Astronomers and other scientists in the United States and Europe are working on ways to make Earth avoid asteroids that may lie on collision course with it.

Experts from NASA and the European Space Agency (ESA) are looking at ways to change the paths of two asteroids – Didymos and Didymoon.

“An asteroid is very large – almost the size of a mountain – and if we hit it with a spacecraft, we only make a tiny change in the velocity. But over time that’s the difference between hitting Earth and not hitting Earth,” said Andy Cheng, lead investigator for the Asteroid Impact and Deflection Assessment (AIDA) study at NASA.

The cosmic masses aren’t heading our way for another couple of centuries, but the scientists say it is a “very important” mission.

“We want to have the capability to do this as soon as possible because we know these things are out there. But to find all the things that are potential threats is a very big goal,” Denton Ebel of the American Museum of Natural History said.

“This particular mission is to take the next step, which is something very important. What are you going to do about it if something actually has your name on it.”

The project is still in its theoretical phase, awaiting approval for launch. As of 2016, the mission is still in the conceptual phase with a proposed launch for AIM in October 2020, and for DART in July 2021. The impact of DART would be on October 2022 during a close approach to Earth.

65803 Didymos (1996 GT) is a Near-Earth Apollo asteroid discovered on April 11, 1996 by Joe Montani at Spacewatch at Kitt Peak observatory. It has a moon, whence the appellation “Didymos”, meaning “twin”.

The primary is about 800m in diameter while its moon is 150m in diameter. The moon is in an orbit about 1.1 km from the primary and with an orbital period of 11.9 hours. Didymos is the most easily reachable asteroid its size from Earth.