NASA spacecraft will slam into asteroid in first planetary-defence test

NASA is about to launch a multimillion-dollar spacecraft — and slam it into an asteroid. Rather than being a catastrophic error, however, it will be the first test of a way to protect Earth from killer asteroids.

The asteroid that NASA is smashing into, called Dimorphos, is not a threat to Earth. But researchers want to see whether they can change its trajectory, long before they might need to use such a strategy to deflect a truly dangerous asteroid.

“The odds of something large enough to be a problem, that we would have to deflect, are pretty slim in our lifetimes,” says Andy Rivkin, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory (JHU-APL) in Laurel, Maryland, which built the spacecraft for NASA. “But sometimes your number comes up when you don’t expect it, and it’s good to have an insurance policy.”

Slated to launch from California on 23 November, the spacecraft is called the Double Asteroid Redirection Test (DART). Its target is a pair of asteroids that travel together through space, one orbiting the other as they circle the Sun (see ‘A not-so-gentle nudge’). Dimorphos, the smaller of the two at 160 metres wide, orbits Didymos, which is nearly 5 times larger and is named after the Greek word for ‘twin’.

If DART launches successfully, in late September or early October of next year it will slam headlong into Dimorphos at 6.6 kilometres per second. The impact should shrink Dimorphos’s orbit so that it circles Didymos at least 73 seconds faster than before. (Dimorphos is named after the Greek for ‘having two forms’, to signal NASA's intent to change the asteroid’s orbit.) Astronomers using telescopes on Earth will watch Didymos for signs of that orbital change — which would be evident in the way its brightness changes over time, as Dimorphos passes in front of and behind it.

This complicated choreography is meant to test the idea that smashing into an asteroid can give it enough of a nudge to keep it from hitting Earth, says Nancy Chabot, a planetary scientist at JHU-APL who works on the mission. Using the non-threatening pair Dimorphos and Didymos is “a really smart and a safe way to do this first test”, she says. The impact will occur when the asteroids are 11 million kilometres from Earth.

Battling asteroids

Small asteroids and asteroid fragments hit Earth all the time, but most of them disintegrate in the atmosphere or fall harmlessly to the ground as meteorites. NASA has identified more than 27,000 asteroids with trajectories that bring them close to Earth. The worry is that some new asteroid could appear, headed directly towards the planet — and that it would be large enough to cause serious consequences when it hits, just as with the asteroid that helped to kill off the dinosaurs and other life on Earth 66 million years ago.

Space scientists have floated all sorts of ideas to battle incoming asteroids, the most dramatic of which involves blasting them with nuclear weapons. Other, less cinematically worthy strategies involve altering the asteroid’s trajectory by flying a spacecraft alongside to tug on it using gravitational forces, or smashing into it as the US$330-million DART mission will.

Depending on the angle at which DART hits the asteroid, it could kick up a small cloud of dust and rubble. The impact will probably leave a crater that could be around 10 metres across. At the same time, bits of the spacecraft’s wreckage might scatter across the asteroid’s surface, but exactly how DART will break apart remains to be seen. “Just from a pure crime-scene sense, a lot of us are curious about that,” Rivkin says.

Researchers will have a chance to get answers, because minutes later, a tiny probe funded by the Italian Space Agency will fly by to photograph the aftermath. Named LICIACube, it will travel aboard DART and is the agency’s first autonomously guided deep-space mission. LICIACube will be released from DART 10 days before impact, and come within 55 kilometres of Dimorphos. As it whizzes past, its cameras should spot the dust cloud, if the impact kicks one up, and possibly the resulting crater. “We might be surprised by the images we collect,” says Elisabetta Dotto, an astronomer at the National Institute for Astrophysics in Rome, which is leading the collaboration of Italian universities and institutions involved in LICIACube.

In 2026, a follow-up spacecraft, the European Space Agency’s Hera mission, will visit Dimorphos to take more detailed pictures of the impact site.

Data collected by the DART mission should help scientists to understand how impacts affect asteroids, says Megan Bruck Syal, a physicist at the Lawrence Livermore National Laboratory in California, who will model what happens to Dimorphos. But DART is just one test involving one kind of space rock. There could be scenarios in which planetary defenders want to hit an asteroid with more speed than DART will reach when it hits Dimorphos, or in which they need to pummel an asteroid with several impactors to change its course. “We need to do more experiments like this,” Bruck Syal says.

Although many other spacecraft have been deliberately smashed into celestial objects at the ends of their lives, DART promises to be the first to hit a planetary body in the name of saving Earth.

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