As evening settled over Baikonur Cosmodrome in Soviet Kazakhstan on 10 November 1970, a Proton rocket thundered to life and began its climb toward space. Six days later, the rocket's payload, the automated Luna 17 moon lander, soft-landed on broad, flat Mare Imbrium. A team of five operators in the Crimea then remotely drove the Lunokhod 1 rover (image above) down ramps protruding from the lander's sides onto the moon's dusty surface.

The solar-powered (but nuclear-heated) 756-kilogram rover, measuring 1.35 meters tall and 2.15 meters across its tub-shaped body, rolled on eight metal wheels at a top speed of 0.1 kilometers per hour. A hinged, bowl-shaped lid lined with electricity-generating solar cells opened to expose a thermal radiator atop the tub; as night approached, Lunokhod 1's operators commanded it to close the lid to hold in heat and protect its delicate electronics.

Lunokhod 1 had originated in the Soviet manned moon program, though this would not be revealed until the late 1980s. Its role initially had been to scout out the landing site selected for the piloted lunar landing, then stand by until a lander bearing a single cosmonaut arrived. If his lander became damaged so that it could not return him to lunar orbit, the Lunokohod operator team on Earth would drive the rover to pick him up for transfer to a waiting, pre-landed backup lander. The United States, incidentally, had in the early 1960s considered launching site-survey rovers to Apollo landing sites, and had studied long-range automated rovers that visiting astronauts could board and drive.

Even before the successful Apollo 11 landing (20 July 1969), the Soviets had claimed that they never intended to land cosmonauts on the moon. This was, of course, untrue, but it found a receptive audience among those who opposed manned lunar exploration or who favored the Soviet Union in the Cold War. Through their official media, the Soviets declared that they had opted instead for robot explorers that cost much less than Apollo and placed no human life at risk. They told the world that Lunokhod 1 and automated Luna sample returners presaged a new era of extensive robotic lunar and planetary exploration.

U.S. space planners took notice. In a report called An Exploratory Investigation of a 1979 Mars Roving Vehicle Mission, completed a timely three weeks after Lunokhod 1 commenced its Mare Imbrium traverse, a 12-man design team at the Jet Propulsion Laboratory (JPL) in Pasadena, California, described a U.S. Mars rover mission in 1979. Billed as a "logical follow-on" to the Viking landings planned for mid-1976, JPL's 1127-pound rover would include six wire wheels akin to those on the Apollo Lunar Roving Vehicle, which at the time was scheduled to be driven by astronauts on the moon for the first time in 1971. Mobility would enable "extended" Viking objectives: for example, while Viking would land on a safe, flat plain and seek living organisms only within reach of its three-meter-long robot arm, the 1979 rover could land in a flat area, then enter rugged terrain to seek out biologically promising sites.

The Mars rover would leave Earth on a Titan III-C rocket with a Centaur upper stage - the same rocket planned for the 1975 Viking launches - between late October and mid-November 1979, sealed within a Viking-type aeroshell and bioshield cap attached to a Viking-type orbiter. The orbiter's rocket motor would perform a course correction burn 10 days after launch. Assuming a 3 November 1979 launch, Earth-Mars transfer would need 268 days. During the voyage, a door would open in the top of the aeroshell and the rover's cylindrical electricity-generating Radioisotope Thermal Generators (RTGs) would extend into space on a boom. The plutonium-powered RTGs would continually generate heat; if kept sealed within the aeroshell during the flight to Mars, heat build-up would damage the rover.

JPL's 1979 Mars rover inside its Viking-type aeroshell with twin RTGs (arrow) extended. Image: JPL/NASA

Mars arrival would occur in August 1980. The orbiter's rocket motor would slow the spacecraft so Mars's gravity could capture it into orbit. Two days later, it would tweak its orbit so that it would pass over its primary landing site. The JPL team estimated that its rover could reach sites between 30° north and 30° south latitude. Five days after Mars orbit arrival, the orbiter would cast off the bioshield cap to expose the aeroshell with the rover inside. The aeroshell would then separate and fire thrusters to slow down and fall toward Mars.

