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BepiColombo: a duo of spacecraft to orbit Mercury

Dr Amanda Doyle, Feature writer
Mar 23, 2015, 16:28 UTC

Sen—Mercury is the least explored terrestrial planet in our Solar System, and after the completion of NASA's successful MESSENGER mission, a new era of discovery will dawn with the ESA/JAXA mission BepiColombo.

The mission is named after Professor Giuseppe (Bepi) Colombo, a mathematician and engineer who made numerous contributions to planetary science and celestial mechanics.

BepiColombo is set to launch in January 2017, setting off on a long and arduous journey to the innermost planet. "The spacecraft must brake against the Sun's gravity, which increases with proximity to the Sun, rather than accelerate away from it, as is the case with journeys to the outer Solar System," explains project scientist Johannes Benkhoff. "Sending a spacecraft to Mercury requires more fuel than sending a spacecraft to Pluto!"

BepiColombo will slam on the brakes using the gravity of planets. After one flyby of Earth, two of Venus, and four of Mercury itself, the interplanetary explorer will finally settle into orbit around Mercury in 2024.

There are actually two space probes that make up BepiColombo; the ESA led Mercury Planet Orbiter (MPO) and the JAXA led Mercury Magnetospheric Orbiter (MMO). Both will sit atop the Mercury Transfer Module (MTM), which controls the propulsion during the journey to Mercury. Upon arrival, MTM will be jettisoned and MPO and MMO will separate.

BepiColombo has many scientific objectives, including understanding the origin and evolution of a planet which is in close proximity to its parent star—vital for planet formation theories. The probe will investigate the composition and geology of Mercury, the unusually dense interior, the magnetosphere and how the planet interacts with the solar wind, along with a plethora of other things. To facilitate these investigations, there are eleven instruments on board MPO, and five on MMO.

The gravitational field of Mercury still isn't understood very well. Both the Mercury Orbiter Radio science Experiment (MORE) and Italian Spring Accelerometer (ISA) instruments aboard MPO will investigate Mercury's gravitational field in detail. ISA also has the task of testing Einstein's theory of general relativity to a greater precision than ever managed before. Relativistic effects are enhanced so close to the massive body of the Sun, making it the ideal location for such experiments.

The BepiColombo Laser Altimeter (BELA) instrument is a laser altimeter that will be used to measure the topography and surface morphology of Mercury. BELA principal investigator Nicolas Thomas tells Sen more. "We should be able to get a very good comparison between the gravity field and the topography. This will allow us to measure the mass associated with the topography and the crust density. The tidal deformations measured by BELA and the radio science instrument will constrain global models of the interior structure."

While the main goal of MMO is to observe energetic particles, and the structure and dynamics of the magnetic environment surrounding Mercury, it is also equipped with an instrument for measuring dust. Observations by the Mercury Dust Monitor (MDM) are crucial for understanding Mercury's tenuous atmosphere. The dominant element in Mercury's atmosphere is sodium, but exactly how this is released is still a mystery. One theory is that interplanetary dust which impacts the planet becomes vaporised, and by comparing the distribution of the surrounding dust to the density of the atmosphere it will be possible to test this theory.

Both MPO and MMO will have polar orbits, with MMO in a more distant orbit in order to sample the magnetosphere. MPO will be nadir pointing, meaning that it is always facing the surface. At low altitudes, this can be quite a challenging feat due to the intense heat radiating from the planet.

"A rotation of Mercury around its own axis will last about 56 days," Benkhoff tells Sen. "Due to this slow rotation and the close distance to the Sun maximum temperatures could become as high as 700 Kelvin [427°C]." 

As such, both craft will be equipped with the latest heat shielding technology, comprised of thermal insulation blankets made up of more than 60 layers.

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NASA's MESSENGER spacecraft took this image of Mercury in January 2008. Image credit: NASA /Johns Hopkins University Applied Physics Laboratory /Carnegie Institution of Washington

Learning from MESSENGER

NASA's MESSENGER mission spent the last four years at Mercury, busily deciphering the planet's mysteries. So how does BepiColombo improve on this?

A key advantage is in the dual spacecraft. As both MPO and MMO can make measurements of the magnetosphere, different regions can then be studied at the same time. MESSENGER's highly elliptical orbit meant that the southern hemisphere was not as well studied as the northern, where as MPO will be in a much tighter orbit around Mercury. "The orbit means we should get excellent coverage of the southern hemisphere, thereby complementing the MESSENGER results," says Thomas.

"The global scientific objectives have not changed," adds Benkhoff. "However, results from MESSENGER provided many unexpected findings, which need to be followed up by BepiColombo. For example, MESSENGER found much more volatiles than expected on the surface of Mercury. BepiColombo will provide global spectral mapping of the surface with a better resolution and in a different (not covered by MESSENGER) wavelength range."

Another interesting twist is how BepiColombo can learn from MESSENGER's demise. MESSENGER ran out of propellant and crashed into Mercury's surface on April 30, 2015, leaving a crater which could be up to 15 metres in diameter. MESSENGER did not detect any newly formed craters, which would be an interesting find as the rate of space weathering could then be observed. For instance, if a fresh crater in 2015 looked relatively eroded by 2024, then it is obvious that the weathering is harsh on Mercury.

The tiny crater left behind by MESSENGER will be difficult to find, but pictures sent back by MESSENGER prior to the impact will help to find its final resting place later. The camera aboard MPO is quite capable of resolving such a crater.

MESSENGER taught us much about Mercury, but also showed us that there is still much more to learn. BepiColombo will take what MESSENGER has observed and use it home in on Mercury's secrets, providing us with fascinating discovereries for years to come.

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