Flying Brickyard Postponed: A 1972-1973 Study of an Interim Ablative Space Shuttle Heat Shield

The Space Shuttle Orbiter as conceived in July 1972. The following month, NASA would make Rockwell International the Space Shuttle Orbiter prime contractor. Image credit: NASA
Launch, ascent to orbit, and Earth atmosphere reentry are the most risk-fraught phases of most piloted space missions to date. They are also the mission phases that most tax the ingenuity of engineers who design reusable spacecraft.

Aerodynamic heating creates challenges during reentry and, to a lesser degree, during ascent to orbit. Before the Space Shuttle, almost all piloted spacecraft designed to operate for some portion of their mission in an atmosphere withstood such heating by employing single-use ablative heat shields. (The only exception was the X-15A-2 rocket plane, which, for part of its career, included a replaceable ablative heat shield — see "More Information" at the end of this post.) During reentry, ablative heat shields char and break away, carrying away heat.

The Space Shuttle, approved for development by President Richard Nixon on 5 January 1972, marked a dramatic departure in heat shield technology. Originally conceived as a fully reusable, economical Space Station resupply and crew rotation vehicle, Nixon's partially reusable Shuttle had as its only approved goal a dramatic reduction in the cost of launching things into space. A reusable heat shield was believed to be essential for achieving that objective.

Over the decades, engineers have considered many reusable heat shield concepts, typically in combination. High on the list was a layer of overlapping "shingles" made of exotic metal alloys. Other approaches included liquid or solid heat sinks, thick metal or composite adjoining plates, or even an "active" system with cooling fluid circulating through a network of tubes behind a metal-alloy hull.

Unfortunately, all of these concepts would be heavy. To compensate for a heavy heat shield, engineers could design a more powerful booster system or could cut back on payload capacity (or both). Both approaches would boost development and operations costs. The Nixon White House had made clear that the Shuttle development budget of $5.15 billion was carved in stone, leaving NASA with little choice but to find new approaches — including some that accepted a significant increase in eventual operations cost.

Image credit: NASA
For the Shuttle Orbiter, NASA and contractor engineers chose a lightweight combination of fabrics and brittle silica ceramic tiles, which they dubbed Reusable Surface Insulation (RSI). The tiles could withstand temperatures of up to 2300° Fahrenheit. Reinforced Carbon-Carbon composite panels would protect the Orbiter's wing leading edges, nose, and other areas subject to the highest reentry temperatures (as high as 3000° Fahrenheit).

Though RSI was meant to block almost all heat, enough would get through that, combined with aerodynamic buffeting, the Orbiter's mostly aluminum skin would tend to warp and flex ("flutter"). This meant that large ceramic panels affixed to the skin would crack, leaving it vulnerable to reentry heating.

Shuttle engineers sought to avoid damage by gluing RSI ceramics to a flexible fabric "strain isolator" layer glued to the Orbiter's skin and by making individual ceramic elements small in size. By resorting to many small "tiles" in place of a relatively few large panels, engineers designed an RSI heat shield that was in effect "pre-cracked."

The tiles, each milled to conform to its place on the Orbiter's complexly curvaceous hull, would number in the tens of thousands. By late in the 1970s decade, when their number hovered around 31,000, the tiles earned the Orbiter the nickname "The Flying Brickyard."

Some engineers harbored doubts about RSI; enough that NASA Langley Research Center in Hampton, Virginia, paid the Denver Division of Martin Marietta Corporation (MMC) to examine an alternative. Between May 1972 and August 1973, MMC engineers sought to determine whether Space Shuttle Orbiters could employ an ablative heat shield.

The ablative shield was seen as a stand-in system meant to provide NASA with more time for RSI development should problems arise. In his October 1975 report on the ablative heat shield study, Rolf Seiferth, who managed the MMC study between 5 September 1972 and its conclusion on 31 August 1973, envisioned that the ablative shield might fill in for RSI for five years. Based on a November 1972 NASA-generated Space Shuttle traffic model, this meant that 151 flights between 1979 and the end of 1983 would rely on the stand-in ablative system.

Seiferth noted that, in past programs, ablative heat shield materials had been glued directly to the spacecraft hull. This was, he explained, a cost-saving, weight-saving approach; scraping away a used directly applied ablative shield would, however, add time to Orbiter refurbishment between flights and generate considerable debris, including invasive dust.

In addition to the directly applied heat shield, MMC examined three types of "mechanically attached" ablative panels. These had ablative material glued to panels made of aluminum, magnesium, graphite composite, or beryllium/aluminum "Lockalloy" sheet or honeycomb.

The panels would be joined to oversized holes in the Orbiter's skin using nut-and-bolt fasteners, enabling entire panels to be replaced as necessary. The oversized holes would allow for thermal expansion of the heat shield components.

The simplest mechanically attached ablative panel would see ablative material glued to a metal or composite sheet. Adhesive and sheet would together measure only about 0.06 inches thick. Attachment points for the sheet panel design would typically occur five inches apart over much of the Orbiter, though larger spacings (up to 20 inches) were also possible.

The two more complex mechanically attached ablative panels substituted metal or composite "honeycomb" for the metal or composite sheet. One had ablative material glued to the honeycomb, which was then bolted to oversized holes in the Orbiter's skin.

The other — to which MMC gave considerably less attention — added rib-like standoffs to the Orbiter's skin. The honeycomb was then mechanically attached to oversized holes in the standoffs, leaving a gap between the underside of the honeycomb and the Orbiter skin.

Honeycomb panel attachment points would typically occur 10 inches apart over much of the Orbiter. Larger (up to 20 inches) and smaller (down to five inches) spacings were possible.

Seiferth's team used computer models to determine required ablator thickness, which would vary depending on its location on the Orbiter. All models assumed a maximum reentry deceleration equal to 2.5 times Earth's surface gravity (that is, 2.5 G) and a maximum allowable Orbiter aluminum skin temperature of 350° Fahrenheit, variables which indicated a relatively benign reentry environment (as compared to an Apollo lunar-return reentry, for example).

MMC used for its calculations properties of several types of ablative material it had developed for other missile and space projects (notably, the Titan missile family and the Viking Mars lander). It found that, for most locations on the Orbiter, its least robust ablator would be sufficient.

