At 01:45 GMT on Saturday, October 19th, 2018, the European / Japanese BepiColumbo mission lifted-off from the European Spaceport in Kourou, French Guiana at the start of a 7-year voyage to Mercury, the innermost planet of the solar system.
Named after Giuseppe “Bepi” Colombo, an Italian scientist, mathematician and engineer, who took a particular interest in Mercury, and first formulated the use of the gravity-assist as a part of an interplanetary mission (Mariner 10, 1973/75).
The mission actually comprises four elements. There are two individual satellites, the Mercury Planetary Orbiter (MPO) and Mio (Mercury Magnetospheric Orbiter, MMO), an propulsion / power unit called the Mercury Transfer Module (MTM) and a Sun shield designed to protect the more sensitive instruments on Mio.
Built by the European Space Agency, MPO weighs 1,150 kg (2,540 lb), and carries a payload of 11 instruments, comprising cameras, spectrometers (IR, UV, X-ray, γ-ray, neutron), a radiometer, a laser altimeter, a magnetometer, particle analysers, a Ka-band transponder, and an accelerometer. It also carries the smaller Mio, and will supply it with power until such time as the two separate once in orbit around Mercury.
Mio, built primarily in Japan, masses of 285 kg (628 lb) and carries five groups of science instruments with a total mass of 45 kg (99 lb). The is a spin-stabilised platform, meaning that prior to detaching from MPO, it will be set spinning at 15 rpm so it can remain stable as it operates in a polar orbit around Mercury.
The overall goal of the mission is to carry out the most comprehensive study of Mercury to date, examining its magnetic field, magnetosphere, interior structure and surface, with a primary mission period of one year. In addition, during the flight, BepiColombo will make the most precise measurements of the orbits of the Earth and Mercury around the Sun made to date as a part of further investigations of Einstein’s theory of general relativity.
As noted above, it will take BepiColombo seven years to reach Mercury. This is because of a couple of reasons. The first is, contrary to what logic might suggest, getting closer to the Sun is actually harder than moving away from it when starting from Earth. The is because a vehicle departing Earth does so with a “sideways” motion relative to the Sun of around 67,000 mph (107,000 km/h), the speed the Earth is orbit the Sun, and this has to be overcome. At the same time, speed has to be managed so that the vehicle can also approach Mercury at a slow enough velocity to allow it to brake its way into orbit.
To achieve both of these goals, the MTM on BepiColombo is equipped with the most powerful ion propulsion system yet flown in space. This is capable for maintaining a low rate of thrust over exceptionally long periods – much long that could be achieved by rocket motors and for far less fuel, given the ion system is electrically powered, using two 14 metre (46 ft) long solar panels to generate the power. The motor will be used to help slow BepiColombo in its flight, acting as a long-slow-burning brake. However, the ion motors aren’t sufficient to get the mission to Mercury; more is required.
This “more” take the form of using no fewer than nine planetary fly-bys. The first of these will happen in April 2020, when BepiColumbo, now in an extended orbit around the Sun, will encounter Earth once more. This will bend the vehicle’s flight path inwards towards the Sun which will swing it past Venus in October of that year, the first of two Venus fly-bys. The second of these will occur in August 2021, and will bend BepiColombo’s orbit further in towards Mercury, which it will reach at the start of October 2021.
But things don’t end there. While planetary fly-bys serve to bend a space vehicle’s trajectory, allowing it to “hop” from planet to planet, it also increases the vehicle’s velocity. Even with the long periods of braking possible using the ion motors, BepiColombo will be travelling too fast to achieve orbit around Mercury at that first encounter. Instead, the spacecraft will be placed in a solar orbit that periodically intercepts Mercury in is orbit, and over a series of five such encounters between June 2022 and January 2025, BepiColombo will use Mercury’s gravity in conjunction with its ion engines to slow itself down to around the threshold at which it can make orbit.
This will occur in December 2025, as the vehicle makes its seventh approach to Mercury. However, with a mass of around 4 tonnes combined, the vehicle will still have too much inertia for the ion motors to bring it into orbit. Instead, the MTM will be jettisoned, and the smaller, lighter MMO will use its own high-thrust conventional motor systems to brake itself into an initial orbit around Mercury. At the same time, Mio will be separated, so it can enter a more distant orbit around the planet.