"Without Hiatus": The Apollo Applications Program in June 1966

Raw material for a post-Apollo space program. Image credit: NASA.
Elsewhere in this blog, I have described how Apollo began in 1959 as a mainly Earth-orbital program (see "More Information" at the bottom of this post). As originally conceived, Apollo included a Mission Module that could serve as a small laboratory in Earth orbit. NASA anticipated that Apollo spacecraft would ferry astronauts, experiments, and supplies to a temporary Earth-orbital laboratory before the end of the 1960s decade; an Apollo spacecraft might also fly around the Moon without stopping in lunar orbit (that is, perform a circumlunar flight). After 1970, a new program would build on experience gained through Apollo, leading to either a permanent Space Station or a piloted lunar landing and voyages to the planets.

President John F. Kennedy's call for a man on the Moon by 1970 (25 May 1961) made Apollo the U.S. lunar landing program. Aware that Kennedy was not enthusiastic about space for its own sake, NASA Administrator James Webb was careful not to assume a national commitment to spaceflight beyond the Apollo lunar program.

Even as NASA came to terms with the Moon goal, however, it sought to keep alive the Space Station option. In April 1963, for example, the space agency tasked Apollo Command and Service Module (CSM) contractor North American Aviation with a study of how the CSM might serve as a Space Station crew rotation and logistics resupply vehicle. Some believed that, if Apollo accomplished a piloted Moon landing in 1967, then NASA might shift to the Space Station track in 1968.

In early 1964, new President Lyndon Baines Johnson called on NASA to declare its plans for U.S. piloted spaceflight after Apollo reached the Moon. In response, Webb formed the internal ad hoc Future Programs Task Group.

In January 1965, the Task Group submitted a report that favored a post-Apollo program built upon a technological foundation of Apollo CSM and Lunar Module (LM) spacecraft and Saturn IB and Saturn V rockets. The Task Group had drawn upon the expertise of Bellcomm, NASA's Apollo planning contractor, which that same month submitted a plan for 55 Saturn-launched Apollo test missions, Apollo lunar missions, post-Apollo lunar and Earth-orbital missions, and Voyager robotic Mars/Venus missions.

Image credit: NASA.
Image credit: NASA.
Image credit: NASA.
The Johnson White House accepted NASA's Apollo-based post-Apollo concept; the NASA Headquarters Office of Manned Space Flight (OMSF) then established the Saturn-Apollo Applications (SAA) Program Office in August 1965. A month later, SAA named the post-Apollo program the Apollo Applications Program (AAP).

Congress did not warm to AAP despite its promise to explore space for the benefit of people on Earth. The forces that would truncate the Apollo Program — for example, the human and fiscal cost of war in Indochina — were building. Though President Johnson went to bat for AAP in Fiscal Year (FY) 1966, he concurred when Congress requested a postponement in major funding for the program. Congressional leaders promised that, if possible, the funding delay would be made up in FY 1967 and subsequent years.

In June 1966, the SAA Program Office described in a memorandum dispatched to officials at the Manned Spacecraft Center (MSC), Marshall Space Flight Center (MSFC), and Kennedy Space Center (KSC) an AAP which, it said, would "continue without hiatus an active and productive post Apollo Program of manned space flight and. . .exploit for useful purposes. . .the capabilities of the Saturn Apollo System." The memo — a snapshot of a program undergoing rapid, chaotic change in response to funding challenges — explained that the plan it outlined was based on proposals NASA had submitted to President Johnson's Bureau of the Budget a month earlier.

AAP objectives fell into two basic areas. The first, Long-Duration Flights, would "measure the effects on men and on manned systems of space flights of increasing duration" and permit NASA to "acquire operational experience with increasingly longer manned space flights" so that it could "establish the basic capabilities required for any of the projected next generation of manned space flight goals (earth orbital space station, lunar station, or manned planetary exploration)."

