Viking on the moons of Mars (1972)

In June 1972, NASA's Langley Research Center (LaRC) in Hampton, Virginia, hired Martin Marietta Corporation to look at using spacecraft based on the planned Viking Mars lander and orbiter designs to explore the martian moons Phobos and Deimos (top and middle images above, respectively; both from NASA's Mars Reconnaissance Orbiter). LaRC managed Project Viking, which aimed to launch two lander/orbiter combinations toward Mars in 1975, while Martin Marietta was prime contractor for the Viking lander (bottom image above). The Jet Propulsion Laboratory in Pasadena, California, built the Mariner-based Viking orbiter.

The proposed missions were in part a response to declared Soviet space plans. The Soviets were active at this time telling the world that they had never meant to land a man on the moon; that they had opted instead for cheaper robots that would not place lives at risk. They claimed that soon they would dispatch robot orbiters, landers, sample-returners, and rovers throughout the Solar System.

Phobos and Deimos revolve about Mars in circular equatorial orbits. Phobos completes one orbit in about 7.5 hours at an altitude of about 5980 kilometers, while Deimos orbits in about 30 hours at 20,070 kilometers. Phobos measures 21 kilometers by 25 kilometers, and Deimos is about half as large. Small size means low gravity; Phobos has only about 0.1% as much the surface gravity as Earth. The Mariner VII spacecraft glimpsed Phobos during its fast Mars flyby in 1969, and the Mariner 9 Mars orbiter returned the first detailed images of both moons in November 1971, while Martin Marietta's study was underway.

LaRC directed the company to assume that its Viking-based Phobos/Deimos missions would depart Earth in the 1979 and 1981 Earth-Mars minimum-energy transfer opportunities. The study report described several Phobos/Deimos spacecraft designs. The first, the baseline Phobos/Deimos landing spacecraft, would comprise a heavily modified Viking lander and a Viking orbiter with tanks carrying 38% more propellants than the Viking 1975 design (Martin Marietta called this a "38% Stretch Orbiter"). Total weight at Earth-orbit departure would be about 3600 kilograms in 1979, of which the lander would account for 482 kilograms.

Upon arrival at Mars, the orbiter would fire its rocket engine to slow down and place itself and the attached lander into an elliptical equatorial "capture orbit" around Mars. The spacecraft would then maneuver into an elliptical, 15-hour "observation orbit." The apoapsis (high point) of this orbit would reach Deimos' orbit, while its periapsis (low point) would dip inside the orbit of Phobos. The spacecraft would repeatedly fly past Phobos and Deimos, gathering data at each encounter so that scientists on Earth could decide which moon most warranted in-depth exploration. Controllers would then command the spacecraft to match orbits with the moon selected.

The lander would separate from the orbiter and move toward its target using Viking lander attitude-control thrusters. It would set down on three spidery legs and deploy 82 kilograms of instruments, including a seismometer, a surface sample auger, and a boom-mounted camera. The lander would be able to hop across the surface in the weak gravity by firing its thrusters; an alternate mobility scheme would employ spindly wheels at the ends of the legs.

Martin Marietta proposed an alternate baseline mission in which the Viking orbiter would land on the target moon. This more efficient "landed orbiter" scenario would land about 500 kilograms of science instruments, the company found. Total cost for a baseline Phobos/Deimos landing mission would come to $324 million.

The company targeted its second design, the baseline Phobos/Deimos sample-return spacecraft, for launch in 1981 "to allow more time for additional mission design and hardware development." The sample-return mission would build on experience gained in the 1979 landing mission. Its 3374-kilogram spacecraft would consist of a 38% Stretch Viking orbiter with four legs and a 260-kilogram Earth-return vehicle based on a proposed Venus Pioneer spacecraft design.

The orbiter would land on the target moon, collect a two-kilogram sample, and transfer it to a sample-return capsule inside the Earth-return vehicle. The Earth-return vehicle would then fire its rocket to separate from the landed orbiter and maneuver into a 1500-kilometer-by-95,000-kilometer Mars orbit. There it would trim its orbital plane so the subsequent Mars departure maneuver could place it on course for Earth.

Near Earth, the saucer-shaped sample-return capsule would separate from the Earth-return vehicle. It would enter Earth's atmosphere at up to 12.8 kilometers per second, slow to subsonic speed, and deploy a parachute for a soft landing. The cost of the baseline sample-return mission would total $446 million.

Martin Marietta's third design, the baseline combined Phobos/Deimos landing and Mars landing spacecraft, would comprise a minimally modified Viking lander and a 26% Stretch Viking orbiter. Total weight at Earth-orbit departure would be 4150 kilograms in 1979. For this "Mars + Phobos/Deimos landing" mission, the orbiter would fired its rocket to place itself and the Viking lander into an elliptical equatorial capture orbit about Mars requiring 97 hours to complete, then would release the lander. De-orbiting from the capture orbit would impose restrictions on the lander - it would be able to set down only within 12 degrees of Mars' equator and would need a beefed-up heatshield to withstand greater Mars atmosphere entry velocity.

The orbiter would then maneuver to a 15-hour observation orbit, match orbits with either Phobos or Deimos, and land bearing 62 kilograms of science instruments. Total cost for the baseline combined mission is $441 million.

Martin Marietta also considered Mars + Phobos/Deimos observation orbit, Mars + Phobos/Deimos rendezvous, and Mars + Phobos/Deimos sample-return missions. These "Mars +" missions would, the company estimated, be more cost-effective than Phobos/Deimos missions without Mars landings. A separate Phobos/Deimos landing mission would, for example, cost 80% as much as a Mars landing mission, while a Mars + Phobos/Deimos landing mission would cost only 14% more than a Mars landing mission.

The company then looked at whether there was sufficient interest in the planetary science community to justify missions to the martian moons. It found that there were "no active and forceful champions" of Phobos/Deimos exploration, but added that

we repeatedly found easily excited curiosity and conjecturing among space scientists about the origin and nature of these tiny bodies. This undercurrent of scientific interest, which has been given impetus by the recent returns of Mariner 9, may be the forerunner of well defined and enthusiastically supported recommendations for exploring the moons of Mars. If this is the case, NASA's decision to conduct this study may prove to be a very timely one.

NASA opted not to fund any missions using Viking technology beyond the original pair of Viking spacecraft. Viking 1 left Earth atop a Titan III-E rocket with a Centaur upper stage on August 20, 1975. Viking 2 launched on September 9, 1975. The spacecraft entered Mars orbit on June 19, 1976, and August 7, 1976, respectively. The Viking 1 lander separated from its orbiter and touched down successfully on July 20, 1976; Viking 2's lander followed on September 3, 1976.

While the landers operated on the surface, the orbiters imaged Mars and its satellites. On October 15, 1977, the Viking 2 orbiter passed just 30 kilometers from Deimos, permitting it to image boulders as small as houses and nearly hidden craters on the little moon's surface (top image below). Viking Orbiter 1 beamed to Earth the images forming the Phobos photomosaic below (bottom image) on October 19, 1978.

A Study of System Requirements for Phobos/Deimos Mission, Final Report, Volume I, Summary, Martin Marietta Corporation, June 1972.