THIS DAY IN SPACE

Galileo Spots Ganymede
On January 7, 1610, continuing his observations of Jupiter through his handcrafted telescope, Galileo Galilei recorded the presence of four 'stars' near the planet. This time, he noticed that one of them behaved in a unique way—it did not appear fixed relative to the others. That object was Ganymede, which would soon be recognized as one of Jupiter’s moons. Although he had initially spotted three moons the previous night, it was only by the seventh that Galileo began to understand the pattern of their movement, realizing they were orbiting Jupiter. This night was crucial because it solidified his suspicion that not all celestial bodies revolved around Earth. Ganymede would later be named and identified as the largest moon in the solar system—larger even than Mercury and Pluto. The observation was not just a significant moment in Galileo’s personal scientific journey but a fundamental challenge to the geocentric universe. The Catholic Church and many scholars were initially resistant to this view, but over time, Galileo’s work would be vindicated. Ganymede itself has remained a major target of planetary science due to its magnetic field and suspected subsurface ocean. The night of January 7 marks the moment when the largest moon in the solar system entered the scope of human knowledge—an achievement that forever altered our understanding of celestial mechanics and laid the groundwork for the eventual acceptance of heliocentrism.

Launch of Sakigake, Japan’s First Interplanetary Probe
On January 7, 1985, the Institute of Space and Astronautical Science (ISAS) of Japan launched *Sakigake*, meaning 'Pioneer', the country’s first interplanetary space probe. It marked Japan’s entry into deep space exploration and was part of an international effort to study Halley’s Comet during its 1986 perihelion. Sakigake’s mission was primarily scientific—it aimed to gather solar wind and plasma data from a safe distance while supporting other probes such as ESA’s Giotto and the Soviet Union’s Vega missions. Although Sakigake did not come as close to Halley’s Comet as the other probes, it provided valuable data on the solar wind conditions ahead of the comet’s arrival, enabling better calibration and timing for the other spacecraft involved. Its trajectory took it on a heliocentric orbit, making it the first spacecraft launched by a country other than the U.S. or the Soviet Union to explore interplanetary space. The successful launch of Sakigake represented a milestone for Japan’s space program, demonstrating its technical capabilities and scientific ambitions. It also set the stage for future Japanese space missions such as *Suisei*, *Nozomi*, and later, the Hayabusa asteroid missions. With Sakigake, Japan not only asserted itself as a player in planetary science but also contributed meaningful data to the global understanding of comets and the solar wind environment.