ESA’s Hera Arrives at the Asteroid Pair NASA’s DART Hit in 2022
Europe's Hera spacecraft reached the Didymos-Dimorphos asteroids on Nov. 11. How it flies around the pair and when ESA will show Dimorphos after DART.
Europe’s Hera spacecraft reached the asteroid Didymos and its small moon Dimorphos on Nov. 11, four years after NASA’s Double Asteroid Redirection Test (DART) deliberately crashed into Dimorphos in 2022. Hera is there to study what that impact left behind, according to Sky & Telescope.
The first close-up image released by the European Space Agency shows Didymos in detail, but Dimorphos sits mostly hidden behind the larger asteroid. ESA plans to unveil its first proper look at the struck moon at a special event in Berlin, Germany, on Nov. 25.
Why Hera cannot simply orbit Didymos
Didymos, formally catalogued as asteroid 65803 Didymos, is only 780 meters (2,600 feet) across. A body that small has too little mass to hold a spacecraft in a stable circular orbit at a safe distance, so Hera travels alongside the pair and watches it from 25 to 30 kilometers (16 to 19 miles) away.
Relative to the center of mass of the two asteroids, Hera drifts at about 12 cm/s (0.3 mph). Sky & Telescope notes that is roughly twice the top speed of a Mars rover. Every three or four days the spacecraft fires a small rocket burn that turns its path by 90º. The result is a series of rectangular loops around the asteroids, with each rectangle taking 2 weeks to complete.
Lighting the asteroids for mapping
The flight path is planned so the asteroids appear mostly, though not completely, lit by the Sun. Hera holds a position slightly off to one side of the Sun, which lets its cameras see surface relief along the terminator, the line between day and night on each body.
Both asteroids keep moving under the spacecraft’s gaze. Didymos spins once every 136 minutes, and Dimorphos circles Didymos once every 12 hours. Over repeated passes, Hera’s images will let scientists build topographic models of both bodies, which will serve as the base for more detailed maps later in the mission.
What this means for the DART experiment
DART was a test of whether hitting an asteroid with a spacecraft can change its path. Hera’s job is to take a close look at the aftermath on Dimorphos. Until ESA releases the new views, the public has only the images of Dimorphos taken before DART struck it.
The image released so far is mainly of Didymos. ESA has not yet shown what Dimorphos looks like after the impact, and no measurements of the crash site from Hera have been published.
What comes next for Hera
Hera is in what ESA calls its Early Characterization Phase. The spacecraft will fly two of its rectangular cycles in this orbit, until Dec. 2. After that it will move closer, to distances of 8 to 20 kilometers from the asteroids, to begin a Detailed Characterization Phase.
The next milestone for the public is the Nov. 25 event in Berlin, where ESA plans to show Dimorphos as it looks today. Sky & Telescope says it will add new images as the agency releases them.
Source: Sky & Telescope
