US space agency NASA launched a satellite early Wednesday that will be steered directly into an asteroid to alter its course, in the first mission of its kind.

The planetary defence probe, powered by a Falcon 9 rocket made by Elon Musk's SpaceX, took off at 10:21 pm Pacific time (0621 GMT) from Vandenberg Space Force Base in California.

"Asteroid Dimorphos: we're coming for you!" the US space agency tweeted.

The satellite - which NASA says is the size of a vending machine - is due to smash into the Dimorphos asteroid in October next year, in what has been dubbed the Double Asteroid Redirection Test (DART).

Dimorphos, a moonlet whose diameter measures some 160 metres, does not pose a danger to Earth, according to NASA's calculations - and the mission is designed to ensure that even after the impact, there would be no risk to the planet.

The European Space Agency mission Hera is scheduled to launch in 2024 to study the impact in greater detail.

NASA hopes the mission, costing some 330 million dollars, will deliver insights about how to protect Earth from any approaching asteroids.

Scientists are not aware of any asteroids currently threatening Earth but have identified some 27,000 asteroids in our planet's proximity, with some 10,000 of them measuring more than 140 metres in diameter.

dpa