Mercury: a tough place to live…

Mercury. The smallest planet in our solar system—only slightly larger than Earth’s Moon and the closest one to the Sun at a distance of about 36 million miles (58 million kilometers) or 0.39 AU. The Sumerians also knew of Mercury since at least 5,000 years ago. It was often associated with Nabu, the god of writing. It was also given separate names for its appearance as both a morning star and as an evening star. Greek astronomers knew, however, that the two names referred to the same body, and Heraclitus, around 500 B.C., correctly thought that both Mercury and Venus orbited the Sun, not Earth.

No evidence for life has been found on Mercury. Daytime Temperatures can reach 430 degrees Celsius (800 degrees Fahrenheit) and drop to -180 degrees Celsius (-290 degrees Fahrenheit) at night. It is unlikely life (as we know it) could survive on this planet. Mercury has a solid, cratered surface, much like the Earth’s Moon. Its thin atmosphere, or exosphere, is composed mostly of oxygen (O2), sodium (Na), hydrogen (H2), helium (He), and potassium (K)—atoms that are blasted off the surface by the solar wind and micrometeoroid impacts create Mercury’s exosphere. Mercury spins slowly compared to Earth, so one day lasts a long time. It takes 59 Earth days to make one full rotation. But a year on Mercury goes fast. Because it’s the closest planet to the Sun, it goes around the Sun in just 88 Earth days.

In 2012, NASA’s MESSENGER spacecraft discovered water ice in the craters around its north pole, where regions may be permanently shaded from the heat of the Sun. The southern pole may also contain icy pockets, but MESSENGER’s orbit did not allow scientists to probe the area. A completely unexpected discovery made by Mariner 10 (the first spacecraft to visit the planet in 1974-75)  was that Mercury possessed a magnetic field. Planets theoretically generate magnetic fields only if they spin quickly and possess a molten core. But Mercury takes 59 days to rotate and is so small — just roughly one-third Earth’s size — that its core should have cooled off long ago. 

The MASCS instrument was designed to study both the exosphere and surface of Mercury. To learn more about the minerals and surface processes on Mercury, the Visual and Infrared Spectrometer (VIRS) portion of MASCS has been diligently collecting single tracks of spectral surface measurements since MESSENGER entered orbit. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

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Mercury in pop culture?

The smallest planet in our solar system never had a big presence in our collective imagination as was the case for the so called “Red Planet”, which raised a lot of interest since 1938, when a radio broadcast really freaked people out and sent them running into the streets looking for cover from invading Martians.

However, Mercury has a few honorable mentions thanks to fiction writers including Isaac Asimov, C. S. Lewis, Ray Bradbury, Arthur C. Clarke and H. P. Lovecraft. And in the 2007 film Sunshine, a crew of eight pilot a colossal nuclear bomb aboard the spaceship Icarus II, with the intent to jump start the Sun. But not without a few wrong misconceptions. A big part of the suspense in the film relates to the astronauts not being able to communicate with Earth because of a “dead zone” around Mercury. In real life, NASA had no problem getting a signal back from the Mariner 10, which observed Mercury in 1975.

Europe’s first mission to Mercury amid coronavirus crisis

Scheduled to begin orbiting Mercury in 2025, BepiColombo is a joint European-Japanese mission to Mercury.

The scientific objectives for the mission are to study the planet’s form, interior structure, geology, composition, as well as its craters, origin, and dynamics of its magnetic field, and dynamics of the vestigial atmosphere. Einstein’s theory of general relativity will be tested too along with the search for asteroids sunward of Earth. BepiColombo is named after Professor Giuseppe (Bepi) Colombo (1920-1984) from the University of Padua, Italy, a mathematician and engineer. He was the first to determine that an unsuspected resonance is responsible for Mercury’s habit of rotating on its axis three times for every two revolutions it makes around the Sun.

The mission is comprised of two spacecraft riding together to Mercury to orbit and to study the planet from unique vantage points. The European Space Agency (ESA) provided the Mercury Planetary Orbiter (MPO), the main spacecraft that will study the planet’s surface and interior while the Japan Aerospace Exploration Agency (JAXA) supplied the second orbiter, the Mercury Magnetospheric Orbiter (MMO) that will study the planet’s magnetic field. Both MPO and MMO will be driven by a Solar-Electric Propulsion Module (SEPM) and a chemical propulsion module (CPM).