The ablative layer for most locations could be surprisingly thin. For the simplest mechanically attached panel design, for example, the MMC computer models indicated that a point on the Orbiter's underside on the fuselage centerline 50 feet aft of its nose would need a layer of ablative material only 1.7 inches thick.

Assessing the cost of the ablative designs relative to RSI was difficult in part because Space Shuttle Program cost estimation was, for want of a better term, eccentric. Seiferth supplied no development or operations cost estimate for RSI in his report, though he did provide estimates for several of MMC's ablative designs.

A system with an ablator glued directly to the Orbiter's aluminum skin would, Seiferth estimated, cost a total of $164.8 million for 151 flights over five years. Of this, installation and removal would account for $27.9 million.

A mechanically attached system comprising an aluminum sheet, adhesive, and an ablator (that is, the simplest mechanically attached ablative system) with attachment points five inches apart would cost $168.3 million with an installation and removal cost of $21.9 million. The aluminum honeycomb system with no standoffs and attachment points five inches apart came in at $187.1 million with $25.7 million for installation and removal.

NASA provided MMC with an RSI weight estimate of 30,240 pounds, enabling an RSI/ablative system weight comparison. The MMC study determined that an ablator directly attached to the Orbiter's skin would weigh 27,199 pounds, while the sheet and honeycomb (no standoffs) mechanically attached systems would weigh 32,577 pounds and 32,158 pounds, respectively.

Seiferth noted that modifications to the Orbiter's aluminum skin design would need to be put in place soon if mechanically attached ablative panels were used. Delaying until after the Orbiter's skin was in place would make prohibitive the cost and difficulty of adopting the ablative Space Shuttle heat shield. By the time Seiferth's report saw print in October 1975 — more than two years after the MMC study concluded — a stand-in ablative heat shield, never high on NASA's list of Space Shuttle priorities, was in fact no longer an option.

Late in the 1970s decade, problems with the Space Shuttle Main Engine, RSI, computers, and systems contributed to delays in STS-1, the Space Shuttle's orbital maiden flight. RSI problems in particular became very public in March 1979, when the Space Shuttle Orbiter Columbia flew from California to NASA Kennedy Space Center (KSC), Florida, atop its 747 carrier aircraft. It was the first Orbiter's first visit to its home base. At the time, Columbia was scheduled to carry out STS-1 in November 1979.

Columbia rolls into the Orbiter Processing Facility at Kennedy Space Center, Florida, on 25 March 1979. Though the image displays only the area around the front of the fuselage, many RSI gaps are evident. Image credit: NASA
For the cross-country flight, about 26,000 permanent RSI tiles were installed on Columbia, along with about 5000 foam "dummy" tiles. By the time the Orbiter/747 combination set down on the Shuttle Landing Facility strip at KSC on 25 March 1979, Columbia had lost more than 200 RSI tiles. Many were lost as more than 4800 of the dummy tiles tore loose, a condition which would not occur during space flight.

Some permanent RSI tiles had, however, fallen off Columbia for other reasons. Close examination revealed tile manufacturing flaws, installation errors, and an overall unexpected degree of fragility. Even as Columbia entered the processing flow for STS-1, NASA conceded that the flight might be delayed until 1980.

Much was made of the "zipper effect," a hypothetical catastrophic failure mode that would begin with the loss of a single tile during reentry. The Orbiter was believed likely to survive loss of a single tile unless it occurred in an especially critical area. Loss of a single tile anywhere would, however, weaken surrounding tiles, potentially leading to a cascading loss of thermal protection. In fact, few tiles fell off Orbiters during the series of 135 Shuttle missions that began with Columbia's first launch on 12 April 1981.

The RSI system did, however, prove prone to impact damage during processing, launch, landing, and transport. The most extreme example before January 2003 occurred during STS-27 (2-6 December 1988), a classified Department of Defense mission. Eighty-five seconds after liftoff, debris broke free from the right Solid Rocket Booster, battering the right wing of Orbiter Atlantis. More than 700 RSI tiles were damaged and one was lost. Because the mission was classified, the near-disaster was not widely known for nearly 20 years.

This closeup of the right wing of the Orbiter Discovery was taken from the International Space Station (ISS) during STS-114 (26 July-9 August 2005), the first post-Columbia "Return-to-Flight" Mission. After the Columbia accident, NASA modified the External Tank design to eliminate the possibility of debris separation; nevertheless, two pieces of icy foam insulation broke free during STS-114, with one striking Discovery. In addition to a tile repair kit, which the STS-114 crew tested during a scheduled spacewalk, Discovery carried a Shuttle Remote Manipulator ("robot arm") extension that enabled its crew to inspect its RSI surfaces; it also performed a slow flip near the ISS so that astronauts on the station could inspect and photograph it. Though no damage was found, NASA prudently grounded the Shuttle fleet for another year after STS-114 returned to Earth so that it could continue its efforts to solve the External Tank debris problem. Image credit: NASA
The Space Shuttle Orbiter Columbia lifted off on 16 January 2003 at the beginning of mission STS-107, its 28th flight and one of the few remaining non-ISS missions NASA had scheduled for the Shuttle fleet. During ascent, a piece of water ice-impregnated insulating foam weighing almost two pounds broke free from the External Tank to which Columbia was mounted. It struck the Reinforced Carbon-Carbon leading edge of the Orbiter's left wing, punching a hole at least 10 inches wide.

The debris strike was captured on video and immediately became the subject of urgent debate within the Shuttle Program. Knowledge of the strike was not shared widely. The viewing angle meant that the strike area was not visible in launch video recorded from the ground and its location meant that the STS-107 crew could not see it. Managers decided that Columbia's wing leading edge was probably intact.

The hole admitted hot gas as Columbia reentered on 1 February 2003. Its internal structure compromised, NASA's oldest Orbiter broke up over east Texas and western Louisiana, killing its seven-person crew and grounding the Space Shuttle fleet for 30 months.

The following January, President George W. Bush declared that the Space Shuttle would be retired after it performed its last International Space Station (ISS) assembly mission. The final Shuttle flight, STS-135 (8-21 July 2011), saw Atlantis, veteran of the STS-27 near miss, deliver supplies to ISS ahead of an anticipated gap in U.S. piloted space flights of indefinite duration.


