For astronomy enthusiasts, here is an article by Emile Bidoux from the Martinique Astronomy Discovery Club, whose news we regularly report on.
On October 19, 2018, the Euro-Japanese space mission BepiColombo[1] took off from Kourou, heading for Mercury, which it will reach in December 2025, after a journey lasting more than 7 years.
It is reasonable to ask the following questions:
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What do scientists expect from the results of the observations to be conducted on the smallest planet?[2] of the solar system? How can they contribute to a better understanding of the world around us?
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Why does it take 7 years to reach Mercury, which is between 46 million and 69 million kilometers from Earth, while a trip to Mars[3], which comes closest to us at 60 million km, takes 6 or 9 months.
Mission Objectives.
Mercury has been studied very little from Earth, unlike most other planets, because it is visible for less than 2 hours before sunrise and after sunset. It is most often obscured by the glow of dawn or dusk. The first map of its surface was created by Antoniadi [4] in 1934.
Because it is difficult to approach—particularly due to its surface temperatures ranging from -180° C to 430° C—few spacecraft have ventured into Mercury's vicinity[5].
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The Mariner 10 probe (NASA), launched in 1973, took the first images of Mercury during its three flybys in 1974 and 1975, but most importantly, it detected an intense magnetic field, similar to Earth’s and unlike Venus and Mars, which have none.
[1] Name of the Italian scientist Giuseppe (Bepi) Colombo (1920–1984). Father of ‘gravitational assist.’ A fundamental technique for space missions to explore objects in the solar system. See below.
[2] Mercury's diameter is 4,868 km, which is 38% of Earth's diameter.
[3] Mars orbits the Sun at a distance of between 200 and 250 million km, ranging from a maximum distance from Earth (apogee) of 400 million km to a minimum distance of less than 100 million km (perigee). The distance at perigee varies because it depends on the orbit’s eccentricity, which is 0.09—5.6 times greater than that of Earth.
- For the record, it was this high value of Mars’ eccentricity that enabled Kepler (1571–1630) to formulate the three laws governing the motions of the planets, based on the highly precise observations of the Danish astronomer Tycho Brahe (1546–1601)
- Reminder: The higher the eccentricity of a planet’s orbit (ellipse), the more flattened it is. A circle has an eccentricity of 0, and a line segment has an eccentricity of 1.
[4] Eugène Antoniadi (1870–1944). Astronomer at the Paris-Meudon Observatory
[5] An upcoming chapter will explore the challenges of a spaceflight to Mercury.
She was followed on Messenger [1] (NASA), launched in 2004 and in orbit around the planet from 2011 to 2015.
The observations made by Mariner 10—particularly the discovery of Mercury’s intense magnetic field—led scientists, more than 30 years ago, to consider another mission to study Mercury.
The BepiColombo mission, which launched from Kourou on October 19, 2018, will arrive near Mercury on December 25, 2025. Two scientific probes will then be placed into orbit around the planet and will remain there until May 2027, or possibly May 2028.
The first probe is called MPO (Mercury Planetary Orbiter) and the second is called MMO
MPO [2], will create a complete map of Mercury and study its composition and internal structure. It is equipped with 11 scientific instruments [3].
Mio [4], will analyze its magnetic field and magnetosphere[5] .The data collected will provide a better understanding of the formation and evolution of the «inner» planets» [6] such as Earth. The spacecraft is equipped with 5 instruments.
But BepiColombo will also be tasked with verifying whether Mercury's orbit confirms Einstein's calculations of general relativity [7].
[1] Messenger (MErcury Surface, Space ENvironment, GEochemistry and Ranging, or in French, «Surface, Space Environment, Geochemistry, and Ranging of Mercury»)
[2] The MPO probe is being developed by the ESA (European Space Agency),
[3] See the end of this document for the biography of Alain Doressoundiram, the designer of one of these instruments.
[4] Mio is under the jurisdiction of JAXA (Japan Aerospace Exploration Agency).
[5] The magnetosphere is the region surrounding a celestial body whose physical characteristics are governed by the magnetic field emitted by that body. Source: CNES
[6] A planet is called an inner planet when it is close to its star
[7] Mercury’s orbit is highly eccentric (see 3); furthermore, its perihelion (the shortest distance from the Sun), which is about 46 million km, orbits the Sun very slowly. Nineteenth-century astronomersth Scientists of that century could find no explanation for this phenomenon. They even considered the possibility of a planet—Vulcan—orbiting close to Mercury. It was Einstein (1879–1958) who provided the solution, set forth in the «general theory of relativity» in 1915.
Seven years of travel.
Why does it take so long to get to Mercury, when, as already mentioned, it only takes between 6 and 9 months to get to Mars?
