The Unrealised I’s: NASA’s Would-Be Lunar Missions

Pictured from the lunar module Falcon, this impressive view reveals the layout of the Scientific Instrument Module bay (SIMbay) aboard the Apollo 15 command and service module, Endeavour. The unrealised I-series of lunar flights would have employed similar instrumentation in their solo orbital voyages. Photo Credit: NASA

Early in August 1971, as Apollo 15 astronauts Dave Scott and Jim Irwin worked on the lunar surface, something unprecedented was going on in orbit around our closest celestial neighbour. In the bowels of the command and service module, named ‘Endeavour’, their crewmate Al Worden was tending to the biggest complement of cameras and remote-sensing equipment ever used to study the Moon. Collectively known as the Scientific Instrument Module bay (SIMbay), these instruments contributed enormously to transforming Apollo 15 into one of the grandest missions of scientific inquiry in history. SIMbays would also fly aboard the final two Apollo lunar voyages in April and December 1972, but what is perhaps less well known is that an entire series of orbital missions were planned, but never reached fruition. They were called ‘the I-series’.

As discussed in yesterday’s History article, Apollo 15 was originally manifested as a so-called H-series mission, lasting around ten days, with a pair of Moonwalks and no more than 33 hours spent on the surface. As Command Module Pilot, Worden had a full plate of scientific investigation, but he could scarcely have imagined how the scope of his mission would change in September 1970. The cancellation of two lunar flights meant that Apollo 15 morphed into the first of the J-series, whose landing crews would spend up to 70 hours on the surface and whose command and service modules would be outfitted with the advanced SIMbay. “Our purpose,” Worden told the NASA oral historian, years later, “changed from getting there and getting back to going out there and collecting all this science. There was an end game here; there was an end purpose to going. It wasn’t just to go and come back. It was to go out there and really do something scientific that was worthwhile.”

Worden’s payload was housed in ‘Sector One’ – originally an empty part of the service module – and consisted of no fewer than eight experiments: an X-ray fluorescence detector, a gamma-ray spectrometer, an alpha-particle spectrometer, panoramic and mapping cameras, a laser altimeter, a dual-beam mass spectrometer and a subsatellite to be deployed into lunar orbit. Main areas of scientific study included measurements of the chemical composition of the Moon’s various terrain types, analysis of its tenuous ‘atmosphere’ and precise observations of the spacecraft’s altitude. The cameras at Worden’s disposal were capable of resolving surface features as small as just a few tens of feet across…and in order to retrieve their film cassettes he would be required to perform an EVA, in cislunar space, during the return journey to Earth. It would be the first time that such a spacewalk had ever been undertaken, so far from home.

Artist’s concept of Al Worden’s cislunar EVA to recover film cassettes from the SIMbay. Crewmate Jim Irwin monitors the proceedings from the command module’s open hatch. The I-series flights would have followed a similar protocol during their return journey. Image Credit: NASA

Planning for lunar orbital science began in May 1968, when Wilmot ‘Bill’ Hess, head of science and applications at the Manned Spacecraft Center in Houston, Texas, requested consideration over the placement of instruments into the empty Sector One. (Indeed, when the Block II service module was defined in 1964, one bay was left empty for precisely this purpose.) Apollo prime contractor North American perceived no major difficulties and in March 1969 a Steering Committee drew up a tentative list of possible instruments. Funding for the formal development of the SIMbay instrumentation was released by NASA’s head of manned spaceflight, George Mueller, in May, with an original expectation that it might make its maiden voyage on Apollo 14 in the summer of 1970. However, in order to work out how to best integrate the experiments and finalise their operation, it was concluded that the first SIMbay would fly aboard Apollo 16 in early 1971. Later, following the cancellation of several later J-series missions, the first SIMbay was advanced to Apollo 15.

Testing the experiments of the SIMbay on the ground, though, had been problematic from the start. Since the instruments were designed to operate in microgravity, they had to be tested on the ground under normal terrestrial conditions. The deployable booms for the spectrometers, for instance, could only be extended using railings which mimicked the space environment as closely as possible, although this was far from satisfactory and they never worked particularly well on the ground. Then, when technicians tried to integrate the whole bay into Apollo 15, data streams failed to synchronise properly and last-minute changes and adjustments were still being made by principal investigators until shortly before launch.

Al Worden’s duties during his solo time in lunar orbit were far from simply being a ‘caretaker’ of the command and service modules. In fact, according to Dave Scott in his memoir Two Sides of the Moon, Worden would be “performing alone all the manoeuvres carried out by some of the earlier three-man Apollo missions”. Together with the principal investigators, Worden actively planned many of the SIMbay activities and integrated them into his flight schedule. Years later, he would take justifiable pride in having mapped around a quarter of the Moon’s surface with the high-resolution imaging gear.

