RBradbury-SPST435-Week 1-Mercury
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Mercury: Current State of Knowledge and Mission Timeline
Reiley Bradbury
American Military University
SPST435: Planetary and Space Exploration
Dr. Kunal Kothari
February 9, 2024
Introduction
Of all the planets in our Solar System, Mercury is of one the least explored and is rather difficult to do so. This is mainly due to two reasons. One is that it is the closest planet to our Sun,
making it a challenge to observe in front of the sun’s brightness. The other reason being that the sun has such a large gravitational pull that probes, and other spacecraft have difficulties entering orbit without having to constantly hit the brakes and defy gravity (Three Reasons Why We Know
so Little about Mercury, n.d.). While exploring Mercury may be challenging, it has not stopped scientists from wanting to learn more about it.
Mercury’s Discovery
Galileo Galilei was the first known person to discovery Mercury through a telescope in the 17
th
century, but the belief among many scientists is that the earliest recorded observations were done by an Assyrian astronomer around the 14
th
century on the Mul.Apin tablets. A telescope is not necessary to see it because it is visible to the naked eye, so there really is no way to know for sure who actually observed it for the very first time. Later in 1639, Mercury’s phases, similar to those of the Moon and Venus, were discovered by Giovanni Zupi. The planet gained its name from a Roman messenger god (a god of speed) due to its rapid orbit (the nine planets, 2019).
What We Already Know
Despite the challenges of exploration, successful past missions have given us a lot of insight into the characteristics of Mercury. We already know that it is the closest planet to the sun
at 57.91 or 0.4 AU, and it takes 3.2 minutes for sunlight to reach. It has a density of 5.43 g/cm
3
, a
radius of 2.439 km and a diameter of 4.879 km, making it the densest and smallest planet, only about 1/3 the size of Earth. What is interesting is that though Mercury sits so close to the sun, it is not the hottest planet due to the lack of an atmosphere to protect it from extreme cold at night. Instead, Mercury has an exosphere composed of oxygen, sodium, hydrogen, helium and potassium. The exosphere is created by solar winds spitting atoms that collide with meteoroids. Mercury reaches temperatures of 430 degrees Celsius during the day, and dips to -180 degrees Celsius at night. Claiming the title of “fastest planet”, it takes Mercury just 88 Earth days to orbit
the sun at a pace of 47 km/s. While it moves rapidly, it spins slowly on its axis, tilted at about 1/30 degrees, and in 59 Earth days it completes one rotation. The planet’s aphelion is 0.44 AU from the sun and perihelion is 0.30 AU, giving it the largest orbital eccentricity (the nine planets, 2019). Earth and Mercury are both structurally similar since they both possess a core, mantle and
crust. The iron-rich core is about 2,074 km thick, accounting for roughly 85% of Mercury’s radius (Mercury: Facts – NASA Science, n.d.). Additionally, Mercury’s magnetic field, about 1.1% the size of Earth’s interacts with solar winds, sometimes resulting in a magnetic tornado directing hot plasma to the surface. The planet has no moons or rings and is also visually very similar to the moon with its grey, cratered surface, indicating that it has not experienced any geological activity in years (the nine planets, 2019). Mariner 10, MESSENGER and BepiColombo
Space agencies like NASA and the ESA have been making efforts to explore our tiny neighboring planet since the 1970s. The Mariner series was the first mission to Mercury, followed by MESSENGER and now, most recently, BepiColombo. NASA’s Project Mercury,
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however, was not a mission to Mercury despite the name. Rather, it was the mission to send the first astronauts into space (What Was Project Mercury? (Grades 5-8) – NASA, n.d.). The first spacecraft to not only reach Mercury as the seventh successful launch of the Mariner series, but also use the advantage of Venus’ gravitational pull to get there and visit two planets in one trip was none other than NASA’s Mariner 10. The mission was focused on measuring the environment, atmosphere, surface and body characteristics of both Mercury and Venus. Gaining experience with dual planet gravity assist, as well as running experiments in the interplanetary medium were among the other goals the team aspired to reach. Mariner 10 was launched out of Cape Canaveral on November 3, 1973, and placed in a parking orbit for about 25
minutes after launch. Following a retrograde heliocentric orbit, the spacecraft completed three Mercury flybys, capturing images and collecting data. It was during this mission that scientists made some major discoveries about Mercury, such as the confirmation that it does not have an atmosphere, the surface is very Moon-like, and that it contains a small magnetic field and iron-
rich core. Before reaching Mercury, Mariner 10 completed a flyby of Venus to gather images and
data, thus successfully utilizing gravity assist to travel from one planet to another (Venus to Mercury) for the first time (NASA – NSSDCA – Spacecraft – Details, 2019).
