Caleb Scharf writes:

Things go bump in the cosmic night all the time. Rocky objects collide in planetary systems across our galaxy, providing astute astronomers with telltale signatures of warmly glowing dust from these grinding impacts. Stellar remnants like neutron stars can crash together unleashing bursts of searing gamma-rays, and even black holes can collide and coalesce in events marked by the gargantuan ringing of spacetime as energy ripples outward in gravitational waves.

On Sept. 26, 2022, another particularly novel kind of cosmic collision called out for attention in the quiet of space. A complicated package of machinery and electronics smashed at high speed into an ancient 200-meter-sized rubble-pile asteroid, known by some of the beings on the planet Earth as Dimorphos. Weighing in at around 600 kilos, this package was the Double Asteroid Redirection Test (DART) spacecraft, an international mission led by NASA to attempt the first deliberate alteration of the orbit of a celestial body. Specifically, the moon-like orbit of Dimorphos around a larger asteroid known as Didymos, all while this tumbling pair tracks around the sun.

The motivation for this mission was to examine our options for diverting, or deflecting, asteroids that might pose an impact hazard to the Earth. To do this the DART spacecraft, launched in late 2021, threaded itself along a carefully chosen trajectory so that it would hit Dimorphos “head on” as it orbited around Didymos. Rather than risk changing the overall orbit of the asteroid pair around the sun, this would simply alter their binary configuration, and allow changes in Dimorphos’ motion to be precisely gauged.

The collision was spectacular, sending up stringy-looking plumes of rubble and dust that could be seen by astronomers back on Earth. The impact also seems to have done the trick, shortening Dimorphos’ orbit around Didymos by a substantial 32 minutes out of its previous 12-hour period.

We have crashed spacecraft into objects before, including deliberate efforts, like NASA’s Deep Impact mission in 2005, to smash into the nucleus of comet Tempel-1 to excavate material using a 360-kilogram impactor. We’ve also crunched into different asteroids in search of samples, or dropped onto the moon and other planetary surfaces. But none of these previous events involved either enough energy to cause significant change in a celestial object’s motion, or quite such targeted precision. The Deep Impact event, for instance, is thought to have modified the comet’s orbital velocity by no more than 0.0001 millimeters per second, and its entire orbital period around the sun by far less than 1 second.

In that sense, the DART mission has been an unqualified success; we now have a proof-of-principle that we can deliberately modify the trajectory of objects orbiting the sun, especially the kinds of objects that pose the greatest threats to our species’ civilization if they were to hit us. Plans are now afoot at the European Space Agency to send a followup mission called Hera to carry out detailed reconnaissance of the Dimorphos-Didymos system in 2026 and to check out more of the DART aftermath.

But the story does not end here. Carl Sagan, writing in 1992 and later in 1994, may have been the first scientist to point out what has become known as the “deflection dilemma,” in which the technology developed to prevent asteroids from hitting the Earth could also be used to cause asteroids to hit the Earth. In fact, with enough skill, asteroids could target very specific locations—from heavily populated cities to the agriculturally critical regions of enemy nations. He reasoned that since the odds of having to deploy the technology to divert a naturally hazardous asteroid in any 50-year period is around 1 in 10,000, the probability of misuse is almost certainly larger. This rather grim assessment was clearly colored by Sagan’s deep concerns about the prospects of nuclear war, where the idea of annihilation brought about by technology looms large. [Continue reading…]