BepiColombo Separates Over Mercury Today

The European Space Agency (ESA) separates the transfer module of the BepiColombo mission from the two science orbiters it has carried across the solar system today at 2 p.m. Central European Summer Time, or 12 p.m. UTC. It is the first in a series of maneuvers meant to place BepiColombo into orbit around Mercury, a planet only two spacecraft have ever visited. Live coverage starts at 1:45 p.m. on ESA’s YouTube channel, at youtube.com/live/oC7lQngNG3o, and confirmation that the separation succeeded is expected to reach Earth at 3:53 p.m.

BepiColombo is a joint mission between the European Space Agency and the Japan Aerospace Exploration Agency (JAXA), launched on October 20, 2018. Since then, the spacecraft has traveled roughly 10 billion kilometers, or about 6 billion miles, with its three parts, the transfer module and the two orbiters, flying joined together as a single stack for eight years. To reach the speed needed to enter orbit around Mercury, the spacecraft made nine flybys of planets, Earth, Venus, and Mercury itself, using their gravity to slow down.

Why is entering orbit around Mercury harder than reaching farther into the solar system?

A spacecraft falling toward the Sun does not slow down, it speeds up, which makes entering orbit around Mercury more energy-demanding than reaching planets that are farther away and far larger. That is also why BepiColombo’s journey to Mercury has taken longer than some earlier missions needed to reach Jupiter.

“You can get to Mercury faster if you go straight for it. But the problem is, we want to actually get into orbit around Mercury,” Frank Budnik, ESA’s flight dynamics manager, told CNN.

ESA’s own team describes the separation maneuver itself as one of the most delicate moments of the entire mission, since it is the first time the transfer module splits from the rest of the stack while all three components are deep in space, near one of the most demanding stretches of the flight path.

“The separation of the transfer module will take place under extremely difficult conditions and we will have to face significant risk,” said Ignacio Tanco, ESA’s spacecraft operations manager for the inner solar system missions.

A months-long sequence of maneuvers follows today’s separation. According to ESA, the European Mercury Planetary Orbiter (MPO) and Japan’s Mio are set to enter orbit around Mercury together in November 2026, then separate from each other in December of the same year. Science observations begin in April 2027. The MPO will study the planet’s surface and interior, while Mio, built by the Japanese space agency, will measure the surrounding space environment and magnetic field. To shield itself from the Sun’s heat, Mio spins on its axis 15 times a minute.

Mercury sits an average of 58 million kilometers from the Sun, completes an orbit every 88 days, and reaches surface temperatures of around 430 degrees Celsius during the day. Only two spacecraft have ever come close to it: the American Mariner 10 in 1974, and MESSENGER, which entered orbit around Mercury in 2011. It was MESSENGER’s data that raised the questions BepiColombo is meant to answer, among them the puzzle of the so-called hollows scattered across the planet’s surface.

“There are hollows on the surface that look as if the surface is being eaten away. We may be able to see changes in those areas over time,” said Geraint Jones, BepiColombo’s project scientist at ESA.

No mission has yet explained what actually creates those hollows, since Mercury has neither wind nor water to shape them. What is scheduled for today is only the first step: the separation of the transfer module on its own does not guarantee that the more demanding maneuvers still ahead in November and December will go as planned, and ESA says openly that it will only know the spacecraft is in good shape once it receives that signal at 3:53 p.m.