Exploration of Mercury
The exploration of Mercury has taken only a minor role in the space interests of the world. It is the least explored inner planet. As of 2011, the Mariner 10 and MESSENGER missions have been the only missions that have made close observations of Mercury. MESSENGER made a flyby of Mercury on 14 January 2008, to further investigate the observations made by Mariner 10 in 1975. A third mission to Mercury, BepiColombo, is to include two probes. BepiColombo is a joint mission between Japan Aerospace Exploration Agency and the European Space Agency. MESSENGER and BepiColombo are intended to gather complementary data to help scientists understand many of the mysteries discovered by Mariner 10’s flybys.
Compared to other planets, Mercury is difficult to explore. The increased speed required to reach it is relatively high, and due to the proximity to the Sun, orbits around it are rather unstable. MESSENGER is the first probe to orbit Mercury.
Interest in Mercury 
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Mercury has not been a primary focus of many space programs. Since the planet is so close to the Sun and spins on its own axis very slowly, its surface temperature varies from 427 °C (801 °F) to -173 °C (-279 °F). There is discussion of the possibility of terraforming Mercury and inhabiting the poles of the planet, but this possibility lies far in the future, being significantly less practical than terraforming Mars or even Venus. The current interest in Mercury is derived from the unexpected observations of Mariner 10. Before Mariner 10, it was thought that the planet simply revolved around the sun in a highly elliptical orbit. The planet had been observed through ground-based telescopes, and scientists felt that they were able to draw a great many inferences from such observations. Mariner 10 provided data that contradicted many of these.
Another reason why so few missions have targeted Mercury is that it is very difficult to obtain a satellite orbit around the planet on account of its proximity to the Sun, which causes the Sun’s gravitational field to pull on any satellite that would be set into Mercury's orbit. Furthermore, spacecraft naturally accelerate as they approach the greater gravitational pull of the Sun, but must slow down in order to orbit Mercury, so this entails considerable fuel requirements. This is different with planets beyond Earth’s orbit where the satellite works against the pull of the Sun. Therefore it requires a great amount of energy to reach and observe the planet. Mercury's lack of an atmosphere poses further challenges because a probe attempting to land on Mercury would not be able to aerobrake or use a parachute type device. Thus a landing mission would have rather demanding fuel requirements.
Mariner 10 
Mariner 10 was a probe whose primary objective was to observe the atmosphere, surface, and physical characteristics of Mercury and Venus. It was a low-cost mission completed for under $98 million. Mariner 10 was launched at 21:45 PST on November 2, 1973, from Cape Kennedy. Since Mercury is so close to the Sun it was too difficult to incorporate an orbit around Mercury in the route so Mariner 10 orbited the Sun. In order to reach its destination, the satellite was accelerated with the gravity 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 encounter Mariner 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 mission Mariner 10 had only photographed 45% of its surface. The mission ended as fuel of the probe ran out in March 24, 1975. As the spacecraft was no longer controllable without fuel, a command was sent to the probe to shut down its transmitter.
The close observations collected two important sets of data. The probe detected Mercury’s magnetic 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 of craters on the surface of the planet. The visual data also allowed scientists to determine that Mercury had “not experienced significant crustal modification”. This also added to the mystery of the magnetic field, as it was previously believed that the magnetic fields are caused by a molten dynamo effect, but since there was little crustal modification this undermined that idea. The visual data also allowed scientists to investigate the composition and age of the planet.
MESSENGER is a NASA probe in orbit of Mercury. MESSENGER stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. It was launched from Cape Canaveral on August 3, 2004, after a two-day delay due to bad weather. 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 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 uses a “chemical bi-propellant system both to reach Mercury and brake into orbit”. MESSENGER's scheduled orbital insertion took place successfully on March 18, 2011. The mission is scheduled to end sometime in 2012, when there will no longer be enough fuel to maintain the probe's orbit.
The information collected by MESSENGER will be used to try to answer six questions about Mercury:
- Why is the planet so dense?
- What is Mercury's geologic history?
- What is the structure of Mercury's core?
- What is the nature of Mercury's magnetic field?
- What are the unusual materials at Mercury's poles?
- What volatiles are important on Mercury?”
This mission to Mercury is to include two satellites: the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO). Each orbiter has a distinct purpose: the MPO is to take images of Mercury, and the MMO is to study the magnetosphere. The European Space Agency and Japan Aerospace Exploration Agency are working in conjunction on BepiColombo and will each provide one of the orbiters. The ESA will provide MPO, while JAXA will provide MMO. The BepiColombo was designed with 12 specific objectives:
- What can we learn from Mercury about the composition of the solar nebula and the formation of the planetary system?