The first module is optimal for slow cruise maneuvers and consists of a set of xenon ion thrusters housed in a rectangular prism powered by two GaAs-cell solar panel wings covering 33 square meters. The second module is a bi-propellant, N2O4-MMH, system for attitude control and orbital insertion. The dry mass of the CPM is 71 kg, with 156 kg of fuel for MPO and 334 kg for MMO. The total launch mass of the entire MPO system will be 1229 kg and for MMO about 1200 kg.

This is the first Mercury mission for the ESA and JAXA. Only two other spacecraft have visited Mercury: NASA’s Mariner 10, which was the first mission to explore Mercury and MESSENGER, the first to orbit the innermost planet.

Launched aboard an Ariane 5 rocket from Europe’s Spaceport in Kourou, French Guiana, on October 20, 2018, BepiColombo continues to be an ambitious, multi-spacecraft mission to explore the mysteries of the “Swift Planet”. The BepiColombo spacecraft swung-by the Earth on April 10, 2020, and approached it at the distance of only 12,700 Km, which is less than half the altitude of Europe’s Galileo navigational satellites. “This is the last time we will see BepiColombo from Earth,” said Joe Zender, BepiColombo Deputy Project Scientist at ESA. “After that, it will head deeper into the inner Solar System.” The operation was performed with limited personnel at ESA’s European Space Operations Centre (ESOC) in Darmstadt, Germany, where engineers had to comply with social distancing rules presently in place all over Europe as a response to the coronavirus pandemic. The coronavirus threat forced the team to work with minimal face to face interaction while ensuring all steps in the process to be properly covered.

During its 7-year long journey to Mercury, Bepi Colombo takes advantage of the gravity of Earth, Venus and Mercury itself to adjust its trajectory to reach the correct orbit around the innermost planet of the Solar System. ESA’s BepiColombo Project Scientist Johannes Benkhoff hopes that, despite the challenging circumstances, the science teams will be able to switch on the MPO instruments to test and calibrate them. “For example, the PHEBUS spectroscope will use the Moon as a calibration target to then produce better data once at Mercury,” says Johannes. “We also want to make some measurements of the solar wind and its interaction with Earth’s magnetic field. The main purpose of having the instruments on at this stage, however, is testing and calibration. If we can use the data for some scientific investigation, it will be a bonus.”

The Earth flyby on April 10 was only the first of nine gravity assist maneuvers awaiting BepiColombo during its 7-year journey to Mercury. In October, the spacecraft will perform the first of two flybys at Venus. The final six orbit-tightening maneuvers will use the gravity of BepiColombo’s destination, Mercury.

BepiColombo will arrive at Mercury in late 2025. The science mission will commence three months later, after Mio and the MPO separate from the transfer module and enter their respective target orbits. Together, the two orbiters will help scientists shed light on the evolution of Mercury, the least explored of the four rocky planets in the Solar System and the one closest to the Sun. Amateur astronomers equipped with small telescopes will be able to observe BepiColombo during the flyby, if located in southern latitudes. Observers in southern Europe might be able to spot the spacecraft briefly. The best view, however, will only be possible from the southern hemisphere.

Mercury significant events in a nutshell

• 1631: Thomas Harriott and Galileo Galilei observe Mercury with the newly invented telescope.
• ​1631: Pierre Gassendi uses a telescope to watch from Earth as Mercury crosses the face of the Sun.
• 1965: Incorrectly believing for centuries that the same side of Mercury always faces the Sun, astronomers using radar find that the planet rotates three times for every two orbits.
• 1974-1975: Mariner 10 photographs roughly half of Mercury’s surface during three flybys.
• 1991: Scientists using Earth-based radar find signs of ice locked in permanently shadowed areas of craters in Mercury’s polar regions.
• 2008-2009: MESSENGER observes Mercury during three flybys.
• 2011: MESSENGER begins its orbital mission at Mercury, yielding a treasure trove of images, compositional data, and scientific discoveries.
• 2015: MESSENGER is deliberately crashed into Mercury after expending all its propellant, ending its mission.​
• 2018: BepiColombo launches with a target date for Mercury orbit insertion of 2025.