Level 1 — Absolute Beginner

Mercury is the planet closest to the Sun. It is small and very hot. Not many spacecraft have visited it.

A spacecraft called BepiColombo is flying to Mercury now. Europe and Japan built it together. It started its journey in 2018, eight years ago.

BepiColombo has three parts. One part is a big engine. On September 3, 2026 the spacecraft let the engine go. It does not need it any more.

Two smaller parts are left. They are called orbiters. In November they will go around Mercury. Then in December they will separate and do different work.

planet

a large round object that goes around a star

Sun

the star that gives light and heat to Earth

spacecraft

a machine that travels in space

engine

the part of a machine that makes it move

journey

a trip from one place to another

orbiter

a spacecraft that goes around a planet

separate

to move apart from each other

hot

having a high temperature

Level 2 — Elementary

BepiColombo is a joint mission of the European Space Agency and the Japan Aerospace Exploration Agency. It launched in 2018 and has spent eight years travelling through the inner solar system, using the gravity of Earth, Venus and Mercury itself to slow down. Reaching Mercury is difficult because the Sun's gravity pulls a spacecraft faster and faster as it falls inward.

Until now the spacecraft has flown as a stack of three parts. The Mercury Transfer Module carried large solar panels and electric thrusters that pushed gently but constantly. On September 3, 2026 that module separated and was left behind, the first major step of the arrival sequence.

Two science orbiters remain. The European Mercury Planetary Orbiter will study the surface and the interior of the planet. The Japanese orbiter, called Mio, will study the magnetic field and the particles around Mercury. They will enter orbit together in November 2026 and then separate in December.

The mission is running about a year behind the original plan. Problems with the electric thrusters meant the spacecraft could not push as hard as engineers expected, so the flight path had to be redesigned. The European orbiter should reach its final working orbit on March 10, 2027.

mission

a planned space journey with a scientific goal

agency

an official organisation, often run by a government

gravity

the force that pulls objects toward each other

solar panel

a flat panel that turns sunlight into electricity

thruster

a small engine that pushes a spacecraft in space

module

a separate section of a spacecraft

magnetic field

the invisible area around a planet where magnetism can be felt

engineer

a person who designs and builds machines

Level 3 — Intermediate

On September 3, 2026, after eight years in transit, the ESA and JAXA BepiColombo mission jettisoned its Mercury Transfer Module, marking the opening move in what the European Space Agency describes as one of the most operationally demanding planetary arrivals it has ever attempted. The module's solar electric propulsion has been thrusting almost continuously since launch, not to accelerate the spacecraft but to bleed off the speed that the Sun's gravity relentlessly adds.

That counterintuitive difficulty explains the mission's design. Falling toward the inner solar system is easy; arriving slowly enough to be captured by a small planet with negligible gravity of its own is not. BepiColombo executed nine gravity assists, including six passes of Mercury itself, each one shaving away orbital energy that no chemical engine could realistically have removed.

With the transfer module gone, two orbiters remain mated. The Mercury Planetary Orbiter, built in Europe, will map the surface, composition and interior structure. Mio, built in Japan, will characterise the magnetosphere and the charged particles that Mercury's surprisingly strong magnetic field traps. The pair will enter Mercury orbit together in November 2026 and separate the following month.

Arrival comes roughly eleven months later than the original schedule. In 2024 engineers discovered the electric thrusters were delivering less power than designed, a fault traced to the power supply, and the trajectory had to be reworked around reduced performance. The planetary orbiter is now expected to settle into its operational ellipse, 480 by 1,500 kilometres above the surface, on March 10, 2027.

jettison

to deliberately throw off or discard something from a vehicle

propulsion

the means by which a vehicle is pushed forward

counterintuitive

opposite to what common sense would suggest

gravity assist

using a planet's gravity to change a spacecraft's speed or direction

magnetosphere

the region of space controlled by a planet's magnetic field

trajectory

the path an object follows through space

ellipse

an oval shape; the usual shape of an orbit

operational

working normally and doing the job it was built for

Level 4 — Advanced

The most demanding thing about reaching Mercury is not getting there but stopping. A spacecraft released from Earth falls inward through a steepening gravitational gradient, gaining velocity the whole way, and must then contrive to be moving slowly enough, relative to a body of negligible mass, to be captured rather than flung past. BepiColombo's answer to that problem was an eight year exercise in patient subtraction, and on September 3, 2026 the instrument of that subtraction was discarded: the Mercury Transfer Module, with its outsized solar wings and its solar electric thrusters, separated from the science stack and was left to its own fate.

The arithmetic behind the mission is worth appreciating. Nine gravity assists, one at Earth, two at Venus and six at Mercury itself, were used to shed orbital energy that no chemical propulsion system could plausibly have removed within a sane mass budget. Ion thrusting supplied the remainder, applied over years at thrust levels comparable to the weight of a sheet of paper. The design is an argument that in deep space, time is a resource that can be traded against propellant, and BepiColombo spent almost a decade of it.

The mission did not proceed as planned. In 2024 controllers found the electric propulsion system delivering meaningfully less thrust than specification, a shortfall traced to the power supply, and the entire arrival had to be recomputed around degraded performance. The resulting slip is about eleven months, an unglamorous outcome that nonetheless represents a considerable feat of operational improvisation: a trajectory conceived for one propulsion budget was rebuilt around another, in flight, without losing the destination.

What remains is a mated pair of orbiters with complementary appetites. Europe's Mercury Planetary Orbiter will address the surface, composition and internal structure of a planet whose disproportionately large iron core has never been satisfactorily explained. Japan's Mio will interrogate the magnetosphere, a structure that has puzzled planetary physicists since Mariner 10 found a global magnetic field where theory said a small, slowly rotating body should have none. Orbit insertion is set for November 2026, separation for December, and the planetary orbiter's final 480 by 1,500 kilometre ellipse for March 10, 2027.

gradient

a steady increase or decrease in a quantity across space

contrive

to manage to achieve something by careful planning

negligible

so small as to be not worth considering

propellant

the fuel or material a rocket expels to produce thrust

shortfall

an amount by which something falls short of what was needed

improvisation

creating a solution from what is available, without prior preparation

complementary

combining in a way that completes or enhances the other