6. April 2020
BepiColombo is on its way to Mercury

Grav­i­ty de­cel­er­a­tion ma­noeu­vre close to Earth – with a view of the Moon

Past Earth and on its way to Venus
Past Earth and on its way to Venus
Image 1/5, Credit: ESA/ATG medialab

Past Earth and on its way to Venus

On 10 April 2020, the ESA Bepi­Colom­bo space­craft will pass Earth at a dis­tance of 12,677 kilo­me­tres and em­bark on its jour­ney to­wards the in­ner So­lar Sys­tem and its fi­nal des­ti­na­tion, Mer­cury. This fly­by, or grav­i­ty as­sist ma­noeu­vre, will be used to slow down the space­craft with­out the need to ex­pend pro­pel­lant and al­so to change the plane of its tra­jec­to­ry to bring it clos­er to the or­bit of Venus. Bepi­Colom­bo will con­duct two Venus fly­bys be­fore ap­proach­ing Mer­cury. The Venus fly­bys will take place on 16 Oc­to­ber 2020 and 11 Au­gust 2021.
Earth as a pivot
Earth as a piv­ot
Image 2/5, Credit: DLR, based on an ESA model

Earth as a pivot

Schemat­ic rep­re­sen­ta­tion of Bepi­Colom­bo's Earth fly­by on 10 April 2020. The Moon's or­bit is shown as a dot­ted line. At 02:27 on 10 April (all times CEST) [1], Bepi­Colom­bo will pass through the bow shock of Earth's mag­net­ic field, a tran­si­tion zone be­tween the mag­net­ic field and space. At 03:14 [2], the space­craft will cross the mag­ne­topause, the bound­ary be­tween space and the plas­ma sur­round­ing Earth. At 03:44 [3], Bepi­Colom­bo will still be nine Earth radii away, and will reach a dis­tance of eight Earth radii at 04:05 (4). At 04:50 [5] it will en­ter Earth's mag­net­ic field at a dis­tance of six Earth radii. At 06:25:23 [6], Bepi­Colom­bo will reach its point of clos­est ap­proach, 12,677 kilo­me­tres from Earth. It will then be­gin its de­par­ture, leav­ing the mag­net­ic field at 08:00 [7], pass­ing eight Earth radii at 08:44 [8], and nine Earth radii at 09:06 [9]. The space­craft will then tra­verse the mag­ne­topause [10] and Bepi­Colom­bo will leave the mag­net­i­cal­ly in­flu­enced zone around Earth at 00:08 on 11 April [11].
MERTIS spectrometer
MER­TIS spec­trom­e­ter
Image 3/5, Credit: DLR (CC BY-NC-ND 3.0)

MERTIS spectrometer

The Mer­cury Ra­diome­ter and Ther­mal In­frared Spec­trom­e­ter (MER­TIS) in­stru­ment com­bines an imag­ing spec­trom­e­ter with a ra­diome­ter, which is used for de­ter­min­ing ir­ra­di­ance. The in­stru­ment has di­men­sions of on­ly 18 by 18 by 13 cen­time­tres and a mass of 3.3 kilo­grams; its pow­er con­sump­tion is very low. The MER­TIS sen­sors are unique – the imag­ing chan­nel us­es an un­cooled mi­crobolome­ter to mea­sure in­frared ra­di­a­tion, the first to be qual­i­fied for space in Eu­rope. MER­TIS will be used to in­ves­ti­gate the min­er­alog­i­cal com­po­si­tion of Mer­cury’s sur­face, as well as rock-form­ing min­er­als. At the same time, the in­te­grat­ed ra­diome­ter will mea­sure tem­per­a­ture and de­ter­mine ther­mal con­duc­tiv­i­ty. The sci­en­tists hope that the da­ta will pro­vide in­sights in­to the for­ma­tion and de­vel­op­ment of Mer­cury.
Giuseppe 'Bepi' Colombo (1920-1984)
Giuseppe 'Bepi' Colom­bo (1920-1984)
Image 4/5, Credit: ESA

Giuseppe 'Bepi' Colombo (1920-1984)

The Ital­ian en­gi­neer and math­e­mati­cian Giuseppe 'Bepi' Colom­bo is re­gard­ed as the in­ven­tor of grav­i­ty as­sist ma­noeu­vres, more com­mon­ly re­ferred to as plan­e­tary fly­bys. Put sim­ply, when a space­craft pass­es close to a plan­et, its speed and tra­jec­to­ry can be al­tered with­out the ex­pen­di­ture of pro­pel­lant. Plan­e­tary fly­by ma­noeu­vres were first used on the NASA Mariner 10 mis­sion, to en­able two more close fly­bys af­ter the first vis­it to Mer­cury. The cal­cu­la­tions were made in 1970 by Colom­bo, a pro­fes­sor at the uni­ver­si­ty of his home­town Pad­ua.
Moon in near infrared
Moon in near in­frared
Image 5/5, Credit: NASA/JPL

