Averting Disaster

In July 2019, astronomers spotted an unknown space rock hurtling toward Earth. Just a day later, the asteroid, about the size of a football field, whizzed by. It narrowly missed our planet while traveling at about 86,000 kilometers (54,000 miles) per hour. The huge rock passed just 73,000 km (45,000 mi) from Earth. That’s five times closer than the moon! 

The fact that this space rock, named Asteroid 2019 OK, went undetected until it was right at our doorstep concerned scientists. If an asteroid this size were to strike Earth, the impact would carry enough explosive energy to flatten an area the size of a large city. 

In July 2019, astronomers spotted an unknown space rock racing toward Earth. It was an asteroid about the size of a football field. Just a day later, it flew by at about 86,000 kilometers (54,000 miles) per hour. The huge rock just missed our planet. It passed only 73,000 km (45,000 mi) from Earth. That’s five times closer than the moon!

Scientists named this space rock Asteroid 2019 OK. They were concerned because it wasn’t spotted until it was so close. What if an asteroid this size hit Earth? The explosive energy from the impact could flatten an area the size of a large city. 

 Asteroid 2019 OK is just one of more than 20,000 known near-Earth asteroids that periodically pass through our planetary neighborhood as they orbit the sun (see Crowded Space). Astronomers around the world scan the skies for these objects, track them to see if they pose a threat, and work with engineers to devise ways to keep our planet safe from these rocks. 

One possible way to protect Earth from an asteroid strike is to redirect the dangerous space rocks away from the planet. To test out this idea, NASA plans to launch the Double Asteroid Redirection Test (DART) next summer. Its mission: to determine whether crashing a spacecraft into an asteroid could prevent the space rock’s collision with Earth. 

More than 20,000 near-Earth asteroids are known. Asteroid 2019 OK is just one of them. These asteroids pass close to our planet as they orbit the sun (see Crowded Space). Astronomers around the world scan the skies for these objects and track them to see if they’re dangerous. They also work with engineers to find ways to keep Earth safe from these rocks. 

One possible way to protect Earth is to move dangerous asteroids away from the planet. NASA is preparing to test this idea. They plan to launch the Double Asteroid Redirection Test (DART) next summer. DART will crash a spacecraft into an asteroid. Its mission is to find out if this could stop a space rock from striking Earth.

Large space rocks rarely strike Earth. That’s because space is so big, says Kelly Fast, NASA’s Near-Earth Object Observations Program manager. An asteroid the size of 2019 OK, estimated to be 60 to 130 meters (195 to 425 feet) wide, hits Earth once every 2,000 years, on average. The last time this happened was in 1908, when an asteroid exploded over Siberia in Russia, leveling 2,000 square km (770 square mi) of forest. Bigger asteroids strike our planet even less frequently. A 10 km (6 mi)-wide asteroid, like the one scientists believe wiped out the dinosaurs, collides with Earth every 50 million to 100 million years. But no matter how rare the occurrence, when a giant space rock is racing toward the planet, “it’s the sort of thing you’d want to know well in advance,” says Fast. 

That’s why astronomers are constantly searching space for previously undetected asteroids. To do this, they take pictures of portions of the sky using powerful telescopes. Then the scientists compare the images over time to see if any objects have changed location. Against a background of stars, which appear static in photos, a moving asteroid stands out. “Eventually, we can start to plot out its orbit and say that yes, this is definitely a new asteroid,” says Amy Mainzer, an astronomer at the University of Arizona. 

Large space rocks rarely strike Earth. That’s because space is so big, says Kelly Fast. She’s NASA’s Near-Earth Object Observations Program manager. Asteroid 2019 OK is about 60 to 130 meters (195 to 425 feet) wide. An asteroid that size hits Earth once every 2,000 years, on average. The last time this happened was in 1908, when an asteroid exploded over Siberia in Russia. It flattened 2,000 square km (770 square mi) of forest. Bigger asteroids strike our planet even less often. Scientists believe that a 10 km (6 mi)-wide asteroid wiped out the dinosaurs. A rock that big hits Earth every 50 million to 100 million years. But even though a giant space rock rarely races toward the planet, “it’s the sort of thing you’d want to know well in advance,” says Fast. 

That’s why astronomers constantly search space for unknown asteroids. They take pictures of parts of the sky with powerful telescopes. Then they compare the images over time to see if any objects have moved. In photos, the background of stars seems to remain still. So a moving asteroid stands out. “Eventually, we can start to plot out its orbit and say that yes, this is definitely a new asteroid,” says Amy Mainzer, an astronomer at the University of Arizona.

Still, with so much space to examine, it is possible that asteroid hunters could miss something dangerous. And an asteroid that’s moving quickly or has an odd orbit— or both, as was the case with Asteroid 2019 OK—can be particularly hard to spot. But so far, astronomers believe they’ve identified more than 95 percent of the asteroids that measure at least 1 km (0.6 mi) across, a size that would be large enough to devastate the entire planet. They’ve also found one-third of those larger than 140 meters (460 feet), which would cause significant damage near the area of impact. 

The goal is to find all of them long before they’re near Earth. “The sooner a threat is identified, the more options you have to move the object safely out of the way,” says Mainzer. 

