Gumdrops on Mars (1966)

Exploring Mars in the happy days before Mariner IV. Image credit: Philco Aeronutronic.

The Mariner IV Mars flyby of 14-15 July 1965, marked a watershed in Mars exploration planning. Prior to Mariner IV, engineers and scientists could legitimately propose lifting-body and winged gliding Mars landers that could set down on the planet using almost no propellants. This was because the prevailing scientific opinion gave Mars an atmosphere roughly 10% as dense as Earth's. After data from doughty 261-kilogram Mariner IV finished trickling back to Earth – a process that lasted until 3 August 1965 – such designs were relegated to the dust-bin.

Mars, it turned out, has an atmosphere less than 1% as dense as Earth's. In such an atmosphere, gliders and lifting bodies might still be used – however, they would reach the martian surface traveling at supersonic speeds, not the easily managed subsonic speeds pre-Mariner IV mission planners had assumed. The Philco Aeronutronic Mars Excursion Module (MEM) pictured at the top of this post, for example, would slow only to Mach 2 (twice the speed of sound) before it reached the surface of Mars.

At such a speed, parachute deployment would be problematic, forcing reliance on rockets to slow the MEM below the speed of sound. This would in turn demand substantial quantities of propellants, greatly increasing the MEM's mass, which would generate knock-on mass increases throughout the Mars expedition design.

Less than a year after Mariner IV, Gordon Woodcock, a young engineer in the Advanced Systems Office at NASA's Marshall Space Flight Center (MSFC) in Huntsville, Alabama, proposed what would become the new standard design for MEMs. His four-man MEM was based on the squat conical Apollo Command Module (CM) shape. Two and a half years after Woodcock published his paper, the crew of the Apollo 9 mission (3-13 March 1969), which tested the Apollo Lunar Module in Earth orbit, would name their Command and Service Module spacecraft Gumdrop with good reason.

The Command Module Gumdrop is hoisted aboard the U.S.S. Guadalcanal after the 10-day Apollo 9 mission in Earth orbit, 13 March 1969. Image credit: NASA.

For his Mars atmosphere entry simulations, Woodcock assumed a surface air pressure of 5.69 millibars – that is, a little more than 0.5% of Earth sea-level pressure. He noted that his independently developed Mars atmosphere model compared well with two models the Jet Propulsion Laboratory published just before his paper went to print.

The "semi-ballistic" Apollo CM shape, the MSFC engineer wrote, would have several advantages over lifting-body and delta-winged glider designs. It would, for example, have a low center of gravity and a "wide footprint," making tipping unlikely. The squat shape would enable installation of propellant tanks and payloads with very little wasted internal space.

Furthermore, the Apollo CM-shaped MEM would descend through the martian atmosphere not nose-first, like lifting bodies and gliders, but rather tail-first. This meant that it would not need to accomplish a problematic 180° turn or "flip" at supersonic speeds to point its braking and landing engines toward the ground.

Perhaps best of all, the Apollo Program would generate a large body of experience with use of the CM shape in Earth's upper atmosphere. Much of this experience could be applied to development of the CM-shaped MEM.

Woodcock's 56.1-ton MEM would comprise a descent stage roughly 33 feet across (the diameter of a two-stage Saturn V rocket) and, hidden beneath a protective nose-cone ("separable cap"), a 27.3-ton ascent stage. The ascent stage mass, determined largely by the amount of energy needed to climb to Mars orbit, would size the descent stage, he explained. His MEM would separate from its mother ship in Mars orbit at an altitude of 1000 kilometers, then would fire a retrorocket package to slow down and begin its fall toward the martian atmosphere.

Gordon Woodcock's Mars Excursion Module (MEM) design. Image credit: NASA.

Woodcock advised against MEM separation from the mothership prior to Mars orbit capture. It would relieve the mothership of the MEM's mass, reducing the quantity of propellants it would need to slow itself so that the gravity of Mars could capture it into orbit – thus reducing the overall mass of the expedition – but it would also introduce unacceptable risk. He noted that 10,000 simulations run on an IBM 7094 computer had shown that the safe Mars atmosphere entry corridor for the MEM would be very narrow and thus hard to target during a high-speed entry from an interplanetary trajectory.

The crew would ride in a spherical capsule atop the ascent stage during descent and landing. MEM atmospheric deceleration would cease at a velocity of 0.5 kilometers per second. The MEM's bowl-shaped heat shield would then detach, landing legs would extend, and four landing engines would ignite. Woodcock's MEM design did not include parachutes.

As the landing engines ignited, solid-propellant rockets would blast the separable cap away from the MEM ascent stage. With the conical cover gone, the MEM pilot would see his prospective landing site for the first time.

He would then have 100 seconds of maneuvering time to steer the MEM to a safe touchdown. If rugged terrain made this too short a time to find a safe spot or if a malfunction occurred, the pilot could abort the landing by blasting the ascent stage free of the descent stage and returning to Mars orbit.

MEM mass at touchdown would total 40.9 tons. Following a safe touchdown, the crew would exit an airlock adjacent to the ascent stage cabin and transfer to a Mars surface crew quarters module in the descent stage. The latter would take the form of a segment of a torus with a rectangular cross section.

The MEM descent stage engines would burn non-cryogenic storable propellants drawn from tanks positioned within the MEM to offset its center of gravity, enabling the spacecraft to generate a modest amount of lift during descent. A similar approach would enhance Apollo CM lift characteristics during Earth atmosphere reentry.

By revolving around its offset center of gravity using small thrusters, the CM could halt its descent and climb before descending again. This technique was used during Apollo missions to reduce the deceleration felt by astronauts during reentry at lunar-return speed (39,000 kilometers per hour).

Following the successful completion of their surface mission, the MEM crew would return to the ascent stage cabin and blast off for Mars orbit. The performance advantages of cryogenic propellants led Woodcock to opt for liquid oxygen oxidizer and liquid methane fuel in his ascent stage.

He envisioned a common propellant tank lined with "superinsulation" with a barrier separating the methane and oxygen. Helium stored under pressure in spherical tanks would drive propellants into the three ascent stage engines, any two of which would be sufficient to launch the MEM to Mars orbit.

Logistics MEM. Image credit: NASA.

Shelter MEM. Image credit: NASA.

Much as Apollo engineers envisioned that the basic Lunar Module design would be modified to give it new capabilities (for example, unmanned delivery of cargo to the lunar surface) as the Apollo Program evolved from initial brief sorties to in-depth lunar exploration, Woodcock envisioned that his MEM would form the basis of a long-term, increasingly capable and complex Mars exploration program.

He proposed a design for a one-way logistics MEM in which cargo and a "camper-type" pressurized rover would replace the MEM ascent stage and the surface operations shelter. A crew would arrive separately in a conventional MEM to unpack the cargo and explore widely in the rover.