"Space Shuttle Orbiter and Subsystems," D. Whitman, Rockwell International Corporation; paper presented at the 11th Space Congress in Cocoa Beach, Florida, 17-19 April 1974.

Ablative Heat Shield Design for Space Shuttle, NASA CR-2579, R. Seiferth, Denver Division, Martin Marietta Corporation, October 1975.

"Space Shuttle Orbiter Status April 1980," S. Jones, NASA Johnson Space Center; paper presented at the 17th Space Congress in Cocoa Beach, Florida, 30 April-2 May 1980.

STS-27R OV-104 Orbiter TPS Damage Review Team, Volume I, Summary Report, NASA TM-100355, February 1989.

More Information

X-15: Lessons for Reusable Winged Spaceflight (1966)

Where to Launch and Land the Space Shuttle? (1971-1972)

What If a Space Shuttle Orbiter Had to Ditch? (1975)

What If a Space Shuttle Orbiter Struck a Bird? (1988)

Revival: A Piloted Mars Flyby in the 1990s (1985)

An Orbital Transfer Vehicle (OTV) carrying a drum-shaped Command Module aerobrakes in Earth's atmosphere in this NASA painting by Pat Rawlings. 
In the 1960s, NASA expended nearly as much study money and effort on piloted Mars and Venus flyby mission planning as it did on its more widely known plans for piloted Mars landings. Italian aviation and rocketry pioneer Gaetano Crocco had described a free-return piloted Mars/Venus flyby mission in 1956. Piloted flyby studies within NASA began with the EMPIRE study the Marshall Space Flight Center (MSFC) Future Projects Office initiated in 1962 and culminated in the NASA-wide Planetary Joint Action Group (JAG) piloted flyby study of 1966-1967.

The Planetary JAG, led by the NASA Headquarters Office of Manned Space Flight, brought together engineers from MSFC, Kennedy Space Center, the Manned Spacecraft Center (MSC), and Washington, DC-based planning contractor Bellcomm. It issued a Phase I report in October 1966 and continued Phase II study work in Fiscal Year (FY) 1967. The Phase I report emphasized a piloted Mars flyby mission in 1975, but included Mars and Venus flyby missions tailored to low-energy mission opportunities through 1981. All would be based on hardware developed for the Apollo Program and its planned successor, the Apollo Applications Program (AAP).

The piloted flyby spacecraft would carry automated probes, including one that would land on Mars, collect a sample of surface material and launch it back to the flyby spacecraft for immediate analysis. A leading point in favor of the piloted flyby mission was, in fact, the ability of the flyby crew to examine a Mars sample for signs of life less than an an hour after it left the martian surface.

Red planet off the port bow: a piloted flyby spacecraft based on Apollo spacecraft hardware releases probes as it passes Mars. Image credit: Douglas Aircraft Company.
According to Edward Clinton Ezell and Linda Neumann Ezell, writing in their 1984 NASA-published history On Mars: Exploration of the Red Planet, 1958-1978, NASA MSC was largely responsible for the demise of 1960s piloted flyby mission planning. On 3 August 1967, the Houston, Texas-based center, home of the astronaut corps and Mission Control, distributed to 28 aerospace companies a Request for Proposal (RFP) for a piloted Mars flyby spacecraft sample-returner design study. By doing this, MSC appeared to disregard warnings from Congress that no new NASA program starts would be tolerated.

In the summer of 1967, NASA was preoccupied with recovery from the 27 January 1967 Apollo 1 fire, which had killed astronauts Virgil Grissom, Roger Chaffee, and Ed White. Many in Congress felt that NASA had been lax in enforcing quality and safety standards at North American Aviation, the Apollo Command and Service Module spacecraft prime contractor, so deserved to be "punished" for the accident. Other members of Congress were angered by NASA's apparent failure to share its concerns regarding NAA's performance so they could exercise Congressional oversight. They did not, however, wish to cut Apollo funding and endanger accomplishment of Apollo's very public goal of a man on the Moon by 1970.

In addition, by August 1967, the Federal budget deficit for FY 1967 had reached $30 billion. Though negligible by modern standards, this was a shocking sum in 1967. The deficit was driven in large part by the cost of fighting in Indochina, which had reached more than $2 billion a month, or the entire Apollo Program budget of $25 billion every 10 months.

After learning of the MSC RFP, long-time House Space Committee Chair and NASA supporter Joseph Karth declared angrily that "a manned mission to Mars or Venus by 1975 or 1977 is now and always has been out of the question - and anyone who persists in this kind of misallocation of resources. . .is going to be stopped." On 16 August, the House cut all funding for advanced planning from NASA's FY 1968 budget bill and slashed the budget for AAP from $455 million to $122 million. Total cuts to President Lyndon Baines Johnson's January 1967 FY 1968 NASA budget request amounted to more than $500 million, or about 10% of NASA's FY 1967 budget total.

Though he opposed the cuts, President Johnson bowed to the inevitable and signed the budget into law. The Planetary JAG and Bellcomm tied up loose ends of the piloted flyby study during FY 1968, but most work on the concept ended within a few months of the Houston center's ill-timed RFP.

It is ironic, then, that NASA's next piloted Mars flyby study took place in Houston, at Johnson Space Center (JSC), as MSC had been re-christened following President Johnson's death in January 1973. Barney Roberts, an engineer in the JSC Engineering Directorate, reported on the study to the joint NASA-Los Alamos National Laboratory (LANL) Manned Mars Missions workshop in June 1985.

The workshop, held at NASA MSFC, was a significant step in the revival of piloted Mars exploration planning within NASA after the long drought of the 1970s. Unfortunately, in their plan for a piloted Mars flyby in the 1990s, NASA JSC engineers demonstrated little sign of awareness of the 1960s piloted flyby studies. As a result, their proposed mission was less credible than it might have been.

Roberts explained that the NASA JSC flyby plan aimed to counter a possible Soviet piloted Mars flyby. He cited a 1984 Central Intelligence Agency (CIA) memorandum that suggested (without citing much in the way of evidence) that the Soviet Union might attempt such a mission in the 1990s — possibly as early as the 75th anniversary of the Bolshevik Revolution in 1992 — in order to garner international prestige. The CIA study had been performed at the request of Apollo 17 moonwalker Harrison Schmitt, whose chief spaceflight interest in the early-to-mid 1980s was a piloted Mars mission.

