Didymos is not on course to hit Earth, and is not a threat to humanity. Instead NASA and the ESA are using this asteroid to practice diverting an asteroid from its path in case a different asteroid does start heading towards Earth in the future.
For the first time ever NASA and the European Space Agency (ESA) are teaming up for a test of their proposed planetary defense plans. The space agencies will be launching a probe to try and divert an asteroid called Didymos out of its current orbit.
Didymos is not on course to hit Earth, and is not a threat to humanity. Instead NASA and the ESA are using this asteroid to practice diverting an asteroid from its path in case a different asteroid does start heading towards Earth in the future. While small asteroids hit the Earth all the time, a large asteroid could cause tremendous damage if it were to hit the planet, and could even possibly cause an extinction level event. An asteroid is believed to be the cause of the extinction of the dinosaurs.
Now Didymos is about the size of the Great Pyramid in Giza. While that's pretty big and would cause considerable damage to wherever it hits the Earth, it's unlikely it would cause an extinction-level event. But scientists want to practice their defense test on Didymos so they'll know how to react if a larger asteroid heads towards Earth.
NASA and the ESA are planning to launch their test in either 2020 or 2021, but the probe would not reach Didymos until October 2022. So hopefully no giant asteroids head towards Earth until then.
Astronomers have been meeting and seriously talking about what might be needed to deflect an asteroid for at least a couple of decades. Those talks have evolved into action; NASA’s DART mission is planned to launch in 2021, with the goal of ramming an asteroid in 2022, and testing the asteroid’s response. Afterwards, if all goes as planned, an ESA mission called Hera – now currently under study – will also visit the asteroid, gathering more detailed information. A February 4, 2019, statement from ESA explained:
The target of is a double asteroid system, called Didymos, which will come a comparatively close 11 million km (about 7 million miles) to Earth in 2022. The 800-meter-diameter main body (about 2,600 feet) is orbited by a 160-meter-diameter moon (about 525 feet), informally called ‘Didymoon’.
Hera manager Ian Carnelli said in an email to EarthSky that both DART and Hera fall under the framework of what scientists call the Asteroid Impact and Deflection Assessment, or AIDA. Carnelli wrote:
Our Hera and Dart mission teams are fully functional and coordinating this joint experiment. An AIDA workshop is planned in September 2019 in Rome. The original ESA part of the mission, called AIM, did not receive full funding. ESA has therefore re-worked the mission (now called Hera) and optimized for reaching Didymos after DART impact, to complete the experiment by 2026.