Hera is beginning to take a close look at what DART changed. The European probe has obtained its first close-up image of the Didymos asteroid system, from a distance of around 60 kilometres. The larger body is clearly visible; its smaller companion, Dimorphos, remains largely in shadow. Yet it was the latter that the American probe crashed into in September 2022, to test a deflection method.
NASA confirmed as early as October 2022 that DART had changed the orbital period. This first view opens a new phase in the investigation. DART had shown that an impact could change Dimorphos’s orbit around Didymos. Hera must now help explain how the target responded, and why the change was so significant. This distinction is essential to moving from a successful experiment to a technique whose effects could be better anticipated.
Two asteroids, two roles in the same experiment
The European Space Agency gives diameters of 780 metres for Didymos and 151 metres for Dimorphos. The smaller body orbits the larger one. By hitting Dimorphos, DART was therefore not targeting the system as a whole in the same way: the experiment focused on the movement of this small moon around its neighbour.
On 26 September 2022, the 580-kilogram American probe struck Dimorphos at around 6.1 kilometres per second. After the impact, its orbital period around Didymos decreased by 33 minutes, to within one minute, the ESA recalls. This figure refers to the time taken to complete one orbit around Didymos. On its own, it is not a measure of the deflection distance that would be achieved by an operation against another asteroid.
The principle tested is kinetic impact: using the momentum of a spacecraft to change that of its target. But the result also depends on the target itself. Its mass, structure and response to the impact all play a part in how its movement is changed. This is where close-up observations become valuable.
A face of Didymos that raises new questions
The image published by the ESA comes from Hera’s Asteroid Framing Camera, a monochrome camera. It shows a different face of Didymos from the one observed by DART before its collision. An equatorial ridge is still recognisable, while boulders, craters and features that may resemble grooves or ridges can be seen on this face.
The wording matters: these features are visible, but their origin has not yet been established. Michael Kueppers, the mission’s scientist at the ESA, raises the possibility that material ejected from Dimorphos travelled through the space between the two bodies and settled on Didymos. Study and mapping should make it possible to investigate this possibility; the first image does not already prove that every feature comes from the impact.
The system is therefore an ensemble to be observed. Understanding the impact on Dimorphos means looking at its surface, but also at how material may have dispersed around it. An image of the larger asteroid may provide clues about what happened on the smaller one.
What remains to be measured on Dimorphos
In a scientific article published by The Conversation in 2024, before Hera’s departure, Patrick Michel explained why DART’s observations were not enough to complete the analysis. Measuring the effectiveness of the deflection requires, in particular, knowing Dimorphos’s mass and better describing its physical properties.
One question concerns its structure: is it an assemblage of boulders with significant voids, or a more compact body beneath its surface? Another concerns the shape left by the impact: a localised crater or a more global transformation of the small moon? These scenarios were presented as questions to be resolved; they do not become established findings with the arrival of the first images.
The mission was designed to bring measurements closer to answering these questions, using the main spacecraft and two small satellites carried aboard it. The programme described in 2024 provides for several types of observation, from imaging to measurements of surface properties. This description sheds light on the scientific objectives; it does not mean that every instrument has already delivered its findings.
Validating a method rather than repeating a single success
The aim of the models is to reproduce the observed effects based on the impact conditions and the asteroid’s actual characteristics. If some properties of the target remain unknown, several explanations may still fit the same observations. Measuring them reduces this margin of uncertainty and makes it possible to test the simulations more thoroughly.
An observed change in orbit therefore provides important evidence, but not yet a set of instructions that applies to every target. The Hera mission must link this result to Dimorphos’s properties to improve our understanding of deflection by impact.
The ESA has announced that detailed images of both asteroids will be presented on 25 November. For now, the first close-up view marks the start of an examination: it reveals shapes and features to study, while also reminding us that the measurements needed for a complete explanation still lie ahead for the mission.
Sources
- Hera’s first close-up view of the Didymos asteroids — ESA
- The Hera mission will join the asteroid deflected by NASA — The Conversation, 2024 presentation
- Confirmation of the orbital change after DART — NASA, October 2022
Composite view of Dimorphos based on images taken by DART before the September 2022 impact. Archive image; this is not Hera’s new photograph.
Photograph: NASA / Eydeet — Wikimedia Commons — Public domain.
