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MESSENGER at Mercury
The MESSENGER spacecraft orbited Mercury between 2011 and 2015. The first Mercury orbiter had been proposed in the 1970s and it took decades before a proposed mission was accepted. (credit: NASA)

Missions to Mercury: From Mariner to MESSENGER

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If all goes to plan, the European Space Agency’s BepiColombo spacecraft will arrive at Mercury in December 2025. BepiColombo consists of two spacecraft that will circle the planet, one focusing on Mercury’s surface and the other, supplied by Japan, studying its magnetosphere. The spacecraft is named after Giuseppe “Bepi” Colombo, an Italian scientist, mathematician, and engineer at the University of Padua in Italy who calculated how to get a spacecraft into a resonant orbit with Mercury enabling multiple flybys. His technique was used for the successful NASA Mariner 10 mission that flew past Mercury in 1975.

Between the mid-1970s and the late 1990s there had been many Mercury orbiter studies and proposals, as various scientists and engineers tried to figure out the best way of getting a spacecraft to a tiny, speedy planet inconveniently close to a very hot ball of gas.

In the mid-1970s, NASA considered a proposal to place a spacecraft in orbit around Mercury but rejected it. In 1990, NASA again evaluated a Mercury-orbiting mission. This would have involved two relatively large spacecraft similar to BepiColombo. That proposed mission would have been very expensive and had no chance of being funded by NASA.

But, in the mid-1980s, a Jet Propulsion Laboratory trajectory specialist figured out a way to get a spacecraft into Mercury orbit with a decent-sized scientific payload. NASA finally accomplished that goal in 2011. Between the mid-1970s and the late 1990s there had been many Mercury orbiter studies and proposals, as various scientists and engineers tried to figure out the best way of getting a spacecraft to a tiny, speedy planet inconveniently close to a very hot ball of gas.

Mariner 10
The Mariner 10 spacecraft that flew past Mercury in 1974 and 1975. (credit: NASA)

Hot summer

In the summer of 1974, the National Research Council’s Space Science Board held a series of meetings of its advisory panels such as the Committee on Planetary and Lunar Exploration, or COMPLEX, which was assigned the task of reviewing possible planetary missions for the next decade.

In preparation for this activity, JPL conducted nearly a dozen individual studies of new planetary missions, some of them based upon existing spacecraft designs adapted for new missions, and others requiring entirely new spacecraft. Two of the studies focused on the planned arrival of the comet Encke in 1980, one mission involved a Venus orbital imaging radar, another a solar electric out-of-the-ecliptic probe. Other missions focused on Mars, including a polar orbiter, sample return, and a Mars rover. One study proposed a Jupiter/Uranus flyby with Uranus atmospheric probe, and another study evaluated a Jupiter orbiter mission. Many of the spacecraft that NASA later built—like Magellan, Galileo, even Curiosity—were either first proposed or substantially refined in the early-mid-1970s.

One of those 1974 mission proposals was for a “Mariner Mercury Orbiter 1978,” a spacecraft that could be launched in July 1978 and enter orbit around Mercury in May 1980 with an orbital lifetime of four months. The spacecraft would have had an injected mass of 3,120 kilograms and an orbited mass of 775 kilograms, with the difference being the massive amount of fuel required to bring the spacecraft into orbit around Mercury. The instrument mass would have been only 68.4 kilograms.

The mission objectives would have been to map Mercury’s surface at a resolution of 500 meters, make extensive measurements of the magnetospheric environment, test the theory of gravitation using three different experiments, determine Mercury’s mass distribution, and determine the abundance of radioactive nuclides on the surface for composition studies. To accomplish this, the spacecraft would be equipped with two imaging cameras, two magnetometers, a 14-channel ultraviolet airglow sensor, a charged particle telescope, plasma science instruments, an infrared radiometer, and a gamma-ray spectrometer.

The spacecraft would have been what JPL referred to as the “Mercury Venus Mission ’73 spare,” otherwise known as the backup to Mariner 10, equipped with a new propulsion subsystem. Mariner 10 was a Venus and Mercury mission and the last of the Mariner missions, although the two Voyagers were initially given Mariner designations.

Launching from Earth towards the Sun was essentially launching downhill, requiring a lot of fuel to slow down to enter Mercury’s orbit rather than shoot past it and burn up.

