3. There’s no risk to Earth
When DART arrives at Didymos in 11 months, the spacecraft will be 6.8 million miles (11 million km) from Earth, according to NASA. Didymos, which is Greek for “twin,” measures 2,560 feet (780 meters) in diameter, while Dimorphos, which is Greek for “two forms,” measures 525 feet (160 meters) in diameter, which is just shy of two football fields long. The moonlet, also known as Didymoon, is in orbit around Didymos, circling it once every 11.9 hours.
To be crystal clear, neither Didymos or Dimorphos pose a risk to Earth. They don’t present a risk now, nor will they after the redirection test. NASA chose this particular system as it was deemed ideal for test purposes. The smash-up “will change the speed of the moonlet in its orbit around the main body by a fraction of one percent, but this will change the orbital period of the moonlet by several minutes—enough to be observed and measured using telescopes on Earth,” as NASA explained.
As it stands, no known asteroid the size of Dimorphos or larger has a significant chance of hitting Earth within the next 100 years. The concern, however, has to do with potentially hazardous objects that suddenly appear out of the blue. Such was the scenario in this year’s asteroid impact simulation, in which participants were told of a fictional 460-foot-wide (140-meter-wide) asteroid with a 100% chance of hitting Earth in just six months. This left them very little time to react and prepare. Asteroids of this size would inflict serious damage across a 120-mile-wide (200-kilometer-wide) radius should they hit Earth.