The Double Asteroid Redirection Test spacecraft, launched on Wednesday, could be the first to alter an asteroid’s path, a technique that may be used to defend the planet in the future.
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It was 2017, and astronomers projected an asteroid the size of a cruise ship would strike Japan sometime in the next decade.
Scientists and government officials from NASA and other space agencies, gathered at an annual planetary defense conference in Tokyo, hastily devised a plan to knock the asteroid off its path toward Earth. The island’s fate relied on a fleet of robotic spacecraft that would launch in the next few years.
In 2020, the world’s space agencies banded together, launching four ships toward the menacing space rock. The ships, known as kinetic impactors, struck their targets head-on. Japan was spared a herculean evacuation effort, its cities and neighborhoods saved from annihilation.
None of these events really happened. It was a simulation, the kind of tabletop role-playing exercise that officials conduct on a regular basis. And deflecting an object from deep space on its way to a deadly rendezvous with Earth has become a preferred solution at these practice drills for protecting the planet. Yet no one knows whether the technique will actually work. Never in human history has our species tried to knock an asteroid away from our world.
That is about to change. On Wednesday at 1:21 a.m. Eastern time, NASA launched the Double Asteroid Redirection Test mission, or DART, from a U.S. Space Force base in California (it was Tuesday local time). A 1,200-pound, refrigerator-size spacecraft will trek around the sun to slam into a small asteroid named Dimorphos at 15,000 miles per hour next year. If the mission succeeds, it could demonstrate for the first time humanity’s ability to punch a potentially hazardous asteroid away from Earth.
“We’re doing this work and testing this DART capability before we need it,” Lindley Johnson, NASA’s chief of planetary defense, said. “We don’t want to be flying an untested capability when we’re trying to save a population on the Earth’s surface.”
The $324 million DART mission is unusual for NASA, a civilian agency that focuses mainly on exploration, climate monitoring and hunting for signs of past life in our solar system. While it coordinates with and relies on the U.S. Department of Defense for some activities, NASA has not traditionally been responsible for leading efforts to protect the United States — or Earth, for that matter — from any security threat.
That changed in 2005, when Congress assigned the agency the imperative of protecting the planet from dangerous objects that orbit the sun and have the bad habit of occasionally crossing paths with our world. That includes tracking tens of thousands of so-called near-Earth asteroids large enough to wreak catastrophic damage. Lawmakers assigned NASA the task of cataloging 90 percent of the total expected amount of these space rocks, but it has missed that goal.
“You’ve got to find them before you can get them, and you want to find them early,” said Kelly Fast, who manages NASA’s Near-Earth Object Observations Program, the agency’s effort to keep an eye on all nearby asteroids that are bigger than a football stadium. “You want to find these things years or decades in advance.”
NASA later set up the Planetary Defense Coordination Office in 2016 after a watchdog report urged the agency to better organize its asteroid tracking efforts. That office, led by Mr. Johnson, is tasked with warning the Defense Department and Federal Emergency Management Agency of any threatening asteroids, which is one of NASA’s few responsibilities leading a national response to a major disaster threat.
The DART mission shows how the agency is embracing this responsibility. NASA has been studying space rocks up close for decades. It has landed robots on the surface of Mars, plucked samples from a large asteroid named Bennu (which may threaten Earth in the 22nd century) and even deliberately crashed spacecraft into a comet and into the moon, all for the sake of science. But striking an asteroid hard enough to alter its orbit in space poses new challenges for the agency’s engineers and scientists.
The DART space probe will visit Dimorphos and another asteroid, Didymos, in September or October 2022. The two asteroids, called a binary system, orbit the sun every two years along an egg-shaped path that stretches out near Mars and back around near Earth. Dimorphos is the smaller of the pair, orbiting Didymos like a moon at a distance of about one mile and completing a revolution of the larger rock every 11 hours and 55 minutes.
Dimorphos, similar in size to one of the pyramids of Giza, is not a threat to Earth. And when NASA’s DART spacecraft makes contact, it will become the smallest celestial body ever visited by a spacecraft. That will be a challenging mission.
“It’s the first time we’ve ever tested a technique to deliberately move an asteroid using our own capabilities and systems,” said Brent Barbee, a member of the DART mission team and an aerospace engineer at NASA’s Goddard Space Flight Center. “It’s kind of a big milestone for our species. Like, the dinosaurs didn’t have a DART mission.”
Because Dimorphos is such a small space rock, DART will need to hit a bull’s-eye when the asteroid system reaches its closest point to Earth along its orbit around the sun, some 6.8 million miles away. It’s a complex orbital choreography involving a precise launch time off Earth and intermittent firings of a dozen tiny onboard thrusters that will refine DART’s path to collide with Dimorphos.
“From an engineering point of view, this is really hard,” said Andy Rivkin, the DART investigation team lead at Johns Hopkins Applied Physics Laboratory, which is managing the mission. DART’s single shot at striking Dimorphos will rely on a fully automated process that begins four hours before impact and uses an onboard navigation system called SMART Nav.
