BepiColombo is due to enter orbit around Mercury on November 21: what happens to its two orbiters

BepiColombo, the joint Mercury mission of Japan's space agency JAXA and the European Space Agency (ESA), is due to arrive at Mercury on November 21, 2026, JAXA's website for its Mio orbiter shows. The date appears in a timeline graphic on the site's mission page, the same graphic that gives the launch as October 5, 2018, while the page's own text and JAXA's launch release, which gives Japan time, put it on October 20, 2018. The text of JAXA's mission pages says only that the spacecraft goes into orbit around Mercury in November 2026. A May 25 post from Mio's official account, @JAXA_MMO, which JAXA shows on the Mio site's home page, gives the same day and the next step. Written in Mio's voice, it says the spacecraft will be "gently caught by Mercury's gravity" and enter orbit on November 21, and that Mio will separate from the European orbiter, MPO, and continue alone on December 10 (our translation from Japanese). The home page also counts down the days to Mio's separation, to December 10 in Japan time. We found no JAXA text page that gives either day, neither source gives a time of day, and both dates are targets until they happen.

Eight years and nine flybys

The spacecraft left Earth on an Ariane 5 rocket from Kourou, French Guiana, at 10:45:28 p.m. local time on October 19, 2018, which was 10:45 a.m. on October 20 in Japan, JAXA said at the time; the rocket released its payload about 26 minutes and 47 seconds into the flight. It carries two science orbiters stacked together: JAXA's Mio, formerly called the Mercury Magnetospheric Orbiter (MMO), and ESA's Mercury Planetary Orbiter (MPO). ESA is responsible for the launcher, the engine for the cruise between the planets, placing BepiColombo into orbit around Mercury and building MPO, according to JAXA.

Reaching the innermost planet takes an enormous amount of energy, so BepiColombo saves propellant by using the gravity of planets it passes. JAXA's timeline lists nine swing-bys: one of Earth, two of Venus and six of Mercury, the last in January 2025. The long trip is not about distance, the Mio site explains: the spacecraft sometimes had to circle the Sun many times to line up for the next swing-by, and the eight years are "down to the waiting times for the swing-bys." Only two spacecraft, Mariner 10 and MESSENGER, have visited Mercury before, JAXA notes.

What happens on arrival

JAXA's mission page outlines the sequence, without dates. As the spacecraft enters orbit around Mercury, Mio comes first into a stretched, elliptical orbit with a high apoapsis, its farthest point from the planet. MPO then separates from Mio's sunshield, known as MOSIF, and gradually lowers itself into a low-altitude orbit. From then on the two orbit in the same plane at different heights, each with its own observations. JAXA calls putting two spacecraft into orbit around Mercury in this way a world first. The two orbiters are to observe Mercury together for about a year, according to JAXA's project page.

Two orbiters, two jobs

Mio was designed for a harsh place. In orbit it will receive sunlight around ten times as strong as at Earth and heat radiated from a surface whose daytime temperature reaches about 430 degrees Celsius, JAXA says. That is about 800 degrees Fahrenheit, and at night the surface falls to minus 290 degrees Fahrenheit (minus 180 degrees Celsius). Mio's sun-facing sides are covered with mirrors to reflect as much heat as possible. Once at Mercury it is to extend four 15-meter wire antennas and two 5-meter masts.

What the mission wants to learn

Much of the science is about magnetism. Mercury's magnetic field is about 100 times weaker than Earth's while the planet faces a far stronger solar wind, the Mio site says; at the surface, the field is just 1% as strong as Earth's. Measuring from two spacecraft at once should help scientists tell Mercury's own field apart from that carried by the solar wind, and so learn more about the interior. One puzzle is why the smallest planet, which should have cooled fastest, still appears to have a liquid core; its large metallic core reaches about 85% of the planet's radius and appears to be partly molten. MESSENGER also found that the center of the magnetic field sits north of Mercury's equator, JAXA notes.

MPO's surface work follows up earlier findings: far more volatile elements in the surface minerals than expected for the planet closest to the Sun, and strange depressions, found by MESSENGER, that may have formed as volatile elements escaped. Those results may indicate that Mercury was once larger than it is now, the Mio site says. JAXA's team also argues that studying the planet exposed to the strongest solar wind in our solar system will help researchers understand planets that orbit close to red dwarf stars, such as those of TRAPPIST-1, and whether they could be habitable.

The last visitor, and the name

NASA's MESSENGER was the first spacecraft to orbit Mercury. Launched on August 3, 2004, it entered orbit in March 2011, nearly seven years later, and orbited for more than four years, determining the surface composition and verifying that deposits at the poles are mostly water ice. With its propellant gone, it struck the surface on April 30, 2015, at about 8,750 miles per hour (14,080 kilometers per hour).

BepiColombo is named after the Italian astronomer Giuseppe "Bepi" Colombo, who proposed that Mercury's rotation and orbit are locked in a 3:2 resonance, pioneered the use of swing-bys and designed the trajectory of Mariner 10, the first probe to explore Mercury, the Mio site says. If the plan holds, the next milestones are the arrival on November 21 and Mio's separation on December 10.