The second emphasis area, Spaceflight Experiments, would emphasize space life sciences, astronomy, space physics, advanced lunar exploration, and space technology applications and development. AAP lunar exploration would support objectives proposed at the July 1965 meeting of space scientists in Falmouth, Massachusetts. The Falmouth meeting was one of a series of important lunar science planning meetings that began with the interdisciplinary Iowa City meeting in 1962.

At the time the SAA Program Office circulated its memo, the first Apollo lunar landing attempt was expected in late 1967 or early 1968. NASA, the memo explained, had ordered from its contractors 21 CSMs, 15 LMs, 12 Saturn IBs, and 15 Saturn Vs for delivery between 1966 and 1970. Most were intended for ground and flight tests.

The SAA Program Office assumed that four Saturn IBs (designated AS-209 through AS-212), six Saturn Vs (AS-510 through AS-515), and their associated CSM and LM spacecraft would remain unused after the first successful piloted Moon landing, and that these would immediately become available for AAP missions. Basic Apollo CSM and LM spacecraft would be modified to achieve new goals by the installation of "overlay kits."

Building upon these assumptions, the June 1966 memo described two possible AAP Program schedules. The Case I schedule assumed that no Saturn-Apollo hardware beyond that ordered for the Moon program would become available before late 1968 and that only enough AAP missions would be flown to accomplish minimal AAP goals. Case I missions would not necessarily serve as a bridge for linking Apollo lunar missions with a new piloted program in the mid-to-late 1970s. Even with these limitations, the SAA Program Office envisioned that Case I would see 21 Saturn IB and 16 Saturn V launches in the AAP by the end of 1973.

The more ambitious Case II schedule would see "an early extensive utilization of Saturn Apollo capabilities, with an earlier focus on a post-Apollo national space objective (such as a prototype of a space station or a planetary mission module)." Case II would see 26 Saturn IB and 17 Saturn V rockets launched from KSC before the end of 1975.

Both the Case I and Case II schedules would begin in 1968 with missions AS-209, AS-210, AS-211, and AS-212. AS-209 and AS-210, concurrent 14-day Earth-orbital life sciences/crew training missions launched on Saturn IB rockets, would kick off AAP. Their CSMs would dock for crew transfer and an orbital rescue test.

AAP spent-stage Workshop comprising (left to right) a Saturn IB rocket S-IVB stage, a drum-shaped Spent Stage Experiment Support Module (SSESM), and a docked Apollo Command and Service Module spacecraft. Image credit: NASA.
The third 1968 AAP mission, AS-211, would see the launch of the first AAP spent-stage Workshop. The crew would detach their CSM from the Saturn IB S-IVB second stage that propelled it into Earth orbit, then would turn and dock with a Spent Stage Experiment Support Module (SSESM) mounted on the front of the stage.

In addition to docking ports, the SSESM would include solar panels for making electricity, an airlock for spacewalks, experiment equipment, and tanks of gaseous oxygen for purging and filling the S-IVB's 20-foot-diameter hydrogen tank so that it could serve as a habitable volume. The astronauts would conduct biomedical and astronomy/space physics experiments on board the CSM and inside the SSESM and hydrogen tank for from 28 to 56 days.

AAP missions in 1968/1969 would re-fly experiment apparatus first flown on short-duration (no more than 2 weeks) Earth-orbital Apollo Moon program test flights in 1966/1967. These would, the memorandum stated, include experiments in particles and fields, ion wake physics, X-ray astronomy, and UV spectroscopy. At the time the SAA Program Office wrote its memorandum, the first of these test flights, dubbed AS-204, was scheduled for liftoff in late 1966 with Mercury/Gemini veteran Gus Grissom, Gemini veteran and first U.S. spacewalker Ed White, and rookie astronaut Roger Chaffee on board.

