BepiColombo mission : next stop, Mercury !

Mer­cu­ry, one of the four tel­lu­ric pla­nets in our Solar Sys­tem, is the smal­lest pla­net, the clo­sest to the Sun, and the only one, along with the Earth, with a magne­tic field. Yet, because of its proxi­mi­ty to the Sun and its speed, Mer­cu­ry is also the least stu­died of all the planets.

After the two Ame­ri­can NASA probes, MARINER 10 (1973–1975) and MESSENGER (2004–2015), the Bepi­Co­lom­bo mis­sion is the third mis­sion to explore the sur­face and envi­ron­ment of Mer­cu­ry. Bepi­Co­lom­bo will shed new light on the struc­ture and inter­nal dyna­mics of the pla­net, how its magne­tic field is gene­ra­ted and how it inter­acts with the Sun and the solar wind. Through com­pa­ra­tive stu­dies, the mis­sion will also improve our unders­tan­ding of our pla­net, for example, the cou­pling bet­ween the ter­res­trial envi­ron­ment and the inter­pla­ne­ta­ry medium.

Bepi­Co­lom­bo also aims to probe the cha­rac­te­ris­tics and che­mi­cal com­po­si­tion of Mer­cu­ry’s sur­face, as well as the pre­sence of water ice in the polar cra­ters, which are per­pe­tual­ly in sha­dow. Indeed, due to the extre­me­ly low tilt of the pla­net’s rota­tion axis, the meteo­rite cra­ter floors at the poles receive no direct sun­light. Ulti­ma­te­ly, Bepi­Co­lom­bo’s obser­va­tions will help us bet­ter unders­tand how our solar sys­tem for­med and how pla­nets near their parent stars evolve.

Bepi­Co­lom­bo is the first Euro­pean mis­sion to Mer­cu­ry. It was deve­lo­ped by the Euro­pean Space Agen­cy (ESA) toge­ther with the Japan Aeros­pace Explo­ra­tion Agen­cy (JAXA). It is also the first pla­ne­ta­ry mis­sion with two orbi­ters (not inclu­ding Earth-orbi­ting satel­lites): the Mer­cu­ry Pla­ne­ta­ry Orbi­ter (MPO), under the res­pon­si­bi­li­ty of ESA, is a three-axis sta­bi­li­sed satel­lite that will orbit near Mer­cu­ry and stu­dy the sur­face, geo­lo­gi­cal com­po­si­tion, and exos­phere (thin atmos­phere) of the pla­net. The Mer­cu­ry Magne­tos­phe­ric Orbi­ter (MMO), rena­med ‘Mio’ under the res­pon­si­bi­li­ty of JAXA, is rota­ting and will orbit at a grea­ter dis­tance in Mer­cu­ry’s magne­tos­phere – the region of space around the pla­net that is domi­na­ted by its magne­tic field. 

Mio will make in situ mea­su­re­ments of the magne­tic field, elec­tric field, and par­ticles (ions and elec­trons) in the her­me­tic envi­ron­ment, but also in the inner helios­phere. The dif­ferent posi­tions of the two orbi­ters will allow for the first time to make obser­va­tions from two dis­tinct angles and to fol­low both spa­tial­ly and tem­po­ral­ly the cou­pling bet­ween the solar wind and Mer­cu­ry’s magne­tos­phere, the exchanges bet­ween the magne­tos­phere and its exos­phere, and the trans­port processes. 

When it arrives near Mer­cu­ry, Bepi­Co­lom­bo will be sub­jec­ted to such an intense radia­tive envi­ron­ment that the satel­lite will expe­rience tem­pe­ra­tures of over 350°C.

