DAMAGE ASSESSMENT

Could we deflect an asteroid to stop it from hitting Earth? The success of NASA's DART mission suggests so, but only after ESA's soon-to-launch Hera mission has checked the results will we know if this approach to planetary defence is a viable possibility

- DR STUART CLARK

Another day, another rocket launch. So many, in fact, it's easy to get blasé. In 2023, almost 200 rockets lifted off from Earth, carrying satellites and other spacecraft into orbit. By early September this year, the number for 2024 had already reached 158, most of them from Elon Musk's SpaceX company which has launched 89 rockets and is aiming for around 150 by year's end.

In October, one of those additional SpaceX launches will carry the European Space Agency's (ESA) Hera mission into space. Although one more launch may seem almost insignificant, Hera could prove to be one of the most important missions ever launched, because it'll tell us how capable we are of deflecting asteroids.

In November 2021, NASA launched the Double Asteroid Redirection Test (DART) mission. It targeted Dimorphos, a small asteroid with a diameter of 177m (580ft) in orbit around a larger one called Didymos. The DART spacecraft would collide with Dimorphos on purpose to see if it could alter its orbit around Didymos. Any change would be reflected in a shift in the small moonlet's orbital period.

The mission was designed to test a deflection technique known as the kinetic impactor essentially smashing one thing into another - and it succeeded spectacularly. The spacecraft impacted Dimorphos at a speed of approximately 6.6km/s (over 14,750mph) in September 2022, changing its orbital period around Didymos by 33 minutes - far more than had been expected.

It was a historic moment, marking the first time we had intentionally altered the trajectory of a celestial body.

"DART has really shown how effective a kinetic impactor can be in moving and diverting small asteroids. It was a complete and utter success of a mission," says Prof Alan Fitzsimmons, an astronomer from Queen's University Belfast, Northern Ireland, who specialises in asteroid research.