Al Worden (centre), pictured during geology training with Farouk el-Baz (right). Photo Credit: Francis French/NASA

In preparation for Apollo 15, Worden and his backup, Vance Brand, were guided by their dedicated scientific mentor, the Egyptian-born geologist Farouk el-Baz. “He made me memorise the name of every crater there was on the surface of the Moon,” Worden recalled, “and where it was and how it got there and what was happening to it. Farouk and I spent endless hours training on the lunar geology. My kind of geology on the Moon was different from surface geology. Dave and Jim were very good at [analysing rocks]…[but] I had to look at major features. I’m looking at a volcanic crater or a meteor impact crater from 60 miles away, but there’s no way I can look at individual rocks. So I looked at ‘macro’ features and Dave and Jim looked at ‘micro’ features.”

During his three days of solo activity in lunar orbit, Worden and the SIMbay had achieved a huge amount of scientific research and, despite some problems with a sensor for the panoramic camera, his work went exceptionally smoothly. Nor was he simply an operator; his training made him an integral part in the proceedings and he was regarded as very much a ‘co-investigator’ in many of the research tasks. In fact, several scientific journal papers, published after the mission, include the name of A.M. Worden in their list of authors.

Since the service module could not survive re-entry, it was Worden’s task to venture out and retrieve the film from the SIMbay’s cameras. This would be humanity’s first ‘trans-Earth EVA’, the first spacewalk conducted in the cislunar gulf, 180,000 miles from home. Yet, as Worden recounted in his NASA oral history, there were originally ‘other methods’ studied for getting these films inside the command module before the agency went ahead and approved a deep-space EVA. “There had already been some preliminary work on how to get this film out of the SIMbay,” he pointed out. “Of course, it had been in the pipeline for several years and there were a lot of schemes to get the film from…the back of the Scientific Instrument Module all the way up into the command module, a distance of about 30 feet. How do you get out there safely so that you don’t lose it, so that you don’t hurt something? One of the schemes was…an ‘arm’ on a hinge that would go out and pick up the film and…bring it back by the hatch, where you could pick it up.” Another suggestion was an ‘endless clothesline’, onto which the film canisters could be hooked and reeled down the length of the service module to Endeavour’s crew hatch.

‘Transearth’ EVAs were a feature of the last three Apollo lunar missions and would have formed an integral part of the I-series flights. In this view, Apollo 17 Command Module Pilot Ron Evans is shown manoeuvring himself along the exterior of the service module. Photo Credit: NASA

“I objected to all of those, once we [were] assigned to the flight,” Worden recalled. “None of them were very practical. We actually proved it with the clothesline. It’s nice to think about something like an endless clothesline, but the truth was, when you’re in space, if that canister started to bounce around, there [would be] nothing to stop it.” During a test in the parabolic aircraft, it proved difficult and potentially damaging. In the end, it was decided that an EVA was the most effective option. By the time he set out to perform it on 5 August, Worden had practiced his every move of the task more than 300 times aboard the parabolic aircraft.

“The EVA itself was kind of unique,” Worden said of the relatively brief, 39-minute excursion, “sort of a unique perspective. I did have a chance to stand up on the outside [of the service module] and look. I could see the Moon and the Earth at the same time; and if you’re on Earth, you can’t do that, and if you’re on the Moon, you can’t do that! It’s a very unique place to be. I guess our biggest concern was that we had everything tied down so that when we opened the hatch, we didn’t have something go wandering off into space! But outside of that, it was pretty easy.”

Obviously, since the cabin was reduced to vacuum for the spacewalk, Dave Scott and Jim Irwin also had to don their suits. Suddenly, as soon as the hatch opened, everything that had not been secured began drifting around. “When we opened the hatch,” wrote Irwin in his memoir To Rule the Night, “it was just like a vacuum cleaner pulling all the loose stuff from the inside out into space. My toothbrush floated by; it had been in hiding. A camera came by; one of us grabbed it. We were all leaping around, trying to catch the important stuff.” It was Irwin’s job to move slightly outside after Worden in order to televise his EVA, but he had ‘goofed’ when hooking up his suit’s umbilicals by wrapping them the wrong way around a strut. This limited his range of movement. “I had to force my hand out to reach the movie camera,” he wrote, “attached to a boom, to turn it on. I saw the green light and thought the camera was on, but it wasn’t working.”

To a great extent, the scientific activities involved in Apollo 15 mirrored some of the objectives of the I-series of missions, planned but unrealised due to the tightening of budgetary belts in the late 1960s. According to space historian Dave Shayler in his book Apollo: The Lost and Forgotten Missions, the I-series would have gone ahead “if the potential scientific return from a non-landing mission was sufficient to balance or outweigh that from a landing mission”. Operating between 45 degrees North and 45 degrees South latitude, the Apollo command and service modules would have over-flown an area of perhaps 200 degrees of longitude, exploring the surface and possibly identifying future landing sites.

In-depth analysis of the Moon’s physical characteristics would have been a crucial element of the science goals of the I-series missions. Photo Credit: NASA

Unlike its lunar landing cousins, an I-series orbital flight would have been subject to a launch window dictated by its assigned scientific instruments. Shayler suggested that orbital inclinations of up to 88 degrees were attainable and the missions would have provided a stable platform for six or seven days of dedicated lunar science. During the return journey, one crew member – presumably the Command Module Pilot – would have performed a cislunar EVA to recover film cassettes.

