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