Woodcock also offered a design for a one-way nuclear-powered MEM that would provide electricity to a long-term Mars surface base built up from one-way shelter MEMs. The nuclear-power MEM would include a shielded reactor, a reactor control room, and a skin-mounted radiator for discarding reactor waste heat.

Each shelter MEM would house five or six astronauts on three levels: communications & control on top; living quarters in the middle; and a laboratory at the bottom. The lab would connect to a "sortie room/decontamination airlock" that would enable access to the surface.

Woodcock calculated that 10.6 tons of water, food, and oxygen with a four-ton reserve could sustain a five-man crew in the MEM on Mars for 500 days. Like the logistics MEM, the power and shelter MEMs would land on Mars unmanned.

The Apollo CM-shaped MEM design became closely identified with piloted Mars missions after NASA MSFC director Wernher von Braun, famous for his 1950s Mars glider lander designs, presented a variation on Woodcock's Apollo-shaped lander theme to President Richard Nixon's Space Task Group in early August 1969. Image credit: NASA.

Sources

"Summary Presentation: Study of a Manned Mars Excursion Module," F. Dixon, Aeronutronic Division, Philco Corporation; paper presented at the Symposium on Manned Planetary Missions, 1963/1964 Status, NASA George C. Marshall Space Flight Center, Huntsville, Alabama, 12 June 1964.

An Initial Concept for a Manned Mars Excursion Vehicle for a Tenuous Mars Atmosphere, NASA TM X-53475, G. Woodcock, NASA Marshall Space Flight Center, 7 June 1966.

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Dyna-Soar's Martian Cousin (1960)

A Forgotten Pioneer of Mars Resource Utilization (1962-1963)

Dyna-Soar's Martian Cousin (1960)

Dyna-Soar spaceplane. Image credit: U.S. Air Force.

In 1960, Philip Bono, a Space Vehicle Design Specialist with the Boeing Airplane Company, envisioned a manned Mars spacecraft which outwardly resembled the X-20A Dyna-Soar single-seat orbital glider his company was at the time developing for the U.S. Air Force. Bono's Mars glider was, however, much larger than Dyna-Soar — large enough, in fact, to hold an eight-man "expeditionary force" and nearly 40 tons of supplies and equipment. The flat-bellied Mars glider measured a whopping 125 feet long and 95 feet across its delta wings.

Though Bono's Mars glider was impressively large, it was part of a Mars expedition plan that was stripped-down and bare-bones by early 1960s standards. It lacked redundancy and provided few abort modes. For those familiar with Wernher von Braun's 1950s plans for Mars expeditions, some of which included 10 or more cargo and crew spacecraft, Bono's plan must have seemed daring, even reckless.

Bono himself acknowledged that his study did not "present the solution to many major problem areas." He nevertheless assured his readers that it was "restricted to the realm of practicality and reflect[ed] a moderate degree of conservatism."

A large crane hoists into place the forward section of Bono's Mars glider. Final assembly occurs on the launch pad. Image credit: Boeing Airplane Company via San Diego Air & Space Museum.

Prior to launch, the forward section of Bono's glider would be lowered into place atop its aft section on the launch pad. All assembly would take place on Earth. In the event of trouble during ascent, the crew would blast free in the glider's forward section. The glider aft section would be mounted atop a living module with an attached small rocket stage which in turn would rest upon a short central booster rocket.

Six tall outboard booster rockets would surround and hide the short booster, living module/rocket stage, and most of the aft section of the glider. Fully assembled, loaded with liquid hydrogen and liquid oxygen propellants, and ready for launch, Bono's massive Mars stack would stand 248 feet tall and weigh in at 4150 tons.

Abort: the forward section of the Mars glider (upper right) blasts free of a malfunctioning booster rocket during first-stage ascent. Image credit: Boeing Aircraft Company via San Diego Air & Space Museum.

Bono, in common with many Mars exploration enthusiasts of the early 1960s, optimistically targeted his expedition for the favorable 1971 Earth-Mars transfer opportunity, when the energy required to reach Mars would be at a minimum. On 3 May 1971, seven plug-nozzle engines — one per booster — would ignite and power up to generate a total of 10 million pounds of thrust. The advanced plug-nozzle engine design would do without large engine bells, in theory largely eliminating engine cooling requirements and reducing engine mass. The crew would feel a maximum acceleration equal to 5.6 times the pull of Earth's gravity during ascent.

During first-stage operation, four of the outboard boosters would supply propellants to all seven engines. The rocket would climb to an altitude of 200,000 feet, where it would cast off the four expended boosters. These would fall to Earth 60 nautical miles downrange of the launch site.

Bono's Mars spacecraft begins second-stage flight by casting off four outer boosters (lower left). Image credit: Boeing Aircraft Company via San Diego Air & Space Museum.

The three remaining engines would continue firing with the two remaining outboard boosters supplying all of their propellants. At 352,000 feet, the two boosters would expend their propellants and detach. The short central booster would continue firing until it placed the glider, living module, and small rocket stage on a trans-Mars trajectory, then would expend its propellants and detach. The Mars spacecraft — two-part glider, living module, and small rocket stage — would have a mass of nearly 138 tons following Earth escape.

Safely on course for Mars, the astronauts would crawl through a tunnel in the glider's aft section to reach the 45-foot-long, 18-foot-diameter living module. They would deploy an inflatable 50-foot dish-shaped antenna for radio communication with Earth (the dish might have been a late addition to Bono's plan, for it is not depicted in any of the illustrations for this post). During the 259-day voyage to Mars, the crew would breathe a 40% oxygen/60% helium air mix, so in their radio reports to Earth they would sound like Donald Duck.

The end of second-stage operation: the remaining pair of outboard boosters exhaust their propellants and separate, leaving to the short central booster the task of placing the glider, living module, and small rocket stage on course for Mars. This image displays the plug-nozzle engines unobscured by exhaust — they are the cones at the bottoms of the two boosters (lower left and lower center). Image credit: Boeing Aircraft Company via San Diego Air & Space Museum.

Its job done, the short central booster stage shuts down and fires thrusters to separate from the Mars spacecraft. Image credit: Boeing Aircraft Company via San Diego Air & Space Museum.

On 17 January 1972, at the end of a 259-day Earth-Mars transfer, the crew would strap into the glider and separate it from the living module. They would discard a 10.4-ton capsule containing human waste accumulated during the voyage to Mars. The small rocket stage, meanwhile, would ignite its four 20,000-pound-thrust Pratt & Whitney-built Centaur engines to slow itself and the living module so that Mars's gravity could capture them into orbit.

After deploying the antenna, the crew would point the glider's nose — which would contain a nuclear reactor for generating the Mars expedition's electricity — at the Sun. This would place the living module in shadow, and would shield the liquid hydrogen/liquid oxygen propellants in the small rocket stage from solar heating. Bono assumed that no course corrections would be necessary so that his spacecraft could maintain its nose-toward-Sun attitude throughout the journey to Mars.

