Rocks from space have walloped Earth for eons, and it’s only a matter of time until our planet lands yet again in the crosshairs of a very large asteroid. But unlike other forms of life—here’s looking at you, dinosaurs—humans have a fighting chance of altering our cosmic destiny. At AGU’s Fall Meeting 2022 held in December, researchers presented a slate of new results from NASA’s Double Asteroid Redirection Test (DART) mission, the first demonstration of asteroid deflection.
Peering at an Orbit
DART’s target, the Didymos-Dimorphos asteroid system, was first discovered in the mid-1990s. But astronomers back then spotted only its larger member, Didymos, which is roughly 800 meters (half a mile) in diameter. It wasn’t until 2003 that scientists realized that a much smaller body, dubbed Dimorphos, was also present. Dimorphos is about one fifth the size of Didymos, and its orbit takes it in front of and behind Didymos as seen from Earth. That’s serendipitous, because by monitoring how the brightness of the Didymos-Dimorphos asteroid system varies over time, scientists were able to precisely determine how long it took Dimorphos to complete an orbit: 11 hours and 55 minutes.
“We needed to understand the Didymos-Dimorphos system before we changed it,” said Cristina Thomas, a planetary scientist at Northern Arizona University in Flagstaff, at AGU’s Fall Meeting 2022.
The primary goals of the DART mission were simple, at least in concept: Hit Dimorphos with the roughly 570-kilogram (half-ton) DART spacecraft to alter the orbital period of Dimorphos around Didymos significantly and measure that change and characterize the physics of the impact. If successful, it would be the first demonstration of deflecting an asteroid using so-called kinetic impactor technology. (In 2005, another NASA mission, Deep Impact, tested kinetic impactor technology with a comet.)
On 23 November 2021, a Falcon 9 rocket lifted off from California’s Vandenberg Space Force Base. By then, the SpaceX-designed rocket had notched more than 100 successful launches, but for members of the DART mission, the event was anything but ordinary: Nestled within the rocket’s nose cone was the spacecraft they’d spent well over a decade designing, building, and testing.
The launch went smoothly, and DART soon entered into orbit around the Sun. For roughly 10 months, the spacecraft largely tracked the orbit of Earth, essentially waiting to catch up to the Didymos-Dimorphos asteroid system, which orbits the Sun between Earth and Mars. “We stayed close to Earth the entire time and just caught up with the Didymos system at its closest approach to Earth,” said Elena Adams, DART mission systems engineer at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.
Approaching the Unknown
It was only around July of 2022 that DART’s onboard camera—the Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO)—caught its first glimpse of Didymos. But Dimorphos wouldn’t come into view until much, much later: Just an hour before impact, at a distance of roughly 25,000 kilometers, the tiny moonlet was still a mere two pixels across in DRACO images.
“We didn’t see Dimorphos until late in the game,” said Adams. To prepare for the uncertainties of impacting a body they knew virtually nothing about, DART team members ran thousands of Monte Carlo simulations beforehand in which they varied the moonlet’s size, shape, albedo, and a slew of other parameters.
The DART spacecraft successfully impacted Dimorphos on 26 September 2022. The event was recorded by a cadre of Earth-based telescopes and also the Light Italian Cubesat for Imaging of Asteroids (LICIACube), a briefcase-sized spacecraft carrying two cameras that launched with DART and was released from the spacecraft 15 days prior to impact.
A Serendipitous Boost
Researchers had calculated that the impact, which occurred roughly head-on, would shorten Dimorphos’s orbital period by just under 10 minutes. That was assuming the simplest case of no ejecta being produced, said Andy Cheng, DART investigation team lead at the Johns Hopkins University Applied Physics Laboratory, at a press conference.
“The amount of momentum that you put in the target is exactly equal to the momentum that the spacecraft came in with.” But if ejecta flies off the asteroid after impact, physics dictates that the asteroid can get an extra boost, said Cheng. “You end up with a bigger deflection.”
That’s good news when it comes to pushing a potentially harmful space rock out of the way, said Cheng. “If you’re trying to save the Earth, that makes a big difference.”
And ejecta there was, in spades—on the basis of detailed follow-up observations of the Didymos-Dimorphos system, scientists discovered that Dimorphos is now traveling around Didymos once every 11 hours and 22 minutes. That’s a full 33 minutes shorter than its original orbital period, a finding that implied that a substantial amount of ejecta was produced. Imagery obtained from ground- and space-based telescopes has borne that out—a plume of debris tens of thousands of kilometers long currently stretches out from Dimorphos. Researchers have estimated that at least a million kilograms (1,100 U.S. tons) of material were blasted off the asteroid by the impact. That’s enough debris to fill several rail cars, said Andy Rivkin, DART investigation team lead at the Johns Hopkins University Applied Physics Laboratory, at a press conference at the Fall Meeting.
Follow the Debris
Interestingly, the ejecta shed by Dimorphos has remained in distinctly more plumelike configurations than the debris shed by comet 9P/Tempel 1 when NASA’s Deep Impact spacecraft intentionally crashed into it in 2005. “The Dimorphos ejecta has a lot of morphological features,” said Jian-Yang Li, a planetary scientist at the Planetary Science Institute in Fairfax County, Virginia, and a member of the DART team, at the Fall Meeting.
The reason is probably the different compositions and surface features of the two bodies, he said. Tempel 1 is rich in volatiles and fine-grained dust; Dimorphos’s surface, on the other hand, is littered with boulders. Scientists plan to continue to monitor Dimorphos’s debris plume through at least March.
The DART mission has also enabled scientists to investigate a fundamental question about the Didymos-Dimorphos asteroid system: Do the two asteroids have the same composition? It’s a common assumption when it comes to binary asteroids, but it’s never been confirmed. Thomas, leader of the DART Observations Working Group, presented new results on the subject at a press conference at the Fall Meeting. She shared near-infrared spectra of the binary asteroid system that astronomers had collected both before and after impact using a NASA telescope in Hawaii.
Observations obtained prior to impact (when the overwhelming majority of the sunlight reflected off the asteroid system came from Didymos) and after impact (when the debris shed by Dimorphos was responsible for more than two thirds of the reflected light) revealed very similar spectra, with characteristic dips at wavelengths of 1 and 2 micrometers in both cases. That’s strong evidence that the two asteroids have similar compositions, said Thomas.
Scientists aren’t yet finished with Didymos and Dimorphos: In 2024, researchers involved in the European Space Agency’s Hera mission plan to launch a spacecraft to the system to further characterize the asteroids—including accurately measuring the mass of Dimorphos—and to study the crater created by the DART impact.
—Katherine Kornei (@KatherineKornei), Science Writer