Three Pegasus satellites were launched to gather data on the the meteoroid environment of low-Earth orbit. Pegasus 3, with a wingspan of 29 meters, included meteoroid-capture panels designed for retrieval by Gemini or Apollo astronauts; it would, however, not receive visitors before it reentered the atmosphere in August 1969. Image credit: NASA. 
The final 1968 AAP mission, AS-212, would see a CSM deliver supplies to the AS-211 spent-stage Workshop. It would then rendezvous with Pegasus 3, a -ton satellite that was launched atop a Saturn I rocket on 30 July 1965. The AS-212 crew would spacewalk to retrieve meteoroid-capture and thermal coating test panels mounted on the satellite.

The Case I AAP schedule had the disadvantage of not permitting continuous rocket and spacecraft production and launch operations between AS-212 and the missions that would follow it. This, the memorandum explained, meant that Saturn-Apollo production and operations would be required to "phase down" during 1969-1970 and build up again in 1971. Case I missions after AS-212 would occur from three to nine months later than in Case II. The SAA Program Office favored and thus provided more details for the Case II schedule than for Case I. For this reason, from here on this post focuses exclusively on Case II.

The first of four 1969 AAP missions, AS-213, would be a near-duplicate of the AS-211 Workshop mission. On the second 1969 mission, AS-214, a CSM and the first LM-derived Apollo Telescope Mount (ATM) would carry out a 14-day solar astronomy mission. The ATM would reach orbit within the Spacecraft Lunar Module Adapter (SLA), the tapered shroud that linked the CSM with the top of the S-IVB rocket stage. AAP flights in 1968-1970 would occur during solar maximum, when activity on the Sun would peak, so in general their astronomy programs would emphasize solar observations. The AS-214 CSM would then undock from the ATM and dock with the AS-213 spent-stage Workshop to provide resupply and crew rotation.

In the June 1966 memorandum, the SAA Program Office assumed that LM-derived ATMs, labs, and carriers would launch with and operate while docked with piloted CSMs. As 1966 progressed, however, AAP planning increasingly emphasized ATM, lab, and carrier dockings with spent-stage Workshops. Such dockings would enable NASA to build up capable interim space stations and gain experience with in-space assembly of multi-modular spacecraft.

AS-214 would include the first Apollo Telescope Mount (ATM). Two astronauts would operate the ATM from the LM ascent stage; astronomical instruments would fill the stripped-out LM descent stage. Image credit: NASA.
The proposed AAP Laser Communications lab was not specifically mentioned as a payload in the June 1966 AAP program plan, though its design was typical of proposed LM-derived AAP labs. Image credit: Perkin Elmer.
The proposed AAP Optics Lab as it would appear stowed for launch in the SLA. Note that experiment equipment (mainly telescopes) and a square "platform" with attachment points at its corners for linking to the SLA completely replace the descent stage. Image credit: NASA.
The third 1969 mission, AS-215, was envisioned as a meteorology-oriented mission dubbed "Applications-A." It would probably have operated in an orbit steeply inclined relative to Earth's equator and employed an experiment/sensor carrier based on the LM design.

The AS-510 mission, the final 1969 AAP mission and the first AAP mission to launch on a Saturn V rocket, would place a CSM into geosynchronous Earth orbit (GEO) for communications, biomedicine, and Earth observation experiments. The rocket's S-IVB third stage, modified to permit two restarts, would ignite in low-Earth orbit to boost the CSM into an elliptical transfer orbit, then would fire again 5.5 hours later to circularize the CSM's orbit at the GEO altitude of 35,870 kilometers.

Five AAP Saturn IB missions would fly in 1970. These would include a 135-day stay on board a spent-stage Workshop in Earth orbit, two resupply visits to the spent-stage Workshop as part of other AAP missions, two solar ATM flights, a Biomed Lab mission, a fluids lab for studying weightless propellant behavior, the Applications-B Earth observation mission, and the introduction of an "Extended Capability CSM" for independent 45-day flights. Extended Capability CSM modifications would include long-life, high-capacity fuel cells for making electricity and water, an oxygen-nitrogen breathing mixture to replace Apollo's pure oxygen atmosphere (this was a concession to aerospace physicians, who worried about the health effects of breathing pure oxygen for long periods), and a long-life C-1 rocket engine in place of the Apollo CSM's Service Propulsion System main engine.