But asteroid hunters have a great deal of space to search. That means they could miss something dangerous. And an asteroid that’s moving quickly or has an odd orbit can be extra hard to spot. Both were true with Asteroid 2019 OK. But so far, astronomers believe they’ve found more than 95 percent of the asteroids that are at least 1 km (0.6 mi) across. That size would be large enough to devastate the entire planet. They’ve also found one-third of those larger than 140 meters (460 feet) across. These asteroids would cause heavy damage near the area of impact.

Scientists want to find all of them long before they’re near Earth. “The sooner a threat is identified, the more options you have to move the object safely out of the way,” says Mainzer. 

If scientists happen to detect a dangerous asteroid, they want to have defensive strategies in place—that’s where DART comes in. It is NASA’s first attempt to use a kinetic impactor to deflect an asteroid. During the test, the DART spacecraft will try to crash into a space rock with enough force to change its speed and path. 

DART is scheduled to launch in July 2021. Once in space, solar panels will extend from each side of the car-sized spacecraft (see Targeting an Asteroid). “They basically roll out like tape measures,” says Andrea Riley, program executive for NASA’s DART mission. These solar arrays will collect energy from the sun to generate electricity. It will power the spacecraft as it makes an 11 million km (6.8 million mi) voyage to reach its target: the asteroid Didymos B. The space rock, which is about the size of a small football stadium, orbits another asteroid, Didymos A, which is about five times larger (see How Big?). 

What if scientists spot a dangerous asteroid? They want to have defensive plans in place. That’s where DART comes in. This test is NASA’s first attempt to use a kinetic impactor to move an asteroid. The DART spacecraft will try to crash into a space rock with enough force to change its speed and path. 

DART’s launch is planned for July 2021. The spacecraft is the size of a car. When it reaches space, solar panels will extend from each side of it (see Targeting an Asteroid). “They basically roll out like tape measures,” says Andrea Riley, program executive for NASA’s DART mission. These solar arrays will collect energy from the sun to make electricity. It will power the spacecraft on its 11 million km (6.8 million mi) voyage. DART’s target is the asteroid Didymos B. This space rock is about the size of a small football stadium. It orbits another asteroid, Didymos A, that’s about five times larger (see How Big?). 

The binary asteroid system doesn’t actually pose a threat to Earth. But since Didymos B is part of a pair, it makes it an ideal test target for DART. Astronomers will be able to detect if the impact successfully shifted Didymos B by measuring how its orbit around its companion Didymos A changes. 

DART should reach Didymos B in the fall of 2022. The spacecraft will then be too far away for scientists on Earth to manually control it. Instead, DART will rely on its autonomous navigation system. “The spacecraft has to figure out how to guide itself and impact Didymos B,” says Riley. A week before the collision, DART will deploy a small, box-shaped satellite, called a CubeSat, built by the Italian Space Agency. It will travel off to the side of the main spacecraft to capture images of its approach and, hopefully, the final moments before, during, and after it slams into Didymos B. 

The binary asteroid system isn’t a danger to Earth. But Didymos B is a perfect test target for DART. That’s because it’s part of a pair. Astronomers will be able to measure how the orbit of Didymos B around the bigger asteroid changes. That will tell them if the impact successfully moved Didymos B.

DART should reach Didymos B in the fall of 2022. Then scientists on Earth won’t be able to manually control it. The spacecraft will be too far away. Instead, DART will use its autonomous navigation system. “The spacecraft has to figure out how to guide itself and impact Didymos B,” says Riley. A week before the impact, DART will release a CubeSat. This small, box-shaped satellite was built by the Italian Space Agency. It will travel off to the side of the main spacecraft to take pictures of its approach. The CubeSat will try to capture the final moments before, during, and after the spacecraft slams into Didymos B.

If DART works, it will demonstrate one possible way to stop a asteroid from wreaking havoc on Earth. But there are other methods on the drawing board. One idea is to maneuver a large spaceship next to an asteroid. The ship’s mass would generate enough gravity—an attractive force between two objects—to deflect the space rock away from Earth. “You could let the slight pull of the spacecraft’s gravity tug on the asteroid until it’s moved out of the way,” explains Mainzer. Another option would be to set off a powerful nuclear explosion near an asteroid. Its energy would blow rocks and dust from its surface and push the giant rock in the opposite direction and out of Earth’s path. 

Most astronomers agree that the chance of an asteroid colliding with Earth isn’t something people should lose sleep over. But because the potential exists, it makes sense to plan for the possibility. That means testing technology like DART and continuing to monitor space. “From my perspective, the reasonable thing we can do as astronomers is to watch the skies,” says Mainzer. 

If DART works, it will show one possible way to stop an asteroid from devastating Earth. But other methods are on the drawing board. One idea is to move a large spaceship next to an asteroid. The ship’s mass would produce gravity—an attractive force between two objects. The force might be enough to shift the space rock away from Earth. “You could let the slight pull of the spacecraft’s gravity tug on the asteroid until it’s moved out of the way,” explains Mainzer. Another idea is to set off a powerful nuclear explosion near an asteroid. Its energy would blow rocks and dust from the asteroid’s surface. That would push the giant rock in the opposite direction and out of Earth’s path. 

The chance of an asteroid striking Earth is tiny. Most astronomers agree that people shouldn’t lose sleep over it. But because it could happen, planning for it makes sense. That means testing technology like DART and continuing to scan space. “From my perspective, the reasonable thing we can do as astronomers is to watch the skies,” says Mainzer.