Mission to mercury
The scientists reveal how Bepicolombo will crack the Solar System’s smallest planet
“BepiColombo will be perfect to follow up on MESSENGER”
Mercury, the closest planet to the Sun, is an unsolved mystery that has left many questions swirling around the minds of scientists. We have recently been blessed with many missions exploring the outer regions of our Solar System, such as NASA’s Juno mission at Jupiter and the New Horizons mission to Pluto and the Kuiper Belt, and the NASA/ESA/ASI Cassini-Huygens mission to Saturn. However, a mission to the inner-most planet poses its own set of challenges and rewards.
Now, the highly anticipated BepiColombo mission, a collaborative mission between the ESA and the Japan Aerospace Exploration Agency (JAXA), will be launched in order to answer the most profound questions about Mercury and our Solar System. Once the BepiColombo stack arrives at Mercury in late 2025 the stack will split apart and send two orbiters into their own unique orbit around the planet. The ESA is responsible for the operation of the Mercury Planetary Orbiter (MPO), and JAXA will operate the Mercury Magnetospheric Orbiter (MMO, or Mio as named by JAXA).
“MPO and MMO complement each other.
While the MPO is more focused on studying the planet itself and its interior, the MMO is focused
Dr Johannes Benkhoff
on studying the plasma, particle and magnetic environment around the planet,” explains Benkhoff to All About Space. “Therefore the BepiColombo mission will provide a rare opportunity to collect multi-point [on two spacecraft] measurements in a planetary environment.”
This ball of metals and silicate materials has a relatively tiny radius of only 2,440 kilometres
(1,516 miles), making it smaller than the moons Ganymede and Titan of Jupiter and Saturn respectively. As Mercury sits at an average distance of 58 million kilometres (36 million miles) from the Sun, which is less than 40 per cent of the SunEarth distance, this tiny planet exhibits a harsh environment due to its close proximity. The planet faces two different extremes; daytime temperatures can reach a toasty 430 degrees Celsius (806 degrees Fahrenheit) while night-time temperatures, due to Mercury's lack of atmosphere, can drop to a chilly -180 degrees Celsius (-292 degrees Fahrenheit), and the Sun peppers the surface in high-energy particles that have enough power to reach the other planets in the Solar System.
An environment such as this is not an easy one to experience as a spacecraft, let alone sustain a working laboratory for a prolonged time. Only two spacecraft have ever visited the planet. Between 1974 and 1975, NASA’s Mariner 10 spacecraft flew by Mercury three times in order to find out about its environment, atmosphere and surface, and it imaged about 45 per cent of its surface.
Fast-forward to March 2011 and NASA’s Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft became the first spacecraft to orbit Mercury, making unrivalled observations about every aspect of the planet for about four years and one month. After travelling over 14 billion kilometres (8.7 billion miles) and completing 4,105 orbits of Mercury, and returning over 250,000 images back to Earth in the process, MESSENGER was deliberately crashed into the surface of Mercury on 30 April 2015. The data collected from this mission reshaped our knowledge of Mercury and the evolution of the Solar System, revealing unknown traits about the planet’s magnetosphere, surface features and composition.
Although MESSENGER was a pioneering spacecraft, it was still limited by its orbit at the time. “Because of the orbital limitation, there is not enough learned from MESSENGER about the southern hemisphere of the planet” says Fujimoto. “BepiColombo will complete the full-hemisphere coverage and will set the ground to obtain deeper understanding of the issues.”
Much like when we look back on any past spacecraft, it was also limited by its technologies of the time, as Benkhoff explains: “BepiColombo, with its comprehensive instrumentation, its sophisticated radio science instrument set, its [thermal and] multi-wavelength spectral imagers, not to mention its much closer orbit allowing full global highresolution coverage, will be perfect to follow up on MESSENGER.”
With a sophisticated instrumental suite of 16 instruments and experiments – five on Mio/ MMO and 11 on MPO – including various cameras, spectrometers over a wide range of wavelengths, particle analysers, a magnetometer, a laser altimeter and more, BepiColombo will leave no stone unturned in the examination of Mercury’s magnetosphere, exosphere, surface and interior, along with an experiment to test Einstein’s theory of general relativity.
The most thought-provoking result that MESSENGER provided was the evidence that Mercury’s surface is more abundant in volatile elements such as potassium and sulphur, which completely contradicts the models of Mercury’s formation, as well as the entire Solar System. When MESSENGER measured the ratio of potassium (the more volatile element) to thorium (the more stable element), they noticed that abundance of potassium was much higher than thorium. It was previously thought that potassium would have been heated up due to the incredibly high temperatures and evaporated away. However, the copious amounts of potassium show the temperatures were not as high as first thought.
Previous analysis of Mercury's interior also showed that the core makes up roughly 85 per cent of the planet's radius. When compared to Earth’s core, which makes up only half of our home planet, it is theorised that Mercury was once a larger planet that had its outer layers cast off in a destructive collision. This is all theory however, and without instruments such as the MPO’s Mercury Orbiter Radio-science Experiment (MORE) to further understand the planet’s core, and the Spectrometers and Imagers for MPO BepiColombo Integrated Observatory System (SIMBIO-SYS), which will analyse the surface composition, the mystery of Mercury’s evolution will remain extremely blurred.
Another important aspect of Mercury that MESSENGER couldn’t quite answer is the state of the planet’s magnetosphere – the planet’s selfdriven, magnetic field. Fujimoto says that Mercury shouldn’t theoretically exhibit its current intrinsic magnetic field, as there needs to be some sort of liquid or molten interior powering the magnetic activity. A planet as small as Mercury should have cooled down to its core a while ago and cut off its magnetic activity, making its current state a keen area of research.
“Mio is the spacecraft that focuses on this magnetic-oriented science theme. It will observe the magnetic field itself and see how it interacts with the solar wind, the super-sonic flow of ionised gas from the Sun,” says Fujimoto. “For this purpose, on board Mio are electromagnetic field instruments, plasma particle detectors and an imaging device for the thin atmosphere that Mercury has.”
Observations of Mercury’s interior structure, geology and composition are just some of the aspects that BepiColombo will observe. In terms of visually inspecting the rocky surface, MESSENGER was able to locate a source of water ice hidden within the craters at the north pole. With the MPO orbit more comprehensive, the south pole can be scouted for more sources of water ice.
MESSENGER also imaged some interesting surface features called ‘hollows’ – shallow, irregular depressions on the planet’s surface. Along with the observed volcanic features and areas of a young, uncratered land, Mercury’s surface could be much more active than originally thought.
The previous MESSENGER mission raised many questions, and even more questions have come along in recent years. With BepiColombo soon to be on its way and scheduled to arrive at Mercury in late 2025, the elusive nature of our most unknown terrestrial planet will be placed under a microscope; this will be another step in understanding the history and formation of Mercury and the inner planets, including Earth. “I strongly believe that the instrumentation of BepiColombo is perfectly suited to obtain our science goals and to deliver answers to the necessary and new questions raised by MESSENGER,” concludes Benkhoff.
“Because of the orbital limitation, there is not enough learned from MESSENGER about the southern hemisphere”
MESSENGER’s revelation of small cliff-like landforms are features that scientists believe are geologically young