Phobos - Viking (1972)

 In the June of 1972, NASA's Langley Research Center, based in Hampton, Virginia, collaborated with Martin Marietta Corporation to explore the feasibility of employing spacecraft based on the planned designs of the Viking Mars Lander and Orbiter. The mission's goal was to investigate the diminutive Martian moons, Phobos and Deimos. Beyond Apollo's blogger delves into these plans, outlining why missions to Phobos and Deimos never came to fruition.

During that June of 1972, NASA entrusted Martin Marietta with the task of exploring the possibility of utilizing technology derived from the Viking Mars Lander and Orbiter plans to investigate the Martian moons Phobos and Deimos. NASA's Langley Research Center (LaRC) managed the Viking Project, aimed at launching two Lander/Orbiter combinations to Mars in 1975. Martin Marietta played a key role as the contractor for the Viking Lander, while the Jet Propulsion Laboratory (JPL) in Pasadena, California, constructed the Viking Orbiter based on Mariner.

Viking emerged as an evolution of the Voyager Program, initially proposed by JPL in 1960. Voyager's primary objective was the detailed exploration and discovery of life on Mars. However, it faced management difficulties, territorial disputes, schedule delays, a comprehensive review, and cost increases due to new data from Mariner IV's Mars mission, coupled with a close association with NASA's plans for crewed Mars/Venus flyby missions derived from Apollo.

After Voyager's cancellation in August 1967, NASA, in an unusual move, sought funding for a replacement in a meeting with Congressional leaders. It was agreed to fund the Viking project starting from Fiscal Year 1969. Like Voyager, Viking's main goal was to search for life on Mars. Additionally, funding was secured for a pair of Mariner-based Mars orbiters scheduled for launch in 1971.

The Mariner Mars 1971 missions, Viking, and proposed Phobos/Deimos missions based on Viking were responses to the declared space plans of the Soviet Union. While NASA astronauts explored the Moon, the Soviet Union claimed they never intended to achieve the same feat and opted to send robots to avoid risking human lives. They highlighted their robotic Luna sample return missions and the lunar rover Lunokhod 1 when announcing plans to send orbiters, landers, sample return missions, and rovers across the Solar System.

Mariner VII spacecraft spotted Phobos during its rapid flyby of Mars in 1969, and the Mariner 9 orbiter returned the first clear images of both Martian moons in November 1971, while Martin Marietta conducted its study. Mariner 8, the planned companion of Mariner 9, had fallen into the North Atlantic Ocean off Puerto Rico after the failure of its Centaur upper stage on May 9, 1971.

Phobos and Deimos were the first non-spherical bodies in the Solar System closely examined by humanity. These moons orbit Mars in circular equatorial orbits. Phobos completes an orbit in about 7.5 hours at an altitude of around 5980 kilometers, while Deimos takes approximately 30 hours to orbit at an altitude of 20,070 kilometers. Phobos measures 21 by 25 kilometers, and Deimos is roughly half its size. Their reduced size implies low gravity; Phobos has only about 0.1% of Earth's surface gravity. Landing on Phobos or Deimos would resemble more of a docking than a traditional landing.

LaRC instructed Martin Marietta to assume that Phobos/Deimos missions based on Viking should depart from Earth during the minimum energy transfer opportunities between Earth and Mars in 1979 and 1981. The study report detailed various spacecraft designs for Phobos/Deimos missions.

The first option, the standard Phobos/Deimos landing spacecraft, would consist of a modified Viking Lander and a Viking Orbiter with tanks carrying 38% more propellant than the 1975 Viking design (referred to by Martin Marietta as a "Stretched 38% Orbiter"). The mass at Earth orbit departure would be around 3600 kilograms during the minimum energy transfer opportunity from Earth to Mars in 1979. The Lander would represent 482 kilograms of that mass.

Upon reaching Mars, the Orbiter would fire its rocket to slow down and position itself and the attached Martian moon Lander in an elliptical equatorial "capture" orbit around the planet. The spacecraft would then maneuver into a 15-hour elliptical "observation" orbit. The apoapsis (highest point) of this orbit would reach Deimos' orbit, while its periapsis (lowest point) would dip into Phobos' orbit.

The spacecraft would repeatedly pass by both moons, collecting data at each encounter so that scientists on Earth could decide which moon deserved a more detailed exploration. Then, controllers would instruct the spacecraft to match orbits with the selected moon.