NASA's piloted Mars flyby would be based on space hardware expected to be operational and readily available in the late 1990s. Space Shuttle Orbiters would deliver to NASA's Space Station an 18-ton Mission Module (MM) and a pair of expendable propellant tanks with an empty mass of 11.6 tons each. The MM, derived from a Space Station module, would carry a 3000-pound solar-flare radiation shelter, 7000 pounds of science equipment, and 2300 pounds of food and water.

Going for a ride: a piloted Mars flyby spacecraft prepares for launch from Earth orbit in the late 1990s. A = twin Orbital Transfer Vehicles (OTVs); B = twin strap-on propellant tanks; C = Command Module; D = Mission Module. Image credit: NASA/David S. F. Portree.
The MM would be docked to a six-ton Command Module (CM) and two 5.75-ton Orbital Transfer Vehicles (OTVs). The OTVs would each include an aerobrake heat shield and two rocket engines derived from the Space Shuttle Main Engine. The JSC engineers had assumed that the CM and OTVs would be in space already as part of a late 1990s NASA Lunar Base Program. The strap-on tanks would be joined to the MM/CM stack by trunnion pins similar to those used to anchor payloads in the Space Shuttle Orbiter payload bay, then Space Station astronauts would perform spacewalks to link propellant pipes and electrical and control cables.

Shuttle-derived heavy-lift rockets would then deliver a total of 221 tons of cryogenic liquid hydrogen and liquid oxygen propellants to the Space Station to fill the piloted flyby spacecraft's twin tanks. The propellants would be pumped aboard just prior to departure from Earth orbit to prevent liquid hydrogen loss through boil off. Mass of the piloted flyby spacecraft at the start of its Earth-departure maneuver would total 358 tons.

As the launch window for the Mars flyby opportunity opened, the piloted flyby spacecraft would move away from the Space Station using small thrusters on retractable arms, then the four OTV engines would ignite and burn for about one hour to put it on course for Mars. The only propulsive maneuver of the baseline mission, the burn would empty the OTV and strap-on propellant tanks. Roberts advised retaining the spent tanks to serve as shielding against meteoroids and radiation for the MM and CM during the year-long flight.

Roberts told the workshop that the flyby spacecraft would spend two-and-a-half hours within about 20,000 miles of Mars. Closest approach would bring it to within 160 miles of Mars. At closest approach, the spacecraft would be moving at about 5 miles per second.

The spacecraft would then begin its long return to Earth. Roberts provided few details of the interplanetary phases of his piloted Mars flyby mission.

As Earth grew from a bright star to a distant disk, the Mars flyby astronauts would discard the twin strap-on tanks. They would then undock one OTV by remote control and re-dock it to the front of the CM. After entering the CM and sealing the hatch leading to the MM, they would discard the MM and second OTV, then would then strap into their couches to prepare for aerobraking in Earth's upper atmosphere and capture into Earth orbit. After the OTV/CM combination completed the aerobraking maneuver, the crew would pilot it to a docking with the Space Station.

Almost home: the piloted Mars spacecraft prepares for the aerobraking maneuver in Earth's atmosphere at the end of its epic year-long interplanetary voyage. A = OTVs; C = Command Module bearing crew; D = discarded Mission Module (attached to discarded OTV). Image credit: NASA/David S. F. Portree.
Roberts told the NASA/LANL workshop that Earth return would be the most challenging phase of the piloted Mars flyby mission. The OTV/CM combination would encounter Earth's upper atmosphere at a speed of 55,000 feet (10.4 miles) per second, producing reentry heating well beyond the planned capacity of the OTV's heat shield. In addition, the crew would suffer "exorbitant" deceleration after living for a year in weightlessness.

Roberts proposed a "brute-force" solution to these problems: use the OTV's twin rocket motors to slow the OTV/CM to lunar-return speed of 35,000 feet (6.6 miles) per second. The braking burn would, however, increase the Mars flyby spacecraft's total required propellant load to nearly 500 tons. He calculated that, assuming that a Shuttle-derived heavy-lift rocket could be designed to deliver cargo to LEO at a cost of $500 per pound (an optimistic assumption, as it would turn out), then Earth-braking propellant would add $250 million to his mission's cost.

Roberts briefly considered partially compensating for the large mass of braking propellants by substituting an MM derived from a five-ton Space Station logistics module for the 18-ton MM. This would mean, however, that the crew would have to spend a year in cramped quarters with no exercise or science equipment.

Planners in the 1960s had wrestled with and prevailed over the same problems of propellant mass and Earth-return speed that NASA JSC engineers faced in their 1985 study. Bellcomm had, for example, proposed in June 1967 that the Planetary JAG's piloted Mars flyby mission conserve propellants through assembly of the flyby spacecraft in an elliptical orbit, not circular Space Station orbit. The elliptical assembly orbit would mean, in effect, that the flyby spacecraft would begin Earth-orbit departure even as it was being assembled.

In addition, returning the crew directly to Earth's surface in a small Apollo-type capsule with a beefed-up heat shield would greatly reduce the quantity of braking propellants required; it could eliminate the braking maneuver entirely. It would also enable an aerodynamic "skip" maneuver that would reduce deceleration stress on the astronauts.

TRW Space Technology Laboratory had proposed as early as 1964, during the EMPIRE follow-on studies, that NASA use a Venus flyby to slow spacecraft returning from Mars. Crocco had described the concept in 1956, in fact, though in a form that turned out to be unworkable because of errors he made when he calculated his flyby spacecraft's orbit about the Sun.

Exploiting a Venus flyby to reduce speed would, of course, limit Earth-Mars-Earth transfer opportunities to those that would intersect Venus on the return leg, but would also eliminate the costly end-of-mission braking burn and enable Venus exploration as a bonus. The Planetary JAG's October 1966 report described Mars-Venus and Venus-Mars-Venus flyby missions in the late 1970s. Bellcomm determined in late 1966 and 1967 that Mars/Venus flyby opportunities are not rare.


"Soviet Plans for a Manned Flight to Mars," C. Cravotta and M. DeForth, Office of Scientific and Weapons Research, Central Intelligence Agency, 2 April 1985.