Decades later in 2004, MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) had its turn to journey to Mercury. This was the seventh Discovery-
class mission (part of NASA’s Discovery Program dedicated to planetary science) to reach Mercury and became the first spacecraft to enter orbit as well. The MESSENGER mission objectives were slightly different than those of previous mission as it aimed to study the geology (with focus on the core), magnetic field and chemical composition of Mercury. Like Mariner 10, MESSENGER was placed in parking orbit after launch on August 3, 2004, and completed not
one, but two flybys of Venus and three of Mercury over a more than 6 ½ year journey (MESSENGER – NASA Science, n.d.). The spacecraft used gravity assist like its predecessor to avoid using too much propellant and lower velocity to safely enter Mercury orbit (John Uri, 2019). On March 18, 2011, the spacecraft finally entered an elliptical orbit at Mercury after being slowed during the flybys due to complications with the vehicle. MESSENGER achieved some more groundbreaking accomplishments at the planet, such as the discovering large concentrations of calcium and magnesium on the night side, revealing the magnetic field was offset from the center of the planet, finding water in the exosphere, as well recovering evidence of past volcanic activity and mapping the entire surface in high-resolution monochrome and color. MESSENGER didn’t just focus on Mercury, however. The spacecraft was also able to complete a flyby of Earth, capturing images of the Earth-moon system, collect data from Venus during the Venus flybys, and acquired a stunning image of the solar system featuring the rest of the planets, excluding Uranus and Neptune. Additionally, during two extended missions, evidence of ice was discovered at Mercury’s poles and the spacecraft even managed to photograph some comets. The MESSENGER mission finally came to an end on April 30, 2015, when the spacecraft ran out of propellant and crashed into the surface of Mercury (MESSENGER – NASA Science, n.d.). Finally, Europe (ESA) and Japan (JAXA) have come together to complete their first and the world’s third mission to Mercury. Named after Professor Giuseppe (Bepi) Colombo from the University of Padua, Italy, BepiColombo is set to reach Mercury orbit in 2025 after a series of Earth, Venus and Mercury flybys as a set of two spacecraft. The Mercury Planetary Orbiter (MPO) and Mercury Magnetospheric Orbiter (MIO) will work together to study Mercury’s surface, internal composition and magnetosphere (BepiColombo-NASA Science, n.d.). More
specifically, scientists want to learn if there is a molten interior and study the planet’s mineralogy
and elemental composition as well (BepiColombo Overview, n.d.). The journey is expected to take seven years and once the spacecraft reaches Mercury in 2025, it will be sent into polar orbit and lowered with thrusters (NASA – NSSDCA – Spacecraft – Details, n.d.). There are no known plans for future Mercury missions after BepiColombo comes to an end, but scientists are eager to
learn more secrets about Mercury through ESA and JAXA’s efforts. To Conclude
Mercury is a small planet laden in mystery and though it is so close to our home planet, it
remains one of the least observed and most difficult to do so. Mariner 10 and MESSENGER have paved the way for exploration and now with BepiColombo taking up the torch, scientists have been and will continue to make exciting discoveries about tiny, mysterious Mercury and furthering our knowledge of the Universe.
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References
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mercury-grades-5-8/
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