Mariner missions included backups in case of launch or spacecraft failures. But Mariner 10 was not planned to have a fully flight-qualified spare. According to Bruce Murray and Eric Burgess in their 1977 book Flight to Mercury, JPL originally planned on building an engineering spare that would not have all the components required for flight. However, they saved sufficient money on the primary spacecraft that they procured a full set of flight hardware for the backup, enabling it to fly if the primary spacecraft failed. When the mission was successful, the backup went into clean room storage.

For the Mercury orbiting mission, the spacecraft would require new thermal protection for its closer proximity to the Sun, an attitude control system developed for the Viking Orbiter, and hot gas attitude control jets. The instrument suite would be slightly different from Mariner 10. A wide-angle field of view television similar to the one developed for Mariner 9, which was launched to Mars in 1971, would have been used and the ultraviolet occultation spectrometer deleted because it was unnecessary. The Viking Orbiter fuel tank would have been stretched about 0.6 meters. The spacecraft would trade out the 300-pound-thrust Viking engine, which was not sufficient to deal with slowing down the spacecraft so deep in the Sun’s gravity well, and instead use a 900-pound-thrust reaction control thruster engine developed for the Space Shuttle.

To go to Mercury, the spacecraft would have been launched atop the most powerful American rocket then in service, the Titan IIIE with the Centaur upper stage. It would conduct two swingbys of Venus a year apart and would then be inserted into a 24-hour elliptical orbit inclined 70 degrees, with the northern-hemisphere periapsis at 500 kilometers altitude but its apoapsis much farther out. This orbit would have allowed the spacecraft to map Mercury’s surface, gravitational field, and magnetosphere during its short lifetime in orbit. The final orbit inclination would have been selected by a science steering group but would probably be between 110 and 135 degrees. Getting an orbit that went over Mercury’s poles was important for several reasons, including that it enabled the spacecraft to view more of the surface.

The proposed trajectory meant a slow transit time to Mercury, but it resulted in the lowest retro velocity that JPL’s trajectory experts could achieve. That was the biggest challenge for any Mercury orbiter: launching from Earth towards the Sun was essentially launching downhill, requiring a lot of fuel to slow down to enter Mercury’s orbit rather than shoot past it and burn up.

In 1975, the National Research Council’s Space Science Board presented the results of its review of possible future activities in the space sciences. COMPLEX’s findings about solar system exploration were incorporated into the SSB’s report “Opportunities and Choices in Space Science.” The primary planetary science recommendation for NASA from the Space Science Board was to pursue a mission to return a sample from the Martian surface—something that remains an unachieved top priority for the planetary science community more than 40 years later. Other top recommendations were about outer planets research. The report did address the inner planets and the Moon, but these were clearly lower priorities. The report included a table that indicated that a Mercury orbiter was part of its “recommended program,” but a Mercury orbiting mission was not explicitly called out in the text of the report.

Mariner 10
The backup spacecraft for the Mariner 10 mission on display in the Smithsonian’s National Air and Space Museum. This spacecraft could have been modified for a Mercury orbiting mission in the 1970s, but it would have been expensive. (credit: Dwayne Day)

Slowing down

In spring 1976, NASA Headquarters commissioned a “Mercury Orbiter Transport Study” from Science Applications Incorporated that was presented to NASA in January 1977. The study was intended to “assist NASA planners to assess the trajectory/payload performance requirements of candidate flight modes in delivering spacecraft systems to orbit the planet Mercury.”

The study looked at three different flight modes, 15- and 21-kilowatt solar electric propulsion options, and a solar sail, and identified a range of transit times and payload masses for Mercury missions. The report’s authors’ primary conclusion was that all three options were preferable to a more straightforward “ballistic trajectory” to Mercury like that proposed in the earlier JPL study.

Three years after COMPLEX’s report de-emphasized Mercury for NASA, in 1978, the COMPLEX committee issued the report “Strategy for Exploration of the Inner Planets.” The report outlined the scientific goals for the exploration of Mercury including mapping, at ground resolution of 100 meters or better, the half of the planet’s surface that had not already been imaged.

The Mariner 10 backup was taken out of clean room storage and donated to the Smithsonian.