The Biomed Lab would be based on the Apollo LM or a "refurbished Command Module." The latter was envisioned as a used Command Module stripped of its heat shield, parachutes, and other systems, fitted out as a small pressurized laboratory, and launched a second time on a Saturn IB with a piloted CSM.

Four AAP Saturn V missions would fly in 1970, of which three would voyage to the Moon. These would be the first lunar missions since Apollo's end. The AS-511 Saturn V would launch a piloted mapping mission to lunar polar orbit. It would orbit for up to two weeks while the Moon rotated beneath it. This would enable the CSM to pass over nearly the entire lunar surface (and fly over half the surface in daylight).

The refurbished Command Module (CM) Lab came in dependent (upper left) and independent (lower right) forms. The "cruciform" was included in the design to provide attachment points linking the dependent CM Lab to the SLA. Image credit: North American Aviation.
Apollo CM pressure vessel. Image credit: NASA.
The AS-512 CSM would transport to lunar orbit an LM Shelter containing living quarters, supplies, and exploration gear (a small rover, a core drill, and an advanced sensor package). Once in orbit about the Moon, the LM Shelter would undock from the CSM and land automatically, then the piloted CSM would return to Earth. Less than three months later, the AS-513 Saturn V would launch an Extended Capability CSM and an LM Taxi to the Moon. The latter would land near the LM Shelter with two astronauts on board, including the first scientist-astronaut to reach the Moon. They would explore their landing site for 14 days.

The year 1970 would end with the AS-514 launch, which would place the first modified ("Mod") S-IVB Workshop into Earth orbit. The Mod S-IVB Workshop was a step up from the spent-stage Workshop; it would launch with no propellants in its tanks and its hydrogen tank outfitted with living quarters, supplies, and experiment gear. The four Saturn IB-launched AAP missions in 1971 would, the memorandum explained, support a one-year stay by a single crew on board the AS-514 Mod S-IVB Workshop.

In 1971, the AS-515 Saturn V would launch an Extended Capability CSM and an ATM on a 45-day mission to GEO to conduct stellar and solar astronomy, relativity, and space physics experiments. AS-516 (the first Saturn V built specifically for AAP) and AS-517 would launch an advanced lunar exploration mission similar to the AS-512/AS-513 pair, and AS-518 would launch a second Mod S-IVB Workshop.

The four Saturn IBs launched in 1972 (AS-225 through AS-228) would support stays on the second Mod S-IVB station. One of these missions would also test Command Module modifications meant to replace Apollo ocean splashdowns with cheaper land landings. Modifications would include steerable parachutes.

AS-512 in 1970 would deliver the LM Shelter to the lunar surface; AS-513 would see two astronauts arrive separately in an LM Taxi and live in the LM Shelter for 14 days. The LM Shelter would include a rover (shown stowed and in release position) and a core drill (shown deployed). This image dates from January 1965 but is applicable to the June 1966 AAP plan. Image credit: NASA.
Apollo CM with deployed parawing. Image credit: North American Aviation.
From 1972 through 1975, the memorandum explained, AAP missions would support a transition to an unspecified post-AAP piloted "follow-on program." NASA would increase its Saturn IB launch rate to six per year by 1973, and would continue to launch Saturn V rockets at a rate of four per year. The latter would launch four missions to GEO to conduct stellar astronomy, physics, and technology applications experiments (1972-1973), the automated Voyager Mars probes (1973), and a lunar mission similar to the AS-512/AS-513 pair each year through 1975. Two of the GEO missions would include ATMs. AS-520/AS-521 would launch the 1972 lunar mission pair and AS-525/AS-526 the 1973 pair.

The SAA Program Office envisioned that ATM missions might lead in late 1973 to an AAP astronomy mission featuring a reflecting telescope with a mirror measuring from 60 to 100 inches across. This, the memorandum explained, might serve to verify the mirror design ahead of its use in planned orbiting National Astronomical Observatories, sophisticated space telescopes expected to reach Earth orbit in the late 1970s.

