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BepiColombo: Hot mission to Mercury
By Matthias Gründer
The Cornerstone missions constitute the ambitious core of ESA's Aurora research programme. In February 2007 the green light was given for the third of these space probes. Its goal will be Mercury, the smallest planet and the one closest to the sun.
The Scientific Programme Committee of ESA took the decision to start the industrial development phase on 23 February 2007, thus placing the Mercury probe BepiColombo alongside the two previous Cornerstone missions, ExoMars and Mars Sample Return. The probe is named after the Italian scientist, Giuseppe “Bepi” Colombo, who in the 1960s had calculated the trajectories on which NASA's Mariner probes would rendezvous with the planets, as a result of which it became possible for the first time to send accurate photographs and data from Mercury to the Earth (Mariner 10 in the year 1974/75). Colombo died in 1984.
Since Mariner 10 no further human scouts have approached Mercury, and the planet remains one of the least explored in the inner solar system. Resembling our Moon on account of its countless craters, less than half of its surface has been photographed. Its inners seem more like the geologically dynamic Earth, and only recently scientists from Earth concluded on the basis of gravitational fluctuations that Mercury must have a molten iron core. This has an estimated diameter of 3600 km, corresponding to about three-quarters of the 4,880 km diameter of the planet. Mercury's atmosphere is as thin as that of our Moon, and the planet has a measurable magnetic field.
The large number of craters provides an indication of the age of the crust, which is believed to be between four and four and a half billion years old. Pictures acquired to date do not give any hint of volcanic activity or plate tectonics, nor have any traces of erosion been detected. Water ice could still exist internally within a crater close to the pole, since despite the searing heat of 470º Celsius on the side facing the sun, icy temperatures of up to -212ºC prevail at the poles. This means, for example, that one could melt lead there by day and liquefy oxygen by night. But that is all we know about Mercury.
Amongst others, we are hoping for answers to the following questions:
- What do the hitherto unknown parts of the surface of Mercury look like?
- Why does the planet have such a high density?
- Is Mercury still active?
- Is there really any ice at the poles?
- Is the planet shielded by radiation belts?
- What effect does the nearby Sun have on the planet's surface?
The idea is that these and other scientific problems will now be clarified with the aid of the space probe which is to set out on its long journey as a European-Japanese tripack. The approximately five metre long, three tonne unit will consist of three components: a European orbiter, a Japanese orbiter and a propulsion module which will transport the two orbiters to Mercury. BepiColombo is scheduled to lift off from the European space launch centre in Kourou in August 2013, with a Soyuz 2-1B with modified Fregat upper stage serving as the launch vehicle. After about six years' flying time, BepiColombo will reach its destination and be able to start its scientific work in August 2019.
The primary probe, the Mercury Planetary Orbiter (MPO), will come from Europe and will be equipped with 11 scientific experiments, one of which is to be contributed by Russian scientists. For at least one year the MPO will investigate the structure and composition of the planet and its atmosphere, create altitude profiles, examine the internal structure and create an imagery atlas from a polar orbit at an altitude of between 400 and 1500 km with the aid of several cameras, spectrometers and accurate acceleration and radiation measuring devices. The atlas will be based on imagery data with a resolution of about 1 metre per pixel.
The Japanese Mercury Magnetospheric Orbiter (MMO) sister probe will focus primarily on the magnetic field of Mercury, using five instruments (four from Japan and one from Europe). These measurements will be taken from a polar orbit at an altitude of between 400 and 12,000 km. Originally the mission was to have included a lander, but this was abandoned in November 2003 for cost reasons. At the launch, the two probes will be joined and mounted on the Mercury Transfer Module (MTM) propulsion module: with MMO at the top, MPO in the middle and MTM at the bottom, the entire structure will constitute BepiColombo.
The MTM has both solar-electric and conventional propulsion units incorporating state-of-the-art technology. The solar-electric or ion propulsion engine with 0.24kN thrust will power the interplanetary flight, while the “normal” engine will be required for the start of the journey and for swivelling into orbit around the planet. The advantages of ion propulsion, of which ESA already has valuable experience from the Smart 1 lunar probe mission which only ended recently, are extremely low fuel consumption and higher speed over great distances. The drawbacks are low thrust, extremely long burning times, a long flight time and the fact that it can only be deployed in a vacuum. Moreover, deceleration on the approach to Mercury would not be possible, which is why the engine technologies have to be combined. In this way the advantages of both forms of propulsion will be adeptly combined.
Centrifugal acceleration from the moon and the inner planets
Following the launch, BepiColombo will be carried with the aid of the Russian Fregat upper stage to a transfer orbit around the Earth, from where its path will take it initially towards our Moon, utilising the on-board propulsion. After that, the ion propulsion engine will be ignited and in the course of several swing-by manoeuvres (past the Moon, the Earth, twice past Venus and twice past Mercury) the probe combination will gather the gravitational momentum necessary to achieve the requisite orbit energy.
Before the probe slips into polar orbit around Mercury in the year 2019, the electrical propulsion module will be detached and the chemical one will then be ignited. This will decelerate BepiColombo sufficiently to enable the two probes, still joined together, to take up the orbit planned for the MMO. After separation, the MPO will then be transported to its trajectory, at which point the second module will also be discarded. From this point on the two spacecraft will then circle the target planet, in the course of which they will be constantly exposed to temperatures well in excess of 350º Celsius. This high temperature poses a major challenge to the designers, as the highly sensitive scientific equipment and structures must be capable of functioning properly despite the adverse conditions. To protect the electronics and the instruments, amongst other measures a new insulating foil constructed out of ceramic fibres will be used, and specially developed solar cells should reliably produce onboard energy even under high temperatures.
The MPO/MTM combination is to be developed and built on ESA's behalf by a European industrial consortium headed by the German company, Astrium GmbH, in Friedrichshafen, while the mission control system will be supplied by ESOC, the satellite control centre of the European Space Agency, in Darmstadt. The contract which was let to Astrium in February 2007 totals about €330 million. Astrium is also responsible for integration of all three modules, while the MTM will be built by Astrium UK.
From page 72 of FLUG REVUE 7/2007
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