How do you identify a cosmic pebble passing across the void, three million kilometers distant, and be certain that it’s not another star? For ESA’s Hera mission, which is now far into its cruise across the asteroid belt, the solution lies in a combination of technical ingenuity and operational flexibility that is steadily pushing the limits of deep-space navigation and planetary defense.
May 11, 2025, Hera’s Asteroid Framing Camera (AFC) was tested to its limits. The subject: asteroid (1126) Otero, a stupendously rare A-type asteroid originally listed almost a century ago. In the spacecraft’s view, Otero was nothing more than a distant, crawling pinpoint against the twinkling emptiness of space. However, as ESA’s European Space Operations Centre Flight Dynamics Engineer Giacomo Moresco explained, the drill was not as straightforward as a publicity photo. “Didymos will also be a tiny, faint point of light among the stars when it first appears,” Moresco noted. “The spacecraft will need to identify Didymos as soon as possible and keep the asteroid in the centre of the camera’s field of view as it approaches.”
The AFC, which is the core of Hera’s navigation and science, is a small, 1.3-kilogram Jena-Optronik camera in Germany with a 5.5-degree field of view and a CMOS active pixel sensor engineered to handle the harshness of deep space. The heritage of the camera dates back to the Dawn mission, in which the camera imaged Vesta and Ceres with historic accuracy. On Hera, the AFC must not only image but also guide, navigate, and control targeting asteroids when they are just a speck and moving to close range navigation as the spacecraft approaches its prey.
Key to the May campaign was requiring ESA’s Flight Dynamics and Mission Analysis departments to sort through orbits of hundreds of thousands of known asteroids. Otero’s well-mapped orbit and relative brightness made it the perfect pick. Within a matter of weeks, the teams choreographed a sequence of spacecraft slews and observation sets a choreographic feat not often seen with deep-space missions. Hera followed Otero for three hours, taking a picture every six minutes. By aligning the star fields within each image, engineers generated a time-lapse that showed Otero’s gentle motion, confirming both the camera’s faint-object detection and the mission’s image-processing pipeline.
This technical demonstration was more than a dry run for Hera’s visit to Didymos and Dimorphos later in 2026. It demonstrated a new paradigm for planetary defense: the ability to rapidly retarget and observe newly discovered near-Earth objects (NEOs) or even interstellar visitors from deep space. Earlier this year, the asteroid 2024 YR4 temporarily caused alarm with an estimated Earth impact in 2032 an event since disqualified. Had a spacecraft such as Hera been in place, an observation similar to that one could have yielded vital information about the object’s trajectory and potential danger.
The flight agility was again tested on July 19, when Hera’s camera pointed at (18805) Kellyday, a target fforty times fainter than Otero. “These observations really pushed the limits of Hera’s faint object detection and of our image processing capabilities. But nonetheless, we spotted it!” Moresco said. The findings are “very encouraging for the performance of the camera during the approach to Didymos.”
Hera’s mission also serves as a test bed for its suite of instruments and software. The AFC’s monochromatic imagery is supplemented by the HyperScout instrument, which records 25 spectral bands of data, and the ASPECT hyperspectral imager on the Milani CubeSat, pushing observations to the infrared. The guidance, navigation, and control system of the spacecraft uses a data fusion algorithm that blends visual tracking, lidar altimetry, and inertial measurements to create a precise, real-time model of the asteroid environment. This is critical to operations in the Didymos system, where gravity is tens of thousands of times weaker than Earth’s and even small navigational mistakes would cause the spacecraft to drift away.
Science risks are high. Hera’s visit to Didymos will be the first in-depth exploration of the effects of NASA’s DART impact, which in 2022 changed the orbit of Dimorphos and launched a plume of debris into space. Hera will map the impact crater, quantify the mass and composition of both asteroids, and inform models of kinetic impactor methods the most advanced technique for deflecting dangerous objects on a collision course toward Earth. As highlighted in a recent technical appraisal, “the design of an efficient mitigation strategy relies on our understanding of the physical properties of threatening objects and their response to a mitigation tool, which is still extremely poor.” Hera’s discoveries are predicted to close vital gaps in this understanding.
In addition to its immediate objective, Hera is establishing the foundation for a new generation of deep-space operations. Its capacity to make quick, accurate measurements of distant targets shows a possible paradigm for future planetary defense missions a model whereby spacecraft can be commissioned short notice to characterize NEOs or interstellar objects, complementing the data that ground-based telescopes cannot provide.
Every faint, fleeting sighting of a far-off asteroid is more than a technological milestone. It’s a step toward a day when human capacity for detecting, following, and if called upon deflecting cosmic threats rests on proven, trusted engineering. Hera’s unassuming achievements in the darkness of the asteroid belt are already rewriting the playbook for planetary defense.