"Concept for a Manned Mars Flyby," Barney B. Roberts, Manned Mars Missions: Working Group Papers, Volume 1, NASA M002, NASA/LANL, June 1986, pp. 203-218; proceedings of a workshop held at NASA Marshall Space Flight Center, Huntsville, Alabama, 10-14 June 1985.

On Mars: Exploration of the Red Planet, 1958-1978, NASA SP-4212, Edward Clinton Ezell & Linda Neuman Ezell, NASA History Office, 1984, pp. 117-118.

Humans to Mars: Fifty Years of Mission Planning, 1950-2000, Monographs in Aerospace History #21, NASA SP-2001-4521, David S. F. Portree, NASA History Division, February 2001, pp. 11-12, 15, 60-62.

More Information

After EMPIRE: Using Apollo Technology to Explore Mars and Venus (1965)

Relighting the FIRE: A 1966 Proposal for Piloted Interplanetary Mission Reentry Tests

Apollo Ends at Venus: A 1967 Proposal for Single-Launch Piloted Venus Flybys in 1972, 1973, and 1975

Triple Flyby: Venus-Mars-Venus Piloted Flyby Missions in the Late 1970s/Early 1980s (1967)

Flyby's Last Gasp: North American Rockwell's S-IIB Interplanetary Booster (1968)

Astronaut Telescope Servicing at Earth-Sun L2 (1999)

Interplanetary space showing the positions of the Sun, Earth, Earth's orbit about the Sun, the Moon, the Moon's orbit about the Earth, and the five Earth-Sun Libration Points. Image credit: NASA.
The Earth-Moon and Sun-Earth Libration (L) points are not places in the sense that one can land on them and pick up rocks. Because of this, some space exploration planners perceive them to be unsatisfying destinations. The L points have, however, long been proposed as space transportation way stations and as radio relay and scientific instrument sites.

In 1999, the Decadal Planning Team (DPT), a secretive NASA-wide study group chartered by President William Clinton's Office of Management and Budget, identified astronomical observatories in "halo orbits" around the Sun-Earth L points as a key NASA goal for the early 21st century. These large and complex instruments would, among other tasks, seek to observe Earth-like worlds around other stars.

The NASA Exploration Team (NExT), the DPT's immediate successor, subsequently sought to incorporate the Sun-Earth L point emphasis into its piloted spaceflight planning. In a 20 December 1999 presentation to the NeXT, for example, NASA Johnson Space Center exploration planner Bret Drake examined ways that the Sun-Earth L points might aid future piloted Mars missions.

An automated solar observatory orbiting the Sun-Earth L1 point, 1.5 million kilometers from Earth, could provide Mars crews with early warning of solar flares, Drake explained. Radio relays in halo orbit about Sun-Earth L4, 60° ahead of the Earth along its Sun-centered orbit, and Sun-Earth L5, 60° behind the Earth along its orbit, could enable continuous radio communication between Earth and crews exploring Mars during superior conjunctions, when the Sun blocks line-of-sight radio contact between the two planets.

Drake hastened to add that the Sun-Earth L points would not be good staging places for piloted Mars missions. He explained that the trip to and from a Sun-Earth L point would add almost two months to the typical duration of a roundtrip Mars voyage that started from low-Earth orbit (LEO).

Piloted missions to Sun-Earth L points might, however, serve as experience-building intermediate steps between piloted LEO missions and piloted Mars missions. Drake suggested that L point missions could enable astronauts to experience interplanetary conditions (for example, solar radiation undiminished by Earth's magnetic field), yet would have one-way trip times as short as 25 days.

Drake proposed that NASA astronauts carry out a 100-day telescope-servicing mission to Sun-Earth L2, 1.5 million miles from Earth. The mission would employ Solar-Electric Propulsion (SEP) technologies and techniques first proposed in 1998 for NASA's Mars Design Reference Mission.

The mission would begin with the unmanned launch to LEO of a 32,975-kilogram telescope-servicing spacecraft comprising a 14,450-kilogram inflatable "mini-Transhab" crew module, a 4271-kilogram Apollo Command Module-shaped Earth Return Vehicle (ERV), and a 14,164-kilogram two-stage Chemical Propulsion Module. The spacecraft would reach LEO within the streamlined shroud of a next-generation expendable rocket called an Evolved Expendable Launch Vehicle-Heavy (EELV-H).

A Space Shuttle Orbiter would rendezvous with the telescope-servicing spacecraft in LEO so that astronauts could oversee inflation of the doughnut-shaped single-deck mini-Transhab and deployment of its twin electricity-generating solar arrays. They would install equipment and furnishings in the mini-Transhab and stock it with supplies, then would return to Earth.

A second EELV-H would place a 33,000-kilogram automated Solar-Electric Propulsion (SEP) Vehicle into LEO, where it would automatically deploy solar-array wings and dock with the telescope-servicing spacecraft. Over the next seven months, the SEP Vehicle would operate its electric-propulsion thrusters at perigee (the low point in its orbit about the Earth) to raise its apogee (the high point in its orbit).

The result of these SEP Boost Phase maneuvers would be a highly elliptical orbit loosely bound to the Earth. The SEP Vehicle would then detach from the telescope-servicing spacecraft and operate its thrusters at apogee to return to LEO for refurbishment and reuse.

Use of the SEP Vehicle to place the telescope-servicing spacecraft into a highly elliptical Earth orbit would dramatically reduce the quantity of chemical propellants required to leave LEO for Earth-Sun L2. SEP thrusters produce little thrust but can do so over long periods and expend little propellant. This approach would greatly reduce overall mission mass and the number of EELV-H and Shuttle Orbiter flights required to place the telescope-servicing spacecraft into LEO.

The telescope-servicing spacecraft would carry no crew during the SEP Boost Phase because it would pass through Earth's radiation belts repeatedly. Over time, this would subject the crew to an unacceptably high cumulative radiation dose.

Drake inserted into his telescope-servicing mission assembly-and-launch sequence an optional piloted mission that would fly only if the telescope-servicing spacecraft needed repairs following the SEP Boost Phase. A Shuttle Orbiter would deliver to LEO a maintenance crew, a small lifting-body Crew Taxi, and a chemical-propulsion rocket stage. The stage would rapidly boost the Taxi into a highly elliptical Earth orbit matching that of the telescope-servicing spacecraft.

