lunar orbiter – Inverting Vision

Lunar exploration has always held a strange position in the history of exploration. For all of human history, people have been staring up at the Moon, and for centuries astronomers used telescopes to study the lunar surface. The telescopic surveying and mapping of the Moon by astronomers can (and should, I think) be considered a form of exploration. From this perspective, the Moon had been thoroughly explored far before the dawn of the Space Age. But on the other hand, because of the nature of the Moon’s orbit, the Moon also possessed some of the most mysterious and inaccessible terrain that ever taunted exploration-minded humans.

The Moon is tidally locked, meaning that only one side of the Moon ever faces the Earth. And so for all those millennia of Moon-gazing, there was an entire half of our natural satellite that no human had ever seen before. We would only get our first look at the end of the 1950s, and it would take even longer for us to complete a full map of the Moon. Here is a visual history of how we did it, designed to guide you through the process, even if you aren’t yet familiar with any lunar features.

First, let’s look at the near side of the Moon. The dark parts are the maria, Latin for “seas.” The brighter parts are, generally, called the highlands.

The maria provide helpful landmarks for understanding how the far side of the Moon was revealed, and I have highlighted a few helpful maria below. In red you can see the Sea of Tranquility, probably the most famous mare, since that’s where Apollo 11 landed. But the other ones I have marked will be the most helpful landmarks for this history. We will especially be tracking Grimaldi, really a crater on the western limb (edge) of the Moon with a mare floor. Over on the eastern limb, highlighted in green, is the Mare Humboldtianum. I’ve highlighted Mare Crisium in blue, since it is large and distinctive. But mainly keep your eye on Grimaldi and Humboldtianum.

*ORANGE: Grimaldi Crater, RED: Sea of Tranquility (Mare Tranquillitatis) , BLUE: Mare Crisium, GREEN: Mare Humboldtianum*

Oddly, even though the moon is tidally locked, we have been able to see small portions of the far side, due to a wobbling motion called lunar libration. You can see libration happening in the animation below. You might be able to notice that Humboldtianum actually disappears over the limb at certain times. Because of libration, we have actually been able to see more than fifty percent of the lunar surface for a long time. In the 17th century, astronomers began using telescopes to study the extent of libration, and to extend lunar maps slightly.¹

File:Lunar libration with phase Oct 2007 HD.gif — *Lunar Libration, from Wikimedia*

William Gilbert and Galileo were some of the first astronomers to detect and measure libration, and to attempt mapping the Moon in detail.² Over time, astronomers began depicting the effects of libration on their maps, which can be seen very well in Johannes Hevelius‘ 1647 map from his Selenographia. The slivers on the northern and southern edges of the map show which areas come in and out of view through libration, and you can see that the effect is slightly more pronounced near the poles. You might even be able to find Humboldtianum on this map.

Hevelius was using some traditional astronomical measuring devices, but he was also using telescopes of his own construction.³

We also see libration depicted on the map below, made by Giovanni Battista Riccioli in collaboration with Francesco Maria Grimaldi in 1651. Riccioli’s naming scheme was one of the most influential, and our maps today retain many of his names. You can see Mare Tranquillitatus and Mare Crisium on this map, for example. You can also see that Riccioli named the crater Grimaldi after his illustrator. He also named a nearby crater after himself, and yet another in the vicinity after Hevelius. Humboldtianum, however, is named Zoroaster on Riccioli’s map.

Below, you can see the effects of libration on the visibility of Zoroaster/Humboldtianum. It’s helpful to note the positions of Aristotle, Hercules, Atlas, and Endymion. They can help you orient yourself around the northeastern limb.

*A close-up of Riccioli’s Map, showing craters from Aristotle to Mercurius. Herclus, Atlas, Endymion form a distinctive pattern. In the libration zone is a dark spot called Zoroaster.*

This view was the one that astronomers had for the next several centuries. Better telescopes allowed observers to add a little more detail near the limbs, but observing features there would always prove extremely difficult. For the time being, the far side of the Moon would remain obscured. We can get a decent idea of the extent of our knowledge using John Russell’s Selenographia, an amazing lunar globe from 1797.⁴

*John Russell’s Selenographia, a lunar globe made in 1797. All images of globe from Science Museum Group. This image is released under a CC BY-NC-SA 4.0 Licence*

On the back of the globe, you can see the blank spots on our map. You can also see clever mechanisms Russell devised to display the movement of the Moon caused by libration.

*The back of Russell’s globe, showing blank space with inscriptions, and mechanisms for displaying libration.*

If we take a closer look at the western limb, we can see Grimaldi clearly, with Riccioli and Hevelius nearby. We can also see that Russell has filled in a lot of the area sometimes hidden through libration to the west of that area. This is where his map became more difficult to align with real features on the lunar surface. This is partly because that area is heavily cratered, and gives us a tantalizing glimpse at one of the largest and most complex surface features on the Moon.

