Out of the four rocky planets of the inner solar system, Mercury is the one least explored. Hot and more difficult for an orbiter to reach than Saturn, the planet long eluded scientists. BepiColombo, a joint mission of the European Space Agency (ESA) and the Japan Space Exploration Agency (JAXA), will be only the second (and the most complex) mission in history to orbit Mercury.
Its two orbiters, the European Mercury Planetary Orbiter (MPO) and the Japanese Mercury Magnetospheric Orbiter (MMO), carry a combined 16 scientific instruments that will help shed light on some perplexing questions: Do the scorched planet's polar craters really harbor water ice? Where does Mercury's magnetic field come from? What are the strange 'hollows' on its surface?
The mission, launched on Oct. 19, 2018, is expected to reach Mercury in 2025, after a complicated seven-year journey, which requires nine flyby maneuvers (one at Earth, two at Venus and six at Mercury itself). These flybys slow down the spacecraft against the gravitational pull of the sun so that it can safely enter the orbit around the innermost planet of the solar system.
During the epic journey, the two orbiters travel stacked on top of an ESA-built Mercury Transfer Module (MTM). The module, with its 15 meter (49 feet) solar arrays, supplies power during the interplanetary cruise. It is also fitted with three "selfie" cameras, originally used to monitor the deployment of the solar arrays and antennas after launch. But these cameras are regularly being used by the BepiColombo team to obtain simple black and white images of the spacecraft's whereabouts. Thanks to these cameras, BepiColombo is sending regular postcards home from its encounters with various planets during the flybys.
In April 2020, the spacecraft waved goodbye to Earth during a flyby at a distance of 7,900 miles (12,700 kilometers). In October 2020 and August 2021, BepiColombo flew past Venus. The first flyby saw the spacecraft zip by at a distance of 6,660 miles (10,720 km) from Venus. The second, however, took it incredibly close, only 340 miles (550 km) above the hot, cloudy planet's surface. This enabled scientists to take some unique measurements of Venus' murky atmosphere.
Related: BepiColombo in pictures: A Mercury mission by Europe and Japan
Once the stacked spacecraft reaches its destination, the MTM will separate from the orbiters. Then, the two spacecraft will each be put in separate orbits to perform scientific investigations of Mercury. Japan's MMO will have an orbit of 9.3 hours, and Europe's MPO an orbit of about 2.3 hours. The mission is expected to last for one Earth year, an equivalent of four Mercury years. It may be extended for an additional Earth year if funding allows, and the two spacecraft remain in good health.
BepiColombo's journey through space is as long as winding as was its journey from paper to the launch pad. The first proposal for a European Mercury mission came in 1993, according to ESA. The agency included a Mercury orbiter as one of three possible new "cornerstone" missions as a part of the Horizon 2000 science program, which also birthed the Cassini orbiter that visited Saturn and its moons, as well as the accompanying Huygens lander for Titan.
BepiColombo and the Gaia mission, which maps our galaxy, the Milky Way, in three dimensions, were approved in 2000. ESA then issued a request for proposal for the BepiColombo MPO payload in 2004. The payload is the part of the spacecraft that performs the main functions of the mission, including scientific investigations. The payload instruments were selected later that year.
In 2007, Astrium GmbH (now Airbus Defence and Space) was selected as prime contractor for BepiColombo. The Mercury spacecraft was originally supposed to fly on a Soyuz-Fregat, but that was switched to a heavier Ariane 5 rocket after the mission's mass was increased during the design phase in 2008. ESA's science program committee gave final approval for BepiColombo's redesigned mission in November 2009.
The launch date was pushed back several times. A target of July 2014 was delayed to August 2015 after Astrium looked at the development of several components of the spacecraft (such as the solar arrays and the electric propulsion system) and determined they couldn't make the earlier deadline. Subsequent delays in different parts of the mission development pushed the launch date back to 2016, 2017 and then 2018.
BepiColombo's April 2018 launch date was then delayed six months to October 2018 after a major electrical problem was found during a test of MTM. When the announcement was made in late 2016, ESA said it forecasted no impact on the science return of the mission.
