The first operation to ever collide a spacecraft with an asteroid is set to launch Wednesday (Nov. 24) and will see whether humans can deflect a potentially disastrous cosmic impact.
For such an ambitious goal, the NASA project — called the Double Asteroid Redirection Test (DART) — had rather humble origins. "I conceived the idea for the mission that became DART on a winter morning in early 2011, while I was doing stretching exercises in my basement," Andrew Cheng, a planetary scientist at Johns Hopkins University's Applied Physics Laboratory in Maryland and a lead investigator for DART, told Space.com.
DART is targeting a binary near-Earth asteroid called Didymos and its moonlet Dimorphos — the larger body is about 2,560 feet (780 meters) across, while its companion is just about 525 feet (160 m) in size. The plan is for the spacecraft to reach the pair in September or October of 2022, when the asteroids are relatively close to Earth, within 7 million miles (11.2 million kilometers).
The mission's aim is to change the course of Dimorphos through what's called a kinetic impact, with DART ramming its target at speeds of about 14,760 mph (23,760 kph), several times the speed of a high-powered rifle bullet. NASA will use ground-based telescopes to monitor the asteroids before and after the collision to see how much the impact changes the orbit of Dimorphos around Didymos.
"DART is the first demonstration of a kinetic impactor on an asteroid, and DART is the first test of an asteroid deflection method," Cheng said. "There has not been another space mission like DART in either respect."
The idea that came to Cheng that winter morning in 2011 was to target a binary asteroid instead of a singleton. A kinetic impactor operation aimed at a single asteroid would likely require two spacecraft — one to hit the target and the other to monitor the collision's relatively tiny effects on the orbit of the rock around the sun.
In contrast, a mission where researchers slammed a probe into the smaller member of a binary asteroid would lead to relatively large changes to that rock's orbit around its larger partner, which scientists could monitor from Earth. Such a design could then require just one spacecraft, proving less difficult and more cost-effective.
"With a kinetic impactor, we could hope to deflect an asteroid and prevent it from hitting Earth, but this technique has never been tested," Cheng said. "We don't know how an asteroid will respond to a high-speed spacecraft impact or how much of a deflection will result. This mission will help us answer these questions, and that's what I find exciting about DART."
Although neither Didymos nor Dimorphos pose a danger to Earth, a rock as large as Dimorphos would, "if it hit the Earth, be capable of devastating a region the size of a small state," Cheng said. "It's a good thing that DART will test the kinetic impactor approach before any specific asteroid impact threat has been discovered."
DART was initially conceived as only employing a conventional rocket, using chemical reactions to drive propulsion. In 2016, the mission changed to also employ the NASA Evolutionary Xenon Thruster (NEXT-C), which uses electric power to accelerate xenon away from the spacecraft to generate thrust. "DART is the first flight for the NEXT-C technology," Cheng said. NASA added that DART could help pave the way for NEXT-C's use in future missions.
With electric propulsion, the DART spacecraft needed solar arrays large enough to generate the required electrical power, so it acquired Roll-out Solar Arrays (ROSAs), which extend about 62 feet (19 meters) tip-to-tip when deployed. DART is the first deep space mission to use ROSAs, Cheng noted.
In 2018, DART was changed again with the addition of the Italian Space Agency's cubesat LICIACube. This miniature probe will fly by Didymos 167 seconds after the DART impact to provide images of the impact and its aftermath.
Technical challenges in the development of the ROSAs as well as the DRACO (Didymos Reconnaissance & Asteroid Camera for OpNav) camera the spacecraft will use to guide itself at its target helped push DART's impact window from October to November 2022.
"DRACO needed to be reinforced to ensure it withstands the stress of launch, and the ROSAs were delayed due to supply-chain impacts resulting from, but not limited to, the COVID-19 pandemic," Cheng said.
If DART proves successful, the next step is hitting a variety of different asteroids.
"There are many different kinds of asteroids, not only in terms of compositions, but also in terms of their surfaces and interior structures," Cheng said. "Does the effectiveness of a kinetic impactor depend very much on these differences?"
In addition, there are other methods to deflect an asteroid. These include nuclear explosives, as well as gravity tractors, in which a robotic probe flies alongside a space rock for months or years, gradually nudging it off course via a slight gravitational tug. "What is the best method to use if an asteroid impact threat is one day discovered?" Cheng said.
Finally, successors to DART should explore the speed at which Earth can respond to imminent threats. "How much time is needed to actually mount an asteroid mitigation space mission, and how will that compare to the amount of warning time we may have?" Cheng noted.
DART and its successors may one day help answer these vital questions, and in doing so, help save the planet.