While Dimorphos resemble many near-Earth asteroids, the DART team chose it as a target because it is the youngest member of the asteroid duo. It’s called the “little moon,” and it orbits Didymos, its larger partner, like a clockwork every 11 hours 55 minutes. DART will hit Demorphos at an angle of about 17 degrees with respect to its orbit, and the scientists plan to measure how much its orbit has shifted as a result. In other words, they can easily compare its motion to that of another nearby object. If they choose to hit a single asteroid, the tiny deviation from its orbit won’t be apparent for years, until it passes close to Earth. But thanks to its partner’s proximity, any changes in Dimorphos’ orbit can be identified in a matter of days.
“It’s really a smart, smart way, and it’s cost-effective. And it’s also safe: you’re pushing this moon a little bit closer to the asteroid that’s already orbiting it,” Chabot says. She and her team expect DART to shorten the asteroid’s orbit by five to 15 minutes, taking it just 11 hours and 45 minutes to circle Didymos. NASA will consider an aberration of 73 seconds or more a successful mission.
The asteroid pair will be close enough to make accurate measurements using telescopes on Earth until March 2023. After that, they will travel far, as part of their path around the Sun extends beyond the orbit of Mars. While the asteroids appear as a single point of light from this distance, scientists will be able to measure how often the brightness of reflected sunlight bounces off Didymus – a clue to the duration of Demorphos’ orbit.
These asteroids, like many other asteroids and some meteorites – space rocks that enter Earth’s atmosphere – are not as dense and solid as billiard balls. Pieces of rock, gravel and ice may be loosely held together in an arrangement called a “rubble pile,” with a rocky composition similar to the asteroids Ryugu and Eros, as well as the meteorite that exploded over Chelyabinsk, Russia, in 2013. In fact, Demorphos may have simply formed By turning on the side of Didymus. If the Dimorphos were full, the DART effect could crater, rather than spew out debris and push the asteroid heavily. But this uncertainty is one of the reasons for doing the job.
For a more detailed examination of the crash site, the European Space Agency’s Hera mission comes next. The spacecraft is scheduled to launch in 2024. When it reaches the asteroid duo in 2026, an optical camera, a lidar instrument, an infrared scanner, and two sides of the CubeSat will work on detailed maps of the Dimorphos’ surface and structure.
If a dangerous asteroid is actually headed toward Earth and hits by a spacecraft – or a “kinetic collider” – that’s just one tool at humanity’s disposal. NASA, the European Space Agency, and other space agencies are also exploring other methods, such as placing a spacecraft nearby like a “gravity tractor” to pull it onto a different path, or detonating a nearby nuclear explosion to force it away. (Exploding the asteroid itself runs the risk of failure, because that could turn it into many “The Kinetic Collider is the most mature of these technologies,” Lori Glaze, director of NASA’s Planetary Science Division, said at a media briefing Sunday.
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