As mentioned above, NASA began AAP amid increasing fiscal pressures. After pushing off a formal start to AAP as requested by Congress in FY 1966, President Johnson submitted a $5.01 billion NASA budget for FY 1967, of which $270 million was meant to fund AAP. Congress slashed the FY 1967 AAP budget to $83 million.

Observers of the U.S. space program were surprised when President Johnson went to bat for AAP again the following year. He requested that NASA's FY 1968 budget total $5.1 billion, with $455 million allotted to AAP. On 27 January 1967, the day after NASA OMSF director George Mueller briefed the press corps on the planned rapid ramp-up in AAP development, fire broke out inside the AS-204 Apollo CSM crew cabin during a test on the launch pad. Fed by the CSM's pure oxygen atmosphere, it immediately became an inferno. A poorly designed hatch trapped astronauts Grissom, White, and Chaffee inside, so they perished.

After the fire, NASA came under close scrutiny and was found wanting. Congress could not "punish" the agency by cutting the Apollo Program budget — to do so would have endangered achievement of President Kennedy's goal of a man on the Moon by 1970 — but it could express its displeasure by cutting programs meant to give NASA a post-Apollo future. The agency's FY 1968 appropriation was slashed to $4.59 billion, with AAP receiving only $122 million.

Under President Richard Nixon, NASA's budget slide accelerated. The Saturn rocket production lines were placed on standby in January 1970. At the same time, AAP became the Skylab Program. NASA Administrator Thomas Paine, who saw Skylab as a step toward a late 1970s 50-to-100-man Earth-orbiting Space Base, cancelled the Apollo 20 Moon mission so that its Saturn V (AS-513) could launch Skylab, a Saturn IB S-IVB-derived Orbital Workshop (OWS) resembling the AAP Mod S-IVB Workshop. Two years later, in January 1972, Nixon called for new-start funding for the Space Shuttle, which became NASA's main post-Apollo piloted program.

Work toward using Saturn-Apollo hardware in post-Apollo missions continued, though on a much-reduced scale. Apollo 17 (December 1972) saw the sixth and last piloted Moon landing of the 20th century and the last flight of the LM. NASA designated its Saturn V SA-512. On 14 May 1973, SA-513, the last Saturn V to fly, launched Skylab. An ATM for solar studies — the design of which was not based on the LM — reached orbit permanently attached to the OWS, and the Multiple Docking Adapter (MDA) replaced the SSESM. Three Saturn IBs (SA-206 through SA-208) launched three-man crews to Skylab in Apollo CSMs. The final Skylab crew splashed down on 8 February 1974, after 84 days in space.

The SA-210 Saturn IB, the last Saturn rocket to fly, launched the last Apollo CSM to fly. Its July 1975 mission to dock with a Soviet Soyuz spacecraft in low-Earth orbit brought the Apollo era to a close.

Source

"Saturn/Apollo Applications Program Summary Description," memorandum with attachments, MLD/Deputy Director (Steven S. Levenson for John H. Disher), Saturn/Apollo Applications, NASA Headquarters, to George M. Low, Manned Spacecraft Center, Leland F. Belew, Marshall Space Flight Center, and Robert C. Hock, John F. Kennedy Space Center, 13 June 1966.

More Information

A Forgotten Rocket — The Saturn IB

Space Station Resupply: The 1963 Plan to Turn the Apollo Spacecraft Into a Space Freighter

Apollo Extension System Flight Mission Assignment Plan (1965)

"Assuming That Everything Goes Perfectly Well in the Apollo Program. . ." (1967)

The First Voyager (1967)

"Still Under Active Consideration": Five Proposed Earth-Orbital Apollo Missions for the 1970s (1971)

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.

Sources

"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.

Sources

"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)

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
Sources

"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, Space.com, 19 January 2000.

More Information

Solar Flares and Moondust: The 1962 Proposal for an Interdisciplinary Science Satellite at Earth-Moon L4

Lunar GAS (1987)