The maintenance crew would rendezvous and dock with the telescope-servicing spacecraft. After completing the needed repairs, they would undock, fire the Crew Taxi's rocket motors at apogee to lower its perigee into Earth's atmosphere, perform reentry, and glide to a landing.

If, however, flight controllers on Earth determined that the telescope-servicing spacecraft in highly elliptical Earth orbit was healthy and that no repairs were needed, the Crew Taxi would deliver a four-person crew to the telescope-servicing spacecraft. After casting off the Taxi, they would ignite the telescope-servicing spacecraft's first chemical-propulsion stage at perigee to escape their loosely bound highly elliptical orbit and begin the 25-day voyage to Sun-Earth L2. They would then cast off the spent stage.

In the Sun-Earth L2 Operations Phase, the telescope-servicing spacecraft would enter a "halo parking orbit" centered on Sun-Earth L2. For 50 days the astronauts would service large next-generation telescopes in halo orbits around Sun-Earth L2, much as Space Shuttle crews in 1993, 1997, 1999, 2002, and 2009 serviced the Hubble Space Telescope in LEO. Drake suggested that during their down time between servicing calls they might conduct unspecified scientific research.

Their mission completed, the astronauts would ignite the second stage of the telescope-servicing spacecraft's Chemical Propulsion Module to begin return to Earth. About 25 days later, they would strap into the ERV capsule, undock from their home of the previous 100 days, reenter Earth's atmosphere, and parachute to a landing. The other components of the telescope-servicing spacecraft would burn up in Earth's atmosphere.

Even as Drake presented his Earth-Sun L2 servicing mission concept, NASA engineers conceived of a Gateway space station in halo orbit about Earth-Moon L1 as a base for observatory servicing and as a stepping stone to points all over the lunar surface. They envisioned that observatories needing servicing would ignite small thrusters to begin a slow transfer from their Earth-Sun L1 and L2 halo orbits to the vicinity of the Gateway. Once at Earth-Moon L1, they would be serviced by spacewalking astronauts, "cherry picker" booms, and teleoperated systems.

Flying formation with teleoperated systems, an advanced space telescope arrives in the vicinity of the Earth-Moon L1 Gateway. The twin red spheres carry imagers that supply information on the telescope to astronauts inside the Gateway. As they escort the telescope, a boxy teleoperated robot with several jointed appendages moves into the shadow cast by its multi-layer sunshield. Partially silhouetted against the Moon, the Gateway includes six solar arrays, a doughnut-shaped pressurized mini-Transhab habitat module, multiple docking ports, servicing equipment, and three rocket stages for unspecified missions. Please click on the image to enlarge. Image credit: NASA
Cislunar space showing the positions of Earth, the Moon, the Moon's orbit about Earth, and the five Earth-Moon Libration Points. Image credit: NASA
In January 2004, in the aftermath of the STS-107 Columbia Space Shuttle accident (1 February 2003) and at the start of the 2004 election cycle, President George W. Bush called for a new NASA program to take humans to the Moon and Mars. At first, the Vision for Space Exploration (VSE), as it became known, incorporated many elements of DPT/NExT.

Soon after Michael Griffin became NASA Administrator on 13 April 2005, however, the VSE veered away from DPT/NExT and toward the Constellation Program, which Griffin called "Apollo on steroids." Bush showed little interest in the VSE after he announced it, so did not intervene to keep his program on track.

Constellation and the VSE were mostly abandoned in 2009-2010 under President Barack Obama. The global economy was in crisis following the collapse of the U.S. housing market in 2008 and the near-collapse of the global financial system. Spaceflight, rarely a high priority, took a back seat to repairing the U.S. economy.

When Obama unveiled a new space plan in 2010, it resembled DPT/NExT more than Constellation. The Bush Administration's decision to cancel the Space Shuttle led to the most significant deviation from the DPT/NExT architecture: retention of Constellation's large rocket under the name Space Launch System. Resembling an oversized EELV-H, SLS replaced the Shuttle Orbiter and the solar-electric tug of the DPT/NExT plan. The Orion Crew Exploration Vehicle (CEV) replaced the lifting-body taxi.

Meanwhile, China launched a program to explore the Moon using robots. Chang'e 1 orbited the Moon in 2007-2009; Chang'e 2 orbited the Moon in 2010-2012 before leaving lunar orbit for a flyby of the Near-Earth Asteroid 4179 Toutatis; and Chang'e 3 landed on the Moon in late 2013.

Chang'e 4, targeted for the lunar farside hemisphere, landed successfully in January 2019. It transmits radio signals to Earth via the Queqiao satellite, which reached a halo orbit around Earth-Moon L2 in June 2018. In addition to relaying signals from Chang'e 4 and its rover to Earth, Queqiao also serves as a radio observatory remote from the radio noise of Earth.

A radio-relay satellite in Earth-Moon L2 halo orbit enables communication with spacecraft out of line-of-sight radio contact on the hidden farside hemisphere of the Moon. Image credit: NASA

"Future Missions for Libration-point Satellites," R. Farquhar, Astronautics & Aeronautics, May 1969, pp. 52-56.

"Strategic Considerations for Cislunar Space Infrastructure," IAF-93-Q.5.416, W. Mendell and S. Hoffman; paper presented at the 44th Congress of the International Astronautical Federation, 16-22 October 1993.

"Representative Human Missions to the Sun-Earth Libration Point (L2) '100' Day Class Mission," SEL2 Ver. R, Bret G. Drake, NASA Johnson Space Center, presentation materials, 20 December 1999.

"'Invisible Planets' Gain Favor as Real Estate in Space," L. David,, 19 January 2000.

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Apollo Science and Sites: The Sonett Report (1963)

Apollo 17 Lunar Module Pilot Harrison Schmitt, a geologist, was the only professional scientist to reach the Moon. Image credit: NASA.
The Apollo Program was driven by the perceived national need to decisively demonstrate American technological prowess in the face of early Soviet space victories. Scientific lunar exploration was a secondary concern. In fact, some engineers saw lunar science as a distraction from the already daunting task of landing a man on the Moon and returning him safely to Earth.