As NASA’s budgets shrank from 1967 onwards, effectively eliminating lunar landing missions and a plethora of exciting and dramatic Apollo Applications mission proposals, the I-series flights breathed their last. Then, in September 1971, an in-house NASA study proposed sending Apollo 18 (a mission which had been officially cancelled a year earlier) into polar orbit around the Moon. Alas, the study required a go-ahead to have been given no later than 1 December for the mission to occur in the 1973-74 timeframe. No such go-ahead was forthcoming. The Shuttle, with its promise of cheap, routine access to low-Earth orbit, and a new era of detente with the Soviet Union, rendered other missions considerably more palatable to the incumbent Nixon administration.

Over the past four decades, many historians, observers and enthusiasts have lamented the decision to cancel the Apollo lunar programme. Much focus is given to the ‘lost’ landing missions and the impact upon the astronauts who lost out – Dick Gordon, Joe Engle and others – because actually reaching the lunar soil was perceived to be the ultimate jewel. Yet the lunar orbital missions, and the unrealised I-series flights, carried no less excitement or drama. Apollo 16 Command Module Pilot Ken Mattingly told the NASA oral historian that, whilst he would have loved to walk on the surface, the exhilarating experience of a voyage in lunar orbit was equally important.

“If you were going to devise a programme for personal enjoyment,” Mattingly said, “the only thing you’d change in the way things worked out for me is I would have had another flight to go land on the Moon! Nothing can take the place of being there…but you wouldn’t want to skip the lunar [orbit] part to go to the surface. You need both, because the lunar [orbit] piece, especially solo, was probably more sense of exhilaration. I can’t explain it, but it was really, really something!”

In a sense, the Apollo 15, 16 and 17 missions accomplished a serious amount of the work originally proposed for the I-series flights and, certainly, the haul of lunar orbital science data from those three voyages was significant and substantial. None of them achieved the high inclinations afforded by the I-series flight plans, however, and it remains one of our species’ greatest failures to have completed the most astonishing scientific and technical endeavour in history, by reaching the Moon with a human crew, and gone no further.


This is part of a series of History articles which will appear each weekend, barring any major news stories. Next week’s article will focus on ‘The New Nine’, NASA’s second class of astronauts – including Neil Armstrong – who were selected 50 years ago this month.

Missions » Apollo »


  1. The level of excellence, ingenuity and grandeur of the Saturn V/Apollo capsule never ceases to amaze me! This hardware and the people that created it were actually out of this world! For my money, 1960’s Saturn V/Apollo was a technological marvel equivalent to the ‘Grand Orient’ ship of the 1860’s. And it was the Pyramids of the 20th century and I have it up there with the 7 Wonders of the World. I don’t think that’s an exaggeration at all!

    All of these feats were so unreal for their time, that it’s no wonder that people have created all shorts of stories and conspiracy thinking about them. It shouldn’t surprise as that there are people that view the Pyramids as extraterrestrial artifacts, the same way that they view Saturn/Apollo as a hoax.

    What strikes me the most, is how much Apollo was a Multi-Purpose Vehicle. It could be used for going to the Moon, create space stations in Earth orbit (Skylab anyone?) go for manned fly-bys around Venus and create lunar habitats!(Apollo Applications Program). It’s what the US tries to replicate with Orion, 50 years later! And that just brings home the level of the disaster that the dismantling of the whole Apollo program was! The US had created such a workforce 50 years ago, and throwed it on the garbage can entirely.

    Like Columbus returning from the New World, and Isabelle saying ‘OK, now burn the fleet!’

    Ben, your writing is superlative. I can’t praise it high enough! You should be teaching in college or be part of NASA!

    But you’ll have to excuse me for not reading the entire article. It’s too depressing to compare the ‘Ifs’ of yesterday with the reality of today…

  2. Outstanding article, love this kind of subject matter. Oh, what Apollos 18-20 might have accomplished! Well done, Mr. Evans.

  3. Wow, nice job, Mr. Evans! Must concur with Leonidas and John. I hope our next President can get our economy back up and restore NASA such that we can continue to lead the world in space exploration. I spoke to President Bush in 2002 and urged him to continue funding New Horizons. I even told him, “You will get a lot of Trekkie votes.” Luckily, he must have thought he would need them in 2004 and signed off on the missive all the way and including signing off on the actual launch in 2006, something all Presidents must do when an American rocket is launched.

    I am an artist, among other things, by the way, and did a series of art using raw stills from footage of the dark side of the Moon taken by the crew of Apollo 8, the first images ever taken of the dark side. You can find the entire series on my website, along with art from New Horizons images, et cetera….

    Keep up the good work, Mr. Evans!

  4. Listen to the words of Gene Cernan, the last human to stand on the lunar surface, as he was about to leave the moon. All who were a part of humanity’s first voyage to another world had such hope, such optimism that certainly we would continue to build upon what they had started. I cannot begin to imagine the depths of their disappointment.

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