Scientific American: “NASA’s DART Spacecraft successfully Smacks a Space Rock–Now What?”

Scientists wanted DART’s target to be a binary asteroid, where one asteroid orbits another, because such celestial configurations allow easier measurements of small, impact-induced orbital changes. The deflection is almost instantaneous, says Patrick Michel of the French National Center for Scientific Research, former lead scientist of AIM and now principal investigator of Hera. In 2013 scientists selected the Didymos system as the target. First found in 1996, that larger asteroid gained its name (Greek for “twin”) following the discovery of a small orbiting companion in 2003, which was later dubbed Dimorphos, or “to have two forms”.

Dimorphos completes an orbit of Didymos every 11.92 hours. The asteroids share a similar orbit with Earth yet pose no threat as they never come closer than a few million kilometers to our planet. But their angle of orbit means that Dimorphos regularly “eclipses” in front of Didymos, allowing its orbital period to be precisely measured. Following the impact, a variety of telescopes, including the James Webb Space Telescope and Hubble—and even spacecraft such as NASA’s Lucy probe, which is currently on its way to visit asteroids near Jupiter—will track this eclipse, allowing scientists to work out just how much Dimorphos’s orbit has been changed.

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Upcoming telescopes, such as the Vera C. Rubin Observatory, set to come online in Chile later this decade, will better track these asteroids. If we ever do find one on a collision course with Earth, the outcomes of the DART mission may well dictate what action we take. It is going to validate a tool that we could use, Rivkin says. To divert a hazardous asteroid, perhaps a larger version of DART could be used or even a series of DART-sized spacecraft to slam into the offending space rock, one after another, incrementally deflecting its doom. It depends on how much warning time we have, Rivkin says. Such a perilous event is unlikely to befall humanity anytime soon. But perhaps, far in the future, our distant descendants will have this little spacecraft to thank. If we can deflect Dimorphos, we can most likely deflect any other near-Earth asteroid, Agrusa says.

Jonathan O'Callaghan

I seem to be quite skeptical lately about boisterous announcements of ‘scientific breakthroughs’. Reading about DART felt… underwhelming. The mission was undoubtedly successful for what it set out to do – post-impact measurements showed that the orbital period of the asteroid-moon Dimorphos was reduced by 32 minutes, to 11 hours, 23 minutes (+/- 2 minutes).

Double Asteroid Redirection Test DART Credit: Matthew Twombly; Source: NASA, Johns Hopkins APL

But this goal itself, to alter the orbit of a moon instead of an actual asteroid orbiting the Sun is what I find pointless. If an asteroid were found on a collision course to Earth, nudging its moons around wouldn’t change its trajectory much, you would have to target the primary body directly. Selecting a moon as target does provide some proof of concept, but it tells you little about how a comparable impact would change an orbital path around the Sun, if it would prove sufficient to prevent a collision with Earth. This feels like a situation where scientists defined the mission to get the faster results and make headlines, instead of actual valuable data, which would have taken months to gather after impact.

This deflection method carries many issues that I think make it unpractical: asteroids vary wildly in shape and composition, so it’s hard to assess the orbital shift from an impact without accurately measuring these parameters. The DART impact on the Dimorphos was amplified by surface material blasting off into space, but other asteroids may not blow off similar ejecta, or in the same patterns. This in turn creates large uncertainties when designing asteroid deflection missions: how large should the impactor be? when does it need to strike to maximize its effects? should there be multiple smaller missions or a single big one? None of these questions were answered by the DART results unfortunately.

More reliable methods of altering asteroid orbits would be to attach rockets to it, either by landing on its surface or through a tether or mesh; or have a spacecraft orbit in its vicinity and regularly blast it with lasers, triggering multiple ejections. The resulting orbital alterations can then be measured after each blast and future interventions adjusted accordingly. A one-off slam like DART seems inefficient and risky, if Earth’s future depended on it.