First Thermal Imaging of a Binary Asteroid System

The Hera mission to the binary asteroid system, led by the European Space Agency (ESA) and carrying the Thermal Infrared Imager (TIRI) developed by the Japan Aerospace Exploration Agency (JAXA), was launched aboard a SpaceX Falcon 9 on October 7, 2024 (JST).1 Following launch, Hera conducted observations of the Earth-Moon system,2 and performed a Mars flyby together with observations of Mars and its moon Deimos in March 2025,3 while continuing its journey toward the binary asteroid system consisting of Didymos and Dimorphos. Beginning in October 2026, Hera executed a series of braking maneuvers and arrived at the binary asteroid system on November 4, 2026 (JST).4 For details of Hera’s upcoming observation and mission operations plan, please refer to this article.5

Figure 1 shows thermal infrared images of asteroid Didymos and its moon Dimorphos, acquired by TIRI from a distance of 57 km from the binary asteroid system at 20:19:52 (JST) on October 31, 2026. Because TIRI observes thermal infrared radiation in the wavelength range of 8-14 μm, the brightness (digital number, DN) of each pixel varies according to the surface temperature of the observed body. Using conversion equations derived from ground calibration tests, these DN values can be converted into temperature measurements.

At the time of observation, the binary asteroid system was located approximately 1.2 au from the Sun (1 au, or astronomical unit, is the average distance between the Earth and the Sun, about 150 million km). Analysis of the surface temperature of Didymos is currently ongoing.

The temporal variation of these temperatures can be used to estimate the thermal inertia of the asteroid surfaces, a measure of how easily a material heats up and cools down. Thermal inertia, in turn, provides insights into the physical properties of the surface, including whether it is dominated by fine-grained regolith or large boulders, as well as its porosity. Although the surface of Dimorphos appears relatively cooler than that of Didymos in Figure 1, this is because part of the moon is located within the shadow cast by Didymos.

Figure 2 shows the binary asteroid system imaged by the visible-light Asteroid Framing Camera (AFC) from a distance of 60 km, under nearly the same observation geometry (the relative positions of the spacecraft, the Sun, asteroid Didymos, and its moon Dimorphos) as the TIRI observation.6 At the time of the observation, the solar phase angle (the angle between the Sun, the asteroid, and the spacecraft) was approximately 45°, indicating that only the sunlit side of the asteroid surfaces was visible. In particular, the moon Dimorphos appears as a thin crescent.

In contrast, TIRI is capable of observing the surfaces of the asteroids on their nighttime sides, where direct sunlight does not reach. Because thermal infrared observations detect heat emitted from the surface itself rather than reflected sunlight, TIRI can reveal thermal characteristics even in regions that appear dark in visible-light images. This capability provides important information about the thermal properties and physical state of the asteroid surfaces that cannot be obtained from visible observations alone.

Figure 1. Asteroid Didymos and its moon Dimorphos imaged by TIRI at 20:19:52 JST on October 31, 2026. (Top) DN (Digital Number) image. (Bottom) Temperature-converted image derived from the DN values. © ESA/JAXA

Figure 2. The binary asteroid system imaged by the Asteroid Framing Camera (AFC) from a distance of 60 km. © ESA
Original image: ESA：https://www.esa.int/articles/about-us/directorates/operations/space-safety/hera/hera-s-first-close-up-view-of-didymos-asteroids

Figure 3 shows the motion of the binary asteroid system as observed by TIRI. At the time of these observations, the solar phase angle was approximately 45°, meaning that the asteroids appeared half-illuminated in visible-light images, similar to a half moon. In contrast, because TIRI detects thermal infrared radiation, it can observe the asteroid surfaces even in shadow, making it possible to determine their global shapes.

In the video, the temperature of Dimorphos can be seen to decrease as it passes into the shadow cast by Didymos. This phenomenon cannot be directly observed with visible-light cameras. The successful detection of this temperature variation represents an important observation for estimating the physical properties of Dimorphos, including its thermal characteristics and surface material properties.

Figure 3. Motion of the binary asteroid system as observed by TIRI between 19:20:27 on October 28 and 00:29:01 on October 29, 2026 (JST). (Left) Full-field TIRI images. (Right) Cropped view of the images shown on the left. A 3 × 3 binning process was applied to emphasize the decrease in brightness values as Dimorphos enters the shadow cast by Didymos. © ESA/JAXA

Comment from Tatsuaki Okada
Associate Professor, Department of Solar System Sciences, ISAS; Hera JAPAN Project Team Leader“

Following the thermal imaging observations of Ryugu and Torifune with the TIR (Thermal Infrared Imager) aboard Hayabusa2, I am delighted that asteroid observations are finally beginning with TIRI, the thermal infrared camera that we have been developing for the Hera mission since 2020. I would also like to express my sincere gratitude to everyone who contributed their support and cooperation throughout its development.
We will continue to advance the world’s first rendezvous exploration of a binary asteroid system through the combined use of TIRI and Hera’s other scientific instruments. Hera is a unique mission that contributes to both planetary defense and planetary science, and we look forward to sharing many exciting scientific achievements in the years ahead.”

Comment from Naoya Sakatani
Project Research Assistant Professor, Department of Solar System Sciences, ISAS; Hera JAPAN Project Team Member

“From this point onward, Hera will approach to within 20-30 km of Didymos and begin detailed scientific observations. Beyond investigating the aftermath of the DART spacecraft impact on Dimorphos and its effects on Didymos, the mission marks the beginning of humanity’s first true challenge in planetary defense. It is exciting to think about what kind of future this endeavor may help create. I am truly looking forward to the discoveries and achievements that lie ahead.”

Comment from Yuya Mitsumasu
Leader, Hayabusa2 Extended Mission Team, ISAS

“Congratulations on your arrival at the asteroid system. Seeing Dimorphos orbiting its primary body, Didymos, is truly inspiring and fills me with excitement.
Just imagining the various mission operations ahead is thrilling: approaching close to Dimorphos for detailed observations, accompanying it in its orbit while observing Didymos, or perhaps even venturing inside Dimorphos’s orbit to observe Didymos from entirely new perspectives. These operations sound both challenging and fascinating, and I cannot help feeling a little envious of the opportunities that lie ahead for the Hera team.
I hope you thoroughly enjoy the mission in the months to come. I am also looking forward to the many discoveries and insights that Hera will bring, not only to the field of planetary science but also to the advancement of planetary defense. The mission has enormous potential to deepen our understanding in both areas, and I eagerly await its future achievements.”

References
1. https://www.isas.jaxa.jp/topics/003853.html
2. https://www.isas.jaxa.jp/topics/003865.html
3. https://www.isas.jaxa.jp/topics/003967.html
4. https://x.com/deepbluedot/status/2118114691541315855
5. https://hera.isas.jaxa.jp/wp-hera/4802/
6. https://www.esa.int/articles/about-us/directorates/operations/space-safety/hera/hera-s-first-close-up-view-of-didymos-asteroids