It was one of the biggest events in the exploration of the Solar System last year. The impact of the Dart probe on the moon of the asteroid Dimorphos has been seen by many telescopes and observatories, the James-Webb was also witness to it. For this, he had to go beyond his limits.
On September 26, 2022, the American probe DartDart (Double Asteroid Redirection Test) of NASA impact the lunelune Didymos, which revolves around the asteroid Dimorphos, which is on a orbitorbit not far from that of the Earth. The purpose of the test was to divert Didymos a little from its trajectory. The mission is a success and shows that this kind of operation could save the Earth if it were in the way of a planet-killing asteroid.
Numerous observatories and space telescopes were witnessed, including HubbleHubblebut also the small probe LiciaCube which took pictures on site. The télescope spatial James-Webbtélescope spatial James-Webbin service at that time for only eleven weeks, also has observed the impact. But it is not at all made for that.
« Push it to the limits ! »
The James-Webb is designed to observe objects that move slowly relative to it. This allows him to make long exposures without having to move too much, and thus collect lightslights too shy for everyone else telescopestelescopes existing.
Its speed limit of an object in its sky corresponds to the displacement of Mars, or an angular speed of up to 30 milliseconds of arc per second (0.0083 degrees per second). But the Dimorphos-Didymos duo, whose orbit is much closer to the SoleilSoleil, moves faster. It is more than three times faster than Mars, which is well beyond the tracking limits of the James-Webb.
To track objects movementmovement in its sky, the James-Webb uses the sensorsensor FGS which refers to starsstars “guides”, and goes from one to the other to maintain good guidance. The faster the target object moves, the more complex the guidance. During ground tests, the JWST had never tracked objects as fast as Dimorphos. Two weeks before the impact of Dart, an equivalent test had been carried out with the asteroid 2010 DF1, which moves as fast as Dimorphos.
If this kind of observation was not necessarily planned during the career of the James-Webb space telescope, the test of the impact of Dart made it possible to raise the limit of maximum speed of follow-up to 75 milliseconds of arc, and faster follow-ups can be studied on a case-by-case basis. The James-Webb is definitely beyond all our expectations!