Published August 2018
This October, our attention turns to Mercury and the new mission that sets off on a seven-year journey to visit the smallest terrestrial planet in our Solar System.
BepiColombo is a joint mission between ESA and Japan’s space agency JAXA (under ESA’s overall leadership) that aims to spend a nominal year, with a possible one-year extension gathering data to understand more about Mercury’s composition, structure, atmosphere and history.
The mission is named after the Italian mathematician, Giuseppe ‘Bepi’ Colombo (1920-1984), who first noted Mercury’s 3:2 resonance in 1965 and also calculated how a gravity-assist using Venus could help NASA’s Mariner 10 spacecraft to successfully complete its Mercury flybys in 1974.
THE JOURNEY TO MERCURY
Launch is scheduled for 19 October 2018 when BepiColombo will start its journey from French Guiana on board the Ariane 5. Although Mercury is a relatively short distance from Earth (on average, 91 million km*), Bepi will be taking quite the scenic route to Mercury, covering 18.9 billion km over 7.2 years. For the most part, Bepi will cruise using solar electric propulsion, although it will employ nine gravity-assists (often known as a flyby): one past Earth in 2020, two past Venus and six past Mercury itself, before eventually using chemical and ion propulsion to settle into orbit in December 2025.
*The distance between Mercury and Earth depends on where in their orbital paths they are – it can vary between 77 million km to 222 million km. 1 AU is known as an astronomical unit, the measure of the distance between the Earth and the Sun, which is approximately 150 million km. Astronomers often use AU when talking about distances between the planets.
Unlike other planets, Mercury has a tenuous atmosphere that is insufficient to allow approaching spacecraft to decelerate (known as aerobraking) and enter orbit. The gravity assists allow BepiColombo to reduce its momentum so that it can release the satellites into orbit, rather than crashing into the planet, or being flung off-trajectory far into space.
Bepi Colombo Journey Timeline | Credit: ESA http://sci.esa.int/bepicolombo/60082-bepicolombo-timeline/
WHAT DO WE HOPE TO LEARN?
BepiColombo aims to unlock some of the secrets held by this mysterious planet – how the inner rocky planets, including Earth, formed some 4.5 billion years ago and what events might have contributed to the characteristic features of the terrestrial planets.
ESA outlines its mission objectives as including investigating the origins and evolution of a planet close to its parent star, Mercury’s composition, internal structure, surface features, thin atmosphere, and its magnetic field.
Through this exploration, ESA hopes to answer scientific questions about the solar nebula and planetary formation. Mercury has a higher density than expected for a small planet and its core accounts for over 50% of its mass, suggesting a colourful history of massive impacts that whittled away its outer layers and reduced its size. It remains a wonder how Mercury came to have and more importantly, keep its magnetosphere given its low mass, and how this interacts with the solar wind. Does it have similar features to Earth such as aurorae and radiation belt that are characteristic of a magnetic field?
Named after the winged messenger of the Roman gods, Mercury has always had an air of mystery for astronomers and scientists. Its size, which is no larger than Earth’s moon, coupled with its proximity to the Sun makes it difficult to observe from Earth and even more challenging to send spacecraft to. The mission team has to overcome several environment challenges to get BepiColombo to Mercury in one piece.
BEPI’S EQUIPMENT
Mercury’s proximity to the Sun creates one obvious problem – immense heat. On the day-side of Mercury, temperatures reach in excess of 350°c. However, the night-side in stark contrast can dive to sub-zero temperatures of around -190°c.
To protect it from these harsh and radically opposing environments, bespoke technology and systems have been built, causing severe mission delays. One such bespoke piece is the MMO Sunshield and Interface Structure (MOSIF), designed for thermal protection as well as providing an electrical interface for parts of the spacecraft.
Much of the entire spacecraft is also covered in a hand-stitched, silver insulation blanket made of layers of aluminium, plastic and ceramics, designed to keep the instruments at room temperature. Additionally, a radiator (that is covered in fins to protect it from radiation) will push internal heat out into space. Yup, your protective tools also need their own set of protective equipment.
Although not the first to visit Mercury, BepiColombo is unique because it is deploying two orbiting spacecraft simultaneously: ESA’s Mercury Planetary Orbiter (MPO) and JAXA’s Mercury Magnetospheric Orbiter (MMO). Both are carried by the Mercury Transfer Module (MTM), also built by ESA and the MMO is also protected by a sun shield (MOSIF).
Collectively, all of these modules are known as the Mercury Composite Spacecraft (MCS).
Credit: ESA
During the initial part of the journey (interplanetary cruise) the MPO has overall command and will communicate with Earth, while the MTM will provide electric power. The MMO will remain dormant throughout this part of the journey.
When BepiColombo reaches Mercury in December 2025, the MTM will be jettisoned from the stack as it’s no longer needed. The MPO will continue to provide resources for itself and the MMO, up until the point that the two orbiters head on their separate orbital paths to gather data. Only when the MMO reaches its intended orbit of Mercury, will it assume its own command through JAXA.
By separating into two orbits, the workload can be split. The MPO will study the surface and internal composition, while the MMO will study Mercury’s planetary environment, including the magnetosphere (the region of space around the planet that is influenced by its magnetic field).
FROM THE PAST TO THE FUTURE
Only two spacecrafts have visited Mercury: NASA’s Mariner 10 arrived in 1974 and MESSENGER in 2011. It is from these two missions that we were able to validate some of our theories and assumptions and paint the picture of Mercury that we commonly understand today. Before this, it was not even 100 years ago that many great astronomers were convinced that a hidden planet lay in between Mercury and the Sun. The theory was so convincing that false observations were made, and the planet was even given a name: Vulcan. It wasn’t until Albert Einstein’s theory of general relativity in 1915 that laid the laws of space-time and mass and energy, that the idea of Vulcan was quashed for good. And now we wait to see what more of Mercury’s intriguing history BepiColombo reveals.
Nisha Beerjeraz-Hoyle 