Blowing up asteroids can actually protect Earth

Whenever a near-Earth asteroid is spotted, sensationalism almost always crashes into social media, with people wondering whether they should finally get a start on that doom shelter they were thinking about building. Not necessarily.

What scientists consider the best way to get rid of asteroids that may threaten our planet (and possibly obliterate us like the dinosaurs) is to launch a spacecraft that will push them out of the way by deflecting them out of orbit. This is why NASA will soon be sending up its DART mission to try out this technique on non-threatening asteroid Didymus. But what if an unexpected space rock comes hurtling towards Earth so fast that there isn’t enough time to plan for it?

The backup plan is disruption. It’s basically a euphemism for blowing up asteroids into smithereens. While that sounds like it’s just going to send an epic meteor shower our way, a team of researchers led by physicist Patrick King of the Johns Hopkins Applied Physics Laboratory have found that carrying it out a certain way can actually save Earth from total annihilation. They recently published a study in Acta Astronautica.

"If we have plenty of time, say, 10 years of lead time, which is certainly a possibility given our detection capabilities, a kinetic impactor could handle a wide variety of threats, ranging in size," King tells SYFY WIRE.

Their method is so effective that around 99% of resulting fragments would miss the planet. To do this, the asteroid needs to be broken up into pieces that are as evenly scattered as possible, which is tricky to pull off when you’ve got an object speeding toward you in space. Some of these asteroids may go unnoticed for too long, because they might be too faint for even the most powerful telescope to catch soon enough. Others may just be shooting through space at breakneck speed and appear almost as if they had burst out of nowhere.

Never mind that disruption explodes with so much power (and lightens payloads) because you don't need much when it comes to nuclear explosives. Planetary defense needs high levels of energy density (meaning how much energy is packed into a certain amount of mass) and total energy content (or how high that energy can be scaled up). Higher mass and speed result in a bigger boom. Hit the rock in just the right orbit at the right speed and time, and nuclear explosives beat just about everything else when it comes to getting rid of a threatening asteroid.

"We also have to factor in the entire process of building a spacecraft and launching it at the appropriate time," King says. "Not all orbits can be achieved by launching on some arbitrary date, and launch operations have to be carefully run in order to ensure the launch goes successfully."

More than 19,000 near-Earth asteroids are floating out there. What you might not realize is that 80 to 100 tons of asteroid material, from chunks of rock that burn up in the atmosphere, fall to the surface every day. Disruption of a more dangerous asteroid would still need warning time. There is no way (yet) to blast one that is within minutes or even hours of head-butting the planet. The new research has looked into the behavior of various asteroids and used simulations to predict how their fragments would scatter if they were disrupted.

To blow up an asteroid about a fifth the size of Bennu, some 100 feet in diameter, by using the LLNL team’s method that was tested in the simulations, you would need a nuclear weapon to bombard it with at least 1 megaton of power. That weapon would have to be flown within around 10 feet of the offending rock. The team’s simulations tested five different asteroid orbits and found that a successful disruption as little as two months before Earth would have been pounded is enough to make 99.9 percent of that asteroid’s mass miss us. Whew.

King and his team found out what would happen in different asteroid situations by using Spheral, a type of software developed by himself and colleagues Mike Owen and Cody Raskin, both co-authors of the study. Physics models built into the software showed how and where the hypothetical asteroid pieces being dispersed would go into orbit. 

"Spheral is 'hydrocode', shorthand for a physics simulation code that can handle extreme deformations, shocks, and other high-strain rate phenomena," King explains. "What makes Spheral different from many codes is that it is based on a specific technique well-suited for modeling nuclear disruption."

But what would happen to the debris left over from the explosion? It is possible that some could burn to ash in the atmosphere, but the pieces are most likely to drift off into their own orbits. That would probably happen with the hypothetical Bennu-shaped asteroid a fifth of Bennu’s size. Larger monsters would scatter more unevenly, but the velocities of all those pieces would still take 99% of them away from Earth and into their own separate orbits — so long as the asteroid was apprehended and its disruption planned six months ahead.

After that, the cloud of asteroid debris would continue to evolve depending on the effects of gravity. Simulations also accounted for that. The researchers wanted to see how the fragments would interact with one another as well as how the gravity of the Sun and other planets would influence them. This can get complicated. Just finding the orbit of each piece can be tedious, and in addition to that, the entire cloud of fragments would stretch into something of a curve around the path of destruction the asteroid was riding before it met its end.

"In planetary defense, it is critical to change the orbit of the threat so that the asteroid is not crossing the Earth's orbit at a conjunction, when both are in the same place in their orbit at the same time and are about to collide," King says. "For any that do hit, if you can make the fragments small enough and disperse them enough, most could simply burn up in the atmosphere harmlessly."

Let’s just hope that if an asteroid does come for Earth, we get it first before it gets us.