On the way to Mercury: spacecraft "BepiColombo" flies towards Earth

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On its way to Mercury, ESA's BepiColombo spacecraft will pass Earth on

On its way to Mercury, ESA's BepiColombo spacecraft will pass Earth on 10 April at a distance of 12 677 kilometres and enter the inner Solar System. © ESA/ATG medialab

In the early hours of 10 April, the European Space Agency (ESA) BepiColombo spacecraft will fly towards Earth at over 30 kilometres per second. At 06:25 CEST it will make its closest approach, over the South Atlantic, at an altitude of 12,677 kilometres. The spacecraft will then fly further towards the centre of the Solar System, travelling somewhat more slowly than when it arrived. The main purpose of the so-called Earth flyby is to slow down BepiColombo somewhat without expending propellant, in order to bring the spacecraft onto a trajectory towards Venus.With two subsequent close flybys of Venus (the first flyby will take place on 16 October 2020), BepiColombo will then be on a trajectory that will take it to the destination of the six-year journey, an orbit around Mercury, the innermost planet of the Solar System.

For planetary researchers at the Institute for Planetology at the University of Münster and theGerman Aerospace Center (Deutsches Zentrum für Luftund Raumfahrt; DLR) this is a unique opportunity to conduct a unique experiment, where they will study the Moon. As early as 9 April, with its Earth-facing side illuminated by the Sun, the Moon will be observed for the first time in the thermal infrared and examined for its mineralogical composition using the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) instrument. This will be possible because there will be no absorption by Earth’s atmosphere. At Mercury, MERTIS will investigate the composition and mineralogy of Mercury’s surface and investigate the planet’s interior.

During its flight towards Earth, on its spiral orbit through the inner Solar System, it will travel at a speed of 30.4 kilometres per second. As it moves away from Earth, BepiColombo be travelling at a speed of approximately 25 kilometres per second. Due to the enormous gravitational field of the Sun and the limited transport capacity of the available launchers, planetary missions to the inner and outer Solar System can only be accomplished by following very complex trajectories.

“The Moon and Mercury are not dissimilar in size, and their surfaces resemble one another in many ways,” explains Harald Hiesinger from the University of Münster, Principal Investigator for the MERTIS experiment. After decades of lunar research, he is particularly looking forward to the new measurements. “We will obtain new information on rock-forming minerals and the temperatures on the lunar surface and will later be able to compare the results with those acquired at Mercury. The Moon and Mercury are two important bodies that are fundamental to enhancing our understanding of the Solar System,” Hiesinger adds: “I am anticipating many exciting results from the observations with MERTIS. After about 20 years of intensive preparations, the time will finally come on Thursday - our long wait will be over, and we will receive our first scientific data from space.”

The scientific evaluation of the data will then be carried out jointly at participating institutes in Münster, Berlin, Göttingen and Dortmund, as well as several locations in Europe and the USA.

“Observing the Moon with the MERTIS instrument on board BepiColombo is a one-of-a-kind opportunity,” says Jörn Helbert from the DLR Institute of Planetary Research, who is a Co-Principal Investigator for MERTIS. The researchers will examine the Earth-facing side of the Moon spectroscopically in the thermal infrared for the first time. Without any absorption by Earth’s atmosphere, the view from space will provide a valuable new data set for lunar research. This is also an excellent opportunity to test how well our instrument works and to gain experience in preparation for operations in Mercury orbit. The current situation with the Coronavirus pandemic is also putting the team to the test. The team will support the MERTIS instrument from their home offices and process and evaluate the data there. This has been tested several times over the last few days and “data evaluation at the kitchen table” seems to work well.

Last chance to see ‘Bepi’ - but not in Germany

Space enthusiasts are, of course, interested in knowing whether they will have the opportunity to see BepiColombo one last time, during the flyby, before it leaves on its way to the inner Solar System. The answer is indeed yes. However, this will only be possible south of 30 degrees north over the Atlantic, in South America, Mexico and, with some restrictions, over Texas and California. In Central Europe, the consolation remains that on the night of 7 to 8 April, there will be an exceptionally large full Moon, commonly referred to as a ‘supermoon’.

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