It was one of the biggest events in solar system exploration in the past year. The impact of the Dart probe on the moon of the asteroid Dimorphos was observed by many telescopes and observatories, including James-Webb. For this he had to go beyond his limits.
On September 26, 2022, NASA’s DartDart (Double Asteroid Redirection Test) probe hit the Didymos moon orbiting the Dimorphos asteroid, in an orbit not far from Earth. The purpose of the test was to distract Didymos a bit from his trajectory. The mission is a success and shows that this type of operation could save Earth if it got in the way of a planet-destroying asteroid.
Numerous observatories and space telescopes were witnesses, including HubbleHubble, but also the small probe LiciaCube, which took pictures on site. The James Webb Space Telescope, which was only in operation for eleven weeks at the time, also observed the impact. But that’s not what it’s made for.
“Go to the limit!” »
The James-Webb was designed to observe objects that move slowly relative to it. This allows it to take long exposures without having to move too much, collecting lights that are too timid for all other existing telescopes.
Its speed limit of an object in its sky is equivalent to the displacement of Mars, or an angular velocity of up to 30 milliseconds of arcs per second (0.0083 degrees per second). But the Dimorphos-Didymos duo, whose orbit is much closer to the Sun, moves faster. It’s more than three times faster than Mars, which is well beyond the James-Webb’s tracking limits.
To track moving objects in its sky, the James-Webb uses the FGS sensor sensor, which refers to “guiding stars”, switching from one to the other to maintain its proper guidance. The faster the target moves, the more complex the guidance becomes. In ground testing, the JWST had never tracked objects as quickly as Dimorphos. Two weeks prior to Dart’s impact, an equivalent test was conducted using asteroid 2010 DF1, which is moving as fast as Dimorphos.
If this type of observation was not necessarily planned during the career of the James Webb Space Telescope, the Dart impact test made it possible to increase the limit of the maximum tracking speed to 75 arcms, and faster follow-ups can be checked on a case-by-case basis . The James-Webb definitely exceeds all of our expectations!