Nasa on Tuesday said it had succeeded in deflecting an asteroid in a historic test of humanity's ability to stop an incoming cosmic object from devastating life on Earth.

The fridge-sized Double Asteroid Redirection Test (Dart) impactor deliberately smashed into the moonlet asteroid Dimorphos on September 26, pushing it into a smaller, faster orbit around its big brother Didymos, said Nasa chief Bill Nelson.

"Dart shortened the 11 hour 55 minute orbit to 11 hours and 23 minutes," he said. Speeding up Dimorphos' orbital period by 32 minutes exceeded Nasa's own expectation of 10 minutes.

"We showed the world that Nasa is serious as a defender of this planet," added Nelson.

The asteroid pair loop together around our Sun every 2.1 years, and pose no threat to our planet.

But they are ideal for studying the "kinetic impact" method of planetary defence, in case an actual approaching object is ever detected.

Dart's success as a proof-of-concept has made a reality of science fiction – notably in films such as "Armageddon" and "Don't Look Up."

Astronomers rejoiced in stunning images of matter spreading out thousands of miles in the wake of the impact – pictures collected by Earth and space telescopes, as well as a mini satellite that had travelled to the zone with Dart.

Thanks to its temporary new tail, Dimorphos, which is 160 metres in diameter or roughly the size of a big Egyptian pyramid, has turned into a man-made comet.

But quantifying just how well the test worked required an analysis of light patterns from ground telescopes, which took a few weeks to become apparent.

The binary asteroid system, which was around 11 million kilometres from Earth at impact, is visible only as a single dot from the ground. (AFP)