The report addressed the current limitations of propulsion technology for reaching Mercury orbit. “At present, the U.S. capability is limited to ballistic-type launches, and the opportunities for such launches to insert an appropriately instrumented payload into a circular orbit of Mercury are precluded by one or more” constraints. These included small injected payload mass, few launch windows, long flight times, and the ability to achieve only highly elliptical orbits “which would seriously degrade the resolution and coverage of surface chemistry, surface imagery, and heat-flow experiments.” The report recommended waiting until a “low-thrust” propulsion system—i.e., an ion engine or electric propulsion system—became available. But if such a system became available, the committee recommended that a Mercury mission be adopted late in the period 1977–1987, as long as it did not “detrimentally affect” the study of Earth, Mars, and Venus.

Although there is no direct documentation to indicate it, apparently COMPLEX was not impressed with the proposed orbital mission using the Mariner 10 backup. Without high-level scientific support, the Mercury orbiter mission did not get NASA approval. NASA had higher priority science missions by the later 1970s, and planetary missions were already competing with astrophysics for funding. Of all the possible targets in the solar system, Mercury was not at the top of the priority list for further study, even if it could be accomplished with a spare spacecraft. Soon American planetary science entered a period of stagnation in the 1980s, what some referred to as the “lost decade” when no new planetary missions were launched. The Mariner 10 backup was taken out of clean room storage and donated to the Smithsonian. In 2013, it was finally put on display in the National Air and Space Museum’s “Time and Navigation” gallery in Washington, DC.

Mercury spinner
In 1988, the Jet Propulsion Laboratory and NASA’s Goddard Space Flight Center started a series of studies of a Mercury Orbiter mission. The proposed mission would have consisted of two spacecraft and a total injected mass of 5,000 kilograms at Mercury. This would have been a very heavy payload and required a Titan IV-Centaur launch vehicle. The expensive mission was not approved. (credit:JPL)

Orbital maneuvers in the light

For the next decade, there was no real planning for a NASA Mercury mission. NASA’s planetary science program was focused upon other goals. At the time, NASA was making no progress with solar sails, and was unwilling to commit to solar electric propulsion. Without these new technologies, it seemed unlikely that NASA could choose to build a Mercury orbiter.

In 1985, JPL orbital dynamics expert Chen-Wan Yen turned her attention to Mercury. She crunched the numbers for a range of launch windows and trajectories and demonstrated that it was possible to launch a spacecraft toward Mercury and, by doing multiple flybys of the planet combined with small propulsive maneuvers, put a relatively small spacecraft into orbit. Although the flight times were relatively long—three to five years—the benefit of this approach was that it was possible to place a larger payload into Mercury orbit than previous options. Even better, these trajectories could use the inexpensive and venerable Delta II launch vehicle.

Yen’s work energized people interested in further exploration of Mercury, and soon new studies of Mercury missions were undertaken by various groups. In 1985, the European Space Agency conducted a preliminary Mercury orbiter study. From 1990 to 1992, the Russian Academy of Sciences’ space science department conducted informal Mercury orbiter studies. Also starting in 1992, Japan’s ISAS science agency considered a Mercury orbiter as a candidate for its “Planet C” mission.

In 1988, Yen led a JPL study of a Mercury orbiter mission design and that same year NASA’s Space Physics and Planetary Exploration Divisions supported a joint effort led by the Goddard Space Flight Center (GSFC) that consisted of several workshops and studies over the next several years. By 1990, the joint JPL/GSFC team produced its report for a Mercury Orbiter which was soon followed by a report of the Mercury Orbiter Science Working Team.

The Mercury Orbiter study was conducted when NASA still conceived of planetary missions as large and comprehensive, and the dual-spacecraft mission with a mass much greater than Viking was big even for that time.

The Mercury Orbiter science objectives included both planetology and magnetospheric physics. The planetary objectives were to study the thermal and geological evolution of Mercury’s interior and surface, the origins of the solar system and formation processes, and new constraints on planetary magnetic field dynamos. The physics objectives were to study how particles from the Sun behaved on both the day side and night side of Mercury. Other goals included studying solar flare processes producing neutron and gamma ray emissions, solar energetic particle acceleration, origins of the solar wind and interplanetary magnetic field, and coronal mass ejections and stream structure in the inner solar system. An astrophysics goal was to study significant improvement in relativity parameters and the solar gravity field. To accomplish these science goals, the mission would require two nearly identical spacecraft launched on a single rocket.

