Asteroids and comets could devastate Earth. N.A.S.A wants to stop them

N.A.S.A is studying how to dodge giant space rocks.

Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox’s podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.Many disasters — volcanic eruptions, earthquakes, hurricanes, tornadoes — are unavoidable. Scientists talk about when, not whether, they’ll strike. Though humans make some calamities worse, natural disasters have been happening since long before we were here. They’re a fact of life on Earth.

But one kind of disaster need not be inevitable: a collision between an asteroid or comet and the Earth. “It’s the one natural disaster, essentially, that you could cancel out,” says the science journalist Robin George Andrews on a new episode of Unexplainable, Vox’s science podcast about unanswered questions.

“You can’t stop volcanoes erupting, or an earthquake happening, or hurricanes,” Andrews says. “But an asteroid? If you just flick it out of the way of the Earth, the threat is gone.” This may sound like science fiction, but scientists are already working on it. It’s arguably going a lot better than depicted in the film Don’t Look Up in which two astronomers struggle to convince the world that a deadly comet is coming. The US government is spending time and money to avert a future asteroid disaster.

“We’re laying the groundwork here that would allow a threat to be addressed in the future and save lives,” says Kelly Fast, a N.A.S.A scientist who works in the Near-Earth Object Observations Program, part of the space agency’s Planetary Defense Coordination Office. Her job, she says, “is finding asteroids before they find us.”

Fast and her colleagues still have a lot of work to do, both to find and catalog asteroids and comets that could pose a threat to Earth and also devise a concrete plan of action if one were on a collision course. But the work is well underway, making comets and asteroids a rare example of a risk that’s getting attention before it’s too late.

After disasters strike, people often wonder why humans didn’t do more to prepare for — or even prevent — the worst. We ask that about the pandemic and climate change. Humanity often seems more reactive than proactive. In that light, our preparations for asteroids and comets are a feel-good story. It’s a story of how, when scientists described a threat, the government actually decided to do something about it.

How to prevent an asteroid disaster

Step 1: Find the asteroids

In 1993, scientists spotted pieces of a huge comet heading straight for Jupiter. Before the comet broke apart, it was around a mile wide. If this object had hit the Earth, it could have caused a mass extinction. For astronomers, it was something of a fireworks show. They knew the pieces would strike in July 1994, so they watched via telescope.

“Oh, boy. It was amazing,” says Fast, a planetary scientist at the time. “You could fit a few Earths, or more, across some of the larger impact regions.”

In the bottom left of this infrared video of Jupiter, you can see part of the impact exploding out from the planet’s atmosphere (one of Jupiter’s moons, Io, is the bright sphere on the right).

The scientists were both enthralled and concerned. It became a wake-up call: “This does happen, can happen, in the solar system,” Fast says.

Lawmakers noticed too. In 1998, Congress asked N.A.S.A to find at least 90 percent of the asteroids and comets 1 kilometer wide or larger that would come close to the neighborhood of Earth’s orbit. (Whereas asteroids are pieces of rock and metal in orbit around the sun, comets contain ice and tend to come from farther out in the solar system; when it comes to their potential for destruction, they’re pretty much the same.) Large objects like these strike Earth once every half a million years and could threaten life as we know it.

We can’t do anything about an asteroid or comet if we don’t know it’s coming for us, so identifying them is a good first step to preventing a disaster. Once an asteroid is spotted, scientists can project its trajectory forward in time and see if it’s likely to come dangerously close to Earth.

In 2017, the journal Geophysical Research Letters published an analysis of likely sources of casualties from an asteroid impact. “Effects such as cratering, seismic shaking, and ejecta deposition [i.e., ejected debris] provide only a minor contribution to overall loss,” the study concluded. The biggest source of casualties: wind generated from the impact blast. Vox

The good news: N.A.S.A says it’s found more than 95 percent of huge, doomsday-size asteroids. None seem to pose a threat to Earth.

But there are more out there. In 2005, Congress raised the bar on the mission, directing N.A.S.A to find asteroids 140 meters and larger, or about the size of a skyscraper. They’re sometimes called “city killers” because they could flatten a city and cause massive regional damage, even if they wouldn’t necessarily cause a global apocalypse. There’s around a 1 percent chance of one of these asteroids hitting in any given century.

Here’s what astronomers are looking for in their search for asteroids. These images — taken on January 1, 2014, by the Catalina Sky Survey — show a very small asteroid (circled faintly in red and purple) streaming past static stars. Catalina Sky Survey

“The odds are so slim” of it happening in our lifetimes, Fast says. Then again, an asteroid of around this size passed between the Earth and the moon in 2019, and N.A.S.A didn’t see it coming. Rare things can happen, sometimes with devastating consequences. Rarity is not an excuse for inaction. (Even smaller asteroids — like the 17-meter rock that exploded with the force of a nuclear bomb over Chelyabinsk, Russia, in 2013 — are more common and harder to spot. Luckily, our atmosphere tends to break up these smaller asteroids before they have a chance to reach the ground.)

N.A.S.A has a lot more work to do toward cataloging potential city-killers. “We’ve found about 40 percent of them,” Fast says. At the rate scientists are going, it could take 30 more years to find the rest, though there are some plans to launch a specialized space telescope to speed up the search.

Step 2: Boop the asteroids

Let’s say scientists discover a 200-meter asteroid heading for the Earth. Could they do anything about it? That’s the question N.A.S.A scientists are currently in the process of answering.

Hollywood has had some flamboyant ideas about how to deal with an asteroid, namely using nuclear weapons to break them or knock them off course — this is the basic plot of the ’90s-era epics Armageddon and Deep Impact. There’s some merit to this strategy. But there might be a much easier and safer way to deal with an asteroid on a collision course with Earth: Simply nudge it out of the way.

Last year, N.A.S.A launched the Double Asteroid Redirection Test (DART), which is a car-size box outfitted with solar panels. It’s currently on its way to a 160-meter asteroid called Dimorphos. In the fall, DART will crash into Dimorphos at 24,000 kilometers an hour (about 15,000 miles per hour).

Illustration of the DART spacecraft N.A.S.A

Dimorphos is a “moonlet” asteroid, which means it orbits a larger asteroid named Didymos, much like the moon orbits the Earth. If all goes according to plan, the collision will change its orbit, proving that it’s possible to redirect a big hunk of rock in the middle of space. (The larger asteroid’s gravitational pull will help ensure that Dimorphos doesn’t fly off in a new direction, say, toward Earth.)

DART is the definition of a long shot. “This has never been tried before,” Andrews explains on Unexplainable. If it doesn’t work, that doesn’t mean we couldn’t ever deflect an asteroid. But perhaps it will take more force than DART can supply.

“Basically, this test is answering: How big a wallop do we need to give these city-killer asteroids to save the planet, if we need to?” Andrews says.

If DART works, N.A.S.A can build a similar strategy into its anti-asteroid game plan. If they spot an asteroid heading toward Earth, “they can send a precursor mission to observe it beforehand,” Andrews says. In those initial observations, scientists can study the asteroid’s composition and make calculations for how much force it will take to deflect the rock. “You could then send a DART-like mission.”

The problem of asteroids potentially crashing into the Earth is not yet solved. Still, a couple hundred years in the future, it’s possible that scientists like Fast will be hailed as forward-thinking heroes who laid the groundwork of a planetary defense system. “There’s no greater gift that America’s space agency can give” than protection for humans of the future, Fast says.

Even though these disasters might not arrive in our lifetimes, we can still feel good about the progress being made. It’s a way to be good ancestors to the generations that follow us. On the long list of problems to solve and disasters to mitigate, maybe we can actually solve one.