Bepi­Co­lom­bo car­ries two other modules : the Mer­cu­ry Trans­fer Module (MTM), which uses the solar-elec­tric pro­pul­sion tech­no­lo­gy nee­ded for Earth-Mer­cu­ry tra­vel, and the Mer­cu­ry Magne­tos­phe­ric Orbi­ter’s Sun­shield and Inter­face Struc­ture (MOSIF), which is ins­tal­led on top of the probe to pro­tect Mio from heat flux and infra­red radia­tion during the cruise phase. When it arrives near Mer­cu­ry, Bepi­Co­lom­bo will be sub­jec­ted to such an intense radia­tive envi­ron­ment that the satel­lite will expe­rience tem­pe­ra­tures of over 350°C – high enough to melt any of the pro­be’s com­po­nents or ins­tru­ments. To pro­tect against these tem­pe­ra­tures, a ther­mal control sys­tem has been spe­cial­ly desi­gned for the mis­sion to ensure that the mate­rials can withs­tand the very intense ultra­vio­let radia­tion and the flow of char­ged par­ticles from the solar wind without degradation.

The Bepi­Co­lom­bo mis­sion was laun­ched in Octo­ber 2018 from Kou­rou in French Guia­na and will be inser­ted into orbit around Mer­cu­ry in Decem­ber 2025. “This inser­tion is extre­me­ly dif­fi­cult because the pla­net is close to the Sun and the spa­ce­craft risks being ‘sucked in’ by its gra­vi­ta­tio­nal pull. The chal­lenge is not to go there, but rather to aim at Mer­cu­ry,” explains Lina Hadid, CNRS research fel­low at the Plas­ma Phy­sics Labo­ra­to­ry (LPP¹). In fact, the spa­ce­craft must be slo­wed down consi­de­ra­bly in the inner helios­phere to prevent it from being attrac­ted by the Sun. This is a real space mecha­nics challenge ! 

Des­pite its inno­va­tive and effi­cient ion-elec­tric pro­pul­sion, it is almost impos­sible for a mul­ti-ton mis­sion to reach orbit around Mer­cu­ry by bra­king alone. “To over­come this pro­blem, Bepi­Co­lom­bo per­forms seve­ral fly­bys of other pla­nets to modi­fy its tra­jec­to­ry : this is the prin­ciple of gra­vi­ta­tio­nal assis­tance and is the rea­son why Bepi­Co­lom­bo’s cruise phase is very long,” adds Lina Hadid. “During this cruise phase, the probe bene­fits from nine ‘boosts’ pro­vi­ded by three pla­nets : Earth (1x), Venus (2x) and Mer­cu­ry (6x). Each fly­by allows Bepi­Co­lom­bo to tigh­ten its tra­jec­to­ry, which will even­tual­ly merge with that of Mer­cu­ry in Decem­ber 2025.”

Bepi­Co­lom­bo flew past Mer­cu­ry for the first time in Octo­ber 2021 and for the second time in June 2022, pas­sing within 200 km of its sur­face (an alti­tude never rea­ched by either MARINER 10 or MESSENGER). In doing so, its came­ras pho­to­gra­phed the cra­te­red sur­face of the pla­net. Since its depar­ture, the probe has also flown past the Earth once in April 2020, and Venus twice, in Octo­ber 2020 and August 2021.

“During Bepi­Co­lom­bo’s long cruise, not all the ins­tru­ments are swit­ched on, so we can’t make as many mea­su­re­ments as we’d like. So, we can’t make as many mea­su­re­ments as we would like,” says Lina Hadid. “Howe­ver, among those that are ope­ra­tio­nal during the fly­bys is an ion mass spec­tro­me­ter on board Mio cal­led Mass Spec­trum Ana­ly­ser (MSA), which we have deve­lo­ped at LPP and in which I am invol­ved.” This spec­tro­me­ter will mea­sure the ion com­po­si­tion (char­ged par­ticles) around Mer­cu­ry. Although the FIPS ins­tru­ment on board MESSENGER has done this before, it has not been able to iden­ti­fy hea­vy ions (typi­cal­ly, oxy­gen and beyond) with high mass accu­ra­cy. In addi­tion, the field of view of this ins­tru­ment was very limited. 

