Mariner 10 Uses the Gravitational Pull of One Planet to Reach Another

Date November 3, 1973-March 24, 1975

The concept of gravity-propelled interplanetary space travel was first demonstrated by the U.S. space program in the mission of Mariner 10 to Venus and Mercury. Subsequent missions have successfully utilized the gravity-assist technique.

Also known as Mariner Venus/Mercury mission; MVM73

Locale Earth to Venus and Mercury

Key Figures

• Michael A. Minovitch (b. 1935), mathematician who discovered that interplanetary spacecraft could derive enormous propulsive energy from the gravitational fields of the planets they encountered
• Walter Hohmann (1880-1945), central figure in the German rocket school who published a classic study on rocket-propelled interplanetary voyages
• Gene Giberson (1923-1999), Mariner 10 project manager who directed the construction and flight of the spacecraft

Summary of Ev4ent

The potential for using a planet’s gravitational attraction to obtain significant acceleration and direction changes on an interplanetary spacecraft was not widely recognized before the beginning of the space age. Virtually all astrodynamicists believed that the work of the German rocket scientist Walter Hohmann accurately assessed the minimum energy (and therefore the optimum) trajectories for all possible interplanetary missions. Missions to two or more planets were assumed to require very large chemical or nuclear rockets to provide the energy needed to exit from the encounter trajectory at the first planet, known as a Hohmann transfer orbit, and enter a new Hohmann transfer orbit to the next target.

In the early 1960’s, Michael A. Minovitch, a young graduate student at the University of California, Los Angeles (UCLA), became the first mathematician to solve the vexing restricted three-body problem in celestial mechanics. His solution involved complex three-dimensional vector analysis and required the most powerful digital computers available to civilian research at that time. In the process, Minovitch discovered that a large amount of orbital energy could be exchanged between a space vehicle and a passing planet and that by carefully controlling such a vehicle’s entry into a planet’s gravitational sphere of influence, it would be possible to use that energy to propel the vehicle to virtually any other destination without any additional rocket propulsion. Using large amounts of computer time donated by UCLA and the National Aeronautics and Space Administration’s Jet Propulsion Laboratory (NASA/JPL) at the California Institute of Technology, he then expanded his research to examine a number of possible multiple-planet missions and identified numerous gravity-assisted trajectories and launch windows for missions of interest to planetary scientists.

In the late 1960’s, NASA decided to give high priority to a 1973 Venus-Mercury mission using a gravity-assisted trajectory, since this represented the only practical opportunity to place a spacecraft in proximity to Mercury for the next decade. In 1970, the space agency obtained $3 million in start-up funding for what was then called the Mariner Venus/Mercury mission (MVM73), and a contract to manage the mission was awarded to JPL. Gene Giberson was appointed project manager for the spacecraft by JPL.

From the outset, the MVM73 encounter trajectory was hotly contested between scientists who were most interested in seeing photographs of the planet’s surface and those who wanted to evaluate many nonvisual factors. The objectives of the former would have been best served if the spacecraft passed both Mercury and Venus on their sunlit sides, but virtually all the other high-priority experiments required the spacecraft to pass on their night sides, and it was the latter trajectory that prevailed.

The spacecraft was built around an octagonal body with dual solar arrays. At right angles to the solar panels were a high-gain dish antenna and a 6-meter boom supporting a magnetometer. From the tip of the boom to the end of the dish antenna, the spacecraft measured 9.83 meters and weighed 503 kilograms in flight configuration. The science payload weighed 77 kilograms, including two television cameras, an X-band radio transmitter, a scanning electron spectrometer and scanning electrostatic analyzer, two magnetometers, an infrared radiometer, a charged-particle telescope, and two ultraviolet spectrometers.

MVM73 lifted off from Cape Kennedy on November 3, 1973, atop an Atlas/Centaur booster, which placed the spacecraft in a parking orbit 188 kilometers above Earth. The spacecraft traveled for only thirty minutes in this orbit before the Centaur second stage was refired, accelerating it to 40,900 kilometers per hour on a trajectory toward Venus. After successful trajectory insertion, the mission was officially designated “Mariner 10.”

The trajectory insertion burn was aimed so that the Sun’s gravity pulled Mariner 10 into a long-curving Hohmann transfer orbit to Venus. Hohmann had shown in 1925 that the optimum trajectory for an interplanetary probe was a segment of an elliptical orbit that barely touched Earth’s orbit at one extreme and barely touched the target planet’s orbit at the other. Such orbits require the spacecraft to travel halfway around the solar system from launch to encounter.

When Mariner 10 arrived at Venus, that planet’s gravity was expected to reduce the spacecraft’s orbital energy just enough so that the craft would follow a free-fall trajectory to an encounter with Mercury. Precise guidance was essential, as the window through which Mariner 10 had to pass to receive the desired amount of gravity assistance on its way to Mercury was only 400 kilometers in diameter, and any uncorrected error in aiming would be magnified a thousandfold by the time it reached Mercury.

