BepiColombo

Objectives

The main objectives of the mission are:^[3][12]

• Study the origin and evolution of a planet close to its parent star
• Study Mercury as a planet—its form, interior, structure, geology, composition and craters
• Investigate Mercury's exosphere, composition and dynamics, including generation and escape
• Study Mercury's magnetised envelope (magnetosphere) - structure and dynamics
• Investigate the origin of Mercury's magnetic field
• Verify Einstein's theory of general relativity by measuring the parameters gamma and beta of the parameterized post-Newtonian formalism with high accuracy.^[13][14]

Mission design

The spacecraft will take seven years to position itself to enter Mercury orbit. During this time it will use solar-electric propulsion and nine gravity assists, flying past the Earth and Moon in April 2020, Venus in 2020 and 2021, and six Mercury flybys between 2021 and 2025.^[1]

The spacecraft will leave Earth with an hyperbolic excess velocity of 3.475 km/s (2.159 mi/s). Initially the craft is in an orbit similar to that of the Earth. After both the spacecraft and the Earth have completed one and a half orbits, it returns to Earth to perform a gravity-assist manoeuvre and is deflected towards Venus. Two consecutive Venus flybys reduce the perihelion nearly to Mercury distance with almost no need for thrust. A sequence of six Mercury flybys will lower the relative velocity to 1.76 km/s (1.09 mi/s). After the fourth Mercury flyby the craft will be in an orbit similar to that of Mercury and will remain in the general vicinity of Mercury (see video). Four final thrust arcs reduce the relative velocity to the point where Mercury will "weakly" capture the spacecraft on 5 December 2025 into polar orbit. Only a small manoeuvre is needed to bring the craft into an orbit around Mercury with an apocentre of 178,000 km. The spacecraft then will be lowered using chemical thrusters.^[15][16]

Mission schedule

Animation of BepiColombo's trajectory from 20 October 2018 to 2 November 2025
   BepiColombo ·   Earth ·   Venus ·   Mercury ·   Sun

As of 2018, the planned mission schedule is:^[1]

Mercury Transfer Module

Mercury Transfer Module in The Large Space Simulator at ESTEC.

The Mercury Transfer Module (MTM) is located at the base of the stack. Its role is to carry the two science orbiters to Mercury and to support them during the cruise.

The MTM is equipped with a solar electric propulsion system as the main spacecraft propulsion. Its four QinetiQ T6 ion thrusters operate singly or in pair for a maximum combined throttle of 290 mN.^[17] At the time of launch, the T6 will be the most powerful ion engine ever operated in space. The MTM supplies electrical power for the two hibernating orbiters as well as for its solar electric propulsion system thanks to two 14 meters long solar wings.^[18] Depending on the probe's distance to the Sun, the generated power will range between 7 and 14 kW, each T6 requiring between 2.5 and 4.5 kW according to the desired thrust level.

The solar electric propulsion system has typically very high specific impulse and low thrust. This leads to a flight profile with months long continuous low-thrust braking phases, interrupted by planetary gravity assists, to gradually reduce the velocity of the spacecraft. Moments before Mercury orbit insertion, the MTM will be jettisoned from the spacecraft stack.^[18] After separation from the MTM, the MPO will provide Mio all necessary power and data resources until Mio is delivered to its mission orbit; separation of Mio from MPO will be accomplished by spin-ejection.

Mercury Planetary Orbiter

Mercury Planetary Orbiter in ESTEC before stacking

Radio testing of BepiColombo orbiter

The Mercury Planetary Orbiter (MPO) will have a mass of 1,150 kg (2,540 lb) and will use a single-sided solar array capable of providing up to 1000 watts and featuring Optical Solar Reflectors to keep its temperature below 200 °C (392 °F). The solar array requires continuous rotation keeping the Sun at a low incidence angle in order to generate adequate power while at the same time limiting the temperature.^[18]

The MPO will carry a payload of 11 instruments, comprising cameras, spectrometers (IR, UV, X-ray, γ-ray, neutron), radiometer, laser altimeter, magnetometer, particle analysers, K_a-band transponder, and accelerometer. The payload components are mounted on the nadir side of the spacecraft to achieve low detector temperatures, apart from the MERTIS and PHEBUS spectrometers located directly at the main radiator to provide a better field of view.^[18]

A high-temperature-resistant 1.0 m (3.3 ft) diameter high-gain antenna is mounted on a short boom on the zenith side of the spacecraft. Communications will be on the X and K_a band with an average bit rate of 50 kbit/s and a total data volume of 1550 Gb/year. ESA's Cebreros 35-metre ground station is planned to be the primary ground facility for communications during all mission phases.^[18]

Mio (Mercury Magnetospheric Orbiter)

Mio in ESTEC before stacking

Mio, or the Mercury Magnetospheric Orbiter (MMO), developed and built mostly by Japan, has the shape of a short octagonal prism, 180 cm (71 in) long from face to face and 90 cm (35 in) high.^[3][22] It has a total mass of 285 kg (628 lb), including a 45 kg (99 lb) scientific payload.^[3][23]

Mio is spin stabilized at 15 rpm with the spin axis perpendicular to the equator of Mercury and it will enter polar orbit at an altitude of 590 × 11,640 km (370 × 7,230 mi), outside of MPO's orbit.^[23] The top and bottom of the octagon act as radiators with louvers for active temperature control. The sides are covered with solar cells which provide 90 W. Communications with Earth will be through a 0.8 m (2.6 ft) diameter X band phased array high-gain antenna and two medium-gain antennas operating in the X band. Telemetry will return 160 Gb/year, about 5 kbit/s over the lifetime of the spacecraft, which is expected to be greater than one year. The reaction and control system is based on cold gas thrusters. After its release in Mercury orbit, Mio will be operated by Sagamihara Space Operation Center using Usuda Deep Space Center's 64 m (210 ft) antenna located in Nagano, Japan.^[19]

The Mercury Magnetospheric Orbiter was given the nickname Mio on 8 June 2018. In Japanese, Mio means a waterway for ships, and symbolizes the research and development milestones reached thus far, as well as wishes for a safe travel ahead. The spacecraft will explore the solar wind as it flows through and is interfered by Mercury's magnetosphere, just like a ship voyaging through water currents. Mio was chosen from among 6,494 submissions from the public.^[5]

Mercury Surface Element

The Mercury Surface Element (MSE) was cancelled in 2003 due to budgetary constraints.^[8] At the time of cancellation, MSE was meant to be a small, 44 kg (97 lb), lander designed to operate for about one week on the surface of Mercury.^[15] Shaped as a 0.9 m (3.0 ft) diameter disc, it was designed to land at a latitude of 85° near the terminator region. Braking manoeuvres would bring the lander to zero velocity at an altitude of 120 m (390 ft) at which point the propulsion unit would be ejected, the airbags inflated, and the module would fall to the surface with a maximum impact velocity of 30 m/s (98 ft/s). Scientific data would be stored onboard and relayed via a cross-dipole UHF antenna to either the MPO or Mio. The MSE would have carried a 7 kg (15 lb) payload consisting of an imaging system (a descent camera and a surface camera), a heat flow and physical properties package, an alpha particle X-ray spectrometer, a magnetometer, a seismometer, a soil penetrating device (mole), and a micro-rover.^[24]