![]() ![]() When the timing was critical, it was necessary for the probe to pilot itself. It takes 38 seconds for human operators to send signals to DART-or for the probe to send images back to Earth. Those images aren’t just important for research they’re key for navigation. As it approached, its camera continually took images of the asteroid as it grew in the frame, sending them to Earth via the Deep Space Network, an international system of antennas managed by NASA’s Jet Propulsion Laboratory. Unlike most space probes, DART didn’t slow down before reaching its target. Its makeup could affect a number of variables scientists want to study: How much the crash will alter the asteroid’s trajectory, if it’ll leave an impact crater, rotate the asteroid, or eject rock fragments. “Is this moon a single giant rock, or is it a collection of pebbles or particles? We don’t know,” said Carolyn Ernst, a JHU researcher and DRACO instrument scientist, speaking before the impact. In fact, until today, scientists weren’t really sure whether the asteroid would be more like a billiard ball or a dust ball. “It’s really only within the last 30 seconds that we’ll resolve surface features on the asteroid.” “Because you’re coming in so fast, it’s only within the last few minutes that we’ll get to see what Dimorphos looks like: What is the shape of this asteroid we’ve never seen before?” said Nancy Chabot, planetary scientist at Johns Hopkins University and DART’s coordination lead, in an interview a few days before the impact. Even this much more close-up camera was able to see Dimorphos only as a separate object a few hours before impact. The craft’s final approach was captured by its optical camera, called DRACO, which is similar to the camera aboard New Horizons, which flew by Pluto. The smaller asteroid is so tiny it can't be seen from Earth telescopes-but astronomers can track it by measuring how often it dims the already faint light from its bigger sibling as it orbits around it. Until the very end of DART’s flight, astronomers could see Dimorphos and Didymos only as a single dot of light. The mission also depends on observatories in Arizona, New Mexico, Chile, and elsewhere astronomers are keeping their telescopes focused on Dimorphos and Didymos to measure the post-impact deflection as precisely as possible. DART is a major part of AIDA, the Asteroid Impact and Deflection Assessment, a collaboration between NASA and the European Space Agency. It was designed, built, and operated by Johns Hopkins University’s Applied Physics Laboratory, with support from many NASA centers, and launched last November. The DART probe-the name is short for the Double Asteroid Redirection Test-has been in the works since 2015. That confirmed the spacecraft’s collision, and the room rang out with the shouts of researchers: “Oh wow!” “Oh my goodness!” “We got it!” The images quickly grew in size and then-the screen went blank. The last shots from the craft’s camera revealed Didymos to be a slightly egg-shaped rock, littered with boulders and pockmarked with craters. This is so cool,” said Lori Glaze, NASA’s Planetary Science Division director, two minutes before impact. “It went from a collection of individual pixels, and now you can see the shape and shading and texture of Didymos, and you can see the same thing with Dimorophos as we get closer and closer. As the craft sped along on its final approach, the DART team-watching from mission control-met each milestone with cheering and applause. The DART spacecraft is about the size of a vending machine, and it was hurtling at a ludicrous 14,000 miles per hour as it smashed into Dimorphos. It’s actually the diminutive member of an asteroid pair: It’s a moon of its much larger sibling, Didymos. This particular test subject is called Dimorphos, and it’s about 6.8 million miles from Earth. It’s just a test, an effort to determine whether an asteroid can be nudged off its course-a strategy that could be used to divert a near-Earth object on a collision course with us if it’s spotted well enough in advance. A team of scientists has now deliberately plowed a craft into a tumbling space rock at high speed. But this time, with DART, it’s different. NASA is usually pretty careful with its space probes.
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