Swiss-assisted asteroid defence mission arrives at Dimorphos

Asteroid defence: Mission involving Bern achieves its objective Keystone-SDA

Four years after a NASA probe struck the asteroid moon Dimorphos in the first planetary defence crash test, Europe’s Hera space probe is in place to measure the impact.

The forensic investigation, with Swiss participation, is now underway and Hera’s data is set to reveal the effects of the impact.

“The result will provide a better basis for determining how much energy and momentum are required to deflect an asteroid of a given mass,” astrophysicist Martin Jutzi from the University of Bern, who is involved in the mission, told the Keystone-SDA news agency. “This will then allow us to determine how efficiently an asteroid can actually be deflected by colliding with it.”

The DART spacecraft, launched by the United States space agency NASA, crashed into Dimorphos in September 2022 at a speed of around 6.1 kilometres per second – just under 22,000 kilometres per hour.

This celestial body, approximately 160 metres in size, orbits the larger asteroid Didymos like a moon. The targeted impact shortened Dimorphos’ orbital period from 11 hours and 55 minutes to around 11 hours and 22 minutes.

To what extent did the impact alter Dimorphos as a whole, and how efficiently was the probe’s kinetic energy transferred to the asteroid? To answer these questions, the Hera space probe set off in October 2024 for the asteroid system, which will be around 195 million kilometres from Earth upon its arrival.

Data can now be collected.

Hera is now approaching its target. The braking manoeuvre began last week. “Arrival is a challenging phase for the mission,” says Jutzi.

The so-called ‘Early Characterisation Phase’ will then begin in November. By then, Hera will be only around 30 kilometres from Dimorphos – close enough to provide researchers with the first usable images. “We hope to be able to make a rough assessment of the effects of the Dart impact from these images,” said the astronomer. By December, the distance will have been reduced to around 10 kilometres.

A Bern-based research team led by Martin Jutzi and John Wimarsson from the University of Bern, along with Sabina Raducan from the International Space Science Institute (ISSI) in Bern, has simulated the impact of the US probe Dart on a computer.

Their calculations suggested that Dimorphos is not a solid rock but a loosely bound pile of debris. The Bern simulations therefore predicted that the impact would not merely create an ordinary crater, but would fundamentally deform the entire asteroid moon. The images that Hera is set to deliver should now reveal whether these predictions are correct.

Furthermore, another key parameter is missing to be able to quantify more precisely the efficiency of asteroid defence missions involving a fly-by. “Only when we know the mass can we accurately quantify how great the deflection efficiency was – in other words, how much momentum and energy were transferred when Dart struck,” said Jutzi.

According to Jutzi, while asteroids striking the Earth do not pose an immediate threat, they are nonetheless a realistic scenario. The likelihood of an impact depends largely on the size of the celestial body.

Statistically speaking, smaller objects with a diameter of 10 to 20 metres strike the Earth every few decades. However, it is the larger ones that pose a real threat to humanity: “An object 100 or 200 metres in size might strike the Earth every 10,000 to 20,000 years,” Jutzi explains. Such an impact would, however, have dramatic consequences: “These are precisely the objects that can have devastating regional or even global consequences.”

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