“This is the first time we’ve attempted to move something in our solar system with the intent of preventing a [potential] natural disaster that has been part of our planet’s history from the beginning,” says Statler.
The DART probe—the name is an abbreviation of the Asteroid double redirect test– has been in the works since 2015. It was designed, built, and operated by the Johns Hopkins University Applied Physics Laboratory, with support from many NASA centers, and launched last November. DART is a major component of AIDA, the Asteroid Impact and Deflection Assessment, a collaboration between NASA and the European Space Agency. The mission also depends on observatories in Arizona, New Mexico, Chile and elsewhere; astronomers keep their telescopes focused on Dimorphos and Didymos to measure post-impact deflection as accurately as possible.
Until the very end of the DART flight, astronomers could only see Dimorphos and Didymos as a single point of light. The smallest asteroid is so small it can’t be seen from ground-based telescopes, but astronomers can track it by measuring how often it attenuates the already faint light of its big brother as it orbits around it.
The craft’s final approach was captured by its optical camera, called DRACO, which is similar to the camera on board New Horizons, which flew close to Pluto. Even this much closer camera could only see Dimorphos as a separate object hours before impact.
“Because you’re coming so fast, it’s only in the last few minutes that we’ll be able to see what Dimorphos looks like: what is the shape of this asteroid that we’ve never seen before?” Nancy Chabot, a planetary scientist at Johns Hopkins University and head of DART coordination, said in an interview days before impact. “It’s really only in the last 30 seconds that we will resolve the surface features of the asteroid.”
In fact, until now, scientists weren’t sure if the asteroid would look more like a billiard ball or a dustball. “Is this moon a single giant rock, or is it a collection of pebbles or particles? We don’t know,” said Carolyn Ernst, JHU researcher and DRACO instrument scientist, speaking before impact. Its composition could affect a number of variables scientists want to study: how much the crash will alter the asteroid’s trajectory, whether it will leave an impact crater, spin the asteroid or eject rock fragments.
Unlike most space probes, DART did not slow down before reaching its target. As it approached, its camera continuously took images of the asteroid as it grew through the frame, sending them to Earth via the Deep Space Network, an international antenna system. operated by NASA’s Jet Propulsion Laboratory.
These images are not only important for research; they are essential for navigation. It takes 38 seconds for human operators to send signals to DART or the probe to send images back to Earth. When timing was critical, the probe had to pilot itself. For the past 20 minutes, its automated SMART Nav system performed a “precision lock” on the target and used these images to adjust the spacecraft’s trajectory with propulsion engines.