*The western edge of Russell’s globe, with Grimaldi highlighted in orange.*

If we look really closely at the western limb as libration brings more westward features into view, we see what looks like some layered mountains, interspersed with dark surfaces. Russell looks to be depicting some of these mountains, and the darker areas between them. These seemed to go unnoticed by other astronomers until the late nineteenth century, when Russell’s darker areas begin to appear on maps again. But there is another dark mare beyond those mountains, and in the early twentieth century, German astronomer Julius Franz gave it the name Mare Orientale.⁵

*Grimaldi near the edge of the lunar limb.*

We wouldn’t be able to get a better look at the far side of the Moon until we invented a way to send cameras there. At the dawn of the Space Age, rockets gave us the ability to do just that. In 1959, Soviet engineers created a series of robotic probes, and launched them toward the Moon. One of these managed a lunar flyby, and was named Luna 3. Engineers equipped Luna 3 with a film camera, capable of developing the exposed film, scanning the images, and transmitting them back to Earth by radio. For a fantastic look at the technology involved here, and some of the images, check out Don P. Mitchell’s website. You should also read Sven Grahn’s work on how Jodrell Bank radio observatory in England intercepted the images as they returned.

*Luna 3 mockup at the Memorial Museum of Astronautics in Moscow, from Wikimedia*

In October of 1959, in a cramped room on the Crimean Peninsula, Russian engineers Boris Chertok, Sergei Korolev, and colleagues watched as images materialized slowly on heat-sensitive paper.⁶ Here is the first image we have of the far side of the Moon:

It is not the highest quality, but it is fairly amazing that it was possible at all in 1959, given the early state of the technology. Below, I’ve highlighted our landmark maria for reference–we’re seeing around the eastern limb. While distinct craters are hard to make out, we can easily see some new maria. The ones to the right and south of Mare Crisium are Mare Smythii and Mare Marignis (we had some glimpses of them on the eastern limb before Luna). But way out there on the right hand side of the image are two entirely new, entirely distinctive maria. The large one in the north was named Mare Moscoviense. The one in the south with the peak in the middle is the crater Tsiolkovskiy, named for the pioneering Russian rocket scientist.

Russian astronomer Yuri Naumovich Lipsky led the effort to interpret the images and begin a map of the lunar far side. His efforts led to the following maps:

*Schematic chart. You can probably spot Crisium and Humboldtianum. From Zdeněk Kopal’s Mapping of the Moon: Past and Present.*

This globe was produced, which is a fantastic glimpse at how much of the surface Luna 3 managed to add to our map. But you can clearly see the massive blank spot, and Mare Orientale remains frustratingly in mystery.

*Sternberg Moon Globe 1961, based on Luna 3 data. From Austrian National Library Globe Collection via Digital Museum of Planetary Mapping.*

It would remain that way until 1965, when Soviet engineers sent another robotic spacecraft past the Moon. Zond 3, which may have been destined for Mars, flew by the Moon equipped with another film-based system, and captured a handful of images. To see more about the technical details of Zond 3 and its flight, you should check out Andrew LePage’s website. And once again, Don P. Mitchell has an incredible collection of Soviet lunar photographs.

undefined — *Zond 2 – Zond 3 was part of the same series of spacecraft. From Wikimedia.*

And, for the first time, we see Mare Orientale in its entirety:

*Zond 3, Frame 3, the first full look at Mare Orientale, originally processed by Yu. N. Lipsky (1965). From Don P. Mitchell’s excellent website.*

Here’s a reminder of where we are, to orient ourselves. Zond 3 is giving us a glimpse over the western limb, past Grimaldi:

*Zond 3, frame 18. From Don P. Mitchell (seriously, go check out his website).*

And once again, Lipsky went to work putting all this new data on the map. You should be able to see Crisium, Humboldtianum, and Grimaldi for reference. We can now see Orientale, Muscioviense, and Tsiolkovskiy in their proper positions. But notice, there are still blank spots on the map.

Then, in 1966 and 1967, NASA sent five robotic probes to the Moon: the Lunar Orbiters. The Eastman Kodak company adapted camera systems they had designed for the Air Force to the task of mapping the Moon in preparation for the Apollo program.⁷ The teams behind Lunar Orbiter took thousands of incredibly high resolution images of the lunar surface. The first several Lunar Orbiters were mainly used to scout out landing sites for Apollo on the near side. But even on Lunar Orbiter I, they managed to capture some images of the far side. One of them was this shot, with Tsilkovskiy sitting in the foreground of the first “Earthrise” image ever captured.

What made Lunar Orbiter distinct from the Soviet efforts was not only the number of images, but the level of detail that each image contained. Two lenses allowed the simultaneous capture of both medium-resolution and high resolution images, like this one showing the central peak of Tsiolkovsky and its inner walls.

