(Image: Artist concept of NASA's Double Asteroid Redirection Test (DART) spacecraft.)

A devastating large asteroid headed right for Earth—it's unlikely in our lifetimes, but inevitable in the very long term. NASA and other space institutions are starting to think more about how to deal with this fact, and one idea with widespread support is to build a spacecraft that could simply slam into an asteroid while it is still far away from Earth, altering its trajectory just enough to safely fly around our home planet.

That's exactly what the Double Asteroid Redirection Test (DART) impactor spacecraft would do. This NASA project would target an asteroid not headed toward Earth as a test. The spacecraft concept was approved on June 23 to enter the design phase of the project. A target launch date has not been announced, but NASA mentioned their target asteroid, Didymos, will be making distant approaches to Earth in 2022 and 2024, suggesting a launch soon enough to hit the asteroid in the early 2020s is possible.

The target asteroid is actually a binary asteroid with two bodies, hence the name Didymos, Greek for "twin." Didymos A, the larger asteroid at about a half-mile wide (780 meters), has a smaller asteroid orbiting around it. Didymos B is only about 530 feet (160 meters) wide. DART, about the size of a refrigerator, would impact the smaller of the two, Didymos B, at 3.7 miles per second, or 5 to 9 times faster than a modern rifle bullet.

"A binary asteroid is the perfect natural laboratory for this test," said Tom Statler, program scientist for DART. "The fact that Didymos B is in orbit around Didymos A makes it easier to see the results of the impact, and ensures that the experiment doesn't change the orbit of the pair around the sun."

Of course, for something like DART to save the planet someday, we would need to hit a large asteroid while it is still far away. The bigger the asteroid, the earlier we need to find it, and if we can hit it while it is still months or years out, just a little nudge would be enough to alter its trajectory safely around us. If we are surprised by a large asteroid that would arrive in a matter of weeks, we would face the difficult decision of nuking it to smithereens as a last ditch effort.

Of all near-Earth objects (NEOs) that are over 1 kilometer in average diameter, we have located an estimated 95 percent. However, of NEOs around 20 meters in diameter or smaller, we have located less than 1 percent.

A meteor strike in Chelyabinsk, Russia, occurred in 2013 when an asteroid about 20 meters in diameter unexpectedly exploded over the city of 3.5 million in a 400 to 500 kiloton blast, releasing roughly 30 times as much energy as the atomic bomb over Hiroshima. Even though the primary force of the explosion was 19 miles (30 kilometers) up in the atmosphere, over 1,400 people were injured and significant infrastructure damage occurred in an area roughly ten miles wide.

The Chelyabinsk meteor, brighter than the sun at its brightest point.

If an asteroid like the Chelyabinsk meteor were on a collision course for, say, New York City, it would be nice to be able to knock it out of the way with an impactor spacecraft. But we would need a system like DART on hand, and we would need to find the incoming asteroid—the Chelyabinsk meteor was obscured by the light of the sun, and it struck entirely without warning.

The WISE space telescope was repurposed in 2013 to search for NEOs, and telescopes coming online in the 2020s could greatly expand the catalogue of potentially hazardous asteroids and other planetary bodies.

And of course, we need to worry about the big one, the life-annihilating chunk of rock and ice somewhere out in the void. From studying impact craters, we know meteors around 6 miles (10 km) strike the Earth every 50 to 100 million years. The last one that size struck 65 million years ago, and it wiped out the former kings of this planet. If our species survives long enough, humans will confront a similar rock of doom. Let's hope we're ready for it.

Source: NASA

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