Mercury is the closest planet to the Sun and it orbits around the Sun for a period of 88 days. It is the smallest of the so called rocky terrestrial planets (the others being Venus, Earth and Mars). These inner planets (as planets closer to the Sun are called as) are made up of rocky solid material in contrast to the gigantic gas giants like Jupiter and Saturn which are less denser. Mercury and Earth have similar average densities, and about five and a half times that of water. However, with a diameter of 4,889 km, Mercury is the smallest of the terrestrial planets. While planets such as Venus and Mars have been explored by many space probes, such as the Russian Venera probes and the American Mariner craft, over the past 50 years, Mercury has been studied by only two space missions. Mariner 10 flew past Mercury in 1975, while the Messenger spacecraft, after a voyage of many years, went into orbit (around Mercury) in 2011 and studied the planet intensely till 2015.
The European Space Agency (ESA) is now ready to launch the BepiColombo probe to Mercury next year. It is a joint European-Japanese venture, a collaboration between ESA and the Japanese space agency, JAXA. It has been in development for the past two decades. It comprises two satellites that will be launched together: the Mercury Planetary Orbiter (MPO) by the ESA and the Mercury Magnetospheric Orbiter (MMO) by the JAXA. They will sit atop one rocket along with the propulsion module and the Mercury Transit Module (MTM) which will control the cruise to Mercury.
During this phase, the sun shield protects the MMO, and after reaching Mercury, the satellites will go into different polar orbits around Mercury to conduct their own studies. The Japanese orbiter would dispense with the shield. The launch date was repeatedly put back while engineers struggled to find equipment that could cope with the intense heat and radiation on Mercury. Mercury’s surface is searing hot and has a temperature of 500° Celsius. Moreover, the gravity of the Sun would pull hard on any spacecraft travelling close to it. Hence, BepiColombo will have to fire its thrusters in the direction of travel to ensure it does not overshoot Mercury.
Surprising discoveries
Mercury is the least explored of the rocky planets, as it is both difficult to get there and more difficult to survive searing temperatures close to the Sun. As Mercury is close to the Sun, it is deep in the solar gravitational well. Hence, the spacecraft must accelerate an additional 18 km/s to match Mercury’s much faster orbital velocity and also escape the gravitational attraction of the Sun. That is why the earlier Messenger probe used gravity assists of Venus and Mercury by making two flybys of both of them. In the two Venus flybys the craft came to within 3,000 km of Venus. After the launch, the Messenger spacecraft took nearly six years before finally going into orbit around Mercury in 2011. To match Mercury’s orbital velocity, the spacecraft needed four flybys and two firings of on-board rockets.
The MMO and MPO would now look forward to extend deeper understanding of the considerable information provided by Messenger. When it completed its operation in 2015, Messenger took around 270,000 images of the planet’s surface. It made the surprising discovery that despite the torrid temperatures, there are shadowed craters which are cold enough to support iced water. It found hollowed out surface features indicating presence of volatiles. It also recorded the composition and structure of Mercury. The largest feature is the Caloris basin, which is 1,500 km wide. There is water ice in the planet’s permanently shadowed crater. Temperatures can be as cold as -180° Celsius, making Mercury both a torrid and frigid planet.
Mercury’s large iron core takes up nearly 60% of the planet’s mass and the puzzle is why the planet has such an outsized core with only thin layers of silicate rocks. The earlier popular theory was that Mercury was stripped of its outer layers either by collision with another planet or by the erosion effects of solar wind and radiation owing to its proximity to the Sun. The theory was untenable when Messenger probe observed large abundance of volatile substances which would not be present if such violent events had happened.
Another paradox about the surface is that there is seemingly not much iron on it. To examine this in detail, the MPO would be using its x-ray spectrometer, MIXS. The MPO will have 11 instruments on board. The MMO will have five instruments and hopes to investigate the planet’s magnetic field. Apart from Earth, Mercury is the only terrestrial planet to have a magnetic field. However, the field is very peculiar. It is roughly three times stronger in the northern hemisphere than it is in the south.
Apart from the discovery of water bonanza at Mercury’s pole (raising remote idea of some kind of biological life) the other aspect is that Mercury appears ‘pained black’ presumably by several comet impacts. Comet crashes could have brought both ice and concentrations of organic compounds which overlie the ice deposits beneath. The cometary dust can be one-fourth carbon by weight and such trapped carbon can produce a darkening effect, explaining why the surface on an average reflects very little light. Mercury has a wrinkled and shrinking history, presumably. It has towering cliffs as a result.
After spending four years studying Mercury, the Messenger spacecraft ended its mission with an explosive impact on Mercury’s surface, slamming into it at four km per second. At this speed, the half-tonne spacecraft could have blasted out a new crater on Mercury, about the size of a football field, but too tiny to be visible from Earth.
For better clarity
Will the new mission also survive the punishing heat and extreme radiation doses? Materials have been tested under high temperatures and other extreme conditions. The original launch date was 2009. After the contract to build MPO was signed, even a 2013 date was considered plausible. Coincidentally, the James Webb Space Telescope and BepiColombo are scheduled to be launched at same time. As it is not possible to launch them together, one would have to stand aside. Which goes first is likely to be decided in September. After the launch, it will take several years for the spacecrafts to reach Mercury.
The new mission with its more advanced technology like high resolution, sharper clearer images in all bands would clarify several issues raised by Messenger. New insights and discoveries into the planet’s structure and composition, the problem of its iron core and the bizarre magnetic field, the icy polar deposits, its wispy atmosphere and shrinking topography are all waiting to be studied more to deepen and extend earlier knowledge.