Moon in near infrared

These four im­ages of the Moon were cre­at­ed us­ing in­frared im­age da­ta ac­quired in De­cem­ber 1992 dur­ing the fly­by of the Earth-Moon sys­tem con­duct­ed by NASA's Galileo space­craft. They were ac­quired with the Near-In­frared Map­ping Spec­trom­e­ter (NIMS). The view is di­rect­ed to­wards the Moon's North Pole; the coloured ar­eas show the north­ern hemi­sphere of the Earth-fac­ing side of the Moon. The spec­tral chan­nels of the in­stru­ment ranged from vis­i­ble light to near-in­frared wave­lengths (5.2 mi­crome­tres). The dif­fer­ent (false) colours give in­for­ma­tion about the geo­chem­i­cal and min­er­alog­i­cal com­po­si­tion of the sur­face. The MER­TIS spec­trom­e­ter on board ESA's Bepi­Colom­bo Mer­cury mis­sion will now be the first to im­age Earth's satel­lite with two sen­sors, cov­er­ing wave­lengths down to the ther­mal in­frared – be­tween 7 and 14 mi­crome­tres and 7 and 40 mi­crome­tres, re­spec­tive­ly, as it flies past the Earth and the Moon.
  • BepiColombo to conduct Earth flyby before heading towards Venus
  • MERTIS spectrometer will observe the Moon during the flyby
  • New information on rock-forming minerals and temperatures on the lunar surface
  • Focus: Space exploration

Space exploration missions require precision of the highest order. In the early hours of 10 April 2020, the European Space Agency's (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. This is a unique opportunity for planetary researchers and engineers at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) and the Institute for Planetology at the Westphalian Wilhelms University of Münster 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, developed and built at DLR. 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. 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.

The main purpose of the Earth flyby is to slow down BepiColombo somewhat without expending propellant, in order to bring the spacecraft onto a trajectory towards Venus. 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. 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 final destination of the six-year journey, an orbit around Mercury, the innermost planet of the Solar System. 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.

Unique possibility to observe the Earth-facing side of the Moon

"Observing the Moon with our 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. "We 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. "Our team will support the MERTIS instrument from our home offices and process and evaluate the data there," Helbert adds. "This has been tested several times over the last few days and 'data evaluation at the kitchen table' seems to work well."

MERTIS has two uncooled radiation sensors. Its spectrometer covers a wavelength range from seven to 14 micrometres, and its radiometer to a wavelength range from seven to 40 micrometres. It will identify rock-forming minerals in the mid-infrared at a spatial resolution of 500 metres.

"We will not be able to obtain such a detailed resolution when observing the Moon," explains Gisbert Peter, MERTIS Project Manager at the DLR Institute of Optical Sensor Systems, which was responsible for the design and construction of MERTIS. "Having the Moon in the spectrometer's field of view before the flyby is partly an astronomical or geometric 'coincidence' and, above all, due to good planning. MERTIS will observe the Moon from distances of between 740,000 and 680,000 kilometres for four hours." Here, the instrument, which is very compact at 3.3 kilograms, will be able to demonstrate its unique optical properties for the first time in orbit. Three small cameras on the exterior of the BepiColombo spacecraft will also acquire images of Earth during the approach.

"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."

Third mission to Mercury

BepiColombo was launched on 20 October 2018 on board an Ariane 5 launch vehicle, which lifted off from the European spaceport in French Guiana. It is the most extensive European project to explore a planet in the Solar System. The science component of the mission consists of two spacecraft that will orbit Mercury at different altitudes – the ESA Mercury Planetary Orbiter (MPO) and the Japan Aerospace Exploration Agency (JAXA) Mercury Magnetospheric Orbiter (MMO). Two NASA missions – Mariner 10, in the mid-1970s, and MESSENGER, which orbited Mercury from 2011 to 2015 – are the only other spacecraft to have studied the planet that is closest to the Sun.

While MPO is designed to analyse the planetary surface and composition, MMO will explore its magnetosphere. Further goals of the mission are the investigation of the solar wind, the internal structure and the planetary environment of Mercury, and its interaction with the near-solar environment. Scientists also hope to gain insights into the formation of the Solar System, and Earth-like planets in particular. Until they reach Mercury orbit, the two spacecraft will travel as part of the Mercury Composite Spacecraft (MCS). This includes the Mercury Transfer Module (MTM), which supplies the orbiters with power and protects them from the extreme temperatures as they approach and fly past Mercury. MCS is also equipped with the Magnetospheric Orbiter Sunshield and Interface Structure (MOSIF), which will further protect the MMO before it enters orbit. Surface temperatures on Mercury range between 430 degrees Celsius on the day side and minus 180 degrees Celsius on the night side. On 5 December 2025, after six flybys of Mercury, the MPO, MMO and MOSIF will enter an initial capture orbit.

