Seeking Mercury’s secrets
AFIRST for Europe and Japan took place last weekend, when the BepiColombo spacecraft made its first fly-by of Mercury, the closest planet to the sun.
The mission is being jointly conducted by the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA).
There have been two missions to Mercury in the past, both by the US. In 1974-75, Mariner 10, the last in the Mariner series of spacecraft, flew past the planet three times without orbiting it.
Much later, for four years beginning in April 2011, the MESSENGER spacecraft orbited the planet, returning spectacular images and telling us more about Mercury than we had ever known. We have even seen pictures showing scarps that have resulted from the contraction of Mercury as it has cooled.
To conduct its studies, BepiColombo is composed of a pair of spacecraft called the Mercury Planetary Orbiter and the Mercury Magnetospheric Explorer. On arrival at Mercury they will separate from a third section, called the Mercury Transfer Module, which will provide propulsion and electrical power for the journey.
There is much still to learn about Mercury, which superficially appears rather like a somewhat larger version of our moon: a rocky object covered in craters.
One of the biggest objectives is to learn more about Mercury’s magnetic field, detected by the Mariner 10 spacecraft nearly 50 years ago. This surprised scientists at the time because neither Venus nor Mars, which are larger planets, has one, and it was thought that Mercury would have cooled sufficiently to have a solid core. (Magnetic fields suggest the presence of liquid iron in the core or outer core.)
Connected with this study is the curious absence of iron on Mercury’s surface, even though, overall, the planet must have a high iron content.
Another question to be answered is whether Mercury has water ice on its surface. Because the sunlit surface of Mercury is hot enough to melt lead, that may seem highly improbable. However, there would be permanently shadowed regions near its poles that never receive sunlight. We know this is the case with our moon, which has polar water ice.
There is a more general reason to visit and study Mercury. Since 1995 we have been discovering planets orbiting stars other than our sun, and some of these are remarkably close to their “parent” stars. Making use of the existence of Mercury to study a planet so close to its star will help us understand more about these systems, and even answer questions we haven’t yet considered.
However, we have more than four years to wait before BepiColombo finally settles into orbit around Mercury — and it will fly past five more times before that happens.
This is because placing a spacecraft into orbit around Mercury has particular problems. The laws of physics dictate that objects closer to the sun orbit more quickly, and therein lies the problem: we need to have the spacecraft in the vicinity of Mercury without zipping past at great speed.
Starting from Earth, about three times farther from the sun than Mercury orbits, a spacecraft naturally gains speed as it gets closer to the sun. It’s a bit like a car getting faster as it travels further downhill.
To accomplish a slower approach, necessary to enter orbit, a spacecraft needs to shed some energy. The method for doing this was worked out more than 50 years ago by the Italian scientist Giuseppe (“Bepi”) Colombo, after whom the craft was named.
Colombo devised a method of having a spacecraft fly past the inner planets several times in order to be able to visit Mercury without whizzing past too quickly.
BepiColombo’s path has already taken it past Earth once since its 2018 launch, and past Venus twice. However, that was just the beginning. Last weekend was the first of five fly-bys of Mercury, and it took place on Saturday at 9.34am Tasmanian time. The other five will be in June 2022, June 2023, September and December 2024, and January 2025.
Insertion into orbit will be in December 2025, and the scientific observations will start a few months later.
It’s worth all this trouble and calculation because of the secrets still held by the solar system’s smallest planet.