Dimorphos: Nasa flies spacecraft into asteroid in direct hit

The American space agency's Dart probe has smashed into an asteroid, destroying itself in the process.

The collision was intentional and designed to test whether space rocks that might threaten Earth could be nudged safely out of the way.

Dart's camera returned an image per second, right up to the moment of impact with the target - a 160m-wide object called Dimorphos.

What had been a steady image stream cut out as the probe was obliterated.

It will be some weeks before scientists on the Nasa-led mission know whether their experiment has worked.

Nasa will determine success, or otherwise, by studying the changes to the orbit of Dimorphis around another asteroid known as Didymos.

More than two dozen telescopes on Earth will make precise measurements of the two-rock, or binary, system.

Before the collision, Dimorphos took roughly 11 hours and 55 minutes to circle its 780m-wide partner.

This ought to reduce by a few minutes following the crash.

Certainly, on the evidence of the pictures coming back from 11m km from Earth, everything appeared to go exactly to plan.

The Dart probe, moving at the relative velocity of 22,000km/h, had to first distinguish the smaller rock from the larger one. Onboard navigation software then adjusted the closing trajectory with thruster firings to ensure a head-on collision.

Dart is an acronym for Double Asteroid Redirection Test.

It was designed to do "exactly what is says on the tin", mission lead Dr Andy Rivkin, from the Johns Hopkins University Applied Physics Laboratory, told BBC News.

"This technique, called the 'kinetic impactor technique' could be used if there were to be an asteroid that was incoming at some point in the future. It's a very simple idea: You ram the spacecraft into the object you're worried about, and you use the mass and the speed of your spacecraft to slightly change the orbit of that object enough so that it would miss the Earth instead of hitting the Earth."

Dimorphos and Didymos were carefully chosen. Neither was on a path to intersect with Earth before the demonstration, and a small alteration in their orbital relationship will not have increased the risk.

But there are rocks out there that could potentially pose a danger to us.

Although sky surveys have identified more than 95 percent of the monster asteroids that could initiate a global extinction were they to collide with Earth (they won't; their paths have been computed and they won't come near our planet), this still leaves many so-far undetected smaller objects that could create havoc, if only on the regional or city scale.

An object of Dimorphos's scale would dig out a crater perhaps 1km across and a couple of hundred metres deep. The damage in the vicinity of the impact would be intense. Hence the desire to see if an asteroid can be nudged into going slightly slower or faster. The change in velocity wouldn't have to be great, especially if done many years in advance of the expected intersection with Earth.

"An analogy is if you're wearing a wristwatch and you damage it, and it starts running fast by a little bit," explained Dart mission scientist Dr Nancy Chabot, also from JHU-APL. "You might not notice the error in the first day or two, but after a few weeks you will begin to notice that the watch is just not keeping correct time anymore. It's running fast; it's ahead of where it should be."

Dart's image stream may have ended abruptly at impact, but we should get additional pictures from a spectating spacecraft.

A small Italian cubesat followed three minutes behind the main probe, snapping away at the safe distance of 50km.

The LiciaCube's data will be beamed back to Earth over the next few days.

It ought to have caught sight of the plume of debris dug out by Dart.

Four years from now, the European Space Agency (Esa) will have three spacecraft - collectively known as the Hera mission - at Didymos and Dimorphos to make follow-up studies.

- BBC