In a remarkable stroke of serendipity, two Japanese weather satellites, Himawari-8 and Himawari-9, designed primarily to monitor Earth's atmospheric patterns, have been unknowingly collecting valuable data on Venus for nearly a decade. This unexpected treasure trove of information has allowed scientists to observe long-term changes in Venus's cloud-top temperatures and atmospheric dynamics, providing crucial insights into our mysterious planetary neighbor.
The discovery was made by a team led by Gaku Nishiyama, a visiting researcher at the University of Tokyo and the German Aerospace Center. Nishiyama's initial focus was on lunar science using the Himawari satellites, which launched in 2014 and 2016 respectively. However, during this work, he and his colleagues stumbled upon images of other solar system bodies, including Mercury, Mars, Jupiter, and most significantly, Venus, within the satellite datasets.
While the Himawari satellites orbit Earth, their imaging range extends into space, occasionally capturing glimpses of other celestial objects as they drift through the field of view of their Advanced Himawari Imagers (AHIs). The team meticulously analyzed these "accidental" Venusian observations, using the precise imaging schedule and position of the satellites to isolate the faint pixels corresponding to Venus.
Despite Venus appearing as a tiny dot in the vast satellite imagery, the researchers were able to extract meaningful scientific data. Between 2015 and 2025, they identified 437 instances where Venus was visible. The Himawari satellites' ability to detect 16 different "colors" across visible and infrared light, combined with their frequent calibration and low noise levels, proved invaluable. This allowed them to track subtle changes in the intensity of light Venus was emitting across multiple infrared wavelengths.
This unique dataset has provided one of the longest multiband infrared records of Venus ever assembled. The observations revealed subtle, year-to-year changes in the planet's cloud-top temperatures, as well as signs of phenomena called thermal tides and Rossby waves. These waves are crucial for transporting heat and momentum through Venus's thick, carbon dioxide-rich atmosphere, which is characterized by clouds of sulfuric acid. Understanding how these waves change over time helps scientists better grasp the planet's atmospheric dynamics, especially given observed variations in wind speeds and cloud reflectivity over several years.
The timing of this discovery is particularly significant. With Japan's dedicated Venus probe, Akatsuki, experiencing communication problems and no new dedicated Venus orbiters planned until around 2030, the Himawari satellites offer a unique and continuous bridge for atmospheric monitoring of Venus. The new observations help fill a crucial gap in our understanding of Venus's dynamic upper atmosphere.
Furthermore, the research has a significant technical implication. By comparing images taken by Himawari with data from Akatsuki and the European BepiColombo mission (during its Venus flyby), the team found discrepancies, suggesting that Akatsuki's long-wave infrared camera (LIR) may have been underestimating Venus's radiance by 15-17% due to calibration issues. This finding is crucial for re-evaluating past Akatsuki data and ensuring accuracy in future analyses.
This serendipitous scientific endeavor showcases how existing infrastructure, even if not specifically designed for planetary science, can serve multiple purposes. The success with Venus opens a new frontier in planetary monitoring from Earth orbit, with the potential for other weather satellites and data archives to reveal further hidden insights into other solar system bodies, including the Moon and Mercury, which Nishiyama's team is also now analyzing. The findings were reported in the journal Earth, Planets and Space.