On Lunar Orbiter IV and V, scientists got the opportunity to play a more direct role in target selection. In the shot below from Lunar Orbiter IV, you can see Grimaldi, and down there near the edge, coming into full view: Mare Orientale. Just a few shot later…

Mare Orientale.

Between all the Lunar Orbiters, we gained imagery of 99 percent of the lunar surface. This allowed cartographers to create a fully detailed map of the lunar far side.⁸

Apollo astronauts orbiting around the Moon would add to our imagery and understanding of the far side, as would more Soviet probes. Later robotic probes would fill in the few remaining gaps, which were mainly near the poles. Over those years, our knowledge and understanding of the lunar surface would only continue to grow. For centuries, the far side of the Moon had been one large blank spot on a map. Then, in less than ten years, largely due to the hard work of engineers and scientists working with robotic probes, we filled in blank spots. We have become more fully acquainted with our Moon, but we still have more to learn.

Special note for posterity:
This post was written the day before the crew of Artemis II flew around the Moon. One reason for the focus on Mare Orientale in this history is a quirk in the history of direct observation of the lunar far side by astronauts. On all the Apollo missions, their orbits and the lighting conditions never allowed them to view Mare Orientale under sunlight. The closest they got was an image of Orientale under Earthsine on Apollo 17. Tomorrow, people will see Mare Orientale in fully glory for the first time.

*Mare Orientale in Earthshine on Apollo 17. From NASA/LPI.*

Update 4/12/2026:
On Monday, April 6 – they did it.

There are a lot of great histories of the early mapping of the Moon, but I won’t linger long on the details in this post. One of the most definitive is Ewen Whitaker’s Mapping and Naming of the Moon. There is also a good section on the topic in Mapping of the Moon: Past and Present by Zdeněk Kopal and Carder. Much of the information in this post comes from these sources. ↩︎
Stephen Pumfrey argues Gilbert was definitively first: https://adsabs.harvard.edu/full/2011JHA….42..193P ↩︎
Janet Vertesi takes a look at where Hevelius’ instruments fit into the technological culture of the time: https://www.jstor.org/stable/40731030 ↩︎
The Linda Hall Library has a digital copy of Russell’s pamphlet, in which he describes the workings of the device: https://catalog.lindahall.org/permalink/01LINDAHALL_INST/1oon2h5/alma99455413405961 ↩︎
Mare Orientale means “Eastern Sea,” which is confusing, considering it is on the western limb. The name was correct at the time Franz observed it–the directions changed in 1961. Ewen Whitaker and Richard Baum have a great history of Mare Orientale, which highlights the contributions of Russell, and explains the shift in directions. ↩︎
See Boris Chertok’s Rockets and People, Volume II (PDF), translated by Asif Siddiqi. 519-538 ↩︎
An older version of this article stated that the camera systems were designed for the CIA, but actually the ancestral tech of the LO cameras was originally designed for the Air Force as a part of WS-117L. Kodak camera systems in WS-117L did become a part of the CIA project CORONA. But the film readout system that was incorporated into Lunar Orbiter was a part of SAMOS (PDF), a separate component of WS-117L that was not transferred to the CIA. Both USAF and CIA satellite reconnaissance systems were soon were soon bundled into the organization that became the National Reconnaissance Office (NRO). The author caught this mistake on a re-read. ↩︎
For more on Lunar Orbiter, you can find my master’s thesis on the about page, which focuses especially on the Kodak camera systems and the role of science in Lunar Orbiter. There is also NASA’s official history by Bruce Byers, and Farouk El-Baz’s The Moon as Viewed by Lunar Orbiter. The Lunar and Planetary Institute has an amazing page hosting images from Lunar Orbiter. ↩︎

The Odysseus lunar lander built by Intuitive Machines (IM) recently became the first U.S. robot on the Moon’s surface since the Surveyor landers in the 1960s. Earlier this week, IM worked with NASA to get pictures of the lander from orbit. The resulting image is impressive, showing the lander as a tiny speck in the vast grey landscape near the Moon’s south pole. The image is also an echo of the first time NASA managed this feat, 57 years ago. In 1967, NASA’s third Lunar Orbiter spacecraft snagged a photograph of Surveyor I. The story of how engineers acquired that photograph (and it is a literal analog photograph) is fascinating, and the image itself played an important role in getting Apollo astronauts to the Moon. First, here’s the image of Odysseus along with the historic photograph of Surveyor:

Odysseus in the South Polar region of the Moon. Taken with the Lunar Reconnaissance Orbiter. Credit: NASA/Goddard/Arizona State University

Surveyor I in Oceanus Procellarum on the Moon. Taken with Lunar Orbiter III in 1967. Credit: NASA

It’s a bit easier to see Odysseus in the new image than it is to see Surveyor in the Lunar Orbiter (LO) photograph. But both of them are pretty difficult to spot, beyond the telltale shadow. And making out any detail is impossible. So what’s the point? For NASA in the 1960s, it was all about safety.