“They had to build an algorithm that does it by itself; there’s no joysticking it in,” he said.
Tom Statler, a DART program scientist at NASA, agreed.
“In one sense, DART is fairly simple. There’s only one instrument on board,” he said, referring to the spacecraft’s camera. “But on the other hand, the precision of the navigation is really beyond what we’ve done before.”
Ten days before impact, DART will deploy a small satellite built by the Italian Space Agency called LICIACube, which carries two cameras. This traveling companion will witness DART’s self-destructive mission from 34 miles away and measure the amount of debris kicked up from the impact. The DART spacecraft’s onboard camera, called DRACO, will snap photos of the asteroid as it approaches, streaming them back to Earth up until 20 seconds before impact.
To test whether DART has succeeded, scientists at NASA and the Applied Physics Laboratory will measure how much Dimorphos’s orbit around Didymos changes after the spacecraft’s impact. To ground-based telescopes, the asteroids are tiny dots of light. After impact, the scientists will track the duration of Dimorphos’s orbit by measuring the time between flickers of reflected light that signal that Dimorphos has crossed in front of Didymos, then passed behind it half an orbit later.
If the asteroid’s orbit is sped up by at least 73 seconds, shortening the time it takes to revolve around Didymos, the mission was a success, Dr. Rivkin said. But he expects a more significant change to the asteroid’s orbit.
“We think the actual amount will be something like 10 minutes,” he said.
Whether the impact will leave a large crater and spew a cloud of debris is as much of a mystery to astronomers as the asteroid’s surface is.
Very little is known about the composition of Dimorphos. If the space rock is hard and dense, little debris will be kicked up from the impact. If Dimorphos turns out to be soft, like a tightly packed cluster of smaller space rocks, then DART’s impact will leave a large crater and create a plume of debris that mission managers hope will help push the asteroid farther off its orbit with Didymos. The debris will give scientists an early sense of this momentum transfer: The more that is ejected from the crater, the more the asteroid is likely to move.
“But how that ejected crater material behaves is a wild card,” Mr. Barbee said. “DART’s going to provide our very first data point on that behavior.”
The test, if successful, will give NASA a confirmed weapon in its planetary-defense arsenal. But for asteroids bigger than Dimorphos, other conceptual deflection methods might be more fitting, astronomers say. For instance, a fleet of several impactor spacecraft, like the mission design used by astronomers in the Tokyo simulation, could be used to alter a bigger asteroid’s course.
For any potentially hazardous asteroids that would be at least 10 years from impacting Earth, another untested deflection method involves a “gravity tractor” spacecraft that would orbit or hover close to the asteroid for years and impart a small gravitational influence, gradually tugging the rock’s course away from Earth.
A spacecraft tipped with a nuclear explosive device could try to deflect larger or closer asteroids that pose a more imminent threat. Such a nuclear spacecraft, which has not been tested, would park itself near the asteroid and detonate to potentially blow it in another direction. Using a nuclear bomb, though, has its challenges.
Used the wrong way, a nuclear weapon could create a number of smaller, still-damaging rocks that will continue toward Earth. And some agreements, including the 1967 Outer Space Treaty, ban the placement or use of nuclear weapons in space, which suggests a country’s use of a nuclear weapon to deter an asteroid’s impact with Earth would amount to a treaty violation. That legal predicament could be resolved, however, in an emergency meeting of the United Nations Security Council.
NASA still has plenty of other preparatory work to do, including filling in gaps in scientific knowledge about the asteroid threats we face.
The Wide-field Infrared Survey Explorer, a space telescope NASA launched in 2009, has helped the agency log roughly 27,000 near-Earth asteroids to date. But that is only 40 percent of the total number Congress requires NASA to track, Dr. Fast said. With NASA’s current capabilities, “it will take about 30 years, at the rate we’re going right now,” to complete the near-Earth asteroid catalog, she said. A future spacecraft with a massive infrared telescope, called NEO Surveyor, is scheduled to launch in 2026 to help speed up that task.
“This is the only natural disaster that we could prevent,” Dr. Fast said of asteroid impacts. “DART is really important to have in the toolbox, but we have to find them before we can do something like DART.”
In a news conference on Monday, Thomas Zurbuchen, NASA’s associate administrator for science, said he was confident that DART’s impact would have an effect on the asteroid’s orbit.
“The odds are 100 percent,” he said. “It’s very hard to imagine a scenario in which you come with a lot of momentum into a body like this, create an interaction and nothing happens on the other side.”
But that all depends on DART’s ability to hit its target.
“We keep working harder and harder to not miss,” Ed Reynolds, the DART project manager at the Applied Physics Lab, said. But he acknowledged that “there are limitations” to DART’s onboard sensors, and that it could go off course.
Still, the spacecraft will have about 90 percent of its fuel left when it reaches the asteroid next year, and it could aim for other space rocks if it fails to connect with Dimorphos.
“Right now, we’re not keeping a list of backup dual asteroids to retarget against,” he said. “But the possibility is there.”
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