Nasa successfully bashes asteroid in key test of planetary defences

The US space agency Nasa has successfully attempted a feat humanity has never before accomplished. Its DART mission deliberately smashed a spacecraft into an asteroid to deflect its orbit. It is now up to the European Space Agency's Hera mission to investigate the "crime scene".

An artist's illustration provided by Nasa shows the DART spacecraft prior to impact in the Didymos binary system. © AFP/Handout

Updated Tuesday 27 September

The planned collision occurred in the early hours of Tuesday morning, over 9.6 million kilometres away, with the spacecraft named DART (Double Asteroid Redirection Test) plowing into the small asteroid moonlet Dimorphos at 22,500 kilometres per hour.

Nasa scientists expected the impact to carve out a crater, hurl streams of rocks and dirt into space and, most importantly, alter the asteroid’s orbit.

While Dimorphos poses no threat to Earth, the mission is a test run in case the world someday needs to deflect an asteroid from heading our way.

Though the impact was immediately obvious — Dart’s radio signal abruptly ceased — it will be days or even weeks to determine how much the asteroid’s path was changed.

The $325 million mission was the first attempt to shift the position of an asteroid or any other natural object in space.

Live: Join senior leaders from NASA and @JHUAPL working on the #DARTMission, the world's first test of planetary defense. Preview mission milestones between now and impact with a non-threatening asteroid on Sept. 26. https://t.co/Icq23Hb49a

— NASA (@NASA) September 22, 2022

Now, the European Space Agency's Hera mission, named after the ancient Greek queen of the gods, will follow in its footsteps.

The Hera spacecraft is planned to launch in October 2024, aiming to arrive at Dimorphos in 2026 to measure the exact impact DART had on the asteroid.

But scientists are not only excited to see DART's crater, but also to explore an object that is very much out of this world.

'A new world'

Dimorphos, which orbits a larger asteroid Didymos as they hurtle together through space, provides not only a "perfect testing opportunity for a planetary defence experiment", it is also a completely new environment, ESA's Hera mission manager Ian Carnelli said.

Hera will be loaded up with cameras, spectrometers, radars and even toaster-sized nano-satellites to measure the asteroid's shape, mass, chemical composition and more.

Nasa's Bhavya Lal said that it was critically important to understand the size and composition of such asteroids.

"If an asteroid is made up of, for example, loose gravel, approaches to disrupt it may be different than if it was metal or some other kind of rock," she told the International Astronautical Congress in Paris this week.

• Nasa launches Dart planetary defence mission to knock asteroid off course

So little is known about Dimorphos that scientists will discover "a new world" at the same time as the public on Monday, Hera mission principal investigator Patrick Michel said.

"Asteroids are not boring space rocks – they are super exciting because they have a great diversity" in size, shape and composition, Michel said.

And because they have low gravity compared to Earth, matter there could behave completely differently than expected.

"Unless you touch the surface, you cannot know the mechanical response," he said.

'Behaved almost like fluid'

For example, when a Japanese probe dropped a small explosive near the surface of the Ryugu asteroid in 2019, it was expected to make a crater of two or three metres. Instead, it blasted a 50-metre hole.

"There was no resistance," Michel said.

"The surface behaved almost like a fluid," rather than solid rock, he added. "How weird is that?"

One way the Hera mission will test Dimorphos will be to land a nano-satellite on its surface, in part to see how much it bounces.

• 'Potentially hazardous' asteroid speeds past Earth

Binary systems like Dimorphos and Didymos represent around 15 percent of known asteroids, but have not yet been explored.

With a diameter of just 160 metres – around the size of the Great Pyramid of Giza – Dimorphos will also be the smallest asteroid ever studied.

Learning about the impact of DART is not only important for planetary defence, Michel said, but also for understanding the history of our Solar System, where most cosmic bodies were formed through collisions and are now riddled with craters.

That's where DART and Hera could shine a light not just on the future, but on the past.

(with wires)