When the European Space Agency’s Hera spacecraft photographed Didymos and its moonlet Dimorphos on October 5, the pair appeared as a single point of light 270,280 kilometers away. By October 27, Hera was close enough to photograph the system from 360 kilometers. ESA released the approach sequence on November 5.
The images came from Hera’s main Asteroid Framing Camera, which serves both navigation and science. During approach, the position of the target in successive frames helps navigators track the spacecraft’s path relative to it. As the distance closes, the same camera can begin to distinguish the bodies that shared a single point in the first frame.
Approaching an asteroid for an extended survey also requires matching its motion. During the second half of October, Hera carried out braking maneuvers to reduce its speed relative to the pair. The spacecraft and asteroids continue to move around the Sun. The speed that matters for close-range observations is the difference between their motions.
The Orbital Change Hera Will Investigate
Dimorphos is approximately 151 meters across and orbits Didymos, which is approximately 780 meters across. On September 26, 2022, NASA deliberately struck the smaller body with its Double Asteroid Redirection Test spacecraft, known as DART. The impact shortened the time Dimorphos takes to orbit Didymos by approximately 33 minutes.
An orbiting pair offered a practical way to measure the effect. From Earth, astronomers could time the repeating changes in brightness as the two bodies passed in front of or behind one another. Comparing that cycle before and after the collision gave researchers a measure of the change in orbital period.
The Hera mission examines the physical outcome at close range. Its planned measurements include the mass and structure of Dimorphos and the shape of its surface after impact. Mass matters because the same change in velocity corresponds to a different transfer of momentum for a heavier or lighter body. Together, the orbital timing and Hera’s survey will connect a change measured from Earth to the properties of the asteroid that DART struck.
