US: Nasa gears up to deflect asteroid the size of two Statues of Liberty in key test of planetary defence

Bet the dinosaurs wish they'd thought of this.

On Monday, Nasa will attempt a feat humanity has never before accomplished: deliberately smacking a spacecraft into an asteroid, in order to slightly deflect the cosmic object from its orbit — a key test of our ability to stop cosmic objects from devastating life on Earth.

The Double Asteroid Redirection Test (DART) spaceship launched from California last November and is fast approaching its target, which it will strike at roughly 23,000km/h.

To be sure, neither the asteroid moonlet Dimorphos, nor the big brother it orbits (called Didymos) pose any threat as the pair loop the Sun, passing about seven million miles from the Earth at the nearest approach.

Yet, the experiment is one Nasa has deemed important to carry out before an actual need is discovered.

"This is an exciting time, not only for the agency, but in space history and in the history of humankind quite frankly," Lindley Johnson, a planetary defense officer for Nasa told reporters in a briefing on Thursday.

If all goes to plan, impact between the car-sized spacecraft, and the 160m-big asteroid (equivalent to two Statues of Liberty) should take place at 7.14pm Eastern Time (23.14 GMT), and can be followed on a Nasa live stream.

By striking Dimorphos head on, Nasa hopes to push it into a smaller orbit, shaving ten minutes off the time it takes to encircle Didymos, which is currently 11 hours and 55 minutes — a change that will be detected by ground telescopes in the days that follow.

The proof-of-concept experiment will make a reality what has before only been attempted in science fiction — notably films such as Armageddon and Don't Look Up.

As the craft propels itself through space — flying autonomously for the mission's final phase like a self-guided missile — its main camera system, called DRACO, will start to beam down the very first pictures of Dimorphos.

"It's going to start off as a little point of light and then eventually it's going to zoom and fill the whole entire field of view," said Nancy Chabot of the Johns Hopkins Applied Physics Laboratory (APL), which hosts mission control, in a recent briefing.

"These images will continue, until they don't," added the planetary scientist.

Minutes later, a toaster-sized satellite called the LICIACube — which will separate from DART a couple of weeks prior to the collision — will make a close pass of the site to capture images of the collision and the ejecta, which refers to the pulverised rock thrown off by impact.

LICIACube's picture(s) will be sent back in the weeks and months that follow.

Also watching the event will be an array of telescopes, both on Earth and in space — including the recently operational James Webb — which might be able to see a brightening cloud of dust.

Finally, a full picture of what the system looks like will be revealed when a European Space Agency mission four years down the line, called Hera, arrives to survey Dimorphos' surface and measure its mass, which scientists can only estimate at the moment.

Very few of the billions of asteroids and comets that visit our solar system are considered potentially hazardous to our planet, and none in the next hundred or so years.

"I guarantee to you that if you wait long enough, there will be an object," declared Thomas Zurbuchen, Nasa's chief scientist.

If this succeeds, then it will be a first step towards a world capable of defending itself from a future existential threat, said Chabot.