Analysis of the trajectory achieved by the Centaur’s second burn showed that if no corrections were made, the spacecraft would pass Venus on the sunward side at a distance of 48,300 kilometers. Mariner 10 relied on its own maneuvering engine to make a series of refinements in its course, called trajectory correction maneuvers (TCMs). A TCM was performed on November 13 to refine the flight path to within 1.5 percent of optimum, and a second TCM occurred in late January of 1974.

Mariner 10 missed hitting the exact center of its Venus encounter point by only 17 kilometers. It rounded the planet on February 5, 1974, with gravity bending the trajectory exactly as expected, and exited Venus’s gravitational sphere of influence on the new course to Mercury. As it skimmed about 5,800 kilometers above the cloud tops, the cameras recorded more than four thousand pictures, while other instruments probed deeply into the atmosphere. From these data, scientists were able to gain clues to Venus’s atmospheric composition and movement.

The gravity assist provided by Venus placed Mariner 10 into an elliptical orbit that had its perihelion (closest point to the Sun) approximately at the point it encountered Mercury’s orbit, and its aphelion (farthest point from the Sun) at about the distance of Venus’s orbit. The time it would take for Mariner 10 to circle this orbit would be almost twice as long as the eighty-eight days it takes Mercury to orbit the Sun. Mission planners realized that this would cause the planet and the spacecraft to return to each other about every six months, allowing for additional “free encounters.”

A TCM to refine the trajectory from Venus was executed on March 16, and Mariner 10 arrived at Mercury on March 29, 1974. After a twenty-one-week, 402-million-kilometer journey from Earth, it was only 165 kilometers off dead center of its targeted encounter point. Its cameras showed a heavily cratered landscape, and the magnetometer detected a fairly strong magnetic field, implying that Mercury has a huge iron core. Virtually no atmosphere was detected, nor did the surface features show any evidence of erosion from water or wind.

Mariner 10’s first encounter with Mercury achieved the accuracy needed for a second encounter in six months, but the planet and the spacecraft would have passed at a distance of 805,000 kilometers, precluding any useful science. Another TCM to reduce the miss distance and produce an opportunity for yet a third encounter in about twelve months was approved. A relatively large change in velocity was necessary, and it was decided to make two separate corrections twenty-four hours apart. These were executed on May 9 and 10, leaving Mariner 10 on a course that was within 1 percent of optimum.

Mariner 10 returned to Mercury on September 21, passing 48,000 kilometers above the Southern Hemisphere and returning two thousand high-resolution photographs. After this second flyby, the spacecraft was restored to cruise mode for the six-month journey around the Sun and back to Mercury again. Mariner 10’s third visit to Mercury was its closest. It passed a scant 200 kilometers above the surface on March 16, 1975. Problems with Mariner’s onboard videotape recorder resulted in receiving only 25 percent of each picture on Earth, but the imagery was spectacular. Most important, the third encounter fully substantiated the evidence that Mercury possesses an Earth-like magnetosphere. On March 24, the attitude control system ran out of fuel and communication with Mariner 10 was discontinued. The spacecraft had journeyed more than 1.6 billion kilometers in its 506-day mission, transformed an understanding of the inner solar system, and delivered the highest yield per cost of any interplanetary mission in NASA’s history to that time.

Significance

The gravity-assist technique first demonstrated in the mission of Mariner 10 has been used with brilliant success in several subsequent interplanetary missions and was essential to most of the interplanetary missions planned for the remainder of the twentieth century and the early twenty-first century. Gravity assists offer the possibility of much higher payoff per mission, since more targets can be reached with fewer spacecraft and smaller booster rockets. Also, travel time to the targets is greatly reduced. A spacecraft receiving a gravity slingshot from Jupiter and Saturn can reach Uranus in only nine years, whereas without the slingshot, it would take thirty years.

Pioneer 10 and Pioneer 11 were launched a year earlier than Mariner 10. Pioneer 10’s encounter with Jupiter on December 3, 1973, accelerated the spacecraft to a speed that caused it to escape the Sun’s gravity and to exit the solar system. Pioneer 11 caught Jupiter from behind on December 3, 1974, and as it passed across the bow of the moving planet, gravity pulled it into a tight 270 degree turn and accelerated it back toward a point on the opposite side of the Sun, where it plunged past Saturn on September 1, 1979.