17 January 1972: Arrival at Mars. The unpiloted living module (left) ignites its small rocket stage to slow down so that the planet's gravity can capture it into orbit while the glider bearing the crew enters the martian atmosphere directly. Image credit: Boeing Aircraft Company via San Diego Air & Space Museum.

The waste capsule — the skinny conical object between the living module and the glider in the image directly above — would strike Mars. Needless to say, this peculiar concept would likely have had few fans among scientists; it would certainly have introduced massive amounts of Earth bacteria into the martian environment, greatly complicating studies of any native martian biosphere that might exist.

The glider, meanwhile, would carry the eight-man crew directly into the martian atmosphere with no stop in orbit. If conditions on Mars were not suitable for an immediate landing — for example, if a planet-wide dust storm were raging — then the crew would have no way of aborting atmosphere entry and descent to the surface. (Such a storm did in fact occur in late 1971, though by January 1972 it had mostly abated.)

The Mars glider casts off its drag parachute as it steers toward a smooth area of martian desert. Image credit: Boeing Aircraft Company via San Diego Air & Space Museum.

Vertical descent and touchdown. The artist depicts Mars as smooth and dusty, with no obvious rocks on its surface. Image credit: Boeing Aircraft Company via San Diego Air & Space Museum.

As it descended past 3000 feet of altitude, the glider would deploy a 42-foot-diameter drag parachute to reduce speed. The Mars glider pilot would steer his craft toward a level stretch of ochre desert. At an altitude of 2000 feet — which Bono declared (wrongly, as it turns out) was "adequate to clear the highest mountain of Mars" — three landing engines with a combined thrust of 60,000 pounds would ignite to slow it to a hover. The glider would then lower vertically to the surface in a billowing cloud of yellow dust and sand and touch down on skids with its nose aimed 15° above the horizon. At touchdown, the Mars glider would have a mass of 70.4 tons.

Bono's description of the glider's aerodynamic performance was based on an estimated martian surface air pressure equal to about 8% of Earth's. The true value is, however, less than 1% of Earth's surface pressure. In the actual martian atmosphere, a single 42-foot parachute would not be adequate to slow the heavy glider's descent. In addition, the glider's wing design would not produce sufficient lift to enable effective gliding. In short, Bono's glider would reach the surface while still moving at supersonic speed. Some call this "lithobraking."

Mars Outpost: members of the eight-man crew lower the glider's nose-mounted nuclear reactor onto the expedition truck. In the background (right) stand radio antenna masts and the inflatable dome-shaped shelter. Image credit: Boeing Aircraft Company via San Diego Air & Space Museum.

During the 479-day "Mars Operational Phase," the eight Mars explorers would set up a 20-foot-diameter, 2000-pound inflatable living dome and relocate the glider's nuclear reactor several thousand feet away so that it could safely generate electricity for their encampment. The crew would have at their disposal about 4.2 tons of scientific gear. They would explore and move equipment using a truck-like two-ton rover.

Near the end of their stay on Mars, the astronauts would reconfigure their glider for launch by moving its landing engines so that they could serve as ascent engines and by returning the reactor to its place on its nose. They would also anchor the aft section of the glider to the surface using stakes and cables. The glider's forward section would then blast off at a 15° angle using the aft portion as its launch pad.

Liftoff from Mars: the forward part of Bono's Mars glider begins the climb to Mars orbit. Image credit: Boeing Aircraft Company via San Diego Air & Space Museum.

Bono wrote that his Mars glider's delta wings would provide lift, greatly reducing the quantity of propellant and the size of the engines it would need to attain Mars orbit. In the actual martian atmosphere, however, the glider he described would not reach orbit before it expended its propellants.

The crew would dock the glider forward section tail-first with the waiting living module which would have loitered in Mars orbit throughout their surface stay. Several astronauts would spacewalk to join together the glider and living module and detach the empty torus-shaped propellant tanks on the living module's small rocket stage. The tanks would have been retained after the Mars orbit capture maneuver emptied them so that they could protect the small rocket stage and the precious Earth-return propellants it contained from meteoroid punctures.

Members of Bono's Mars crew cast off empty torus-shaped propellant tanks on the small rocket stage (upper right) attached to the aft end of the living module (center) in preparation for Mars orbit departure. Image credit: Boeing Aircraft Company via San Diego Air & Space Museum.

The forward section of Bono's Mars glider separates from the living module ahead of Earth atmosphere reentry. Image credit: Boeing Aircraft Company via San Diego Air & Space Museum.

24 January 1974: the forward section of Bono's Mars glider returns to Earth. Image credit: Boeing Aircraft Company via San Diego Air & Space Museum.

The crew would use the living module rocket stage to depart Mars orbit on 21 October 1973, then would discard it. Four months later (24 January 1974), as the home planet shimmered invitingly ahead, the crew would board the glider forward section once more and cast off the nuclear reactor and living module (they would burn up in Earth's atmosphere). Bono's glider, its weight reduced to just 15 tons, would then reenter Earth's atmosphere directly 997 days after launch and glide to a triumphant desert landing on skids.

Sources

"A Conceptual Design for a Manned Mars Vehicle," Philip Bono, Advances in the Astronautical Sciences, Vol. 7, pp. 25-42; paper presented at the Third Annual West Coast Meeting of the American Astronautical Society, Seattle, Washington, 4-5 August 1960.

San Diego Air & Space Museum Image Collection (http://sandiegoairandspace.org/collection/image-collection — accessed 23 November 2017).

More Information

Space Race: The Notorious 1962 Proposal to Launch an Astronaut on a One-Way Trip to the Moon

A Forgotten Pioneer of Mars Resource Utilization (1962-1963)

A 1964 Proposal for a Small Lifting-Body Shuttle with "Staged Reentry"

NASA Marshall's 1966 NERVA-Electric Piloted Mars Mission

What If a Crew Became Stranded On Board the Skylab Space Station? (1972)

Image credit: NASA.

On 28 July 1973, the Skylab 3 crew of Alan Bean, Jack Lousma, and Owen Garriott lifted off from Launch Pad 39B at Kennedy Space Center, Florida, bound for the Skylab Orbital Workshop in low-Earth orbit. Despite their mission's numerical designation, they were the second crew to visit Skylab; in a move guaranteed to generate confusion for decades to come, NASA had designated as Skylab 1 the unmanned Workshop launched on 14 May 1973, and had dubbed the first crew to visit it Skylab 2.

The Skylab 3 Apollo Command and Service Module (CSM) separated from the S-IVB second stage of its Saturn IB launch vehicle and began maneuvering to catch up with Skylab. During final approach to the Workshop, one of the four steering thruster quads on the CSM began to leak nitrogen tetroxide oxidizer from its forward-firing engine. The crew dutifully shut off the quad and used the three quads remaining to complete docking without further incident.