The JPL engineers described the rover landing sequence in considerable detail. Two hours after separation from the orbiter and 300 seconds before landing (that is, at L minus 300 seconds), the aeroshell would encounter Mars's thin upper atmosphere. Entry deceleration would peak at about 12 times the force of Earth's gravity. At L minus 80 seconds, moving at a speed of Mach 2.5, the aeroshell would deploy a small ballute ("balloon-parachute") 21,000 feet above Mars. Three seconds later, at 19,000 feet and a speed of Mach 2.2, a single parachute would deploy and the ballute would separate. At L minus 73 seconds, moving at Mach 2, the parachute would fill with thin martian air. Six seconds later, the lower aeroshell would separate, exposing the rover's underside and twin landing radars. Three terminal descent rocket motors on the rover would begin firing at L minus 33 seconds. Three seconds later, at an altitude of 4000 feet and a speed of 300 feet per second, the parachute and upper aeroshell would separate from the rover. It would touch down gently on Mars directly on its wheels 30 seconds later.

Mars surface operations would span one Earth year, from August 1980 to August 1981. JPL's rover would consist of three compartments, each with one wheel pair. The forward compartment (the "science bay") would include a Viking-type soil sampler arm with an attached magnetic properties experiment, a new-design "chisel and claw" arm, four biology experiments (the same number NASA planned to launch on the Viking landers at the time JPL completed the rover report), a mass spectrometer, a weather station, and a seismometer. The forward compartment's wheel hubs would carry one terminal descent rocket motor each, and the front wheel pair would be steerable.

The middle compartment (the "electronics bay") would house the 95-pound dual-purpose (science and rover control) computer and would sport a telescoping stalk supporting a dish-shaped high-gain antenna, a low-gain antenna, a fascimile camera capable of generating a 360° panorama, and a vidicon camera with rangefinder. The rear compartment (the "power bay") would include the twin externally-mounted RTGs, landing radars on its wheel hubs, and a rear-mounted terminal descent rocket motor. The rear wheel pair would, like the front pair, be steerable.

Flexible connectors would link the three compartments. From some time before Earth launch to its second day on Mars, the three compartments would be squeezed together tightly with their wheels touching. This would enable the rover to fit within the confines of its Viking-type aeroshell. Controllers on Earth would check out the rover during the first day after touchdown. On Day 2, they would spread out its compartments, deploy its appendages, and eject the terminal descent motors and landing radars. They would begin science operations on Day 3. JPL looked briefly at retaining the terminal descent rockets to enable the rover to "hop" over obstacles, but rejected this capability as being too risky.

JPL's 1979 Mars rover in deployed configuration with landing rockets (arrows) still attached. Image: JPL/NASA

Controllers on Earth would guide the rover through its daily program, so that operations would occur only during the martian daylight hours, when line-of-sight radio contact with Earth would be possible. Time available for operations during each 24-hour, 39-minute martian day would vary over the rover's one-Earth-year mission, as would radio signal travel time. On August 9, 1980, for example, a rover on the martian equator would be in contact with Earth for 10.93 hours per martian day, while radio signals would need about 21 minutes to cross the gulf between the planets. In May 1981, signal travel time would reach its maximum value of 41 minutes, then would decrease.

Typically, the rover would move from 50 to 100 meters at a time, then stop, image its surroundings, perform a scientific experiment, transmit its data to Earth, and then await new commands. JPL assumed that science sites would be about 14 kilometers apart, and estimated that early in its mission the rover would travel about 300 meters per day, enabling it to traverse the distance between two science sites in 47 days. Distance traversed would, JPL optimistically assumed, rapidly increase as controllers gained confidence in their remote driving ability; the team estimated that in one Earth year its rover might traverse up to 500 kilometers.

