gravitational attraction between the particles to hold them together. But for faster-spinning asteroids, centrifugal forces would overwhelm the gravitational pull and rip the rocks apart.
1950 DA is fast, rotating once every 2.1 hours. If gravity were all that kept it together, it could only spin once every 2.2 hours, scientists calculated. The van der Waals force, the same force that holds geckos to walls, must play a role, they surmised.
Should 1950 DA or another rubble-pile asteroid embark on a collision course with Earth, this new finding favors deflection with a soft touch, like a gravity tug that would subtly alter the space rock’s path. Impacting the asteroid directly would just send its weakly-bound pieces flying out in unpredictable, and potentially deadly, trajectories.