On 2 August, a second thruster quad began to leak, raising fears that tainted nitrogen tetroxide might have damaged both quads. If this were the case, then the Skylab 3 CSM's remaining two quads and Service Propulsion System (SPS) main engine might also have been compromised; though the individual quads and the SPS had independent plumbing, all contained oxidizer from the same batch. If the leaks continued and spread, moreover, nitrogen tetroxide might contaminate the inside of the CSM's drum-shaped Service Module, potentially damaging other spacecraft systems.

The leaks did not catch NASA off guard. As was common in the 1960s and early 1970s, NASA had considered potential Apollo and Skylab failures - however unlikely - and had planned ahead. Within hours of the second leak, The U.S. civilian space agency put into motion a variant of a plan Kenneth Kleinknecht, Skylab Program Manager, and Lawrence Williams, Apollo Spacecraft Program Office, had described less than a year earlier at the Fifth Annual Space Rescue Symposium in Vienna, Austria.

In their paper, Kleinknecht and Williams explained that Skylab would provide the first true opportunity for space rescue in the U.S. space program. One-seat Mercury and two-seat Gemini spacecraft had been too small and limited in capability to serve as rescue spacecraft. Apollo lunar CSMs were much more capable; even so, they each carried only a little more breathing oxygen, fuel cell reactants, and food than were needed to support a three-man crew for the duration of a lunar mission (about 10 days). If an Apollo CSM had become stranded in lunar orbit — by an SPS failure, say — then its crew would have perished long before NASA could have attempted a rescue.

The Skylab Orbital Workshop. The red arrow points to the Multiple Docking Adapter's radial port. Image credit: NASA.

If astronauts needed to evacuate Skylab, they could board their CSM docked at Skylab's front port, undock from the Workshop, and splash down in the ocean in less than a day. If, on the other hand, a crew's CSM became unusable while they lived and worked on board Skylab, then the astronauts could await rescue.

Stranded astronauts were unlikely to run out of supplies. Kleinknecht and Williams noted that the Orbital Workshop would be launched with enough oxygen, food, water, and other supplies on board to support three men for eight months. At the time they presented their paper, NASA planned three three-man Skylab visits lasting 28, 56, and 56 days — that is, a total of a little less than five months.

NASA, meanwhile, would prepare and launch a rescue CSM with a crew of two. Skylab, Kleinknecht and Williams explained, had a second, radial docking port on its Multiple Docking Adapter. The rescue CSM would dock at the radial port to pick up the stranded crew.

They proposed that the CSM intended for the next Skylab crew should become the rescue CSM. This would presumably reduce by one the number of long-duration Skylab missions that could be flown. A fourth CSM, which would serve as the backup CSM throughout the Skylab program, would serve as the rescue CSM for Skylab 4, the third and final planned Skylab crew.

Image credit: NASA.

Kleinknecht and Williams estimated that stripping out the rescue CSM's aft bulkhead lockers to make room for a "rescue kit" would require about a day. The rescue kit would include a pair of special astronaut couches, connectors and hoses for linking two additional space-suited astronauts to the rescue CSM's life support and communications systems, and an experiment-return pallet for bringing home a select few of the stranded crew's science results. The rescue CSM's two-man crew would recline in the left and right CSM couches; the three rescued Skylab crewmen would return to Earth in the center couch and in the two special couches mounted below the others in place of the lockers.

The rescue CSM would bring along a special Apollo probe-and-drogue docking unit that would enable astronauts inside Skylab to manually undock and cast off the crippled CSM. This would clear the Workshop's front port for any future CSM dockings. Kleinknecht and Williams did not explain what would happen to the unmanned CSM after it was discarded.

Though the time needed to install the rescue kit was minimal, the time needed to refurbish Pad 39B and prepare the rescue CSM and Saturn IB rocket for launch would depend upon when NASA declared that a rescue was necessary. After each Skylab Saturn IB launch, ground crews would need about 48 days to refurbish Pad 39B and prepare the next Skylab CSM and Saturn IB.

If a rescue were judged to be necessary at the beginning of the 28-day first manned Skylab mission (Skylab 2), then the mission would be extended by 20 days, making the total duration about 48 days. If a rescue were declared to be necessary late in Skylab 2 — say at the time of planned return to Earth — then preparations for the next Skylab CSM launch would be farther along, but would have started later. The rescue CSM and Saturn IB would thus need 28 days before they could lift off, bringing the total Skylab 2 mission duration to about 56 days, or double the duration planned at launch.

Activation of the Skylab rescue capability early in the Skylab 3 or Skylab 4 mission might permit a rescue before the return time planned when the stranded crew left Earth, Kleinknecht and Williams found. A failure near the planned conclusion of Skylab 3 or Skylab 4 would see a rescue CSM launched as little as 10 days after the rescue plan was activated.

Skylab rescue crewmen Vance Brand (left) and Don Lind. Though he never flew to Skylab, Brand would reach space as part of the Apollo-Soyuz Test Project mission in July 1975 and as Commander of Space Shuttle missions STS-5 (November 1982), STS-41-B (February 1984), and STS-35 (December 1990). Lind would reach space as a Mission Specialist on Shuttle mission STS-51-B (April-May 1985). Image credit: NASA.

The 2 August 1973 failure of the second Skylab 3 CSM thruster quad unleashed a storm of activity. NASA prepared the backup Skylab CSM, not the Skylab 4 CSM, as its rescue vehicle, and tapped Skylab 3 backup crewmen Vance Brand and Don Lind to pilot it.

NASA had made other changes to Kleinknecht and Williams' rescue plan. The special probe-and-drogue docking unit for casting off the malfunctioning CSM had become a concave drogue unit that would be installed over the front port. It was launched with Skylab, not in the rescue CSM. After they installed it, the stranded astronauts would "trigger" the drogue to manually release their balky CSM. The rescue CSM would then dock at the front port, not the radial port.

Almost as soon as NASA activated the rescue plan, laboratory analysis on Earth showed that the batch from which the nitrogen tetroxide in the Skylab 3 CSM's propulsion systems had been taken was not tainted. As unlikely as it might seem, the two thruster quad malfunctions lacked a common cause.

Working in the CSM simulator in Houston, astronaut Brand demonstrated that the Skylab 3 crew could maneuver their spacecraft adequately even if they lost a third thruster quad. That is, if they were left with only one functioning quad when time came for them to return home, they could still safely deorbit their CSM.

Though rescue preparations continued as a precaution, by 10 August NASA managers had cleared the Skylab 3 crew for the full duration of their planned 59-day mission on board the Workshop. On 25 September 1973, Bean, Lousma, and Garriott returned to Earth as originally planned, in the CSM that had launched them to Skylab.

Sources

"Skylab Rescue Capability," Kenneth S. Kleinknecht and Lawrence G. Williams; paper presented at the Fifth Annual Space Rescue Symposium Organized by the Space Rescue Studies Committee of the International Academy of Astronautics, 23rd Congress of the International Astronautical Federation, Vienna, Austria, 9-12 October 1972.