The community of lunar scientists was small in May 1961, when President John F. Kennedy put the U.S. on the road to the Moon. Nevertheless, lunar science had its energetic proponents. In early 1962, they saw to it that NASA's Office of Manned Space Flight (OMSF) asked NASA's Office of Space Science (OSS) to outline an Apollo science program. OSS appointed NASA physicist Charles Sonett to head up an ad hoc working group and OMSF provided the group with guidelines for its deliberations.

The Sonett group's 12 members and nine consultants included U.S. Geological Survey geologist (and aspiring astronaut) Eugene Shoemaker, astronomers Gerard Kuiper and Thomas Gold, NASA geophysicist Paul Lowman, and chemist (and Nobel Laureate) Harold Urey. They circulated their July 1962 draft report at the National Academy of Science's 10-week Iowa City meeting (17 June-31 August 1962) and within NASA, receiving, they reported, "general endorsement" for their recommendations.

The final version of the Sonett report, published in December 1963 and labeled "for internal NASA use only," was the first in a series of influential Apollo planning documents that called for ambitious scientific exploration of the Moon. Its recommendations touched on many aspects of Apollo mission planning.

The Sonett group called for all proposed Apollo landing sites to be photographed by automated Lunar Orbiter spacecraft before final site selection. Lunar Orbiter photographs would be used to make detailed geological maps of planned landing sites. This, the Sonett group's members argued, would save precious time during Apollo landing missions, because it would enable astronauts to begin geological field work without first mapping their landing site.

They urged that every two-person Apollo landing crew include a scientist-astronaut with a Ph.D. in geology and from five to 10 years of field experience. Geologists on Earth would explore the Moon vicariously through his descriptions and through real-time television transmitted from a camera mounted on his space suit.

They acknowledged that Kennedy's end-of-decade deadline for reaching the Moon meant that Apollo scientist-astronauts would probably be drawn from the community of scientists already at work in 1962-1963. They assumed, however, that Apollo would be merely the first U.S. program of piloted lunar exploration, so urged that "graduate students and young post graduate scientists. . .be brought into the field of lunar science as potential astronauts as soon as possible."

OMSF had advised the Sonett group that the Apollo lunar surface space suit would "limit the crew's ability to act, particularly in performing precise manipulations." In their final report, the group's members, undaunted by anticipated technological limitations, urged early development of surface suits that would "permit a close approximation to unsuited limb, arm, and digital [finger] movements."

Sonett working group member Eugene Shoemaker models a pressure suit proposed for advanced Apollo Extension System lunar exploration missions. He stands outside the hatch of a mockup long-range lunar rover. Image credit: U.S. Geological Survey.
OMSF also told the group that a space-suited Apollo astronaut would probably be unable to walk more than a half-mile from his lunar lander, but raised the possibility of a rover or other mobility aids. The Sonett group declared that
. . .reconnaissance beyond a one-half mile radius of the spacecraft will be a necessity. . .For example, a lunar ray, a feature of great interest, is probably a poor place to land, yet the capability of traveling to a ray area is clearly indicated. . .For scientific purposes, therefore, there should be the capability of reaching areas some 50 miles from a landing site.
In 1962-1963, OMSF considered development of an automated lander capable of delivering to the Moon up to 15 tons of equipment and supplies. In addition to a beacon for guiding an Apollo Lunar Excursion Module (LEM) piloted lander to a safe touchdown nearby, it would carry one or more rovers and expendables — for example, liquid and gaseous oxygen — for extending LEM electricity-generation and life-support capabilities. The Sonett group urged OMSF to proceed with cargo lander development, noting that the LEM as planned would carry supplies and equipment inadequate to accomplish "even the modest scientific program recommended."

The Apollo LEM lander and lunar surface space suit as envisioned in 1964. Image credit: NASA.
OMSF informed the Sonett working group that the first Apollo lunar surface mission would probably spend only four hours on the Moon. The group urged OMSF to double that stay time so the astronauts could budget four hours for operational activities (for example, checking out their LEM before departing the Moon) and four hours for exploring the lunar surface. During their lunar traverses, they would take turns moving beyond the immediate vicinity of the LEM, collect up to 100 pounds of rocks, test soil strength, and study whether solar heating caused Moon dust to flow like a highly viscous liquid, as hypothesized by Sonett group member Thomas Gold.

The group acknowledged that "an accident" might limit surface exploration during the first Apollo landing mission to one hour. In that case, a single moonwalker would hurriedly collect about 50 pounds of geological samples near the LEM.

The group's members envisioned a five-day Apollo mission with four days of uninterrupted exploration, during which the two astronauts would drive a rover up to 10 miles from their landing site. They would also drill a hole up to 20 feet deep and insert a heat probe, collect samples "for biological purposes," and emplace a seismometer, a micrometeorite detector, and other instrument packages. They expected that the instruments would be linked by cables to a "central station" containing a radio transmitter. This would use a nuclear source to generate electricity so that it could relay data from the instruments to Earth for months or years.

OMSF asked the Sonett working group to assume "more than one but less than ten" Apollo landings. Apollo landings would, OMSF explained, be limited to sites near the equator on the side of the Moon that faces Earth. The Sonett group recommended that the first Apollo piloted lander set down near Copernicus crater.

Sonett group member Eugene Shoemaker was probably behind the Copernicus site choice; he had spent a great deal of time studying the crater starting in the late 1950s as part of his effort to resolve the debate over whether lunar craters were primarily the result of volcanism or of asteroid impacts and to establish the stratigraphic sequence of the Moon's geologic units. The latter was a requirement if the history of the lunar surface would be deciphered.

Copernicus, a leading Sonett group Apollo landing site candidate, as portrayed in an early 1960s map. Moon maps in this series, based on photos from Earth-based telescopes, were the best available at the time the Sonett Group wrote its report. Image credit: Lunar and Planetary Institute. 
In keeping with their conviction that lunar exploration should continue beyond Apollo, the Sonett group scientists offered two lists with a total of 28 candidate landing sites. All sites were selected using photographs taken using Earth-based telescopes.

