Despite being so close to us, Mercury is one of the least explored places in the Solar System. Issues like thermal management and great propellant usage plague any journey to the small body, making its exploration a great challenge. Only two NASA missions have visited the innermost planet so far: Mariner 10 in the 1970s and MESSENGER in 2011-2015. A joint Euro-Japanese mission is about to change that: BepiColombo. En route since its launch in October 2018, the spacecraft is set to reach Mercury in 2025.
The aim of BepiColombo is to get two spacecraft into Mercury’s orbit: the European Mercury Planetary Orbiter (MPO) and the Japanese Mercury Magnetospheric Orbiter (MMO). The former will study the planet itself from a low polar orbit. Its low-altitude orbit will allow data collection with unprecedented precision. The latter will remain in a highly elliptical orbit, similar to the one previously used by MESSENGER. It will study Mercury’s magnetic field, and interplanetary space as well. A transfer module (MTM) is also present. It will provide solar electric propulsion during the long flight.
The mission takes its name from Giuseppe “Bepi” Colombo, an Italian mathematician, physicist, and engineer. His work in spaceflight and astrophysics touches on the Giotto mission to the Halley comet, the study of Saturn’s rings, and the tethered satellite concept. The work that made him famous, however, is related to Mercury. He discovered that the planet is tidally locked to the Sun in a peculiar way. Instead of performing one rotation for every revolution around our star, it performs one and a half. Colombo famously contributed to the design of Mariner 10’s trajectory, allowing the spacecraft to perform multiple flybys of the planet.
One of the biggest challenges in Mercury exploration is to get there in the first place. Being so close to the Sun, an arriving spacecraft will have a high relative velocity. This means that a direct transfer trajectory, like the ones used for Mars missions, would require excessive amounts of propellant to enter orbit. As such, BepiColombo, as MESSENGER did before, will use a wealth of gravity assists to slow down as much as possible. One Earth flyby took place in 2020, followed by two Venus flybys in 2020 and 2021. Two Mercury flybys have already taken place in 2021 and 2022. Four more are planned: the first one is coming up on June 20, 2023.
After the flybys are completed, the spacecraft will be captured into a highly elliptical Mercury orbit on December 5, 2025. The MTM will then be jettisoned, and the two spacecraft will proceed to their desired orbits by using four 22 N chemical thrusters on the MPO. This option is significantly quicker than using the ion thrusters. The MMO will be released in its elliptical orbit, and its protective thermal cover will be discarded as well. The spacecrafts are expected to work at least until May 2027 as part of the nominal mission, and until May 2028 as part of the planned extended mission.
What we know (and don’t know)
Our knowledge of Mercury is limited to the data collected by the two probes that have visited it in the past. The planet has an extremely cratered surface, but some areas appear to have been shaped by volcanism. Despite the small size, a substantial magnetic field is present. Traces of an atmosphere can be detected (a trillionth of the surface pressure on Earth). The planet is suspected to have a very thin rocky crust and a metal core. Most amazingly, water ice has been detected in permanently shadowed polar craters.
This data leaves us with many questions. What is the precise internal structure? Is the planet geologically active today? Are there volcanoes? What generates its magnetic field? Why is there an atmosphere, albeit extremely thin? How did the crater ice form? BepiColombo will try to shed light on some of these mysteries.
Mercury Planetary Orbiter
The MPO is shaped like a cube resting on a wider base. This base is actually the radiator, and it will always face away from the Sun so it can dissipate heat. It has to deal with not just the heat emitted by the Sun, but also with that reflected by Mercury. Heat management is also the reason the solar panel is so big. To prevent overheating it has to be angled steeply, and mirrors have to be mixed in with the solar cells. The spacecraft is roughly 2.4 m tall and 3.7 m wide, with a mass of 1200 kg.
Of course, these systems are all there to serve the scientific payload of the spacecraft. There are a total of 11 different instruments that will study the planet with never-seen-before precision. Six of these (MERTIS, MGNS, MIXS, PHEBUS, SIMBIO-SYS, and SIXS) are spectrometers, which will investigate the composition of the surface of the planet, its extremely tenuous atmosphere, and solar particles. Some can also act as cameras, investigating surface features, geology, volcanism, and more. They will also aid in navigation by using land markers as reference points.
MPO-MAG and SERENA, in cooperation with MMO, will study the environment around the celestial body. They will monitor and measure the magnetic field, the solar wind, interplanetary dust, and energetic particles, and the way they all interact. To study the planet’s internal structure, accurately mapping its gravitational field is necessary. Two instruments play a key role in this: MORE and ISA. The former will allow the measurement of the range of the spacecraft, with a precision of up to 15 cm, and of its velocity. This is achieved by having it retransmit a signal received from the ground and measuring its roundtrip time and Doppler shift.
The latter instrument is an accelerometer that will quantify the non-gravitational forces acting on the spacecraft. These are mainly radiation pressures coming both directly from the Sun and from sunlight reflecting on Mercury’s surface. By taking these forces out of the equation, the planet’s pull can be measured with little error. This data will also serve to further test Einstein’s General Relativity. Lastly, the spacecraft is equipped with a laser altimeter, BELA, that will provide an accurate 3D map of the planet.
Mercury Magnetospheric Orbiter
The MMO is a Japanese spin-stabilized spacecraft. Its main body is shaped like an octagonal prism, roughly 1.8 m wide and 40 cm tall. It has a mass of 255 kg, out of which 45 are experiments. Inside it are two decks, upper and lower, where instruments are placed. Additionally, many antennas and masts stick out for up to 15 m, both for communication and scientific purposes. Power generation is provided by solar cells mounted on all eight sides of the spacecraft. There are also cold gas thrusters for attitude control.
MMO carries five instruments. MMO/MGF and PWI will investigate Mercury’s magnetic and electric fields and the complex interactions they have with the solar wind. In particular, by being so close to the sun, the Interplanetary Magnetic Field is much stronger. This makes all processes happen faster, and as such high sampling rates are required for these instruments.
Two other instruments aimed at investigating phenomena of interplanetary origin are MPPE and MDM. They will detect and measure plasma, high-energy particles, neutral atoms, and dust. Their results will of course complement those of other instruments. Lastly, MSASI is a spectrometer meant to investigate the dynamics of sodium in the planet’s thin atmosphere.
A mission to explore Mercury was proposed all the way back in 1993. When the Horizon 2000 program was extended in 1994, it was selected as a cornerstone mission. BepiColombo competed with the Gaia star mapper for this role, but in the end, both were selected. In 2008, the mission suffered mass increases, requiring it to be re-approved in 2009. Launch finally took place on 20 October 2018 on an Ariane 5, taking off from Kourou in French Guyana. We now need to wait just a little longer for the science to begin.