Skylab News Reference, NASA Office of Public Affairs, March 1973, pp. IV-6 - IV-8.

"Skylab: Outpost on the Frontier of Space," T. Canby, National Geographic, October 1974, p. 460.

More Information

What If Apollo Astronauts Became Marooned in Lunar Orbit? (1968)

What If an Apollo Lunar Module Ran Low on Fuel and Aborted Its Moon Landing? (1966)

What If An Apollo Saturn Rocket Exploded on the Launch Pad? (1965)

What If Apollo Astronauts Could Not Ride the Saturn V Rocket? (1965)

A Forgotten Pioneer of Mars Resource Utilization (1962-1963)

Sixty-five years ago: the first rocket launch from Cape Canaveral, Florida. A captured German V-2 with an American WAC Corporal sounding rocket on top begins a low-angle flight to a downrange distance of at least 320 kilometers. Image credit: U.S. Army/NASA.

Most Mars expedition plans of the 1950s and early 1960s made little use of martian resources. Apart from using the planet's atmosphere to slow landers for touchdown, either through use of parachutes or, more commonly in the time period, large wings, Mars spacecraft generally depended little on materials or conditions peculiar to Mars. This was because so little was known of the planet.

The potential benefits of using martian resources to make spacecraft propellants, building materials, and life support consumables were so compelling, however, that some planners chose to incorporate them into their mission designs anyway. Chief among the anticipated benefits was a dramatic reduction in spacecraft mass if raw materials for rocket propellants could be found at Mars. Reducing mission mass meant fewer expensive, temperamental rockets would be needed to launch Mars spacecraft components and propellants into Earth orbit for assembly, which in turn meant reduced mission cost and risk.

The Working Group on Extraterrestrial Resources (WGER) was formed in early 1962. Besides NASA, the group included representatives from the U.S. Air Force, the U.S. Army, the U.S. Geological Survey, the Bureau of Mines, aerospace corporations, and universities. The group, which met throughout the 1960s, focused mainly on lunar resources. A few researchers, however, used the WGER as a forum for discussing eventual exploitation of Mars resources.

One of these forward-thinkers was Ernst Steinhoff, representing the RAND Corporation, a think tank created in 1946 to provide advice to the U.S. military services. RAND had performed Mars studies for the U. S. Air Force as early as 1960. Steinhoff, whose specialty was rocket guidance, came to the U.S. in 1945 with Wernher von Braun, Ernst Stuhlinger, Krafft Ehricke, and other members of the Peenemünde rocket team.

After working to launch captured, sometimes modified, V-2 missiles for the U.S. Army — the image at the top of this post shows the 24 July 1950 launch of the two-stage Bumper 8 rocket — Steinhoff went to work for U.S. industry in 1956. He joined RAND in 1961, and was instrumental in the formation of the WGER the following year. In fact, he became the WGER's first chairman.

Mars pioneer Ernst Steinhoff. Image credit: U.S. Air Force.

Steinhoff summed up his Mars work in papers presented at a March 1962 meeting at NASA's Marshall Space Flight Center in Huntsville, Alabama, and at the pivotal June 1963 American Astronautical Society Symposium on the Manned Exploration of Mars in Denver. George Morgenthaler of Martin Marietta Corporation organized the Denver symposium, the first non-NASA meeting devoted to piloted Mars travel. As many as 800 engineers and scientists heard Steinhoff's paper and 25 others. It was the first time so many science and engineering professionals with an interest in Mars had come together in one place, and the last Mars meeting of its size until the 1980s.

Near the end of 1963, soon after he chaired the second annual meeting of the WGER (23-25 October 1963), Steinhoff could not pass up an offer to become Chief Scientist at the Air Force Missile Development Center at Holloman Air Force Base in Alamogordo, New Mexico. When he assumed his new responsibilities, his involvement in the WGER and his work on Mars subjects suffered. This is unfortunate, for in his Huntsville and Denver papers he anticipated and promoted mission concepts which would, with the passage of decades, emerge as highly significant in Mars exploration planning. Had he continued his work at RAND, he might have further promoted his ideas, and that might have changed the course of Mars mission planning in the 1960s and beyond.

Steinhoff's work focused on "autarchic" — that is, self-sufficient — bases on Mars and Phobos. Self-sufficiency would be achieved through mining and processing of local materials, and by equipping the base with regenerable (recycling) life support systems. The Phobos and Mars bases would support scientific research and serve as transportation "terminals" for spacecraft.

Steinhoff estimated that extraterrestrial water could supply over 90% of the logistical needs of space-faring humans. He wrote that the gravitational pull of the Moon — one-sixth that of Earth — would make it an inefficient "interim space base" for fueling Mars-bound ships. Citing Clyde Tombaugh, who had written that the moons of Mars were probably made of the same water-rich materials as Mars itself, Steinhoff proposed that Phobos supplant the Moon as a stepping stone to Mars. Nuclear systems could cook water out of Phobos rocks, then split it into hydrogen and oxygen chemical rocket propellants.

Image of Mars captured at Mt. Wilson Observatory in 1956, the year of the last close Mars opposition at the time Steinhoff wrote his papers. Because Mars has a decidedly elliptical orbit, the Earth-Mars distance during oppositions varies over a roughly 15-year cycle. The next close opposition would occur in 1971; the next after that was in 1988; and the most recent took place in 2003. Another will take place in 2018. When Steinhoff speculated on the nature of martian resources, this was among the best images of Mars available. Image credit: Mt. Wilson Observatory/NASA.

Steinhoff's early Mars expedition would include 18 astronauts and a convoy of three crew and six cargo spacecraft. They would use a conjunction-class Mars mission profile, traveling to Mars in 256 days, remaining in the Mars system for 485 days, and then returning to Earth in 256 days.

Two chemists and two geologists would prospect on Phobos for water-rich rocks. The little moon's weak gravity would enable space-suited astronauts to easily assemble "ready-to-operate" base modules shipped from Earth. Space construction workers, Steinhoff wrote, would be able to carry and connect 50-ton modules by hand. (He neglected to mention that weightless objects retain their mass. Astronauts can move massive objects, it is true, but only through considerable exertion, and only if they have a firm footing and adequate handholds. Stopping a massive moving object in weightlessness requires as much effort as setting it in motion.)

Reusable winged three-man shuttles would transport explorers between the Phobos terminal and the surface of Mars. In common with most Mars planners of his day, Steinhoff assumed, based on the consensus view of Earth-based planetary astronomers, that the martian atmosphere would be about 10% as dense as Earth's — that is, thick enough to support gliding shuttles requiring minimal landing propellants.

The surface of Mars would be rough, Steinhoff expected, so the first gliding shuttle landing would be a difficult proposition. He proposed that early shuttles drop cargo and astronauts using parachutes, then blast back to Mars orbit without landing. Among the early air-dropped cargoes would be a radio-controlled bulldozer, which astronauts on Phobos would remote-control to build a smooth, level runway for the first Mars shuttle landing. This was probably the first time anyone proposed teleoperation of equipment on Mars from Mars orbit.

