NASA gave the Didymos system a push
Steve Gribben/Johns Hopkins APL/NASA
Humanity has shifted an asteroid’s orbit around the Sun for the first time. This was achieved by NASA’s Double Asteroid Redirection Test (DART) mission in 2022, but the effect has only now been measured.
DART’s target was a small asteroid called Dimorphos, which orbits a larger one called Didymos. The spacecraft crashed into the smaller rock in an attempt to change its orbit around the larger one, testing whether this method, called a kinetic impactor, would be an effective way to alter an asteroid’s trajectory if it was headed for Earth and send it safely past.
The mission was a smashing success, shortening the length of Dimorpho’s orbit by 32 minutes. In the years since then, astronomers have continued to watch the system, and with nearly 6,000 observations, they have been able to calculate the change in the pair’s combined orbit around the sun: it has slowed by 11.7 micrometers per second, or about 40 millimeters per hour. It is expected to reduce the radius of the track by approximately 360 meters.
“It doesn’t sound like much, but the whole idea behind these kinetic impacts is that if you do one early enough, a small impact will make a big change in the overall position,” says Rahil Makadia of the University of Illinois Urbana-Champaign, part of the team monitoring the asteroid’s orbits. “It’s a very small number, but if you let it accumulate over decades, it can grow into a big one.”
The deceleration had two causes: the initial impact from the spacecraft and an additional push from the jet of debris sent up from Dimorphos’ surface in its wake. Makadia and his colleagues calculated that the two effects were roughly equal, which in turn allowed them to calculate the masses and densities of the asteroids. Dimorphos is about half as dense as Didymos, lending credence to the idea that it is a so-called rock pile that was formed from material thrown by Didymos due to its rotation.
All this information will prove useful if we ever need to deflect a dangerous asteroid for real. “We now have a solid anchor point for predicting any future kinetic impact missions,” says Makadia. And the European Space Agency’s Hera spacecraft, headed for Didymos and expected to arrive in November, should give us even more precise measurements that will guide any future efforts to protect Earth from incoming asteroids.
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