“Loss of Signal” is not usually a message that space mission ground control likes to see appear on their screens. Tonight, however, at 1:14 am German time, chief engineer Elena Adams and her team in the control room of the “Dart” probe they oversee broke out in cheers at this very report.
It was 7:14 p.m. on site at the Applied Physics Laboratory (APL) in Baltimore when the transmission from the on-board camera, which until then had transmitted an image to earth every second, broke off. The last showed a field full of meter-sized boulders on the surface of the 163-meter one asteroids dimorphos. The $330 million probe had just crashed on it.
An hour earlier, Dimorphos had not been visible at all. Dart’s onboard camera showed only a few pixels in size. This was not the target, but the 780-meter-wide asteroid Didymos, which Dimorphos orbits at a distance of 1.2 kilometers like a small moon.
danger for big cities
The aim of the mission, which started ten months ago, was to ram this moon head-on and as centrally as possible at its center of gravity in order to slow it down. Hopefully that would put it in a closer orbit around Didymos, which should be detectable with Earth-based telescopes. If successful, this would show that at least asteroids of this caliber can be thrown off course with existing technology should one be discovered whose orbit would cause it to collide with Earth. Because chunks of this size could wipe out a big city.
At least the scientists and engineers have taken the NASA and the APL. Dart missed the exact center of Dimorphos by only 17 meters, according to preliminary calculations. This was by no means a matter of course, because the probe was not steered to its target by a joystick from Baltimore, but plunged autonomously into its planned doom using the on-board computer. Therefore, there was always a residual risk that the algorithms would, for example, confuse a sunlit bump on the irregularly shaped Didymos with its moon, thereby throwing the probe off course. “But when we first saw Dimophos, that was the moment we knew we were going to meet,” Elena Adams said in the subsequent press conference on NASA’s Internet channel.
Had Dart missed, they would have fired up the experimental ion engine installed primarily for testing purposes, would have circled back to the asteroid pair, and would have tried again in two years. At least, that’s one of 21 scenarios that Adams and her team had trained in case of anomalies.
Luckily that had been in vain. Dart is no more, instead a ten to twenty meter wide crater gapes between the boulders on Dimorphos and a cloud of debris may have been visible above it for a long time, because the gravity of the small asteroid is so low that material thrown out only slowly falls back to the surface .
A number of telescopes have been keeping an eye out for such a cloud tonight, including the Hubble Space Telescope and the brand new James Webb Space Telescope. The action was observed at close range by an Italian mini-satellite that Dart had deployed two weeks ago. Finally, in four years, the European spacecraft “Hera” arrive in the Didymos system and then of course also examine the crater hit by Dart. However, she should not find the remains of the probe, explained Carolyn Ernst from NASA’s Jet Propulsion Laboratory at the press conference. “It was such a rapid impact that you shouldn’t be able to see anything of the spacecraft.”
This completes the work of the engineers in Baltimore well done, now astronomers must move on. Telescopic observations of the small asteroid system by several teams around the world will take at least two months to be able to determine with certainty and quantified whether and how much Dimorphos’ orbit around Didymos has changed after the crash. Only then will one know with certainty whether people have moved a celestial body here for the first time in history.