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BepiColombo mission: next stop, Mercury!

Lina Hadid
Lina Hadid
CNRS Research Fellow at the Plasma Physics Laboratory (LPP)
Key takeaways
  • The BepiColombo mission (2018-2028) is the third mission to explore the surface and environment of Mercury.
  • BepiColombo aims to learn more about Mercury and its interactions with the Sun, to which it is so close.
  • Because of the Sun’s attraction, this mission is a real space mechanics challenge: it therefore uses the gravitational assist technique.
  • The Mass Spectrum Analyser (MSA) on board the spacecraft will measure Mercury's ionic composition.
  • Studying Mercury will, among other things, confirm or deny the potential presence of water ice in its polar craters.

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 is named after the Ital­ian math­e­mati­cian and engi­neer Giuseppe (Bepi) Colom­bo (1920–1984). He played a major role in the suc­cess of the MARINER 10 mis­sion, the first mis­sion to Mer­cury, with his orbital mechan­ics cal­cu­la­tions for the deter­mi­na­tion of the first grav­i­ta­tion­al assist by a spacecraft. 

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, the first-ever mission

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.

A real space mechanics challenge

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! 

Bepi­Colom­bo approach­ing Mer­cury (cred­it: ESA).

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.

In search of the ionic composition

“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



1*LPP: a joint research unit of CNRS, École Poly­tech­nique – Insti­tut Poly­tech­nique de Paris, Obser­va­toire de Paris, Sor­bonne Uni­ver­si­ty, Uni­ver­sité Paris-Saclay

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