|Phobos, the inner moon of Mars, as imaged by the Viking 1 Orbiter. Viking images of Deimos, Mars's outer moon, can be found at the bottom of this post. Image credit: NASA.|
In June 1972, NASA Langley Research Center (LaRC) in Hampton, Virginia, contracted with 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. NASA 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. The Jet Propulsion Laboratory (JPL) in Pasadena, California, built the Mariner-derived Viking Orbiter.
Viking was the stepchild of the Voyager Program, first proposed by JPL in 1960. Voyager, which had as its major goal the discovery and detailed study of life on Mars, had suffered from management ineptitude, turf battles, schedule slips, and a comprehensive redesign (and attendant cost increase) resulting from new Mars data gathered during the Mariner IV Mars flyby (July 1965).
|The Voyager spacecraft as envisioned at the time of its cancellation in 1967. The lander is packaged in the dark-colored aeroshell at upper right. Image credit: NASA.|
In an unusual move, top NASA officials met with leaders of Congress shortly after Voyager's cancellation in August 1967 to seek funding for a replacement. The latter agreed to fund Viking starting in Fiscal Year 1969, which began on 1 October 1968. Like Voyager, Viking would seek life on Mars. Congressional leaders also agreed to fund a pair of Mariner Mars orbiters that would launch in 1971.
Mariner Mars 1971, Viking, and the proposed Viking-based Phobos/Deimos missions were all in part a response to declared Soviet space plans. As NASA astronauts explored Earth's Moon, the Soviet Union proclaimed to the world that they had never meant to accomplish the same feat; that they had in fact opted to explore space using robots because they would not place human lives at risk. They pointed to their robotic Luna lunar sample-returners and Lunokhod 1 lunar rover when they claimed that they would soon dispatch robot orbiters, landers, sample-returners, and rovers throughout the Solar System.
The Mariner VII spacecraft glimpsed Phobos during its fast Mars flyby in 1969, and the Mariner 9 orbiter returned the first clear images of both martian moons in November 1971, while Martin Marietta's study was underway. Mariner 8, Mariner 9's twin and planned fellow traveler, had fallen into the Atlantic Ocean north of Puerto Rico after the failure of its Centaur upper stage on 9 May 1971.
Phobos and Deimos were the first non-spherical Solar System bodies humankind examined close up. They revolve about Mars in circular equatorial orbits. Phobos completes one orbit in about 7.5 hours at an altitude of about 5980 kilometers (3715 miles), while Deimos orbits in about 30 hours at 20,070 kilometers (12,470 miles).
Phobos measures 21 by 25 kilometers (13 by 16 miles) and Deimos is about half as large. Small size means low gravity; Phobos has only about 0.1% as much surface gravity as Earth. Because of this, setting down on Phobos or Deimos would be more like docking than landing.
|Viking Orbiter (top) shortly after release of the aeroshell containing the Viking Lander. Image credit: NASA/Don Davis.|
|A Viking Lander samples the surface of Mars in this artist's concept from March 1974. Image credit: U.S. Geological Survey/Don Davis.|
NASA LaRC directed Martin Marietta 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"). Mass at Earth-orbit departure would total about 3600 kilograms (7940 pounds) during the 1979 minimum-energy Earth-Mars transfer opportunity. The Lander would account for 482 kilograms (1063 pounds) of that mass.
Upon arrival at Mars, the Orbiter would fire its rocket motor to slow down and place itself and the attached Mars moon Lander into an elliptical, equatorial "capture orbit" about the planet. The spacecraft would then maneuver into an elliptical, 15-hour "observation orbit." The apoapsis (high point) of this orbit would reach the orbit of Deimos, while its periapsis (low point) would dip inside the orbit of Phobos.
The spacecraft would repeatedly fly past both moons, 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 gently on three spidery legs and deploy 82 kilograms (181 pounds) 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 briefly firing its thrusters; an alternate mobility scheme would employ spindly umbrella-shaped wheels at the ends of the landing legs.
|Viking Lander-derived Phobos/Deimos rover. Image credit: Martin Marietta/NASA.|
Martin Marietta proposed an alternate baseline mission in which the Viking Orbiter would land on the target moon. This more efficient "landed orbiter" scenario could land about 500 kilograms (1100 pounds) of science instruments, the company estimated. 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 (7438-pound) spacecraft would consist of a 38% Stretch Viking Orbiter with four legs and a 260-kilogram (573-pound) drum-shaped Earth-return vehicle based on a proposed Venus Pioneer spacecraft design.
The Orbiter would land on the target moon, collect a two-kilogram (4.4-pound) 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-by-95,000-kilometer (930-by-59,000-mile) Mars orbit. There it would trim its orbital plane so that 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 (8 miles) per second, slow to subsonic speed, deploy a parachute, and lower to a soft landing. The baseline sample-return mission would cost a total of $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 come to 4150 kilograms (9150 pounds) in 1979.
For this "Mars + Phobos/Deimos landing" mission, the Orbiter would fire 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 a latitude band extending 12° north and 12° south of Mars's equator and would need a beefed-up heat shield to withstand a 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 (137 pounds) of science instruments. The baseline combined mission would cost $441 million.
Martin Marietta also considered "Mars + Phobos/Deimos observation orbit," "Mars + Phobos/Deimos rendezvous," and "Mars + Phobos/Deimos sample-return" missions. Its "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.
Martin Marietta then looked at whether sufficient interest existed in the planetary science community to justify missions to the martian moons. The company found that there were "no active and forceful champions" of Phobos/Deimos exploration. It added, however, that it had
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.
Viking 1 left Earth atop a Titan III-E rocket with a Centaur upper stage on 20 August 1975. Viking 2 launched on 9 September 1975. The twin two-part spacecraft entered Mars orbit on 19 June 1976, and 7 August 1976, respectively. The Viking 1 Lander separated from its Orbiter and touched down successfully on 20 July 1976; Viking 2's Lander followed on 3 September 1976.
While the Landers operated on the surface, the Orbiters imaged Mars and its satellites. On 15 October 1977, the Viking 2 Orbiter passed just 30 kilometers (19 miles) from Deimos (images at bottom of post).
NASA supported studies of a Viking-derived Moon lander, a Viking 1979 Mars rover mission, and other Viking derivatives, but the U.S. opted not to fund new missions based on Viking technology. Much like the $25-billion Apollo Program, Project Viking — which had cost over $1 billion in 1975 dollars (about $5 billion in 2023 dollars) — ended with its potential barely exploited.
This occurred because the Soviet Union did not keep its promise to explore the Solar System, because NASA's budget shrank to half its Apollo-era value, and because public interest slumped after the Viking search for life on Mars returned equivocal results. The U.S. would launch no new spacecraft toward Mars until 1992, two decades after Martin Marietta completed its study.
|Deimos as imaged by the Viking 2 Orbiter. Image credit: NASA.|
|Individual house-sized boulders and craters submerged in dust are visible in this Viking 2 Orbiter close-up of Deimos. Image credit: NASA.|
A Study of System Requirements for Phobos/Deimos Mission, Final Report, Volume I, Summary, Martin Marietta Corporation, June 1972.
On Mars: Exploration of the Red Planet, 1958-1978, NASA SP-4212, Edward Clinton Ezell and Linda Neumann Ezell, NASA, 1984, pp. 83-153.