Nasa asteroid strike unleashes boulder storm ‘as deadly as Hiroshima’

Nasa Dimorphos asteroid experiment
Nasa’s test knocked the asteroid Dimorphos, in blue, off course, but accidentally released a swarm of boulders, circled - Nasa/ESA/David Jewitt (UCLA)/Alyssa Pagan (STScI)

A storm of boulders “as deadly as Hiroshima” was accidentally unleashed by Nasa during tests to change the trajectory of an asteroid, scientists have found.

Last September, the agency crashed a spacecraft into the asteroid Dimorphos in the first planetary defence experiment aimed at finding ways to protect humanity from an extinction-level event.

Now astronomers have found that although the impact succeeded in knocking Dimorphos slightly off course, it also dislodged 37 boulders, which are currently zipping through space at 13,000mph.

Experts said it showed that deflection strategies could have unintended consequences that leave smaller rocks on a collision course with Earth.

David Jewitt, a professor of Earth and planetary sciences at UCLA, said: “The boulder swarm is like a cloud of shrapnel expanding from a hand grenade. Because those big boulders basically share the speed of the targeted asteroid, they’re capable of doing their own damage.”

Nasa Dimorphos asteroid experiment blunder boulder storm space
This graphic showed the effect of Dart’s impact on the orbit of Dimorphos - Nasa/Johns Hopkins APL

The boulders, which were spotted by the Hubble Space Telescope, range in size from three feet to 22ft across, and are drifting away from the asteroid at little more than a half-mile per hour – roughly the walking speed of a giant tortoise.

Prof Jewitt said that given the high speed of a typical impact, a 15ft boulder hitting Earth would deliver as much energy as the atomic bomb that was dropped on Hiroshima, Japan, during the Second World War.

The rocks are not shattered pieces of the asteroid, but were already scattered on the surface and knocked off by the shock of the impact from the Dart (Double Asteroid Redirection Test) spacecraft, astronomers believed.

A close-up photograph taken by Dart just two seconds before the collision showed a similar number of boulders sitting on the asteroid’s surface – and of similar sizes and shapes.

Nasa Dimorphos asteroid experiment blunder boulder storm space
The last complete image of Dimorphos, as seen by Nasa’s Dart spacecraft, two seconds before impact - NASA/APL

Experts think that the boulders may have been flung off the surface when a seismic wave from the impact rattled through the asteroid – like hitting a bell with a hammer – shaking loose the surface rubble. They may also have been ejected in the impact plume.

The current estimate is that about 1,000 tonnes of debris were blasted away, enough to fill 60 train carriages.

Dimorphos, which was about the size of one of the Great Pyramids of Giza, was chosen because it posed little threat to Earth, so there is no danger from the boulders.

The asteroid is part of a binary system and orbits a larger mountain-sized asteroid called Didymos. Although the system is technically classified as potentially hazardous, it is still six million miles away from Earth and unlikely to pose a threat in the near future.

However, experts warned that if rubble from a future asteroid deflection were to reach our planet, it would hit at the same speed the asteroid was travelling — fast enough to cause “tremendous damage”.

Experts hope that future Hubble observations will help them pin down the precise trajectories of the boulders.

“If we follow the boulders in future Hubble observations, we may have enough data to pin down the boulders’ precise trajectories,” added Prof Jewitt.

“And then we’ll see in which directions they were launched from the surface and figure out exactly how they were ejected.”

The European Space Agency is planning an in-depth study of the aftermath of the impact with its Hera mission, due to launch in 2024 and scheduled to reach Dimorphos by Christmas 2026.

Science’s biggest mistakes
Science’s biggest mistakes

Early studies suggested that the Dart mission was a success, with the impact causing the orbit of Dimorphos to slow by about 0.1 inches per second.

Patrick Michel, Hera’s principal investigator, said: “It is likely Dimorphos was tidally locked before Dart’s impact, but is now probably either rotating or ‘librating’ – wobbling – as it orbits Didymos.”

The research was published in the journal Astrophysical Journal Letters.

Associate professor Cristina Thomas, the Dart Observations Working Group Lead, of the University of Arizona, said: “These boulders are definitely something to take into consideration as we look forward to ESA’s Hera mission.

“As for planetary defense missions in general, it is certainly something for us to consider to improve our understanding of the kinetic impact technique, but on Earth there is nothing to be concerned about from objects this size.

“A kinetic impact would be used many years in advance of any potential Earth impact, so boulders of this kind would have a lot of time to move away from the potentially hazardous asteroid.

“We did expect boulders to be part of the material ejected from the surface of Dimorphos after the Dart impact. It’s still incredibly exciting to see these predictions come to life.”

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