Inspired, perhaps, by Lunokhod 1, the JPL team concluded its study by looking briefly at a lunar variant of its Mars rover design. The team found that the basic design of both rovers could be much the same, though the lunar rover launch vehicle would not need to be as large and powerful (a Titan III/Centaur without strap-on boosters would suffice) and a solid-propellant braking rocket would need to replace the Mars rover's aeroshell, ballute, and parachute because the moon has no atmosphere. In addition, the lunar version would be able to tote an additional 150 pounds of science payload.

As the team's study circulated to a limited JPL audience, Lunokhod 1 continued its slow traverse of dusty Mare Imbrium. The Soviet rover was designed to function for three months, but did not officially cease operations until the 14th anniversary of the launch of Sputnik 1 on 4 October 1971, some 10 months after JPL completed its report (radio contact with Lunokhod 1 was, however, lost on 14 September 1971). During its 11-month, 10.54-kilometer traverse, it beamed to Earth more than 20,000 images of its surroundings and analyzed lunar surface composition at 25 locations.

The Soviets followed up this success a few weeks after Apollo 17 (7-19 December 1972), the final manned moon mission. On 17 January 1973, Luna 21 landed inside rugged Le Monnier Crater bearing the Lunokhod 2 rover.

On 9 May, after traversing some 37.5 kilometers, Lunokhod 2 rolled into a dark-floored crater. There its open bowl-shaped solar array/thermal cover apparently brushed against the crater wall, becoming partly filled with lunar dirt. When ground controllers commanded the array/thermal cover to shut at lunar sunset, the dirt fell on Lunokhod 2's thermal radiator. Two weeks later, as the Sun rose again at Le Monnier, controllers commanded the array/thermal cover to open in preparation for a new day of lunar driving. The dirt-covered radiator could no longer reject enough heat, and soon afterwards Lunokhod 2 ceased to function. The Soviets declared its mission ended on 3 June 1973.

This Lunar Reconnaissance Orbiter image shows the crater where Lunokhod 2 accidentally took on a load of moon dust (black arrow), the tracks it left as it moved across the surface (narrow white arrows), and the rover itself parked at its final resting place (thick white arrow). Image: NASA.

In March 2010, NASA released high-resolution images of the moon's surface showing the Lunokhod 1 and Lunokhod 2 rovers and the Luna 17 and Luna 21 landers. The images, beamed to Earth by the Lunar Reconnaissance Orbiter, clearly show the extended Luna 21 ramps and the dark tracks Lunokhod 2 left on the lunar surface.

Proposals for a Viking follow-on robot rover mission would occur throughout the 1970s, but none would move beyond the stage of proposals and studies. In part, this was because the Soviet Union failed to follow through on its promise (or threat) to launch robot sample returners and rovers to the planets. Lunokhod 2 was the last rover to operate on another world until Mars Pathfinder's Sojourner minirover in 1997.

JPL's proposed 1979 rover bears a passing resemblance to the Mars Science Laboratory (MSL) Curiosity rover launched on 26 November 2011. Both have six wheels, rear-mounted nuclear power sources, stalk-mounted cameras, and front-mounted arms. Curiosity has a single body, however, solid wheels, and a more complex suspension system. Curiosity is also larger and heavier (about 2000 pounds), and will depend on a more complex landing system known as the Sky Crane to set down gently on the surface of Mars late in the evening U.S. Pacific Time on 5 August 2012. Perhaps the most profound difference has to do with expectations: whereas in 1970 JPL engineers assumed that their rover might cover 500 kilometers in an Earth year, Curiosity is planned to cover only from five to 20 kilometers in one martian year (687 days).

References:

An Exploratory Investigation of a 1979 Mars Roving Vehicle Mission, JPL Report 760-58, J. Moore, Study Leader, Jet Propulsion Laboratory, 1 December 1970.