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Topics: Space Exploration - 1.2.c Mercury Missions
1.2.c Mercury missions
* Mariner 10
Reaching Mercury from Earth poses significant technical challenges, since the planet orbits so much closer to the Sun than does the Earth. A Mercury-bound spacecraft launched from Earth must travel over 91 million kilometers into the Sun’s gravitational potential well. Mercury has an orbital speed of 48 km/s, while Earth’s orbital speed is 30 km/s. Thus the spacecraft must make a large change in velocity (delta-v) to enter into a Hohmann transfer orbit that passes near Mercury, as compared to the delta-v required for other planetary missions.
The potential energy liberated by moving down the Sun’s potential well becomes kinetic energy; requiring another large delta-v change to do anything other than rapidly pass by Mercury. In order to land safely or enter a stable orbit the spacecraft would rely entirely on rocket motors. Aerobraking is ruled out because the planet has very little atmosphere. A trip to Mercury actually requires more rocket fuel than that required to escape the solar system completely. As a result, only two space probes have visited the planet so far. A proposed alternative approach would use a solar sail to attain a Mercury-synchronous orbit around the Sun.
The first spacecraft to visit Mercury was NASA’s Mariner 10 (1974–75). The spacecraft used the gravity of Venus to adjust its orbital velocity so that it could approach Mercury, making it both the first spacecraft to use this gravitational “slingshot” effect and the first NASA mission to visit multiple planets. Mariner 10 provided the first close-up images of Mercury’s surface, which immediately showed its heavily cratered nature, and revealed many other types of geological features, such as the giant scarps which were later ascribed to the effect of the planet shrinking slightly as its iron core cools. Unfortunately, due to the length of Mariner 10's orbital period, the same face of the planet was lit at each of Mariner 10’s close approaches. This made observation of both sides of the planet impossible, and resulted in the mapping of less than 45% of the planet’s surface.
On March 27, 1974, two days before its first flyby of Mercury, Mariner 10's instruments began registering large amounts of unexpected ultraviolet radiation in the vicinity of Mercury. This led to the tentative identification of Mercury's moon. Shortly afterward, the source of the excess UV was identified as the star 31 Crateris, and Mercury's moon passed into astronomy's history books as a footnote.
The spacecraft made three close approaches to Mercury, the closest of which took it to within 327 km of the surface. At the first close approach, instruments detected a magnetic field, to the great surprise of planetary geologists—Mercury’s rotation was expected to be much too slow to generate a significant dynamo effect. The second close approach was primarily used for imaging, but at the third approach, extensive magnetic data were obtained. The data revealed that the planet’s magnetic field is much like the Earth’s, which deflects the solar wind around the planet. However, the origin of Mercury’s magnetic field is still the subject of several competing theories.
Just a few days after its final close approach, Mariner 10 ran out of fuel. Since its orbit could no longer be accurately controlled, mission controllers instructed the probe to shut itself down on March 24, 1975. Mariner 10 is thought to be still orbiting the Sun, passing close to Mercury every few months.
A second NASA mission to Mercury, named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004, from the Cape Canaveral Air Force Station aboard a Boeing Delta 2 rocket. It made a fly-by of the Earth in August 2005, and of Venus in October 2006 and June 2007 in order to place it onto the correct trajectory to reach an orbit around Mercury. A first fly-by of Mercury occurred on January 14, 2008, and a second on October 6. A third is scheduled for September 29, 2009. Most of the hemisphere not imaged by Mariner 10 has been or will be mapped during these fly-bys. The probe will then enter an elliptical orbit around the planet in March 2011; the nominal mapping mission is one terrestrial year.
The mission is designed to shed light on six key issues: Mercury’s high density, its geological history, the nature of its magnetic field, the structure of its core, whether it really has ice at its poles, and where its tenuous atmosphere comes from. To this end, the probe is carrying imaging devices which will gather much higher resolution images of much more of the planet than Mariner 10, assorted spectrometers to determine abundances of elements in the crust, and magnetometers and devices to measure velocities of charged particles. Detailed measurements of tiny changes in the probe’s velocity as it orbits will be used to infer details of the planet’s interior structure.
The European Space Agency is planning a joint mission with Japan called BepiColombo, which will orbit Mercury with two probes: one to map the planet and the other to study its magnetosphere. A Russian Soyuz-2 rocket will launch the bus carrying the two probes in 2013 from ESA's Guiana Space Center to take advantage of its equatorial location. As with MESSENGER, the BepiColombo bus will make close approaches to other planets en route to Mercury for orbit-changing gravitational assists, passing the Moon and Venus and making several approaches to Mercury before entering orbit. A combination of chemical and ion engines will be used, the latter thrusting continuously for long intervals. The spacecraft bus will reach Mercury in 2019. The bus will release the magnetometer probe into an elliptical orbit, then chemical rockets will fire to deposit the mapper probe into a circular orbit. Both probes will operate for a terrestrial year.
The mapper probe will carry an array of spectrometers similar to those on MESSENGER, and will study the planet at many different wavelengths including infrared, ultraviolet, X-ray and gamma ray. Apart from intensively studying the planet itself, mission planners hope to use the probe's proximity to the Sun to test the predictions of General Relativity theory with improved accuracy.
The mission is named after Giuseppe (Bepi) Colombo, the scientist who first determined the nature of Mercury’s spin-orbit resonance and who was involved in the planning of Mariner 10’s gravity-assisted trajectory to the planet in 1974.