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Why does a NASA spacecraft crash into an asteroid?

CAPE CANAVERAL, Fla. (AP) — In the first of its kind, a world-saving experiment, NASA is about to strike a small, harmless asteroid millions of miles away.

A spacecraft named Dart will zero in on the asteroid on Monday, intending to hit it head-on at 14,000 mph (22,500 km/h). The impact should be just enough to push the asteroid into a slightly tighter orbit around its companion space rock – demonstrating that if a killer asteroid ever heads our way, we might stand a chance of deflecting it.

“It’s stuff from science fiction books and really cheesy ‘StarTrek’ episodes from when I was a kid, and now it’s real,” NASA program scientist Tom Statler said Thursday.

Cameras and telescopes will observe the crash, but it will take days or even weeks to find out if it actually changed the orbit.

The $325 million planetary defense test began with the launch of Dart last fall.


The asteroid with the center of the target is Dimorphos, about 7 million miles (9.6 million kilometers) from Earth. It’s actually the tiny sidekick to a 2,500-foot (780-meter) asteroid named Didymos, Greek for twin. Discovered in 1996, Didymos spins so fast that scientists believe it threw material that eventually formed a moon. Dimorphos – about 525 feet (160 meters) in diameter – orbits its parent body at a distance of less than a mile (1.2 kilometers).

“It’s really asteroid deflection, not disruption,” said Nancy Chabot, planetary scientist and mission team leader at Johns Hopkins University’s Applied Physics Laboratory, which is managing the effort. “It’s not going to blow up the asteroid. He’s not going to tear it to pieces. Instead, the impact will carve a crater tens of feet (meters) high and blast some 2 million pounds (1 million kilograms) of rock and soil into space.

NASA insists there is no chance either asteroid will threaten Earth – now or in the future. This is why the pair was chosen.

Asteroid Strike Explainer


Johns Hopkins Lab took a minimalist approach in developing Dart – short for Double Asteroid Redirection Test – given that it is essentially a battering ram and faces certain destruction. It has only one instrument: a camera used to navigate, aim and chronicle the final action. Believed to be essentially a pile of rubble, Dimorphos will emerge as a bright spot an hour before impact, appearing larger and larger in camera images beamed back to Earth. The managers are confident that Dart won’t crash into the bigger Didymos by mistake. The spacecraft’s navigation is designed to distinguish between the two asteroids and, within the last 50 minutes, target the smaller one.

From the size of a small vending machine to 1,260 pounds (570 kilograms), the spacecraft will slam into about 11 billion pounds (5 billion kilograms) of asteroid. “Sometimes we describe it as racing a golf cart through a big pyramid,” Chabot said.

Unless Dart misses – NASA rates the chance of that happening at less than 10% – it will be the end of the road for Dart. If he passes the two space rocks, he will encounter them again in a few years for Take 2.


The small Dimorphos makes a turn around the big Didymos every 11 hours and 55 minutes. Dart impact should reduce this by about 10 minutes. While the strike itself should be immediately apparent, it could take a few weeks or more to verify the moon’s altered orbit. Cameras on Dart and a mini tagalong satellite will record the collision up close. Telescopes on all seven continents, plus the Hubble and Webb Space Telescopes and NASA’s asteroid-hunting Lucy spacecraft, can see a bright flash as Dart senses Dimorphos and sends streams of rocks and dirt cascading into space. The observatories will track the pair of asteroids as they orbit the sun, to see if Dart has altered Dimorphos’ orbit. In 2024, a European spacecraft named Hera will retrace Dart’s journey to measure impact results.

Although the predicted boost will only slightly change the position of the moon, it will cause a major change over time, according to Chabot. “So if you were going to do this for planetary defense, you would do it five, 10, 15, 20 years in advance for this technique to work,” she said. Even if Dart is missing, the experiment will still provide valuable information, said NASA program manager Andrea Riley. “That’s why we’re testing. We want to do it now rather than when there’s a real need,” she said.


Planet Earth is on an asteroid hunting spree. NASA has nearly a pound (450 grams) of rubble collected from asteroid Bennu heading for Earth. The stash should arrive next September. Japan was the first to collect asteroid samples, accomplishing the feat twice. China hopes to follow suit with a mission launched in 2025. NASA’s Lucy spacecraft, meanwhile, is heading for asteroids near Jupiter, after launching last year. Another spacecraft, Near-Earth Asteroid Scout, is loaded into NASA’s new moon rocket awaiting liftoff; it will use a solar sail to fly over a space rock under 18 meters (60 feet) next year. Over the next few years, NASA also plans to launch a survey telescope to identify hard-to-find asteroids that may pose risks. An asteroid mission is grounded as an independent review panel assesses its future. NASA’s Psyche spacecraft was supposed to launch this year to a metal-rich asteroid between Mars and Jupiter, but the team couldn’t test the flight software in time.


Hollywood has produced dozens of killer-space-rock films over the decades, including 1998’s “Armageddon” which brought Bruce Willis to Cape Canaveral for filming, and last year’s “Don’t Look Up” with Leonardo DiCaprio leads an all-star cast. NASA planetary defense officer Lindley Johnson thinks he’s seen them all since “Meteor” in 1979, his favorite “since Sean Connery played me.” Although some sci-fi films are more accurate than others, he noted, entertainment always wins. The good news is that the coast looks clear for the next century, with no known threats. Otherwise, “it would be like in the movies, right?” said NASA Science Mission Chief Thomas Zurbuchen. What is worrying, however, are the unknown threats. Less than half of the 460-foot (140-meter) objects have been confirmed, with millions of smaller but still dangerous objects circling them. “These threats are real, and what makes this time special is that we can do something about it,” Zurbuchen said. Not by blowing up an asteroid like Willis’ character did — that would be a last-minute resort — or begging government leaders to act like DiCaprio’s character did to no avail. Weather permitting, the best tactic might be to push the menacing asteroid out of our way, like Dart.

The Associated Press Health and Science Department is supported by the Howard Hughes Medical Institute Department of Science Education. The AP is solely responsible for all content.

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