This illustration made available by Johns Hopkins APL and NASA depicts NASA's DART probe, center, and Italian Space Agency's (ASI) LICIACube, bottom right, at the Didymos system before impact with the asteroid Dimorphos, left. DART is expected to zero in on the asteroid Monday, Sept. 26, 2022, intent on slamming it head-on at 14,000 mph. The impact should be just enough to nudge the asteroid into a slightly tighter orbit around its companion space rock. Credit: Steve Gribben / Johns Hopkins APL/NASA via AP

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On Monday, about 6.8 million miles from Earth, the U.S. National Aeronautics and Space Administration (NASA) will make the first attempt to divert an asteroid from its orbit. But forget “Don’t Look Up,” “Deep Impact,” “Armageddon” and all the other movies about planet-killing asteroids: this asteroid is barely 1/10 of a mile wide, and it isn’t coming anywhere near us even if the experiment fails.

On the other hand, there are about 18,000 asteroids of that size or bigger in orbit around the Sun. If Dimorphos (the asteroid in the NASA experiment) did hit Earth, then 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, in fact, because Dimorphos orbits a much bigger asteroid called Didymos, which is about half a mile in diameter, and they would arrive together. Now we’re talking about almost nobody surviving in a city the size of Tokyo, and devastation for more than 50 miles.

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 3 million impact craters larger than half a mile in diameter on Earth, although the great majority are buried under subsequent sediment.

The biggest asteroid to hit the planet, Chicxulub on Mexico’s Yucatan peninsula 66 million years ago, was 6.2 miles in diameter. It caused the last great extinction: the world-wide firestorms and the five- or 10-year “asteroid winter” that followed (due to the ash blocking out the sun) killed off all the 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 a 100 every century. Moreover, we don’t even know where 40 percent of those asteroids are.

Drop to asteroids of ⅕ or ⅘ of a mile, 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 percent 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’re now running the first big experiment.

NASA’s Double Asteroid Redirection Test, or DART, is a spacecraft weighing more than 1,000 pounds fully fueled, but it will weigh a lot less than that when it does a kamikaze dive into Dimorphos on Monday. On the other hand, it will be moving at around 3.7 miles per second, so the energy it transfers to the asteroid will not be negligible.

The primary aim of the exercise is to see how much that can shift the smaller asteroid’s orbit around its primary, Didymos. It won’t be a lot, because Dimorphos’s mass is an estimated 10.5 billion pounds, but it should be enough to be detectable within weeks by big telescopes.

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

If they are, they would be a lot easier to move, because then the collision will not just push the asteroid in the desired direction. It will also spurt out a lot of rubble in the reverse direction, which would magnify the total momentum transferred to the asteroid by as much as fivefold.

One step at a time. It will probably be a couple of decades before we can divert even a Dimorphos-sized asteroid from hitting the Earth and be confident that it will go where we want it to instead.

Larger but much rarer ones, which are more likely to be solid rock, will take a lot longer to get a handle on. Nevertheless, before the end of this century we may be able to protect the planet from all but the very biggest asteroids.

A good planetary defense system will probably take a century to build, but at least we are moving from theory to practical experiments.

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Gwynne Dyer, Opinion columnist

Gwynne Dyer is a London-based independent journalist whose commentary is published in 45 countries.