The runway would be built within 25º of the martian equator so that it could be reached with ease from Phobos, which circles Mars in a near-equatorial orbit. The first Mars surface base would be established near the runway. Inflatable modules would provide living space for early explorers. After the Mars base became operational, shuttles would rely on propellants manufactured from Mars water to return to the Phobos base.

The Mars base would use vehicles and building techniques that Steinhoff's RAND colleagues had proposed in their Air Force studies. Rocket turbine engines tailored to the martian atmosphere — which many expected would be made mostly of nitrogen, as is Earth's atmosphere — would power surface rovers, airplanes, and helicopters with low-mass inflatable parts. Astronauts would manufacture cement from martian materials, construct masonry and cinder-block buildings, and inhabit martian caves.

After the propellant needs of the Mars system were met, Phobos would become a fueling station for interplanetary spacecraft. Steinhoff estimated that enough propellant could be manufactured in just 100 days to launch a spacecraft from Phobos to 300-mile-high Earth orbit, and that Phobos propellants could cut the time required for transfer between Mars and Earth in half.

He added that "use of indigenous resources, combined with more advanced nuclear ferry systems, may . . . pave the way to intensive interplanetary exploration within the limitations of our national resources." Phobos could, for example, serve as a refueling stop for Jupiter-bound piloted spacecraft.

Sources

"Powerplants for Atmospheric and Surface Vehicles on Mars," Research Memorandum RM-2529, W. H. Krase, The RAND Corporation, 10 April 1960.

"Vehicles for Exploration on Mars," Research Memorandum RM-2539, T. F. Cartaino, The Rand Corporation, 30 April 1960.

"A Possible Approach to Scientific Exploration of the Planet Mars," Paper #38, Ernst A. Steinhoff, editor, From Peenemünde to Outer Space, "A Volume of Papers Commemorating the Fiftieth Birthday of Wernher von Braun," NASA Marshall Space Flight Center Technical Report, 1962, pp. 803-836.

"Use of Extraterrestrial Resources for Mars Basing," Ernst A. Steinhoff, Exploration of Mars, George Morgenthaler, editor, pp. 468-500; proceedings of the American Astronautical Society Symposium on the Exploration of Mars, Denver, Colorado, 6-7 June 1963.

"Manned Exploration of Mars?" Raymond Watts, Sky & Telescope, August 1963, pp. 63-67, 84.

Report of the Second Annual Meeting of the Working Group on Extraterrestrial Resources on October 23-25, 1963, at the Air Force Missile Development Center, Holloman Air Force Base, Alamogordo, New Mexico, MDC-TR-63-7, no date (1965?)

More Information

Clyde Tombaugh's Vision of Mars (1959)

EMPIRE Building: Ford Aeronutronic's 1962 Plan for Piloted Mars/Venus Flybys

The Challenge of the Planets: Part One - Ports of Call

What If Apollo Astronauts Could Not Ride the Saturn V Rocket? (1965)

At the time a NASA Marshall Space Flight Center artist created this graphic, the first Saturn V test flight was 17 months in the future. The smaller rocket, labeled "Apollo Saturn I," was subsequently renamed the Saturn IB. The first piloted Apollo flight, Apollo 1, was scheduled for launch on a Saturn IB rocket in early 1967, about six months after this graphic was made. Image credit: NASA.

George Mueller left private industry to become NASA's new Associate Administrator for Manned Space Flight in September 1963. He immediately asked John Disher and Adelbert Tischler, two veteran NASA engineers not directly involved in Apollo, for an independent assessment of the Moon program. On 28 September, they told Mueller that it could not achieve President Kennedy's goal of a man on the Moon by 1970. They estimated that NASA might be able to carry out its first piloted Moon landing in late 1971.

Mueller took drastic action. When he joined NASA, the Apollo flight-test plan was based on the philosophy of incremental testing, which meant that untried rocket stages would launch only dummy stages and dummy spacecraft. On 29 October 1963, Mueller informed his senior managers that Apollo test flights would henceforth use complete systems. Mueller's directive meant that, when the Saturn V S-IC first stage flew for the first time, it would be as part of a complete 363-foot-tall three-stage Saturn V. The new "all-up" approach would, it was hoped, slash the number of test flights needed before the Saturn V could launch astronauts to the Moon.

George Mueller. Image credit: NASA.

All-up Saturn V testing, today hailed as a visionary and heroic step, made many Apollo engineers nervous. The Saturn V was the largest rocket ever developed. It had engines of unprecedented scale and power: the F-1 engines in the 33-foot-diameter S-IC first stage, which burned RP-1 kerosene fuel and liquid oxygen, remain today the largest ever flown. The J-2 engines in the top two stages, the 33-foot-diameter S-II second stage and the 22-foot-diameter S-IVB stage, gulped down temperamental liquid hydrogen and liquid oxygen propellants. Cautious engineers could see many opportunities for trouble, and they were aware that problems they could not foresee might be the most difficult to solve. Many believed that NASA should have in place backup plans in case the Saturn V suffered development delays.

Eighteen months after Mueller's announcement, E. Harris and J. Brom, engineers with The RAND Corporation think tank, proposed one such back-up plan. Their brief report, originally classified "Secret," looked at how NASA might accomplish a piloted Moon landing by 1970 if the Saturn V could not be certified as safe enough to launch astronauts.

Harris and Brom's backup plan would see the Apollo Saturn V lift off without astronauts on board. It would expend its S-IC first stage and S-II second stage in turn, then its S-IVB third stage would place itself plus lunar mission Apollo Command and Service Module (CSM) and Lunar Module (LM) spacecraft into parking orbit about the Earth. Because it would carry no crew, the lunar mission CSM would need no Launch Escape System (LES) tower on its nose.

Three Apollo astronauts would reach Earth orbit separately in a ferry CSM launched atop a two-stage Saturn IB rocket. The ferry CSM would carry a special drogue docking unit on its nose for linking up with the waiting lunar mission CSM's nose-mounted probe docking unit. The special drogue, the only new system required for RAND's backup plan, would need about one year and "perhaps several million dollars" to develop.

The top of the Apollo 13 Lunar Module Aquarius. The red arrow points to the concave drogue docking unit. Image credit: NASA.

The astronauts would dock with and transfer to the lunar mission CSM in Earth orbit, then would cast off the ferry CSM. The remainder of their mission would occur as in NASA's Apollo plan. The astronauts would restart the S-IVB stage to perform Trans-Lunar Injection (that is, to leave Earth orbit for the Moon). After S-IVB stage shutdown, they would detach the lunar mission CSM from the Spacecraft LM Adapter (SLA) shroud that linked it to the top of the S-IVB stage. The SLA, made up of four segments, would peel back and separate, revealing the LM. The CSM would then dock with the drogue docking unit on top of the LM and pull the Moon lander free of the spent S-IVB stage.