“The MSA spec­tro­me­ter will allow us to iden­ti­fy dif­ferent ionic spe­cies such as magne­sium (Mg+, ato­mic mass M = 24 u), sili­con (Si+, 28 u), mole­cu­lar oxy­gen (O2+, 32 u), potas­sium (K+, 39 u) or cal­cium (Ca+, 40 u) with a mass reso­lu­tion unmat­ched on a space mis­sion. Ano­ther ins­tru­ment in which the LPP par­ti­ci­pa­ted on board Mio is the Dual Band Magne­tic Flux­me­ter (DBSC) dedi­ca­ted to the mea­su­re­ment of high fre­quen­cy magne­tic fields (100 mHz-640 kHz).”

The first fly­bys of Venus and Mer­cu­ry allo­wed us to cor­rect some pro­blems with the onboard software.

The cruise phase is also an impor­tant time to check that all the ins­tru­ments on board both orbi­ters are wor­king pro­per­ly. “It is very impor­tant for us to pro­per­ly cali­brate the ins­tru­ments in space to make sure they work as expec­ted ! For example, for MSA, the first Venus and Mer­cu­ry fly­bys allo­wed us to cor­rect some pro­blems with the onboard soft­ware, so we were loo­king for­ward to seeing the mea­su­re­ments during the second Mer­cu­ry fly­by in June 2022 !  And indeed, during this second fly­by, MSA revea­led the pre­sence of ener­ge­tic pla­ne­ta­ry pro­tons and helium (He+). We also obser­ved hea­vy ions, but at a lower den­si­ty than pre­vious­ly detec­ted by MESSENGER. We are cur­rent­ly ana­ly­sing these data to bet­ter unders­tand the source of these ions. At the same time, we are loo­king for­ward to the next Mer­cu­ry fly­by in June 2023!”

Final­ly, Bepi­Co­lom­bo may even be able to confirm – or dis­prove – the pre­sence of icy water on Mer­cu­ry, a sub­ject that has been inten­se­ly deba­ted for many years. In the 1990s, resear­chers dis­co­ve­red, thanks to the Are­ci­bo radio teles­cope, that there are regions in the north of the pla­net, at high lati­tudes, that exhi­bit abnor­mal­ly high light reflec­ti­vi­ty. Using its onboard came­ras, the MESSENGER mis­sion obser­ved that these areas coin­cide with the pre­sence of impact cra­ters on the sur­face of Mer­cu­ry. Since the pla­net’s rota­tion axis is prac­ti­cal­ly unin­cli­ned (unlike Ear­th’s), these cra­ters are per­pe­tual­ly in shadow. 

“The high reflec­ti­vi­ty could the­re­fore be due to the pre­sence of icy water at the bot­tom of these cra­ters – a sur­pri­sing conclu­sion given that Mer­cu­ry is so close to the Sun and so hot,” explains Lina Hadid. “If this result is confir­med, the Sun’s rays would never have rea­ched this water ice, which for­med bil­lions of years ago and would the­re­fore never have melted!”

Interview by Isabelle Dumé

Key dates of the mission

• 20 Octo­ber 2018 (01:45:28 UT): Launch from the Guia­na Space Centre
• 13 April 2020 : Earth flyby
• 16 Oct 2020 : Venus flyby
• 11 Aug 2020 : Venus flyby 
• Oct. 1, 2021 : First fly­by of Mercury
• June 23, 2022 : Mer­cu­ry flyby
• June 20, 2023 : Mer­cu­ry flyby
• Sep­tem­ber 5, 2024 : Mer­cu­ry flyby
• Decem­ber 2, 2024 : Mer­cu­ry flyby
• Jan 9, 2025 : Mer­cu­ry flyby
• Dec. 5, 2025 : Inser­tion into Mer­cu­ry orbit
• May 1, 2027 : End of nomi­nal mis­sion phase
• May 1, 2028 : End of mis­sion extension

Refe­rences

https://​www​.esa​.int/​S​p​a​c​e​_​i​n​_​M​e​m​b​e​r​_​S​t​a​t​e​s​/​F​r​a​n​c​e​/​L​a​n​c​e​m​e​n​t​_​d​e​_​B​e​p​i​C​o​l​o​m​b​o​_​v​e​r​s​_​M​e​rcure