14. October 2020
Slowing down at Venus

Bepi­Colom­bo space­craft con­tin­ues to 'fall' on its tra­jec­to­ry to­wards Mer­cury

First Venus flyby of BepiColombo on the way to Mercury
First Venus fly­by of Bepi­Colom­bo on the way to Mer­cury
Image 1/10, Credit: ESA/ATG Medialab

First Venus flyby of BepiColombo on the way to Mercury

At 05:58 CEST on 15 Oc­to­ber 2020, al­most two years af­ter its launch, the Bepi­Colom­bo space­craft, con­sist­ing of one Eu­ro­pean Space Agen­cy (ESA) and one Japanese Space Agen­cy (JAXA) or­biter, will make the first of two fly­bys of Venus. The aim of the ma­noeu­vre, at a dis­tance of 10,720 kilo­me­tres from the sur­face of the plan­et, is to de­cel­er­ate Bepi­Colom­bo slight­ly and reach an or­bit grad­u­al­ly ap­proach­ing the or­bit of the plan­et Mer­cury, the fi­nal des­ti­na­tion of the mis­sion. The sec­ond fly­by of Venus will take place on 11 Au­gust 2021. Dur­ing the fly­by, sci­en­tif­ic ex­per­i­ments will be car­ried out us­ing some of the in­stru­ments on board both probes, in­clud­ing the MER­TIS in­frared spec­trom­e­ter.
Venus from the perspective of the Japanese space probe Akatsuki
Venus from the per­spec­tive of the Japanese space probe Akat­su­ki
Image 2/10, Credit: JAXA/ISAS/DARTS/Damia Bouic

Venus from the perspective of the Japanese space probe Akatsuki

Our in­ner neigh­bour, the plan­et Venus, is of­ten re­ferred to as Earth’s ‘sis­ter plan­et’. It has al­most the same di­am­e­ter and on­ly slight­ly less mass than Earth, but the plan­et de­vel­oped com­plete­ly dif­fer­ent­ly. Its dense at­mo­sphere con­sists pri­mar­i­ly of car­bon diox­ide, which leads to an ex­treme green­house ef­fect. As a re­sult, Venus ex­pe­ri­ences tem­per­a­tures of 470 de­grees Cel­sius both day and night. The dense gaseous en­ve­lope, with clouds of sul­phuric acid, ex­tends to a much high­er al­ti­tude than Earth’s at­mo­sphere and ob­structs a view of the sur­face in vis­i­ble light. The dy­nam­ics and com­po­si­tion of the Venu­sian at­mo­sphere have been the sub­ject of in­ten­sive sci­en­tif­ic re­search for decades, in­clud­ing by the cur­rent Japanese Venu­sian or­biter Akat­su­ki, which ac­quired this im­age of the plan­et in ul­tra­vi­o­let light. The Bepi­Colom­bo space­craft will al­so be car­ry­ing out some ex­per­i­ments to study the Venu­sian at­mo­sphere dur­ing its fly­by of Venus on 15 Oc­to­ber 2020.
Search for traces of volcanism in the Venusian atmosphere
Search for traces of vol­can­ism in the Venu­sian at­mo­sphere
Image 3/10, Credit: NASA/JPL

Search for traces of volcanism in the Venusian atmosphere

The in­frared spec­trom­e­ter de­vel­oped by the Uni­ver­si­ty of Mün­ster and the Ger­man Aerospace Cen­ter (Deutsches Zen­trum für Luft- und Raum­fahrt; DLR) for ESA's Bepi­Colom­bo mis­sion will al­so search for traces of gas­es that could pro­vide ev­i­dence of ac­tive vol­can­ism on Venus, such as sul­phur diox­ide, dur­ing the space­craft’s Venus fly­by on 15 Oc­to­ber 2020. Mea­sure­ments of the plan­et’s at­mo­sphere will be made from dis­tances be­tween 1.4 mil­lion and 200,000 kilo­me­tres away. From radar im­ages, such as those record­ed dur­ing the glob­al map­ping of Venus car­ried out by NASA's Mag­el­lan or­biter, it is known that there are thou­sands of vol­ca­noes on Venus (im­age: Maat Mons, 4000 me­tres high), some of which are be­lieved to still be ac­tive.
BepiColombo on the long journey to Mercury
Bepi­Colom­bo on the long jour­ney to Mer­cury
Image 4/10, Credit: ESA/ATG Medialab