ESA noted that going to Mercury would help scientists not only understand how the planet had formed, but also give more information generally about the solar system's formation. According to the ESA, mission objectives include:
- Investigate the origin and evolution of a planet close to the parent star
- Study Mercury as a planet: its form, interior structure, geology, composition and craters
- Examine Mercury's vestigial atmosphere (exosphere): its composition and dynamics
- Probe Mercury's magnetized envelope (magnetosphere): its structure and dynamics
- Determine the origin of Mercury's magnetic field
- Investigate polar deposits: their composition and origin
- Perform a test of Einstein's theory of general relativity
Europe's MPO carrys the following instruments:
- BELA – BepiColombo Laser Altimeter
- ISA – Italian Spring Accelerometer
- MPO-MAG – Magnetic Field Investigation
- MERTIS – Mercury Radiometer and Thermal Imaging Spectrometer
- MGNS – Mercury Gamma-Ray and Neutron Spectrometer
- MIXS – Mercury Imaging X-ray Spectrometer
- MORE – Mercury Orbiter Radio science Experiment
- PHEBUS – Probing of Hermean Exosphere by Ultraviolet Spectroscopy
- SERENA – Search for Exosphere Refilling and Emitted Neutral Abundances (neutral and ionised particle analyser)
- SIMBIO-SYS – Spectrometers and Imagers for MPO BepiColombo Integrated Observatory - HRIC, STC, VIHI
- SIXS – Solar Intensity X-ray and particle Spectrometer
Japan's MMO carrys the following instruments:
- MERMAG-M/MGF – Mercury Magnetometer
- MPPE – Mercury Plasma Particle Experiment
- PWI – Mercury Plasma Wave Instrument
- MSASI – Mercury Sodium Atmospheric Spectral Imager
- MDM – Mercury Dust Monitor
Past missions to Mercury
In the early days of the space age, scientists were trying to figure out the most efficient way to take pictures of Mercury. Although the planet sometimes closes in to within 48 million miles (77 million kilometers) from Earth – roughly a third of the Earth-Mars distance – it takes an extraordinary amount of energy to brake a spacecraft to go into Mercury's orbit, ESA said.
Studies from the 1950s and 1960s suggested that a spacecraft could use Venus' gravity to slingshot to Mercury, thus getting to the planet without needing to use much rocket fuel. Opportunities to take advantage of this slingshot would come up in 1970 and 1973, according to early 1960s research from Michael Minovich, a University of California at Los Angeles graduate student.
This is where BepiColombo's namesake comes in. Giuseppe "Bepi" Colombo was then a celestial mechanics researcher at Padua University in Italy. He showed that after a spacecraft made one flyby past Mercury, it would go into a 176-day orbit around the sun. Luckily, this is exactly double the 88-day Mercury year, making it easy for the spacecraft to return to Mercury repeatedly with just minor orbital modifications. "The only drawback was that the surface lighting conditions would be the same for each encounter," ESA wrote.
This was the plan that the first Mercury mission followed. Called Mariner 10, the NASA mission launched on Nov. 3, 1973. In February, the mission swung by Venus and safely made it to its first closest approach to Mercury on March 29, 1974. Although the spacecraft approached on the night side, images were sent back showing the sunlit hemisphere as the spacecraft pulled away.
Other encounters happened on Sept. 21, 1974, and March 16, 1975. Highlights of Mariner 10's mission included discovering a magnetic field similar to that of Earth's, and studying the effects of the solar wind (the constant stream of charged particles that come from the sun.)
"The surface was seen to be subdivided into intercrater plains, smooth plains, and heavily cratered terrain that resembled the landscapes on Earth's moon. The most notable individual feature, partially visible on the sunrise terminator, was a giant impact basin that was named Caloris," ESA wrote. "Other surprising features were steep cliffs, hundreds of kilometers in length, that appeared to have been formed by global compression, possibly due to shrinkage of the planet as it cooled."
Mariner 10 remained the only spacecraft to visit Mercury for a generation, until the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission was launched on Aug. 3, 2004. Similarly to BepiColombo, it got to Mercury after repeated gravity assists from Venus, Earth and Mercury itself. The spacecraft performed three flybys in January 2008, October 2008 and September 2009 before orbiting Mercury between March 2011 and April 2015, when it ran out of gas and crashed into the planet.
Some of MESSENGER's major discoveries include evidence of water ice in permanently shadowed craters at the planet's poles, depressions called "hollows" that are only known to exist on Mercury, and extensive volcanic activity across the planet.