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 space agency crashed a spacecraft into the asteroid Dimorphos in the first-ever 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.”
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.
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, they 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.
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.
Broaden your horizons with award-winning British journalism. Try The Telegraph free for 1 month, then enjoy 1 year for just $9 with our US-exclusive offer.