Planetary Defense

September 21, 2022 0 comments

by GWYNNE DYER

Around eleven million kilometers from Earth, the US aeronautics and space agency NASA will make the first attempt to deflect an asteroid from its orbit on Monday. But forget Don’t Look Up, Deep Impact, Armageddon and all the other movies about planet-destroying asteroids: this asteroid is only 160 meters wide and won’t even come close to us, even if the experiment fails.

On the other hand, about 18,000 asteroids of this size or larger orbit the Sun. If Dimorphos (the asteroid in the NASA experiment) had hit Earth, the impact would have the energy of a hundred-megaton hydrogen bomb, enough to devastate a city the size of New York or Lagos.

More than that, because Dimorphos orbits a much larger asteroid called Didymos, 780 meters in diameter, and they would arrive together. Now we’re talking about almost no one surviving in a city the size of Tokyo and devastation within a hundred kilometers.

These things don’t happen often, of course, but they do happen. The Lunar and Planetary Laboratory at the University of Arizona estimates that there are more than three million impact craters more than a kilometer in diameter on Earth, although the vast majority are buried under subsequent sediment.

The largest asteroid to hit the planet 66 million years ago, Chicxulub on Mexico’s Yucatan Peninsula, was 10 kilometers in diameter. It caused the last great extinction event: the global firestorms and ensuing five- or ten-year “asteroid winter” (due to ash obscuring the sun) killed all non-avian dinosaurs and let the mammals take over.

According to the Planetary Society, the odds of an asteroid the size of Dimorphos hitting Earth are one in one hundred every century. Also, we don’t even know where 40% of these asteroids are located.

Drop on asteroids from 30 to 140 meters that are still big enough to kill a city and there are about a million of them out there. We have good data on less than 2% of them, but we know that at least one will hit the planet every century. So NASA and the European Space Agency (ESA) both have “Planetary Defense” offices – and they are now conducting the first major experiment.

NASA’s Double Asteroid Redirection Test (DART) is a spacecraft that weighs about 500 kilograms when fully fueled, but will weigh much less when it makes a kamikaze mission to Dimorphos on Monday. On the other hand, it will be traveling at six kilometers per second, so the energy it transfers to the asteroid is not negligible.

The main goal of the exercise is to see how much this can shift the orbit of the smaller asteroid around its primary star, Didymos. It won’t be much, as Dimorphos’ mass is an estimated 4.8 billion kilograms, but it should be enough to be detectable by large telescopes within weeks.

Then, four years from now, when ESA’s Hera mission reaches Dimorphos, we should know how big and what shape the crater is. This will confirm or refute the growing suspicion that at least most smaller asteroids are really not solid boulders, but just clumps of debris weakly held together by microgravity.

If so, they would be much easier to move because then the collision doesn’t just push the asteroid in the desired direction. It will also spurt a lot of debris in the opposite direction, which would increase the total momentum transferred to the asteroid fivefold.

One step at a time. It will likely be a few decades before we can stop even an asteroid the size of a dimorphus from hitting Earth and be sure it’s going where we want it to go instead.

Larger but much rarer ones, which tend to be made out of solid rock, take a lot longer to get a hold of. Nonetheless, before the end of this century we could possibly protect the planet from all but the very largest asteroids.

Building a good planetary defense system will probably take a century, but at least we’re moving from theory to practical experimentation.