On March 18th, 2018 we learned about

Calculating what kind of push could prevent a large asteroid from colliding with the Earth

Kids supposedly want to know why the sky is blue, but that question doesn’t grip their imaginations like potentially being killed by an asteroid hitting the Earth. It’s not illogical, since knowing that giant dinosaurs were driven extinct by an asteroid strike 65 million years ago makes it clear that such an event is a severe and nearly hopeless scenario. Factor in how hard it is to explain the statistical unlikelihood that a world-ending asteroid would hit the Earth, and it’s easy to see how a kid might think that adults are weird for not worrying about suffering the same fate as the dinosaurs. Thankfully, some adults are thinking about rocks falling from space, and working out possible responses to larger asteroids that might be headed our way.

Bumping asteroids without breaking them

101955 Bennu is an 87-million-ton asteroid that passes by the Earth every six years. It’s close enough that we can track it with some certainty, and have realized that it does stand a chance of hitting our planet on September 25, 2135. At this point there’s only a 1 in 2,700 chance that it will actually collide with Earth, which is four-times lower than your odds of dying in a car crash in the next year, but it’s a good target to explore potential safety measures that could shield us from being hit.

With an object as large as Bennu, there’s already consensus that we need to nudge it, not blow it to pieces. Aside from the difficulty of really obliterating that much mass, exploding a large asteroid would probably just mean the Earth got hit by lots of smaller rocks instead of one big one. That’s arguably better nothing, but an early adjustment to the asteroid’s orbit would be preferable, and given enough time, a tad more practical.

Adjusting orbits with HAMMERs and explosions

To alter Bennu’s orbit, one proposal is to basically launch a large, Delta IV rocket at it, tipped with a 8.8-ton spacecraft called HAMMER (Hypervelocity Asteroid Mitigation Mission for Emergency Response vehicle). As you might guess, HAMMER would fly into an asteroid like Bennu to try to slow it down and alter its orbital path a small amount. If done early enough, even a small push can lead to big shifts in the asteroid’s trajectory years later. It’s a sensible plan until you work through all the math, at which point it becomes clear that 8.8 tons isn’t going push a big asteroid far enough on its own, even if they collide years in advance. One estimate found that 34 to 53 HAMMER spacecraft would be needed to move Bennu to a safer orbit if given a 10 year lead time. If the project started 25 years before 2135, the orbit could be sufficiently adjusted with only 7 to 11 spacecraft, although that still requires an enormous amount of resources with little room for error. Developing HAMMER spacecraft isn’t a totally lost cause though, as one such craft could probably divert a 295-foot asteroid if given a 10-year head start.

If HAMMER doesn’t look practical right now, an alternative idea is to deflect asteroids like Bennu with a nuclear explosion. Again, the goal wouldn’t be to destroy the rock, but to divert it before it gets to Earth. With that in mind, a warhead would be detonated near the incoming rock, hitting one side of the asteroid with radiation. That radiation could vaporize the surface of the asteroid, essentially turning that entire face into a giant, if gentle, thruster. As vaporized rock pushes off the asteroid, it would push Bennu in the opposite direction, hopefully nudging it over just enough to miss the Earth years later.

Planning for all the possibilities

Hopefully this will all be academic by 2135. As that date approaches, astronomers will track Bennu’s orbit and be able to refine their predictions about its eventual path. Even if it never intersects the Earth, figuring out responses is still worth while though. Bennu is one of 10,000 objects that NASA tracks at this point, but they can’t see everything. It’s possible that a ten-year head start to build a response will be all our planet gets, in which case these early planning exercises will save us all a bit of very precious time.

Source: Scientists design conceptual asteroid deflector and evaluate it against massive potential threat by Lawrence Livermore National Laboratory, Phys.org

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