The first list of 15 sites, compiled in June 1962 by Eugene Shoemaker and R. E. Eggleton, another U.S. Geological Survey geologist, took into account "possible landing conditions and trafficability, and prospects of discovering natural shelter and potential water supplies."
  • 9.8° North (N), 20.1° West (W), near the Copernicus central peaks
  • 13.1° N, 31° W, on a "typical lunar dome" near the crater Tobias Mayer
  • 20.4° N, 3° W, on the southeast edge of Mare Imbrium, near Mt. Huyghens in the Apennine Mountains
  • 12.6° N, 2° W, in Alphonsus crater, site of suspected on-going lunar volcanism
  • 7.7° N, 6.3° East (E), within four-mile-wide Hyginus ("one of the largest craters of likely volcanic origin"), located at a potentially significant bend in Hyginus Rille
  • 37.9° N, 16.4° W, near a "possible flow" in Mare Imbrium
  • 40.9° South (S), 11.1° W, on the "rubbly" north flank of the great ray crater Tycho
  • 50.6° S, 60.8° W, in Wargentin, an odd lava-filled crater
  • 85° S, 45° E, in the south polar crater Amundsen, where, it was believed, permanently shadowed areas might preserve ice deposits
  • 12.7° S, 49.8° W, on a "very bright" plateau north of the crater Billy
  • 41.7° N, 57.5° W, on Oceanus Procellarum north of the Rumker Hills
  • 5.6° S, 26.6° W, near a "small irregular depression" 35 miles southeast of the crater Hortensius
  • 5.1° N, 14.2° W, on dark material about 140 miles southeast of the center of Copernicus
  • 35.3° N, 5.5° W, on Mare Imbrium near the "mountainous block" Spitzbergen
  • 9.1° S, 16.1° W, on the north flank of crater Parry A, a natural drill hole exposing ancient dark material
The Sonett working group's second list was compiled by geochemist Duane Dugan of NASA's Ames Research Center.
  • 3° S, 44° W, in the middle of the Flamsteed Ring, a mostly submerged crater north of Flamsteed crater
  • 13° S, 2.3° W, in Alphonsus
  • 23.4° N, 43.3° W, near bright Aristarchus crater, flat-floored Herodotus crater, and sinuous Schröter's Rille (a region of suspected on-going lunar volcanism and many apparent volcanic features)
  • 23° N, 51.45° W, inside Herodotus
  • 20.3° N, 3.4° W, on Mare Imbrium west of Mt. Huyghens
  • 28° N, 12° E, on Mare Serenitatis near the unusual crater Linne, site of suspected on-going lunar volcanism
  • 19.3° S, 40.2° W, on safe, flat ground in Mare Humorum near the south wall of dark-floored Gassendi crater
  • 5.5° N, 14.3° W, in a "black" surface area east of Fauth crater
  • 5° S, 28.1° W, in the Ural and Riphaeus Mountains, near "old ghost rings" (submerged craters)
  • 9° S, 2° W, on the floor of Ptolemaeus crater, site of ridges, "craterlets," and a "crater cone" of "remarkable" whiteness
  • 15° N, 22° E, between crater Plinius and the Haemus Mountains, a place "with access to the color discontinuity between Mare Tranquillitatis and Mare Serenitatis"
  • 24.3° S, 43.4° W, in Mare Humorum east of the crater Liebig (site of "an interesting scarp" that cuts through craters)
  • 4.5° S, 25.5° E, in southern Mare Tranquillitatus, at the base of Theophilus crater rim west of Torricelli crater (a "very complex" region with "shading" reminiscent of "one of the terrestrial continental shelves")
That the Shoemaker-Eggleton and Dugan lists had in common only Alphonsus, the dark region near 5.5° N, 14.3° W, and Huyghens-Appenine reflected the wide range of attractive candidate lunar landing sites. Some of the proposed sites, such as Amundsen, lay beyond the equatorial zone OMSF had said Apollo could reach. The working group asserted that "there is no question that sites of the greatest scientific interest lie outside the equatorial belt," and urged that NASA develop the "capability of landing in the equatorial belt, at the poles, and elsewhere."

Alphonsus crater made both the Shoemaker-Eggleton and Dugan lists of candidate Apollo landing sites. Image credit: Lunar and Planetary Institute.
NASA paid attention to the Sonett report and other advice it received from scientists as it planned Apollo missions, but the complex interplay of competing technical, political, and scientific requirements meant that the space agency could give scientists no more than a small fraction of what they desired. Most notably, only one scientist-astronaut reached the Moon: geologist Harrison Schmitt (image at top of post), who explored the Taurus-Littrow valley east of Mare Serenitatis with Eugene Cernan during the Apollo 17 mission (7-19 December 1972).

Schmitt and Cernan spent three days on the Moon. They wore A7LB space suits which restricted their movements, but which were an improvement over the A7L suits worn by Apollo 11, 12, and 14 moonwalkers. They collected drill cores, deployed instruments and a heat-flow probe attached by cables to a nuclear-powered central station, and drove 35.9 kilometers using a jeep-like Lunar Roving Vehicle. All of their equipment arrived stowed on board the Apollo 17 Lunar Module Challenger; NASA developed no separate automated cargo lander. Apollo 17 returned 110.5 kilograms of geologic samples to researchers on Earth.

Astronauts explored few of the sites Shoemaker, Eggleton, and Dugan selected. The reasons for this were manifold: the U.S. flew only six successful Apollo lunar landing missions; NASA never became capable of landing men very far beyond the Nearside equatorial belt; new knowledge of the Moon from robotic missions and orbiting Apollos made some of the sites appear less scientifically significant than had been believed or less attractive than newly found candidate sites; and no follow-on lunar landing program materialized.

The first Apollo lunar landing mission, Apollo 11 (16-24 July 1969), spent about 21 hours on Mare Tranquillitatis, not in Copernicus; in fact, Copernicus remains unvisited today. Huyghens-Apennine became Hadley-Apennine; visited by Apollo 15 (26 July-7 August 1971), it is widely considered to be the most scientifically significant Apollo site.

Robots explored Alphonsus (Ranger 9, March 1965), the Flamsteed Ring (Surveyor 1, May-July 1966), and Tycho (Surveyor 7, January 1968). The lunar south pole and Aristarchus, as yet unvisited, are frequently mentioned as candidate landing sites for NASA's eventual return to the Moon.


Report of the Ad Hoc Working Group on Apollo Experiments and Training on the Scientific Aspects of the Apollo Program, 15 December 1963. 

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