Juggling gravity and velocity

Flyby manoeuvres, also referred to as 'Gravity Assist Manoeuvres', have become routine for space missions. They are used to change the flight trajectory and speed of spacecraft without using propellant, by employing the gravitational fields of planets. Leaving Earth's gravitational field and reaching a distant destination in the Solar System requires a great deal of energy for acceleration, for changes of direction, and also for deceleration at the destination. Carrying this energy in the form of propellants for engines or thrusters is expensive in terms of mass and would inevitably reduce the science payload that can be carried or simply make the mission technically unfeasible.

Close flybys of planets enable an elegant technical solution. If a spacecraft approaches a planet, that planet’s gravitational attraction prevails over that of the Sun at a certain distance, influencing its movement. In a sense, a flyby is the juggling of two forms of energy – the kinetic energy of the spacecraft and the planet's potential energy, which, with its much greater mass, attracts the small spacecraft during its approach. With this juggling, depending on the spacecraft's velocity and proximity to the planet, energy can be transferred from the planet to the spacecraft, or vice versa. The visitor then either begins to travel faster (and the planet slows down imperceptibly) or, with kinetic energy being transferred from the spacecraft to the planet, the craft slows down (and imperceptibly accelerates the planet in return). The speed of the spacecraft does not change in relation to the planet; its overall velocity and trajectory are only modified. However, since the planet is in orbit around the Sun, this change in the trajectory of the spacecraft causes it (and minimally the planet) to accelerate or slow down on their orbits around the Sun.

The ingenious trajectory solution proposed by Giuseppe 'Bepi' Colombo

For the first time, flyby manoeuvres along a planetary orbit were used on the Mariner 10 mission to allow two more close flybys after the first flyby of the planet Mercury. The calculations were made by the Italian engineer and mathematician Giuseppe 'Bepi' Colombo. In 1970, Colombo, a professor at the university in his hometown of Padua, was invited to a conference at NASA's Jet Propulsion Laboratory in Pasadena, California, held in preparation for the Mariner 10 mission. There, he saw the original mission plan and realised that, with a highly precise first flyby, two more close flybys of Mercury were possible. The current European-Japanese mission to Mercury was named in his honour.

Of the 15 instruments on board the two orbiters, three were largely developed in Germany: BELA (BepiColombo Laser Altimeter), MPO-MAG (MPO Magnetometer) and MERTIS. The DLR laser experiment BELA will only be operated when the spacecraft reaches its destination. The magnetometer is already being used to carry out measurements during the flight through Earth's magnetosphere, which extends far into space. Funded by the DLR Space Administration, it was developed and built at the Institute for Geophysics and Extraterrestrial Physics at TU Braunschweig in collaboration with the Graz Space Research Institute and Imperial College London.

Last chance to see 'Bepi' – but not in Europe

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. The solar panels, illuminated by sunlight, will probably be most visible above the European Southern Observatory in the clear air of the Chilean Andes. 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'.

Close European-Japanese cooperation

Overall management of the mission lies with ESA, which was also responsible for the development and construction of MPO. MMO was contributed by JAXA. The German contribution to BepiColombo is coordinated and mainly financed by the DLR Space Administration, with funds from the German Federal Ministry for Economic Affairs and Energy (BMWi). The two instruments BELA and MERTIS, which were mainly developed by the DLR Institutes of Planetary Research and Optical Sensor Systems in Berlin-Adlershof, were largely financed from DLR's Research and Technology budget. The mission is also being supported by the Westfälische Wilhelms University of Münster, TU Braunschweig and the Max Planck Institute for Solar System Research (MPS) in Göttingen. A European industrial consortium led by Airbus Defence and Space designed and constructed the spacecraft.

  • Falk Dambowsky
    Ger­man Aerospace Cen­ter (DLR)

    Com­mu­ni­ca­tions and Me­dia Re­la­tions
    Telephone: +49 2203 601-3959
    Linder Höhe
    51147 Cologne
  • Jörn Helbert
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Plan­e­tary Re­search
    Rutherfordstraße 2
    12489 Berlin
  • Gisbert Peter
    MER­TIS Project Man­ag­er
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Op­ti­cal Sen­sor Sys­tems
    Rutherfordstraße 2
    12489 Berlin-Adlershof
  • Prof. Dr. Harald Hiesinger
    Prin­ci­pal In­ves­ti­ga­tor for the MER­TIS ex­per­i­ment
    Uni­ver­si­ty of Mün­ster
    In­sti­tute for Plan­e­tol­o­gy
    Telephone: +49 251 83-39057
    Fax: +49 251 83-36301
    Wilhelm-Klemm-Str. 10

  • Ulrich Köhler
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Plan­e­tary Re­search
    Rutherfordstraße 2
    12489 Berlin

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