BepiColombo on the long journey to Mercury

The Bepi­Colom­bo mis­sion is a joint project of the Eu­ro­pean Space Agen­cy (ESA), which con­tributes the Mer­cury Plan­e­tary Or­biter (MPO, cen­tre), and the Japanese Space Agen­cy (JAXA), which con­tributes the Mer­cury Mag­ne­tosh­pher­ic Or­biter (MMO, top). The or­biter duo is pow­ered by an ion en­gine in the Mer­cury Trans­fer Mod­ule (MTM, bot­tom).
MERTIS spectrometer
MER­TIS spec­trom­e­ter
Image 5/10, 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.
BepiColombo’s seven-year journey to Mercury
Bepi­Colom­bo’s sev­en-year jour­ney to Mer­cury
Image 6/10, Credit: ESA

BepiColombo’s seven-year journey to Mercury

Space­flight from Earth to the in­ner So­lar Sys­tem is ex­treme­ly dif­fi­cult be­cause of the Sun's im­mense grav­i­ta­tion­al pull. The Eu­ro­pean-Japanese mis­sion Bepi­Colom­bo will take sev­en years to com­plete its jour­ney. Over the years, and by means of sev­er­al close fly­bys of Earth and Venus (twice), the space­craft will first be slowed down and then, fly­ing by Mer­cury it­self six times, will be guid­ed in­to an or­bit around the Sun that in­creas­ing­ly ap­proach­es Mer­cury's own or­bit. Fi­nal­ly, Bepi­Colom­bo will be steered in­to an or­bit around Mer­cury on 5 De­cem­ber 2025.
An­i­ma­tion: the first of Bepi­Colom­bo’s two Venus fly­bys
Video 7/10, Credit: ESA/ATG Medialab

Animation: the first of BepiColombo’s two Venus flybys

Credit: ESA/ATG Medialab
Length: 00:00:57
On 15 Oc­to­ber 2020, the Eu­ro­pean-Japanese Mer­cury mis­sion Bepi­Colom­bo will fly past Venus at a dis­tance of 10,720 kilo­me­tres in or­der to steer the tra­jec­to­ry of the space­craft to­wards Mer­cury’s or­bit. A sec­ond Venus fly­by will take place on 11 Au­gust 2021, be­fore six Mer­cury fly­bys bring the mis­sion ev­er clos­er to Mer­cury's or­bit around the Sun. Fi­nal­ly, Bepi­Colom­bo will be steered in­to or­bit around Mer­cury on 5 De­cem­ber 2025.
Bepi­Colom­bo’s fly­by of Venus
Video 8/10, Credit: ESA/ATG Medialab

BepiColombo’s flyby of Venus

Credit: ESA/ATG Medialab
Length: 00:00:20
On 15 Oc­to­ber 2020 at 05:58 CEST, the Eu­ro­pean-Japanese Mer­cury mis­sion Bepi­Colom­bo will per­form a fly­by of Venus just 10,720 kilo­me­tres above the sur­face of Earth’s sis­ter plan­et. The aim of the ma­noeu­vre is to de­cel­er­ate the space­craft and grad­u­al­ly align its or­bit around the Sun with the or­bit of Mer­cury, the space­craft’s fi­nal des­ti­na­tion.
Bepi­Colom­bo’s sev­en-year jour­ney through the in­ner so­lar sys­tem
Video 9/10, Credit: ESA

BepiColombo’s seven-year journey through the inner solar system

Credit: ESA
Length: 00:02:02
On 15 Oc­to­ber 2020, al­most ex­act­ly two years to the day af­ter its launch on 20 Oc­to­ber 2018, the Eu­ro­pean-Japanese Mer­cury mis­sion Bepi­Colom­bo will com­plete the first of two fly­bys of Venus. The sec­ond will take place on 11 Au­gust 2021. Pri­or to this, the space­craft passed Earth on 10 April 2020 in or­der to be steered to­wards the in­ner So­lar Sys­tem. Af­ter a fur­ther six fly­bys of Mer­cury from Oc­to­ber 2021, Bepi­Colom­bo will en­ter in­to or­bit around Mer­cury on 5 De­cem­ber 2025.
Ob­serv­ing Venus with the MER­TIS in­frared spec­trom­e­ter
Video 10/10, Credit: MERTIS team