The JPL/Goddard group came up with a list of strawman science instruments including an energetic particle detector, fast electron analyzer, fast ion analyzer, gamma/X-ray spectrometer, ion composition plasma analyzer, line-scan imaging, a magnetometer, and an optimized solar wind analyzer, radio and plasma wave analyzer, and a solar neutron analyzer.

The strawman spacecraft was an octagon with two small solar panels on each of four of its sides capable of providing 415 watts at Mercury. These panels could fold up to partially cover the spacecraft and control heating. The spacecraft would also be protected by forty layers of thermal wrap and spin at 10 rpm, with the spin axis perpendicular to Sun direction so that no side faced the sun for long. The spacecraft dry mass would be 800 kilograms, with 1,600 kilograms of fuel. This would result in a total of 5,000 kilograms injected mass: two spacecraft plus a 200-kilogram launch vehicle adapter. This added up to a very heavy mission. By comparison, each Viking orbiter and lander pair weighed 3,527 kilograms.

The big spacecraft would require a Titan IV-Centaur with multiple gravity assists at Venus and Mercury. The flight time could be a minimum of three years and a maximum of six depending upon how many Mercury flybys were required before orbital insertion. The mission would emphasize magnetosphere survey for the first two Mercury years and planetary imaging for the second two. The two spacecraft would enter different orbits, with one in a 200-kilometer 12-hour polar orbit and the second in a very loose equatorial orbit, eventually changed to a polar orbit similar to the other spacecraft. The mission assumed launch options in 1997, 1999, 2002, 2004, 2005, and 2007.

The Mercury Orbiter study was conducted when NASA still conceived of planetary missions as large and comprehensive, and the dual-spacecraft mission with a mass much greater than Viking was big even for that time. Titan IV-Centaurs were expensive rockets, and there are reports that the design team even considered using a shuttle launch. The total cost of the Mercury Orbiter mission was estimated at approximately $650 million, requiring a major commitment by NASA. But there simply was not sufficient scientific or political support for an expensive Mercury orbiter mission in the early 1990s. Fortunately, another option had emerged.

Mercury spinner
Spacecraft configuration with booms stowed. Lower views include side plates and science booms. The two spacecraft would have been spin-stabilized. (credit:JPL)

The Discovery program

In the early 1990s NASA started a new program of smaller planetary spacecraft named Discovery. To kick off the effort, NASA invited mission proposals for a meeting held in September 1992 in San Juan Capistrano, California. Several teams proposed Mercury missions:

  • Robert Ready of Lawrence Livermore proposed a Mercury Interior, Surface and Environmental Mission Concept.
  • Duane Muhleman of Caltech proposed MIRROR, the Mercury Imaging and Radar Ranging Orbital Reconnaissance spacecraft.
  • Albert Metzger of JPL proposed a Mercury Mapping Orbiter mission.
  • Robert Nelson of JPL proposed the Hermes Global Orbiter.
  • Bruce Bills of NASA’s Goddard Space Flight Center proposed Mallcu.
  • Jacob Trombka of NASA Goddard proposed a Mercury Field and Surface Dynamics mission.
  • Stan Peale of UC Santa Barbara proposed a Mercury Geophysics Mission.
  • C.T. Russell of UCLA proposed the Mercury Magnetic, Gravity and Atmospheres mission.
  • Paul Spudis of the Lunar and Planetary Institute proposed the Mercury Polar Flyby mission.

Of approximately 70 proposed Discovery missions submitted to the San Juan Capistrano workshop, nine of them were for Mercury, demonstrating not only the interest in doing such a mission but also the widely-accepted belief that Chen-Wan Yen’s calculations had proven that it was now possible to orbit Mercury with a Delta II-class spacecraft.

Bruce Bills, who worked in the Geodynamics Branch of NASA’s Goddard Space Flight Center, proposed a Mercury mission named Mallcu, the Aymara name for the Andean Condor, mythic companion of the Sun god. Mallcu would enter polar orbit around Mercury with the goal of determining the planet’s figure and center of mass, characterize its tidal amplitudes and phases, quantify its volcanism, characterize its tectonic history, investigate its impact cratering, estimate its internal density distribution, clarify the origin and nature of its magnetic field, and assess Mercury’s surface reflectivity.

Despite the renewed interest in Mercury, it still took many years for a mission to be proposed, selected, and launched.

