Sonali Tiwary
On September 3, after eight years in space, a spacecraft called BepiColombo shed its final external component and began the last phase of its journey to Mercury. It has traveled 9.9 billion kilometers—nearly the distance to the outer edge of Neptune's orbit and back.
This isn't a simple trip. Mercury orbits closest to the Sun, where reaching and matching its orbit requires substantial orbital energy loss. Instead, BepiColombo took a path that seems counterintuitive: it flew past Earth, Venus, and Mercury multiple times.
The spacecraft launched on October 20, 2018, from Earth. Over the next eight years, it completed nine planetary flybys—one past Earth, two past Venus, and six past Mercury itself. Each close encounter with a planet altered the spacecraft's trajectory and velocity, reducing its orbital energy. Gravity did work that fuel couldn't afford to do. By the time it reached Mercury's vicinity, BepiColombo had lost enough energy that the final orbit insertion became possible.
On September 3, the Mercury Transfer Module—the spacecraft component that provided propulsion and thermal protection—separated from the science orbiters. Two spacecraft remained: the Mercury Planetary Orbiter operated by the European Space Agency, and the Mercury Magnetospheric Orbiter operated by Japan. These are the actual science instruments.
The timeline ahead is compressed. Mercury orbit insertion occurs on November 21, 2026. Then, in December, the two spacecraft will separate into different orbits. Beginning in April 2027, routine science operations start.
Why does Mercury matter enough to send a spacecraft on a 9.9 billion kilometer journey? Because it remains profoundly mysterious. Only two spacecraft have ever orbited Mercury—Mariner 10 in 1974-75 and MESSENGER from 2011-2015. Basic questions remain unanswered.
Mercury has a surprisingly powerful magnetic field, despite being small and rotating slowly. Why? Its composition remains poorly understood. Its geological history is incomplete. The surface shows evidence of complex processes not yet explained. Its magnetosphere—the region where its magnetic field meets the solar wind—is still poorly understood.
The two BepiColombo spacecraft address different mysteries. The European orbiter will map Mercury's surface at high resolution and study its composition. The Japanese orbiter will study the magnetosphere specifically. Together, they'll gather eight years of observations from a planet almost no one has studied closely. The 96P/Machholz comet, which travels inside Mercury's orbit, demonstrates the dynamic complexity of the inner solar system.
The arrival phase is complex. The spacecraft must navigate an environment where solar radiation creates extreme conditions. Orbital maneuvers must be precise. Communication delays exceed 12 minutes from Earth—controllers cannot send corrections in real-time. The spacecraft must execute sequences with autonomy.
The Mercury Transfer Module, having completed its job, will continue orbiting the Sun indefinitely without further communication capability. The science orbiters will begin their actual work, revealing secrets Mercury has kept from us since the solar system formed.