Observing Venus with the MERTIS infrared spectrometer

Credit: MERTIS team
Length: 00:03:46
The in­frared spec­trom­e­ter MER­TIS (MEr­cury Ra­diome­ter and Ther­mal In­frared Spec­trom­e­ter) will record da­ta from the Venus at­mo­sphere dur­ing two se­ries of mea­sure­ments dur­ing Bepi­Colom­bo’s fly­by. The first se­ries will take place be­tween the dis­tances of ap­prox­i­mate­ly 1.4 mil­lion and 670,000 kilo­me­tres, mea­sur­ing for up to 23 hours. Af­ter a pause to check the in­stru­ment, the sec­ond se­ries will be­gin 11 hours be­fore the clos­est ap­proach of the Venus fly­by and take place be­tween the dis­tances of 300,000 and 120,000 kilo­me­tres. The MER­TIS ob­ser­va­tions will be com­plet­ed four hours be­fore the clos­est ap­proach of the fly­by at 10,720 kilo­me­tres. This video was pro­duced us­ing Bepi­Colom­bo's Sci­ence Plan­ning and Op­er­a­tion Tool and shows in its cen­tre how Venus is tar­get­ed and ‘seen’ with the ob­ser­va­tion ge­om­e­try of MER­TIS and scanned dur­ing the two mea­sure­ment se­ries.
  • In an unusual test, experiments designed for Mercury, with its lack of atmosphere, will be tested on the dense gas envelope of Venus.
  • The MERTIS spectrometer (University of Münster and DLR) will acquire approximately 100,000 spectral images during two series of experiments.
  • A second Venus flyby will take place in August 2021 – following this, BepiColombo will be on course for an orbit of Mercury.
  • BepiColombo will arrive at the innermost planet in 2025, after another six Mercury flybys
  • Focus: space, space science

Approaching Venus from its day side, passing the planet, using its gravitational pull to slow down and continuing on its night side on course for Mercury: on Thursday 15 October 2020, at 05:58 CEST, ESA’s BepiColombo spacecraft will fly past Venus at a distance of approximately 10,720 kilometres and transfer some of its kinetic energy to our neighbouring planet in order to reduce its own speed. Two years post launch, the purpose of the manoeuvre is to lower BepiColombo’s orbit around the Sun towards the orbit of Mercury. The two orbiter spacecraft of the European Space Agency (ESA) and the Japanese Space Agency (JAXA) are part of a joint mission that will reach this point after another flyby of Venus in August 2021. Following six close flybys of Mercury, the mission will then enter orbit around the innermost planet at the end of 2025. For planetary researchers and engineers at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) and for the Institute of Planetology at the University of Münster, the Venus flyby is another opportunity to test BepiColombo’s MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS).

View of the Venus gas envelope with infrared sensors

The flyby of Venus and the Earth-Moon-flyby that took place in spring 2020 are spaceflight manoeuvres used to test the functionality of some of the experiments on board both orbiters, and to calibrate the sensors and signal chains with the data obtained. “Scientific measurements will also be carried out during approach and departure and at the closest approach to Venus,” say the two people primarily responsible for the MERTIS instrument, Jörn Helbert from the DLR Institute of Planetary Research and Harald Hiesinger from the Institute of Planetology at the University of Münster. “Our imaging spectrometer MERTIS, which we built together with industry and international partners, will be used again to make these measurements,” says Helbert. MERTIS was primarily developed to measure spectra of rock-forming minerals on Mercury’s atmosphere-free surface. But with its infrared sensors, it can also look into the dense gas envelope of Venus down to a certain depth. “We are already expecting some very interesting findings, with more to follow in 2021, when we will be much closer to Venus,” adds Hiesinger.

MERTIS is an imaging infrared spectrometer and radiometer with two uncooled radiation sensors that are sensitive to wavelengths of 7 to 14, and 7 to 40 micrometres, respectively. During two series of measurements, the first of which begins today, MERTIS will capture almost 100,000 individual images. The first series will begin as the spacecraft approaches from a distance of approximately 1.4 million kilometres from Venus up to a distance of 670,000 kilometres. After a pause to check the instrument, the second series will start at a distance of 300,000 kilometres, 11 hours before the Venus flyby, and will continue until BepiColombo is almost 120,000 kilometres from Venus four hours before the closest approach of the flyby.

Venus as the focus of planetary research

Venus is almost as large as Earth, but has developed in a completely different way. Its atmosphere is much denser, consisting almost entirely of carbon dioxide, and thus the planet experiences a very strong greenhouse effect. This results in a permanent surface temperature of around 470 degrees Celsius. There is no water and therefore it is thought that no life could survive on the surface. It is quite possible that volcanoes are still active on Venus. “These would be detected, for example, through the sulphur dioxide that they emit,” says Helbert. “Following the first measurements made in the 1960s and 1970s, about ten years ago, ESA's Venus Express mission recorded a massive reduction, by more than half, of sulphur dioxide concentrations. Venus literally 'smells' of active volcanoes! MERTIS could now provide us with new information." The experiments will be complemented by simultaneous observations from the Japanese Venusian orbiter, Akatsuki, and from a dozen professional telescopes as well as information from amateur astronomers on Earth.