Actually, these two cases are different.
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To reach Mars, we will follow a trajectory that can be described as «direct.» This involves the spacecraft following an elliptical curve (a half-ellipse) that is tangent to both Earth and Mars. See figure. The merits of this transfer trajectory were demonstrated in 1925 by the German engineer Walter Hohmann (1880–1945).
The constraint of this orbit is the need to stay within the «launch window.» In fact, to follow this trajectory, the rocket must be launched on a specific date—with a tolerance of just a few days—and this window opens only occasionally.
Thus, in the case of Mars, the departure must take place about 3 months before an «opposition.» [1]. This configuration occurs only once every 26 months.
Philippe LABROT (www.nigral.net)
If we assume, as a first approximation, that the orbits of Mars and Earth are circular, the transfer orbit is the arc of an ellipse that is tangent to Earth’s orbit at the departure point AT and tangent to Mars’s orbit at the arrival point BM. (The navy blue curve in the diagram above)
The last opposition of Mars occurred on July 27, 2018 (Earth-Mars distance: 57.7 million km), and the next one will occur on October 13, 2020 (Earth-Mars distance: 62.7 million km).
Note: The European Mars 2018 mission, originally scheduled for 2018, was postponed due to a technical failure. As of the date of this article (September 2019), there are still doubts about whether it will launch in 2020.
[1] An outer planet is in opposition when it is behind the Earth relative to the Sun.
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We could also use Hohmann's direct transfer orbit to travel to Mercury. The trip would take about 106 days, and the launch window occurs every 116 days.
The main difference from Mars is that Mercury lies between Earth and the Sun, and so the probe must not be accelerated but, on the contrary, must be brake at the start and at the finish, to counteract the Sun's increasingly strong gravitational pull.
However, this braking maneuver—which is difficult to perform—also consumes a great deal of fuel, requiring large fuel tanks. It takes 1.5 times as much fuel as it does to leave Earth, and that fuel must be carried from Earth. This leaves very little room for scientific instruments. The cost of a A direct flight is therefore prohibitively expensive.
We prefer to use the more economical—though time-consuming—method of «Gravitational assistance" »[1] or «sling effect.».
This involves using the gravitational pull of a celestial body (planet, Moon) to alter the direction and speed of a spacecraft’s trajectory (space probe, artificial satellite, etc.). The goal is to conserve the fuel that would otherwise have been consumed by the vehicle’s rocket engine to achieve the same result. All space probes headed for celestial bodies far from Earth use this method.
Using this method, it is possible to both accelerate and decelerate a spacecraft, such as during a journey to Mercury or Venus.
BepiColombo will use 9 gravity assists:
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Earth, April 13, 2020
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Venus on October 16, 2020, and August 11, 2021
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Mercury 6 times: the first on October 2, 2020, and the last on January 9, 2025
[1] Bepi Colombo is the pioneer of this method, which is widely used in space exploration. It was first tested by Marner 10, which flew by Venus and Mercury in 1973 and 1975.
The next article will focus on Mercury's transit across the Sun on November 11, 2019
Afterword
A reminder to readers of the February 1, 2019, conference, held in the deliberation room of the Territorial Collectivity of Martinique, to’Alain Doressoundiram, (See his biography), during which he brilliantly shared previously unknown details about this exceptional mission with us.
I am reproducing the text of the announcement for this conference here: «BepiColombo is proving to be the most sophisticated exploration mission ever undertaken, with no fewer than three spacecraft to manage simultaneously. Upon its arrival in 2025, it is expected to answer long-standing questions such as the origin of the magnetic field, the interaction of the magnetosphere with the solar wind, and the planet’s volcanic history…»
Biography: Alain DORESSOUNDIRAM is an astrophysicist at the Laboratory for Space Studies and Astrophysical Instrumentation at the Paris Observatory.
A specialist in small bodies in the solar system and infrared observation, he is in charge of an instrument on the BepiColombo space mission to Mercury. He teaches astrophysics in the observatory’s Education and Training Unit, participates in the digital campus initiative, and is deeply involved in efforts to promote scientific literacy, particularly among young people. He is the author of several scientific books.
Every year, he regularly organizes «Astronomy Week» in Martinique. He shares his passion and knowledge through lectures and workshops for middle school students, college students, teachers, and the general public.
He was one of the discoverers of Saturn's moons, Siarnaq and Tarvos.
The asteroid (7456) was named «Doressoundiram" » to honor his work as a researcher.
BIDOUX, September 2019
Bibliography:
«L’Astronomie» magazines from September 2019 and December 2018
Various websites, including those of the ESA and CNES
