Departing from Cairo on October 19, 2018, the European-Japanese probe Bepiclombo will arrive on Mercury in December 2025 for an in-depth investigation that will last at least a year. Until then, it will have to fly several times over its destination before placing its two satellites in orbit. Mercury orbiter And Mercury magnetosphere orbiter. They will be able to study geology, magnetic fields and the environment of the planets closest to the Sun.
The next flyby will take place on June 23, 2022 – an opportunity to test the accuracy of the instruments and get the first data. These will necessarily be limited, because, during overflight, Bepiclombo’s configuration is not ideal: the satellites are assembled with propulsion modules and hidden behind a thermal shield between them, which significantly limits the instrument’s visual fields.
Nevertheless, the first flyby of Mercury in October 2021, for example, revealed the periodic “puffs” of ions, as well as the first sign of electrons in the planet’s near atmosphere – two sources of spontaneous and rapid reconstitution of Mercury’s magnetic field, which is still quite mysterious.
In fact, although Mercury’s magnetic field is much weaker than Earth’s, its existence is astonishing: the planet is the size of the moon and, like the latter, it must be cool and no longer have a molten iron core. Leading the production of a magnetic field.
Mercury’s new flyby by BepiColombo in June 2022 is an opportunity for us to look back at various missions that have made it possible to better understand the planet over time.
Mercury’s magnetic field was discovered in the 1970s
The Bepiclombo mission is actually the third mission to Mercury. It all started in the 1970s: in 1974 and 1975, NASA’s Mariner 10 probe performed three flybys on the planet. To do this, Italian professor Giuseppe “Bepi” Colombo proposed changing the trajectory of the Mariner 10 probe using Venus’s gravitational field as it passed through the planet. As the word implies, “flying over” means crossing the probe. There is only one orbit around the planet and so there is no systematic observation in orbit around it.
During every ten minutes during Mercury’s three flybikes (and especially the two that are closest to the planet, hundreds of kilometers away), Mariner 10 made brief but decisive observations that accelerated research on Mercury over many years. One of Mariner 10’s discoveries was that the planet has a magnetic field similar to Earth’s.
Today’s observations suggest that Mercury’s convective center occupies a significant part of the planet’s interior. The existence of such a magnetic field is extremely important. In its absence, the solar wind (ionized gas emitted by the sun) can have a direct effect on the atmosphere and slowly erode it, thus making it gradually disappear in the case of Mars. Earth must have a magnetic field that deflects wind from the sun, protects its atmosphere, and makes life possible.
Mariner 10 also mapped the surface of Mercury and highlighted the existence of a thin atmosphere (or exosphere) covering the planet with elements such as hydrogen or oxygen.
In the 2000s, focus on the “magnetic field”
The second probe to Mercury was launched in 2004 as part of NASA’s Discovery Program. After a seven-year cruise and three flybikes to Mercury, the probe has done much more than Mariner 10 to observe the planet since it was in Mercury’s orbit from March 2011 to April 2015, when it “crashed” on its surface. During these four years of systematic observations, Messenger was able to map the entire surface of the planet, revealing the existence of defects, signs of volcanoes or even the existence of ice beneath polar holes that have never been exposed to solar radiation.
Messenger also confirmed Mariner 10’s measurement of the planet’s magnetic field, but these new observations indicate that the field has moved north, a shift that is difficult to explain today. In addition to a magnetometer for measuring magnetic fields, the probe carries two instruments for measuring charged particles. Mercury contains an internal magnetic field, developed around a small bubble-shaped magnetic cavity (called the “magnetosphere”), forming space around different regions of the plasma – space is no longer “isotropic”, meaning it is no more. All aspects are the same!
This type of magnetism acts as a shield against the solar wind. It is relatively stable in the case of Earth, and even more so in the case of gas giants like Jupiter and Saturn which are far away from the Sun and have very strong internal magnetic fields. In the case of Mercury, due to both the weak internal magnetic field and the proximity to the Sun (thus a dense solar wind exerts a strong pressure), the planet’s magnetic field becomes more “fragile” and highly variable, even as it may erupt. Or “shrinks” so strongly that, uniquely, the solar wind directly bombs or “screens” the planet’s surface. This may explain, for example, the bluish hollow observed in certain places on the surface.
In this way the elements of the planet emitted into space can then revolve around the planet and, if it is ionized by the sun’s ultraviolet radiation, can be filled by the magnetic field guided by the magnetic field. Within the magnetosphere, acceleration processes take place that move the object to a specific area of space and in particular can cause it to flow towards the planet’s surface. In the case of Earth or gas giants, this type of precipitation leads to the occurrence of aurora borealis and australis, which are produced by the interaction of magnetic electrons with the upper atmosphere. Due to the absence of atmosphere in Mercury, this type of aurora may not occur, but the interaction of magnetic matter with the surface may create another type of aurora on X-rays.
Two probes for better observation
Although the Bepiclombo mission’s European MPO (Mercury Planetary Orbiter) satellite is primarily dedicated to planetary observations, the Japanese MIO satellite (primarily, MMO or Mercury Magnetospheric Orbiter) aims to study the planet’s environment. And especially the magnetosphere.
A particularly interesting aspect of this mission is that it includes two probes (never before have two observation satellites of the same celestial object carried on a planetary mission), which makes it possible to observe from two angles. In the case of a single satellite such as Messenger, we are either inside or outside the magnetosphere, which makes assumptions about the region where the satellite is not located. Conversely, in the case of Bepiclombo, one satellite will be able to act as a “sentinel” upstream of the planet’s solar wind, while the other will observe the planet’s response and its environmental disturbances. Solar wind.
The existence of the Hermione magnetic field (“Hermione” means “Mercury” since, for the Greeks, Mercury is Hermes) thus raises many new questions from the process of formation of this field to its role in the environment. The planet is evolving near its star. Several missions have explored the environments of other Telluric planets (Venus, Mars) or gas giants (Saturn, Jupiter), but relatively little information is available for Mercury. BepiColombo will fill this void and provide a completely new arrangement for very specific conditions. Researchers will not only study Mercury but also try to compare scientific results with other astronomical conditions.