The RAND engineers declined to recommend whether the Saturn V or the Saturn IB should be launched first. They noted that liquid hydrogen fuel in the Saturn V S-IVB stage would boil and escape at a rate of 700 pounds per hour; the stage would thus need to be restarted within 4.5 hours of reaching parking orbit if it was to retain enough propellants for Trans-Lunar Injection. They noted that deletion of the 2900-pound LES would make the lunar mission Saturn V that much lighter, so its S-IVB stage could be loaded with an extra 2900 pounds of liquid hydrogen; that is, enough to permit it to loiter in low-Earth orbit for nearly 10 hours. Extending the loiter time further would demand a complex and costly S-IVB stage redesign.

Launching the crew first would avoid the S-IVB stage loiter-time constraint. Harris and Brom noted that, though the Apollo lunar mission was scheduled to last only from seven to 10 days, NASA planned a 14-day Earth-orbital Gemini mission by the end of 1965 to certify that astronauts could withstand long space flights. (That mission, Gemini 7, flew in December 1965. Astronauts Frank Borman and James Lovell returned to Earth after 14 days in good health and high spirits.)

Assuming that the Gemini flight confirmed that humans could endure 14 days in weightlessness, then the ferry CSM crew could in theory wait for from four to seven days for the unmanned Saturn V to join them in Earth orbit. Harris and Brom recommended that, in the event that launch of the Saturn V was delayed so that the astronauts waiting in orbit could not accomplish a lunar mission and return to Earth within 14 days of reaching space, then they should carry out an unspecified backup Earth-orbital mission in the ferry CSM so that their flight would not be wasted.

NASA officials did not take up the Harris and Brom proposal, though for a time in 1968 they might have wished that they had. The first Saturn V test flight, Apollo 4, lifted off without a crew on 9 November 1967. In keeping with Mueller's 1963 directive, it included complete S-IC, S-II, and S-IVB stages, plus a CSM with LES. Because LM development had hit snags, a dummy LM rode inside its SLA. The eight-hour Earth-orbital mission was an unqualified success.

Troubled flight: Apollo 6 Saturn V test, 4 April 1968. Image credit: NASA.

Apollo 6, was, however, another story. On 4 April 1968, two minutes into its automated flight, the second Saturn V to fly began to shake back and forth along its long axis. Dubbed "pogo" by engineers, the violent oscillations tore pieces off the SLA and damaged one of the S-II's five J-2 engines. Following S-II ignition, the engine under-performed and shut down prematurely, then a control logic flaw caused a healthy S-II engine to shut down. The remaining three S-II engines burned for a minute longer than planned to compensate for the two failed engines. The S-IVB's single J-2 engine then burned for 30 seconds longer than planned to reach a lopsided Earth orbit. Two orbits later, the engine refused to restart despite repeated radioed commands from flight controllers.

The pogo oscillations might have injured astronauts, had any been on board the Apollo 6 CSM; the S-IVB failure would certainly have scrubbed their flight to the Moon. Post-flight analysis showed, however, that the pogo and engine failures had relatively simple fixes. After intense internal debate, NASA announced on 12 November 1968 that the third Saturn V would launch Apollo 8 astronauts Frank Borman, James Lovell, and William Anders to the Moon. The giant rocket performed flawlessly, placing the Apollo 8 CSM on course for lunar orbit on 21 December 1968.

Sources

"Apollo Launch-Vehicle Man-Rating: Some Considerations and an Alternative Contingency Plan (U)," Memorandum RM-4489-NASA, E. D. Harris and J. R. Brom, The RAND Corporation, May 1965.

The Apollo Spacecraft: A Chronology, Volume II, NASA SP-4009,  Mary Louise Morse & Jean Kernahan Bays, NASA Scientific and Technical Information Office, 1973, pp. 104-106.

Stages to Saturn: A Technological History of the Apollo/Saturn Launch Vehicles, NASA SP-4206, Roger Bilstein, NASA, 1980, pp. 347-363.

Apollo: The Race to the Moon, Charles Murray & Catherine Bly Cox, Simon & Schuster, 1989, pp. 153-162.

More Information

What If Apollo Astronauts Became Marooned in Lunar Orbit? (1968)

What If an Apollo Lunar Module Ran Low on Fuel and Aborted Its Moon Landing? (1966)

What If An Apollo Saturn Rocket Exploded on the Launch Pad? (1965)

A 1974 Plan for a Slow Flyby of Comet Encke

So close: the CONTOUR spacecraft. Image credit: NASA.

Comet Halley is often called "Humankind's Comet" because it has appeared throughout much of recorded human history and because its orbital period of about 76 years is roughly equivalent to a human lifespan. Given the often frustrating nature of spaceflight planning, Comet Encke could be nicknamed "Spaceflight's Comet."

It has made the short list of targets for comet-exploring spacecraft for half a century. With one of the shortest orbital periods of any comet — just 3.3 years — and an inclination relative to the plane of the Solar System of only about 10°, Encke is among the comets most easily accessible to spacecraft. Yet despite being named the target of many proposed comet missions, Encke has never received a visitor from Earth.

Humans came closest to exploring Comet Encke nearly two decades ago. Following its launch on 3 July 2002, NASA's 775-kilogram COmet Nucleus TOUR (CONTOUR) spacecraft moved through a series of elliptical phasing orbits about the Earth designed to position it for a solid-propellant rocket motor burn on 15 August 2002. The burn would have launched it into solar orbit near the Earth. CONTOUR would then have re-encountered Earth in August 2003. The gravity-assist kick it was meant to receive from our planet would have put it on course for a Comet Encke close flyby on 12 November 2003.

Instead, the CONTOUR spacecraft disintegrated during its Earth-departure burn. Observers visually tracked three objects where there should have been one CONTOUR.

The CONTOUR Mishap Investigation Board determined that the most likely cause of the failure was an obvious-seeming design flaw: that the spacecraft's solid-propellant rocket motor, embedded at its center, produced enough heat that it weakened CONTOUR's structure, causing the spacecraft to break apart under acceleration. The Board cautioned, however, that lack of telemetry during the Earth-departure burn left open the possibility of several other causes, including rocket motor casing rupture, meteoroid or human-made space debris collision, or attitude-control failure leading to a destructive tumble.

If engineers and scientists at NASA Goddard Space Flight Center (GSFC) had gotten their way, Comet Encke would have received its first visitor as early as 3 December 1980. In fact, it would have received two visitors at the same time, for they envisioned launching two spacecraft to Comet Encke on a single rocket. The Encke probes, near twins, would have flown by the comet at a relatively slow speed compared with other proposed comet spacecraft; hence, in the November 1974 NASA Technical Note they wrote to describe it, they dubbed their mission a "ballistic slow flyby."

The twin Comet Encke ballistic slow flyby spacecraft stacked within their streamlined Centaur launch shroud. The adapter would join with the top of the Centaur upper stage. Image credit: NASA.