Mallcu would carry only a few instruments. The primary one would be an instrument similar to the Mars Observer Laser Altimeter (or MOLA)—the big and expensive Mars Observer spacecraft would soon fail on its way to Mars. The Mercury version would be smaller to prevent overheating of large aperture optics so close to the Sun. Other instruments would include a magnetometer and a camera. Observations of the spacecraft’s orbit around Mercury would allow scientists to develop a gravity model for the planet. Mallcu would have an unusual thermal protection system, deploying two large shades, looking sort of like parasols, on either side of the spacecraft. Made of beta cloth, one would shield from the Sun, the other would shield from sunlight reflected by Mercury.

The Hermes Global Orbiter mission would use a Delta II rocket to launch a 300-kilogram three-axis stabilized spacecraft based upon TRW’s Eagle satellite that the company was providing for several Air Force low Earth orbit missions. The spacecraft would enter a 12-hour polar orbit around Mercury with a low point of 200 kilometers and a high of 15,000 kilometers. The spacecraft would use a shade and passive radiation to stay cool. Hermes would carry a visible camera, laser, laser detector unit, ultraviolet spectrometer, and magnetometer. It had an estimated cost of $146 million.

Duane Muhleman of Caltech proposed a spacecraft that, like Hermes, used TRW’s Eagle satellite bus. His proposal was called MIRROR, the Mercury Imaging and Radar Ranging Orbital Reconnaissance spacecraft. Jacob Trombka of NASA Goddard proposed a scaled-down version of Goddard’s big Mercury Orbiter mission. The Mercury Field and Surface Dynamics mission would consist of only one spacecraft, launched on a Delta II rocket, but other details are unavailable.

Paul Spudis of the Lunar and Planetary Institute proposed a Mercury Polar Flyby mission that would fly past the planet two or three times, approximately every six months, making observations of its poles, which were suspected of containing water ice. The primary science questions were to determine if the polar caps are composed of water ice and to measure the temperature, extent, and purity of the polar ice. The spacecraft would also study the geology of Mercury. The minimum requirement was for one polar pass and one equatorial pass. The spacecraft would have a dry mass of 375 kilograms and an injected mass of 880 kilograms. It too would use a Delta launch vehicle.

Spudis’ spacecraft would be three-axis stabilized and include articulated solar arrays, a modular construction with eight avionics bays, a gamma-ray spectrometer boom, and a rigidly mounted pre-deployed shade. According to Spudis, these design features were proven on Mariner 10. The instruments would include a neutron/gamma ray spectrometer, a thermal emission spectrometer, a narrow-angle camera, and a radar sounder-scatterometer. The goal was to build the mission for $140 million.

The MESSENGER spacecraft was part of NASA's lower-cost Discovery program. Launched in August 2004, it attained orbit in 2011 and operated for four years. (credit:NASA)

Discovery to Mercury

Despite the renewed interest in Mercury sparked by Chen-Wan Yen’s 1985 trajectory calculations, and despite the numerous Mercury mission studies of the late 1980s and early 1990s, it still took many years for a mission to be proposed, selected, and launched.

In 1996, NASA held a Discovery mission competition. One of the competitors was the Mercury Surface, Space Environment, Geochemistry and Ranging mission, otherwise known as MESSENGER. In October 1997, NASA selected the Genesis and CONTOUR missions, but not MESSENGER. CONTOUR later suffered an in-flight failure, but Genesis was successful in returning dust samples to Earth. In 1998, the agency held another Discovery competition, and MESSENGER competed again. In July 1999, NASA selected Deep Impact and MESSENGER as its next two Discovery missions. Both missions had a cost cap of $300 million and late 2004 launch dates.

MESSENGER reached Mercury orbit in 2011, and for the next several years it extensively studied the planet, providing data that is still being analyzed. The Applied Physics Laboratory built MESSENGER and equipped it with a heat shield that kept it cool throughout its mission, and also gave APL sufficient experience to later build the Parker Solar Probe, which will operate in an even more hellish environment. In 2015, its fuel depleted, MESSENGER crashed into Mercury’s surface.

MESSENGER’s design, and its science, helped inform the development of the BepiColombo mission. Assuming that all goes to plan, BepiColombo will orbit Mercury in 2025 with a set of instruments more advanced than the ones carried by MESSENGER. When it reaches Mercury, hopefully, in addition to Giuseppe Colombo, some people will give a little thanks to Chen-Wan Yen, who 40 years earlier made the calculations that inspired people to think about orbiting Mercury.

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