BepiColombo is a European Space Agency (ESA) Cornerstone mission to Mercury. The mission is still in the planning stages so changes to the current description are likely over the next few years. Due to budgetary constraints the lander portion (The Mercury Surface Element, or MSE) of the mission has been cancelled. The mission as currently envisioned involves two components: the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO). The two components could be launched together (on an Ariane 5) but the more likely baseline scenario at present is two launches on Soyuz-Fregat boosters in 2011 to 2012. The spacecraft will have 3.5 year interplanetary cruises to Mercury using solar-electric propulsion and Moon, Venus, and Mercury gravity assists. The MPO and MMO will be captured into polar orbits, 400 x 1500 km for MPO, 400 x 12000 km for MMO.
|Bepi Colombo MCS tests – Courtesy ESA|
The scientific objectives for the mission are to study Mercury’s form, interior structure, geology, composition, and craters, origin, structure, and dynamics of its magnetic field, composition and dynamics of the vestigial atmosphere, test Einstein’s theory of general relativity, search for asteroids sunward of Earth, and to generally study the origin and evolution of a planet close to a parent star. Collaboration on the mission with the Japanese Space Agency ISAS is being discussed. ISAS might contribute the Mercury Magnetospheric Orbiter and exchange of hardware might also be possible.
Spacecraft and Subsystems
The MPO and MMO spacecraft will each be driven by a Solar-Electric Propulsion Module (SEPM) and a chemical propulsion module (CPM). The SEPM is optimal for slow cruise manuevers and will consist of a set of xenon ion thrusters housed in a rectangular prism powered by two GaAs-cell solar panel wings covering 33 square meters. The panels will be progressively tilted away from the Sun as the spacecraft gets closer from 0.6 AU to 0.32 AU in order to provide approximately constant power, about twice the 5.5 kW available at 1 AU. Three thrusters will be available, one or two to be used at any given time. The nominal thrust is planned to be 0.17 or 0.34 N. The total dry mass of the unit is 365 kg with 230 kg of xenon fuel for the MPO and 238 kg for the MMO. The CPM is a bi-propellant, N2O4-MMH, system for attitude control and orbital insertion. Attitude control will be achieved by eight 20_N thrusters, other maneuvers will be achieved with a single 4000 N engine. The dry mass of the CPM is 71 kg, with 156 kg of fuel for MPO and 334 kg for MMO. The total launch mass of the entire MPO system will be 1229 kg and for MMO about 1200 kg.
Mercury Planetary Orbiter (MPO)
The Mercury Planetary Orbiter will be a 357 kg spacecraft in the shape of a flat prism with three short sides slanted at 20 degrees covered with solar cells providing 420 W at perihelion. A radiator with an area of 1.5 square meters is mounted on one side to provide thermal control. The radiator is always pointed away from the Sun and is protected from planetary IR with a 3.4 square meter shield. High efficiency insulation is also used. A 1.5 m diameter high gain antenna is mounted on a short boom on the zenith side of the spacecraft. The MPO will be 3-axis stabilized and nadir pointing with a planned lifetime of over 1 year in Mercury orbit. Communications will be on the X/Ka band with an average bit rate of 50 kb/s and a total data volume of 1550 Gb/year. A UHF dipole antenna mounted on the nadir side will be used for possible communications with the MSE. Navigation knowledge is provided by 3 star sensors.
The MPO will carry an imaging system consisting of a wide-angle and narrow angle camera, an infrared spectrometer, an ultraviolet spectrometer, gamm, X-ray, and neutron spectrometers, a laser altimeter, a Near Earth Object telescope and detection system, and radio science experiments.
Mercury Magnetospheric Orbiter (MMO)
The Mercury Magnetospheric Orbiter has the shape of a flat cylinder with a mass of 165 kg. The MMO is spin stabilized at 15 rpm with the spin axis perpendicular to the equator of Mercury. The top and bottom of the cylinder act as radiators with louvers for active temperature control. The side is covered with solar cells which provide 185 W and second surface mirrors and protected by thermal blankets. Communications with Earth are amintained through a despun 1-meter diameter high-gain offset antenna and two medium-gain antennas operating in the X-band. Telemetry will return 160 Gb of data per year at about 5 kb/s over the lifetime of the craft, which is expected to be greater than one year. A microstrip UHF patch antenna will be used for communication with the MSE. The reaction and control system is based on cold gas thrusters. Deployable booms and wire antennas are stowed until orbit is achieved. The MMO will carry a set of fluxgate magnetometers, charged particle detectors, a wave receiver, a positive ion emitter, and an imaging system.
Mercury Surface Element (MSE)
The Mercury Surface Element has been cancelled due to budgetary constraints. The description which follows gives the general plan for the MSE at the time of cancellation. MSE is a small (44 kg) lander designed to operate for about one week on the surface of Mercury. The MSE is a 0.9 m diameter disc which is designed to land at a latitude of 85 degrees near the terminator region. Following the release of the MMO, a burn of the 4 kN thruster will put the MSE into a 10 km orbit. Another braking maneuver controlled by gyros/accelerometers and an optical range/range-rate sensor will bring the MSE to zero velocity at an altitude of 120 meters at which point the propulsion unit will be ejected, the airbags inflated, and the module will fall to the surface with a maximum impact velocity of 30 m/s. If the landing occurs in sunlight a thermal protection cover will Since 40% of the terrain at the landing point will be in shadow, primary power is supplied by a 1.7 kWh battery. Scientific data will be stored onboard and relayed via a cross-dipole UHF antenna to either the MPO or MMO at a data rate of 8.7 kb/s providing for a total of 75 Mb over 7 days, assuming 18 contact periods of 480 seconds each. The MSE will carry a 7 kg payload consisting of an imaging system (a descent camera and a surface camera), a heat flow and physical properties package, an alpha X-ray spectrometer, a magnetometer, a seismometer, a soil penetrating device (mole), and a micro-rover.
BepiColombo is named for Giuseppe (Bepi) Colombo (1920-1984), scientist, mathematician and engineer at the University of Padua, Italy.