Arriving at the most difficult planet in the solar system

The European Space Agency’s BepiColombo mission has entered its final phase ahead of its planned orbital insertion around Mercury, the closest planet to the Sun. On Thursday, the spacecraft separated from its propulsion module, marking a critical step toward achieving its goal of entering orbit around the smallest and hottest planet in the solar system. The separation occurred shortly after 14:00 Central European Summer Time, following nearly eight years of travel and over nine billion kilometers through the inner solar system. The mission, a joint effort between Europe and Japan, aims to study Mercury's magnetic field, surface composition, and internal structure in unprecedented detail. The BepiColombo probe consists of two separate orbiters: the Mercury Planetary Orbiter (MPO) developed by the European Space Agency and the Mercury Magnetospheric Orbiter (Mio) built by Japan's space agency, JAXA. These two components were launched together aboard a single rocket but will operate independently once they reach their respective orbits around Mercury. The propulsion module, known as the Mercury Transfer Module (MTM), which had been responsible for transporting the craft across the solar system using ion thrusters, was detached on Thursday. It had been shut down earlier this summer and will remain in space as debris. Mercury presents unique challenges due to its proximity to the Sun, making it difficult to achieve a stable orbit. To slow down enough to enter orbit, the spacecraft performed multiple gravity-assist maneuvers, flying past Earth, Venus, and Mercury itself several times since its launch in 2018. This complex trajectory required careful planning and execution to ensure the probe could decelerate sufficiently against the Sun’s gravitational pull. According to ESA mission manager Santa Martinez, reaching Mercury is “very difficult,” and only two previous missions have studied the planet in depth. BepiColombo is the first European-led mission to attempt such a detailed exploration. The upcoming months will see a series of intricate maneuvers to position both orbiters into their designated paths. Starting in late November, chemical thrusters will fire for a total of 16 braking maneuvers to adjust the spacecraft’s velocity. In December, Mio will separate from MPO, with the European orbiter scheduled to reach its final orbit around Mercury in March 2027. ESA engineer Ignacio Tanco described the mission as high-risk, noting that many aspects might not proceed exactly as planned. However, he emphasized that the team is well-prepared for potential complications. Thermal management poses one of the greatest technical challenges for the mission. The extreme temperature variations on Mercury, ranging from 450 degrees Celsius during the day to -180 degrees Celsius at night, require advanced thermal insulation systems. Components such as the attitude control system for electric thrusters and the electronics managing the solar panels were developed by Beyond Gravity Austria based in Vienna. Proper alignment of the solar panels is crucial to ensure the scientific instruments receive sufficient power while avoiding overheating caused by direct sunlight exposure. Scientific instruments aboard both orbiters include magnetometers designed to measure Mercury’s magnetic field. The Graz-based Institute for Space Research (IWF) of the Austrian Academy of Sciences played a leading role in developing these sensors. The data collected by BepiColombo will provide new insights into Mercury’s geological history, magnetic properties, and interactions with the solar wind. Scientists hope to uncover how the planet formed and evolved under intense solar conditions, offering clues about planetary formation processes throughout the solar system. As the spacecraft continues its approach to Mercury, engineers and scientists are monitoring all systems closely. The success of the upcoming maneuvers will determine whether the mission can achieve its primary objective of establishing a stable orbit. With the separation of the propulsion module complete, the focus shifts to ensuring the two orbiters perform their respective functions effectively once they arrive at their target destination.