- Why is Mercury's normalized density markedly higher than that of all other terrestrial planets, as well as the Moon?
- Is the core of Mercury liquid or solid?
- Is Mercury tectonically active today?
- Why does such a small planet possess an intrinsic magnetic field, while Venus, Mars and the Moon do not have any?
- Why do spectroscopic observations not reveal the presence of any iron, while this element is supposedly the major constituent of Mercury?
- Do the permanently shadowed craters of the polar regions contain sulfur or water ice?
- What are the production mechanisms of the exosphere?
- In the absence of any ionosphere, how does the magnetic field interact with the solar wind?
- Is Mercury's magnetised environment characterized by features reminiscent of the aurorae, radiation belts and magnetospheric substorms observed on Earth?
- Since the advance of Mercury's perihelion was explained in terms of space-time curvature, can we take advantage of the proximity of the Sun to test general relativity with improved accuracy?
Like Mariner 10 and MESSENGER, BepiColombo will use gravity slingshots from Venus and Earth. BepiColombo will use solar electric propulsion and then also use similar manoeuvres at the Moon, Venus, and Mercury. These techniques will slow 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.
BepiColombo is set for launch in August 2015. It is scheduled to enter orbit around Mercury in January 2022. It will then gather data for one, or possibly two years.
Comparison of MESSENGER and BepiColombo 
BepiColombo was designed to complement the findings of MESSENGER and is equipped with far more measuring equipment than MESSENGER. BepiColombo is being sent to obtain a larger range of data. The orbit patterns of BepiColombo and MESSENGER are significantly different.
The MPO will have a circular orbit much closer to Mercury. The reason for this orbit is that the MPO will be taking photos of Mercury, and the close orbit will aid picture quality. On the other hand, the MMO and MESSENGER will take largely elliptical orbits. This is because of the stability of the orbit and the lower amount of fuel required to obtain and maintain the orbit. Another reason for the different orbits of MMO and MESSENGER is to provide complementary data. The data of the two combined satellites will provide more accurate measurements.
These constitute the major differences in the two missions. It is notable that some of the questions that BepiColombo is seeking answers for are very similar to those of MESSENGER. The inexplicably strong magnetic field, for example, is of interest to both missions. A more detailed discussion of differences is provided by An international program for Mercury exploration: synergy of MESSENGER and BepiColombo.
See also 
- JHU/APL (2006). MESSENGER: MErcury Surface, Space ENvironment, GEochemistry, and Ranging Retrieved on 2007-01-27
- Munsell Kirk-editor (November 06, 2006). NASA: Solar System Exploration: Missions to Mercury. Retrieved on 2007-01-27.
- Munsell Kirk-editor (November 06, 2006). NASA: Solar System Exploration: Planet Mercury. Retrieved on 2007-01-27.
- Shirely, 2003
- Dunne, James A. (1978). The Voyage of Mariner 10: Mission to Venus and Mercury (NASA SP-424). U.S. Government Printing Office. p. 45. ASIN B000C19QHA.
- Dunne, 1978, p. 74
- Dunne, 1978, p. 101
- Dunne, 1978, p. 103
- Rayl, A.J.S. (2004). MESSENGER Launch to Mercury Postponed. Retrieved 2007-02-04.
- McNutt, Ralph L.; Solomon,Sean C.;Grard,Rejean;Novara,Mauro;Mukai,Toshifumi.(2004). An international program for Mercury exploration: synergy of MESSENGER and BepiColombo, Advances in Space Research, Volume 33, Issue 12, Mercury, Mars and Saturn, 2004, Pages 2126-2132.
- Planetary Society (2007). Space Topics: MESSENGER. Retrieved 11/9/2010
- ESA (2007). BepiColombo. Retrieved 2007-02-01.
- Mukai, T.; Yamakawa, H.; Hayakawa, H.; Kasaba, Y.; and Ogawa, H (2006). Present status of the BepiColombo/Mercury magnetospheric orbiter. Advances in Space Research, Volume 38, Issue 4, Mercury, Mars and Saturn, 2006, Pages 578-582.
- Shirley, Donna L. (August 2003). The Mariner 10 mission to Venus and Mercury. Acta Astronautica, Aug 2003, Vol. 53, Issue 4-10, p375, 11p; (AN 11471527).