The exploration ofMercury has long had a minor role in the space interests of the world. It is the least exploredinner planet.[1] As of 2025, theMariner 10,MESSENGER, andBepiColombo missions have been the only spacecraft that have made close observations ofMercury.MESSENGER made threeflybys before entering orbit around Mercury.[2]BepiColombo, a joint mission between theJapan Aerospace Exploration Agency (JAXA) and theEuropean Space Agency, is to deliver twoprobes into Mercury orbit in 2026, having previously made six flybys of the tiny planet.MESSENGER andBepiColombo are intended to gather complementary data to help scientists understand many of the mysteries discovered byMariner 10's flybys.
Compared to other planets, Mercury is difficult to explore. The speed required to reach it is relatively high, and its proximity to theSun makes it difficult to maneuver a spacecraft into a stable orbit around it.[3] As a result, all spacecraft sent to visit Mercury as of 2025 have required a complex trajectory of gravity assists, highly efficiention propulsion, or both. The thermal environment around Mercury is also challenging, with probes utilizing sunshields, specially designed materials, or radiators to survive the heat so close to the Sun.
Few missions have targeted Mercury because it is very difficult to obtain asatellite orbit around the planet. Mercuryorbits the Sun very quickly (between 24.25 miles per second (39.03 km/s) and 30 miles per second (48 km/s)), so spacecraft must be travelling very fast to reach it. Mercury's close proximity to the Sun means that spacecraft are accelerated even further by the Sun'sgravitational pull, requiring significant fuel expenditure in order to decelerate fororbit insertion.[4] Mercury's lack of any significantatmosphere[5] poses further challenges because it precludesaerobraking. Thus a landing mission would have even more demanding fuel requirements.[6]
Mariner 10 was aNASAprobe whose primary objective was to observe the atmosphere, surface, and physical characteristics of Mercury andVenus. It was a low-cost mission completed for under$98 million.[7]Mariner 10 was launched at 12:45 amEST on November 3, 1973, fromCape Canaveral.[8] Since Mercury is so close to the Sun it was too difficult to incorporate an orbit around Mercury in the route soMariner 10 orbited the Sun. In order to reach its destination, the satellite wasaccelerated with thegravity field of Venus. It then passed close to Mercury on March 29, 1974, as it flew towards the Sun. This was the first observation made of Mercury at close range. After the encounterMariner 10 was in an orbit around the Sun such that for every one of its orbits Mercury made two, and the spacecraft and the planet would be able to meet again. This allowed the probe to pass by Mercury two additional times before completing the mission; these encounters were made on September 21, 1974, and March 16, 1975. However, since the same side of Mercury was illuminated during each of the flybys, at the conclusion of the missionMariner 10 had only photographed approximately 45 percent of its surface.[9] The mission ended when the probe'sattitude control gas ran out on March 24, 1975. As the spacecraft was no longer controllable without its nitrogen gas thrusters, a command was sent to the probe to shut down its transmitter.[10]
The close observations collected two important sets of data. The probe detected Mercury'smagnetic field, which is very similar to Earth's. This was a surprise to scientists, because Mercury spins so slowly on its axis. Secondly, visual data was provided, which showed the high number ofcraters on the surface of the planet.[11] The visual data also allowed scientists to determine that Mercury had "not experienced significantcrustal modification”.[12] This also added to the mystery of the Mercurian magnetic field, as it was previously believed that the planet's magnetic fields are caused by a dynamo effect of a molten core, but the lack of crustal change brought this idea into question. The visual data also allowed scientists to investigate the composition and age of the planet.[13]
MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) was aNASA orbital probe of Mercury. It was launched fromCape Canaveral on August 3, 2004, after a one-day delay due to bad weather.[14][15] It took the probe about six and a half years before it entered orbit around Mercury. In order to correct the speed of the satellite it undertook several gravitational slingshot flybys of Earth, Venus and Mercury. It passed by the Earth in February 2005 and then Venus in October 2006[2] and in October 2007. Furthermore, the probe made three passes of Mercury, one in January 2008, one in October 2008 and one in September 2009, before entering orbit in 2011. During these flybys of Mercury, enough data was collected to produce images of over 95% of its surface.