The Comet Encke probes were meant to depart Earth between 16 and 30 August 1980 atop a Titan rocket with a Centaur upper stage. Ironically, given CONTOUR's fate, the GSFC team rejected an additional solid-propellant "kick" rocket motor as too risky. The probes would travel on a curving ballistic path directly from Earth to Encke; hence the term "ballistic" in the mission's description.

Robert Farquhar led the four-person GSFC team. In 1972-1973, he had participated in GSFC's 35-member Cometary Explorer Study Group, which aimed to explore Comet Grigg-Skjellerup in April 1977 and Comet Giacobini-Zinner in February 1979 using a single 450-kilogram spinning spacecraft. The NASA-appointed Comet and Asteroid Science Advisory Committee had endorsed Cometary Explorer as the first step in a logical program of comet exploration leading to a NASA Comet Halley mission in 1985-1986.

Unfortunately, the U.S. civilian space agency, faced with rapidly declining budgets and bearing the heavy burden of Space Shuttle development, had been unable to fund Cometary Explorer. The 1980 Encke slow flyby mission would, it was hoped, put NASA comet exploration back on track to Halley.

Technicians at Cape Canaveral lower the launch shroud over the West German-U.S. Helios B solar probe spacecraft. Image credit: NASA.

Farquhar's team modeled its Comet Encke mission on the German-U.S. Helios A/Helios B Sun probe missions. Helios A left Earth in late 1974 (about a month after the GSFC group published its Technical Note, in fact). The Helios probes were designed to survive an approach to 0.3 times the Earth-Sun distance, which is inside the orbit of the planet Mercury. The Encke probes, for their part, would pass their cometary target as it neared perihelion (the point in its orbit where it was nearest the Sun) at 0.34 times the Earth-Sun distance. The Helios probes would orbit in the plane of the Solar System; the Encke probes would match their target's modest orbital tilt.

The GSFC team's Encke probes, which would spin to create gyroscopic stability, would move apart immediately after they separated from their launch vehicle's Centaur stage. Farquhar's team dubbed them the "tail probe" and the "coma probe." Each would resemble the lower half of a hourglass-shaped Helios spacecraft. Solar cells on their sides would power spacecraft systems and a suite of science instruments.

If necessary, a course-correction rocket burn would take place 10 days after launch. A second burn 50 days after launch would aim the tail probe at a point in the Comet Encke's wan tail about 10,000 kilometers behind the nucleus and would aim the coma probe at a point immediately in front of the nucleus. A third, very modest, course-correction burn was scheduled for Launch +85 days. The two spacecraft would encounter Comet Encke at about Launch +102 days.

Depending on their launch date, the Comet Encke spacecraft would reach their target between 3 December and 8 December 1980 moving at between 7.6 and 9.03 kilometers per second. Comet Encke would reach perihelion on 6 December. The Encke flybys would occur at around 1000 hours Greenwich Mean Time on all days of their arrival window so that the 100-meter dish-shaped antenna at Effelsberg, West Germany — the same antenna used to communicate with the Helios probes — could receive data for as long as possible before the twin probes set below the local horizon.

Image credit: NASA.

Farquhar and his colleagues envisioned that their two probes would carry slightly different science payloads. The 375-kilogram coma probe, which would linger within 1000 kilometers of the sunlit side of the nucleus for nearly 42 minutes, would include a despun platform bearing its radio dish antenna, TV camera, neutral mass spectrometer, UV spectrometer, and Lyman-alpha spectrometer. The 325-kilogram tail probe would include a despun antenna, but would lack the coma probe's despun platform with its four instruments. Both probes would include on their spinning main sections an ion mass spectrometer, a DC magnetometer, an AC magnetometer, an electron analyzer, a plasma analyzer, an electric field detector, a dust detector, and a dust composition instrument.

The GSFC team was not the only group in 1974 that planned a 1980 Comet Encke mission. The GSFC scientists and engineers made a point of comparing their mission plan with its main rivals. They explained that, in their comparison, "the primary evaluation criteria [would] be the science value and realism of attaining mission objectives."

Their plan's leading rival, a mission design advocated mainly by the Jet Propulsion Laboratory and its contractors, was based on solar-electric propulsion. Launch would take place on 17 December 1978 and a Comet Encke flyby would occur on 6 November 1980. The GSFC team noted that the mission's 30-centimeter-diameter solar-electric (ion) propulsion thruster had yet to be developed, let alone tested; nevertheless, it would be expected to operate flawlessly for 690 days.

In addition, the thruster would interfere with the spacecraft's particle-and-fields instruments. Interference would not cease when the thruster was switched off.

Assuming that its untried thruster functioned as hoped, however, the solar-electric spacecraft would pass Comet Encke moving at only four kilometers per second, which constituted an advantage over GSFC's ballistic slow flyby. It would do so, however, more than a month before perihelion, when Comet Encke was still about 0.5 times the Earth-Sun distance from perihelion. At that point in its orbit, the nucleus would be relatively inactive: if past observations were any guide, Comet Encke would have almost no tail.

The ballistic slow flyby's lesser rival was a ballistic fast flyby advocated mainly by NASA Ames Research Center and its contractors. A spin-stabilized spacecraft similar to the Pioneer 10 and Pioneer 11 outer Solar System spacecraft would launch on 18 August 1980 atop a relatively cheap Atlas/Centaur rocket with a solid-propellant kick stage. After a voyage of just 92 days, the spacecraft would whiz past Comet Encke on 18 November 1980 at a blistering 20.1 kilometers per second.

Farquhar's group noted that high-speed impacts with Comet Encke dust particles could easily destroy the ballistic fast flyby spacecraft, and that its camera would likely return only motion-blurred images (assuming that it had time to locate the nucleus or any other important comet features). It would remain within 1000 kilometers of the nucleus for a mere nine minutes.

The GSFC team concluded that, compared with the solar-electric and ballistic fast flybys, the ballistic slow flyby was "superior in every respect." This assertion may well have been correct; the rivalry between the slow flyby, solar-electric, and fast flyby groups split the small community of comet exploration advocates, however, helping to ensure that no spacecraft explored Comet Encke in 1980.

Comet Encke as observed by the MESSENGER Mercury orbiter on 17 November 2013. Encke passed the planet Mercury at a distance of just 3.7 million kilometers and reached perihelion four days later. Image credit: NASA/JHUAPL/Carnegie Institution of Washington.

Sources

Mission Design for a Ballistic Slow Flyby of Comet Encke 1980, NASA Technical Note D-7726, R. Farquhar, D. McCarthy, D. Muhonen, and D. Yeomans, NASA Goddard Space Flight Center, November 1974.

Comet Nucleus Tour CONTOUR Mishap Investigation Board Report, NASA, 31 May 2003.

More Information

Cometary Explorer (1973)

Missions to Comet d'Arrest and Asteroid Eros in the 1970s (1966)