It’s a popular movie plot: an asteroid is heading for earth and a team of scientists - or Bruce Willis - emerge as heroes, saving the day and the planet.

But this storyline is close to becoming reality: a NASA strategy to deflect asteroids that threaten to hit Earth has been moved into the design phase and it’s a strategy that involves a spacecraft smashing into the oncoming rocks.

NASA’s Double Asteroid Redirection Test or DART, designed by scientists at Johns Hopkins Applied Physics Laboratory, will first create a deliberate collision between a spacecraft the size of a refrigerator and an asteroid that currently poses no threat to Earth. The idea is that the crash will cause the asteroid to gradually shift its orbit and that this technique could be used against larger asteroids on a collision course with the planet.

The asteroids chosen for the test are known as Didymos, Greek for twin, so called because they are in fact an asteroid binary system consisting of two bodies. They have been selected because they are of a size and composition that would wreak devastation if they hit Earth. The first, Didymos A, measures about 780 metres in diameter, while Didymos B is approximately 160 metres.

The Didymos system will make a distant and safe approach to Earth in October 2022 and then again in 2024. The space agency plans to crash a spacecraft travelling at 3.7 miles a second into Didymos B during this timeframe.

This will be the first time that NASA has deployed what is known as the “kinetic impactor technique” and while we won’t be able to see much of the collision from Earth, the data recorded will help scientists understand how many kinetic impacts would be necessary to effectively shift the trajectory of a single asteroid.

“DART is a critical step in demonstrating we can protect our planet from a future asteroid impact,” said DART co-leader Andy Cheng from the Johns Hopkins Applied Physics Laboratory. “Since we don’t know that much about their internal structure or composition, we need to perform this experiment on a real asteroid.”

While small asteroids hit the Earth on a daily basis these tend to break up in the upper atmosphere, rendering them harmless. It is much rarer for those big enough to cause damage to hit the planet. The space agency has already located 93% of the larger objects that fall into this category. With DART they may have the definitive means to neutralize the threat they pose.