MESSENGER used a chemicalbi-propellant system both to reach Mercury and achieve orbit.[16]MESSENGER's scheduled orbital insertion took place successfully on March 18, 2011. In November 2012, NASA reported thatMESSENGER had discovered a possibility of bothwater ice and organic compounds in permanently shadowed craters in Mercury's north pole.[17] The mission was scheduled to end sometime in 2012, when it was estimated that there would no longer be enough fuel to maintain the probe's orbit.[18] The primary mission was completed on March 17, 2012, having collected close to 100,000 images.[19]MESSENGER achieved 100% mapping of Mercury on March 6, 2013, and completed its first year-long extended mission on March 17, 2013.[20] In February 2013, NASA published the most detailed and accurate 3D map of Mercury to date, assembled from thousands of images taken byMESSENGER.[21] The probe continued collecting scientific data until April 30, 2015, when under a decaying orbit, the probe was allowed to crash onto the surface of Mercury.[22]
TheMESSENGER mission was designed to study the characteristics and environment of Mercury from orbit. Specifically, the scientific objectives of the mission were:[23][24]
BepiColombo consists of two spacecraft: the Mercury Planetary Orbiter (MPO) and Mio (Mercury Magnetospheric Orbiter, MMO). Each orbiter has a distinct purpose: the MPO is to acquire images in several wavelengths to map the surface and exosphere composition of Mercury, and Mio's is to study themagnetosphere. It is a joint mission of the European Space Agency and Japan Aerospace Exploration Agency; ESA provided the MPO, while JAXA provided Mio.[3] TheBepiColombo mission will attempt to gather enough information to answer these questions:
LikeMariner 10 andMESSENGER,BepiColombo usedgravity slingshots from Venus and Earth.BepiColombo usessolar electric propulsion (ion engines) and similar maneuvers at the Moon, Venus, and Mercury. These techniques slowed the orbiters as they approach Mercury. It is essential to avoid using fuel to slow the orbiters as they get closer to the Sun to minimize the gravitational influence of the Sun.[2]
TheBepiColombo mission was approved in November 2009,[25] and successfully launched on October 20, 2018. It was scheduled to enter orbit around Mercury in December 2025, but a loss of performance of its ion thrusters have delayed this to November 2026.[26] Its primary mission will last until April 2028, with a possible extension to April 2029.[27]
Mercury-P (Меркурий-П/Mercury-Landing) is a proposed lander mission to Mercury by the Russian Space Agency. The probe would utilize solar-electric propulsion to reach Mercury orbit, before deploying a semi-hard lander based on those developed for theMars-96 project.[28]
In August 2020, theApplied Physics Laboratory proposed the Mercury Lander for NASA'sNew Frontiers program. If selected, it would launch in March 2035 and land in April 2045.[29]
Mercury Observer was a cancelled proposal in thePlanetary Observer program, which was terminated after the loss of its first mission,Mars Observer.
BepiColombo was designed to complement the findings ofMESSENGER and is equipped with far more measuring equipment thanMESSENGER to obtain a larger range of data. The orbital patterns ofBepiColombo andMESSENGER are significantly different.[16]
The BepiColombo mission comprises two satellites launched together: the Mercury Planetary Orbiter (MPO) and Mio (Mercury Magnetospheric Orbiter, MMO). The MPO will have a circular orbit much closer to Mercury. The reason for this orbit is that the MPO will be measuring the composition of the surface and exosphere, and the close orbit will aid on data quality. On the other hand, Mio (MMO) andMESSENGER took largelyelliptical orbits. This is because of the stability of this orbit and the lower amount of fuel required to obtain and maintain this orbit.[30] Another reason for the different orbits of Mio andMESSENGER was so that the data collected by Mio would complement that of MESSENGER, providing more accurate measurements.[16]
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Solar System →Local Interstellar Cloud →Local Bubble →Gould Belt →Orion Arm →Milky Way →Milky Way subgroup →Local Group→Local Sheet→Local Volume→Virgo Supercluster→Laniakea Supercluster →Pisces–Cetus Supercluster Complex →Local Hole →Observable universe →Universe | |||||||||||||||||||||
