The launch window will open on November 24, 2021. The goal of the DART mission is to strike a small asteroid and minutely change its orbit. In recent decades, it’s become clear that asteroids do have the potential to strike Earth and cause damage. The DART mission is a test run for when Earth is faced with an incoming asteroid that’s threatening our planet. DART will arrive at its target asteroid in late 2022 with a impact, becoming the first Earth mission in our history to deflect an asteroid.
DART’s target asteroid is a moonlet of a larger asteroid. The large asteroid is Didymos, 2,500 feet (780 m) in diameter. Its companion, Didymos B (or, sometimes, Dimorphos) is 525 feet (160 m) in diameter. That’s more typical of the size asteroid that might unexpectedly threaten Earth, these astronomers said. That’s because the larger asteroids are easier to see, and their orbits are better known.
Didymos is itself classified as a Potentially Hazardous Asteroid (PHA). By definition, such asteroids have a minimum orbit intersection distance of 0.05 AU (Earth-sun units of distance) or less and an absolute magnitude of +22 or less. In other words, all asteroids that can’t get any closer to the Earth than 0.05 Earth-sun distances – roughly 4,650,000 miles (7,480,000 km) – or are smaller than about 500 feet (140 m) in diameter are not considered PHAs.