Ten months after launch, NASA's asteroid-deflecting DART spacecraft neared a planned impact with its target on Monday in a test of the world's first planetary defense system, designed to prevent a doomsday collision with Earth.
The cube-shaped "impactor" vehicle, roughly the size of a vending machine with two rectangular solar arrays, was on course to fly into the asteroid Dimorphos, about as large as a football stadium, and self-destruct around 7 p.m. EDT (2300 GMT) some 6.8 million miles (11 million km) from Earth.
The mission's finale will test the ability of a spacecraft to alter an asteroid's trajectory with sheer kinetic force, plowing into the object at high speed to nudge it astray just enough to keep our planet out of harm's way.
It marks the world's first attempt to change the motion of an asteroid, or any celestial body.
DART, launched by a SpaceX rocket in November 2021, has made most of its voyage under the guidance of NASA's flight directors, with control to be handed over to an autonomous on-board navigation system in the final hours of the journey.
Monday evening's planned impact is to be monitored in real time from the mission operations center at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland.
DART's celestial target is an asteroid "moonlet" about 560 feet (170 meters) in diameter that orbits a parent asteroid five times larger called Didymos as part of a binary pair with the same name, the Greek word for twin.
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Neither object presents any actual threat to Earth, and NASA scientists said their DART test cannot create a new existential hazard by mistake.
Dimorphos and Didymos are both tiny compared with the cataclysmic Chicxulub asteroid that struck Earth some 66 million years ago, wiping out about three-quarters of the world's plant and animal species including the dinosaurs.
Smaller asteroids are far more common and pose a greater theoretical concern in the near term, making the Didymos pair suitable test subjects for their size, according to NASA scientists and planetary defense experts.
Also, their relative proximity to Earth and dual-asteroid configuration make them ideal for the first proof-of-concept mission of DART, short for Double Asteroid Redirection Test.
Robotic suicide mission
The mission represents a rare instance in which a NASA spacecraft must ultimately crash to succeed.
The plan is for DART to fly directly into Dimorphos at 15,000 miles per hour (24,000 kph), bumping it hard enough to shift its orbital track closer to its larger companion asteroid.
Cameras on the impactor and on a briefcase-sized mini-spacecraft released from DART days in advance are designed to record the collision and send images back to Earth.
DART's own camera is expected to return pictures at the rate of one image per second during its final approach, with those images streaming live on NASA TV starting an hour before impact, according to APL.
The DART team said it expects to shorten the orbital track of Dimorphos by 10 minutes but would consider at least 73 seconds a success, proving the exercise as a viable technique to deflect an asteroid on a collision course with Earth - if one were ever discovered. A small nudge to an asteroid millions of miles away could be sufficient to safely reroute it away from the planet.
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The test's outcome will not be known until a new round of ground-based telescope observations of the two asteroids in October. Earlier calculations of the starting location and orbital period of Dimorphos were confirmed during a six-day observation period in July.
DART is the latest of several NASA missions in recent years to explore and interact with asteroids, primordial rocky remnants from the solar system's formation more than 4.5 billion years ago.
Last year, NASA launched a probe on a voyage to the Trojan asteroid clusters orbiting near Jupiter, while the grab-and-go spacecraft OSIRIS-REx is on its way back to Earth with a sample collected in October 2020 from the asteroid Bennu.
The Dimorphos moonlet is one of the smallest astronomical objects to receive a permanent name and is one of 27,500 known near-Earth asteroids of all sizes tracked by NASA. Although none are known to pose a foreseeable hazard to humankind, NASA estimates that many more asteroids remain undetected in the near-Earth vicinity.
NASA has put the entire cost of the DART project at $330 million, well below that of many of the space agency's most ambitious science missions.