Venus only recently came under the spotlight of science and the media when a group of astronomers used telescopes in Hawaii and Chile to prove beyond doubt the presence of the trace gas phosphine (or monophosphane, chemical formula PH3) on Venus. Phosphine is industrially manufactured on Earth for use in pest control, but is also produced by biological processes in sapropel or in the digestive tract of vertebrates. Phosphine is a very short-lived molecule, so there must be a current source of the molecule on Venus or in its atmosphere.

Previous modelling of natural phosphine sources such as volcanism, chemical reactions following meteorite impacts or lightning discharges have not been able to explain the measured concentrations. This is why the possibility that the phosphine is produced by microorganisms high up in Venus’s atmosphere is frequently debated by planetary researchers. This finding could suggest that life exists in the temperate ‘flying carpets’ of sulphuric acid clouds that exist at altitudes of 40 to 60 kilometres. The authors of the study themselves question this idea, however, and indicate the need for further measurements in the future. In the future, Venus will be the target of ESA and NASA missions.

Venus, an exoplanet on our doorstep

MERTIS and the other five activated instruments on board the Mercury Planetary Orbiter will not be able to detect any phosphine molecules from the distance of the flyby. Nevertheless, the flyby is scientifically interesting, as the spacecraft can be used to study Venus as if it were a distant, Earth-like extrasolar planet with a solid surface and dense atmosphere. “During the Earth flyby, we studied the Moon, characterising MERTIS in flight for the first time under real experimental conditions. We achieved good results," says Gisbert Peter, MERTIS project manager at the DLR Institute of Optical Sensor Systems, which was responsible for the design and construction of MERTIS. "Now we are pointing MERTIS towards a planet for the first time. This will allow us to make comparisons with measurements taken prior to the launch of BepiColombo, to optimise operation and data processing, and to gain experience for the design of future experiments." All experiments will focus on measuring the composition, structure and dynamics of the atmosphere of Venus, the ionosphere of the planet and – using the instruments on the Japanese MMO (Mercury Magnetospheric Orbiter) – the induced magnetosphere of Venus.

Saving fuel with planetary flybys

Following BepiColombo's first flyby of Earth on 10 April 2020, its second flyby, this time of Venus is designed to continue to slow the spacecraft down without using any fuel. This is necessary in order to compress the spacecraft's orbital ellipse towards a circular orbit that is ultimately almost geometrically identical to the orbit of Mercury. The spacecraft 'falls' towards Venus on its spiralled orbit through the inner Solar System at various speeds depending on its distance from the Sun. At Venus, BepiColombo will reduce its heliocentric speed by 37 kilometres per second (133,500 km/h). The flyby will take place at a distance of 116 million kilometres from Earth. Venus is currently ahead of Earth in its orbit and is visible in the eastern sky just before dawn.

Due to the Sun’s strong gravitational pull, planetary missions to the inner Solar System can only be achieved with very complex trajectories. With the manoeuvre on Thursday, the spacecraft’s relative speed compared to Mercury will be reduced to 1.84 kilometres per second. At the end of its spiralled flight between the orbits of Earth and Mercury, BepiColombo will orbit the Sun at almost the same speed as Mercury. It will then easily be captured by the gravity of the smallest planet in the Solar System on 5 December 2025 and will manoeuvre itself into a polar orbit. BepiColombo was launched on 20 October 2018 on board an Ariane 5 launch vehicle from the European spaceport in Kourou.

The use of flyby manoeuvres was first implemented during NASA’s Mariner 10 mission, enabling the spacecraft to make two additional close flybys of Mercury after it had already travelled past the planet once. The calculations were made by Italian engineer and mathematician Giuseppe ‘Bepi’ Colombo, a professor at the University of Padua. Colombo was invited to a conference in preparation for the Mariner 10 mission at NASA’s Jet Propulsion Laboratory in Pasadena, California, in 1970. After seeing the original mission plan, he realised that a highly precise first flyby could allow for two additional flybys of Mercury. The current European-Japanese Mercury mission was named in his honour.

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
    Head of Me­dia Re­la­tions, Ed­i­tor
    Ger­man Aerospace Cen­ter (DLR)

    Cor­po­rate Com­mu­ni­ca­tions
    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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