In the late 1970’s and early 1980’s, the planets Jupiter, Saturn, Uranus, Neptune, and Pluto (now considered a “dwarf planet”) were spaced about the solar system in a configuration that offered several opportunities for a single spacecraft to visit most of them using the gravity slingshot for retargeting itself at each successive encounter. NASA’s plans to seize this opportunity involved two spacecraft—Voyager 1 and Voyager 2—launched in 1977. Voyager 1’s trajectory took it within 277,000 kilometers of Jupiter on March 5, 1977. There it got a gravity assist to continue to Saturn, arriving on November 13, 1980. Voyager 2 was launched on a slightly different trajectory and passed through the Jovian moon system at a distance of about 650,000 kilometers from the planet on July 9, 1977. Jupiter’s gravity redirected Voyager 2 on a course to Saturn in 1981 and then, with additional gravity slingshots, to Uranus in 1986 and Neptune in 1989.

The flight path for Galileo, the next generation of Jupiter probe, was like a complicated aerial trapeze act. Galileo began its journey in late 1989 by going in the opposite direction, to Venus. Venus tossed it back to Earth, and as it streaked past Earth five months later, gravity accelerated it into a long orbit toward the asteroid belt, from which it returned in two years. Passing Earth again, it got a third gravity kick, which was enough to send it on a three-year trip to Jupiter, where it performed thirty-five orbits between 1995 and 2003 that relayed a wealth of information about Jupiter and its moons.

The Ulysses probe was successfully launched in October, 1990, from the International Space Station in a joint venture between the European Space Agency and NASA. It became the fastest artificially accelerated object up to that time, a record that was later surpassed by the New Horizons probe, launched in early 2006 and expected to reach Pluto by 2015. On reaching Jupiter in February, 1992, Ulysses was freed from the ecliptic orbit by a successful swing-by maneuver, which allowed it to investigate the polar regions of the Sun. These explorations, which were undertaken in 1994-1995 and 2000-2001, produced new knowledge of the Sun, including the discovery that the Sun’s southern pole has a variable location.

Bibliography

Cattermole, Peter, and Patrick Moore. Atlas of Venus. New York: Cambridge University Press, 1997. Filled with photography from telescopes and the Mariner, Pioneer Venus, and Magellan spacecrafts, this work provides a complete atlas of Venus and a gazetteer of Venusian place names.

Chapman, Clark R. Planets of Rock and Ice: From Mercury to the Moons of Saturn. Rev. ed. New York: Charles Scribner’s Sons, 1982. Chapman’s work puts the discoveries made by Mariner 10 and other planetary probes into the context of an emerging understanding of the solar system and the processes that have shaped the planets individually and collectively.

Cross, Charles A., and Patrick Moore. The Atlas of Mercury. New York: Crown, 1977. Based entirely on the images and data reported by Mariner 10, this is an excellent source for any study of the planet’s surface features. Includes discussion of the history of observations of Mercury, background on the Mariner 10 spacecraft and its mission, and information on the magnetic field and atmosphere of the planet.

Gatland, Kenneth. The Illustrated Encyclopedia of Space Technology. 2d rev. ed. New York: Crown, 1990. Referenced for its discussion of interplanetary trajectories. Includes color diagrams of the Hohmann transfer orbit and the Voyager 1 and 2 encounter flight paths at Jupiter and Saturn. The discussion of Mariner 10 is superficial.

Grinspoon, David Harry. Venus Revealed: A New Look Below the Clouds of Our Mysterious Twin Planet. New York: Perseus Books, 1998. Witty, anecdotal work that balances narrative and science. Focuses on the discoveries of the Magellan mission.

Murray, Bruce C., and Eric Burgess. Flight to Mercury. New York: Columbia University Press, 1976. Comprehensive account of the Mariner 10 mission. Murray, a leading planetologist, was in charge of the Mariner 10 imaging experiments. Burgess has written numerous articles and books on the space program, with emphasis on planetary exploration. Illustrated with more than one hundred photographs.

Strom, Robert G., and Ann L. Sprague. Exploring Mercury: The Iron Planet. New York: Springer-Praxis, 2003. Detailed history of our changing understanding of the planet closest to the Sun. Includes Mariner 10 data and describes the goals of the MESSENGER mission.

Von Braun, Wernher, Frederick I. Ordway III, David Dooling, et al. Space Travel: A History. Rev. 4th ed. New York: Harper & Row, 1985. Contains an excellent summary of the history of astronautics and the origin of ideas concerning how to accomplish interplanetary flight. Features a comprehensive bibliography.

Washburn, Mark. Distant Encounters: The Exploration of Jupiter and Saturn. New York: Harcourt Brace Jovanovich, 1983. Comprehensive discussion of the insights into the nature of Jupiter and Saturn made possible by the Voyager spacecraft, but with several valuable discussions of the trajectories chosen for the missions, including choices that were considered but not adopted. Illustrations are adequate and include drawings of the flight paths of the two spacecraft as they encountered the target planets.