The Lander would detach from the Orbiter and move towards its target using the attitude control thrusters of the Viking Lander. It would gently land on three spider-like legs and deploy 82 kilograms of instruments, including a seismometer, a surface sample drill, and an arm-mounted camera. The Lander could move across the surface in the weak gravity by briefly firing its thrusters; an alternative mobility scheme would employ umbrella-shaped wheels on the ends of the landing legs.

Martin Marietta proposed an alternative reference mission where the Viking Orbiter would land on the target moon. This more efficient "landed orbiter" scenario could carry around 500 kilograms of scientific instruments, estimated the company. The total cost of a reference Phobos/Deimos landing mission would be $324 million.

The company aimed its second design, the 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 the experience gained from the 1979 landing mission. Its 3374-kilogram spacecraft would consist of a stretched 38% Viking Orbiter with four legs and a 260-kilogram Earth return vehicle shaped like a drum, based on a proposed design from the Pioneer Venus spacecraft.

The Orbiter would land on the target moon, collect a two-kilogram sample, and transfer it to a sample return capsule within the Earth return vehicle. The Earth return vehicle would then fire its rocket to separate from the landed Orbiter and maneuver into a Mars orbit ranging from 1500 kilometers to 95,000 kilometers. There, it would adjust its orbital plane so that the subsequent Mars departure maneuver would set it on course for Earth.

Near Earth, the saucer-shaped sample return capsule would detach from the Earth return vehicle. It would enter Earth's atmosphere at speeds of up to 12.8 kilometers per second, decrease speed to subsonic levels, deploy a parachute, and gently descend to a soft landing. The total cost of the reference sample return mission would be $446 million.

Martin Marietta's third design, the combined Phobos/Deimos landing and Mars landing reference spacecraft, would consist of a minimally modified Viking Lander and a stretched 26% Viking Orbiter. The total weight at Earth orbit departure would be 4150 kilograms in 1979.

For this "Mars landing + Phobos/Deimos" mission, the Orbiter would fire its rocket to place itself and the Viking Lander in an equatorial elliptical "capture" orbit around Mars that would require 97 hours to complete, then release the Lander. Deorbiting from the capture orbit would impose restrictions on the Lander: it could only land within a latitude band extending 12° north and 12° south of the Martian equator and would need a reinforced heat shield to withstand a higher entry velocity into the Martian atmosphere.

Next, the Orbiter would maneuver into a 15-hour observation orbit, match orbits with Phobos or Deimos, and land carrying 62 kilograms of scientific instruments. The total cost of the combined reference mission would be $441 million.

Martin Marietta also considered "Mars observation + Phobos/Deimos," "Mars encounter + Phobos/Deimos," and "Mars sample return + Phobos/Deimos" missions. Its "Mars +" missions would, according to the company's estimates, be more cost-effective than Phobos/Deimos missions without Mars landings. A separate Phobos/Deimos landing mission, for example, would cost 80% as much as a Mars landing mission, while a "Mars landing + Phobos/Deimos" mission would only cost 14% more than a Mars landing mission.

Then, Martin Marietta examined whether there was enough interest in the planetary science community to justify missions to the Martian moons. The company found that there were no "active and energetic proponents" of Phobos/Deimos exploration. However, it added that there was repeatedly curiosity and easily aroused speculation among space scientists about the origin and nature of these small bodies. This background of scientific interest, driven by recent results from Mariner 9, could be the precursor to well-defined and enthusiastically supported recommendations for exploring the moons of Mars. If this is the case, NASA's decision to undertake this study could prove to be very timely.

Viking 1 departed from Earth aboard a Titan III-E rocket with a Centaur upper stage on August 20, 1975. Viking 2 launched on September 9, 1975. The twin two-part spacecraft entered orbit around Mars on June 19, 1976, and August 7, 1976, respectively. Viking 1 Lander separated from its Orbiter and successfully landed on July 20, 1976; Viking 2 Lander followed suit on September 3, 1976. While the Landers operated on the surface, the Orbiters captured images of Mars and its satellites. On October 15, 1977, Viking 2 Orbiter passed within just 30 kilometers of Deimos (image at the bottom of the post).

NASA supported studies of a lunar lander derived from Viking, a 1979 Viking Mars rover mission, and other Viking derivatives, but the United States chose not to fund new missions based on Viking technology. Like the $25 billion Apollo Program, Viking, which cost over $1 billion in 1975 dollars (around $5 billion in 2013 dollars), was retired with its potential barely tapped. This happened because the Soviet Union failed to fulfill its promise to explore the Solar System, NASA's budget was halved from its Apollo-era value, and public interest waned after Viking's search for life on Mars returned equivocal results. The United States would not launch new spacecraft to Mars until 1992, two decades after Martin Marietta completed its study.