At the time, the push toward the Apollo landings was quickly accelerating. One of the top priorities was to find suitable landing sites. Telescopic imagery of the Moon was fairly comprehensive, but had some serious limitations, so NASA initiated the Lunar Orbiter program. Engineers put robots into orbit around the Moon, equipped with Kodak cameras and film, and took high-resolution images of potential Apollo landing sites.¹ Meanwhile, they Surveyor robots soft-landed on the surface, took pictures, and used scoops to dig into the soil. Knowledge about the nature of the lunar surface grew rapidly. It began to quell doubts that some scientists held about the potential of landing people on the Moon.² The imaging of Surveyor landing sites was an important part of this process.

For scientists in the 1960s, seeing the lander wasn’t as important as seeing the area around the it. Images from the ground could help scientists understand what they were seeing from above. At the time, orbital imagery was pretty difficult to interpret. Shadows were used to figure out the height or depth of some features, but other patterns in the orbital imagery were harder to make sense of. Scientists used aerial imagery of Earth to get started, since you could easily compare pictures of mountains and canyons taken from airplanes to the real thing.³ But the forces that shaped features on Earth weren’t necessarily the same as those that shaped features on the Moon, so the Earth-analog method was not always a reliable guide. What they really wanted were images from the lunar surface. That’s what Surveyor landers were able to provide.

If scientists could compare orbital images with ground-based images of the real lunar surface, they could be more confident in their interpretations. This could make it easier to select Apollo landing sites with confidence. And that’s exactly what they did using a combination Surveyor and Lunar Orbiter imaging. The story of Surveyor III gives us a great example of this.

Surveyor III. Credit: NASA/Lunar and Planetary Institute

In the same mission that took the photograph of Surveyor I, engineers also took photos of the planned landing area for Surveyor III (which launched while LOIII was still in orbit around the Moon). They hoped that a successful Surveyor III mission would then provide images from the ground that scientists could compare to orbital imagery. The plan was a complete success. Using pictures from Surveyor III, they were able to isolate the exact position of Surveyor III in the orbital imagery.⁴

An image from Boeing’s contractor report on Lunar Orbiter III photography. The final Surveyor landing site is shown, along with features that later seen in Surveyor pictures, included below. Credit: NASA/Boeing

Mosaic images of the area to the north of Surveyor III, including one with labels that correlate to those on the Lunar Orbiter photograph above. Credit: NASA/LPI

A mosaic of images taken by Surveyor III, with features labeled A, B, C, D, which correspond to labels on orbital images. — Mosaic images of the area to the north of Surveyor III, including one with labels that correlate to those on the Lunar Orbiter photograph above. Credit: NASA/LPI

An image taken by the Surveyor lunar lander, showing two boulders that were labeled "C" on orbital images. — A close-up of boulders, labeled “C” on the orbital image above. Taken by Surveyor III in April 1967. Credit: NASA/LPI

Scientists got a lot of great data from the robots. Apollo planners analyzed the images and data, and used the information to plan Apollo landing sites. They were able to find places that were both safe for landing, and scientifically interesting. For scientists, that generally meant trying to land Apollo astronauts in places that were geologically distinct.

This wasn’t really something that many of the astronauts were particularly interested in, at least at first. They were something of soldiers in the Cold War, and neither they nor the government officials directing the program thought that science was the main priority. The priority was getting a man to the Moon before the Soviet Union.⁵ The selection of later Apollo sites based on scientific interest was, at least in part, a concession to the scientists who were integral to the safety and success of the mission’s primary objective. But this isn’t to say that these groups saw no use for science within Apollo. Science itself could also serve Cold War goals, as it became a source of prestige–a pattern in scientific exploration going back centuries.

With Apollo 12, the story of Surveyor III came full circle and we got one of the coolest pictures ever taken from the lunar surface. Out of scientific and engineering interest, Apollo 12 landed in the same site as Surveyor III. Al Bean and Pete Conrad got to see the robot up close, which is how we have the image from earlier showing Surveyor sitting on the Moon. They took pictures, and even grabbed pieces of the robot to bring back home for analysis. Right now, the TV camera of Surveyor III sit in the Smithsonian, where you can visit and see actual hardware returned from the Moon. The Apollo astronauts also took what I think are some of the most incredible photographs from the history of exploration–human space explorers interacting directly with their robot counterparts.

Apollo astronaut Pete Conrad “jiggles” the spacecraft to see how firmly it’s rooted to the ground. Credit: NASA

CORRECTION 11/21/2024: The original version of this post identified the astronaut in the last picture as Alan Bean. It’s actually Pete Conrad, and Alan Bean is the one taking the photograph.

Footnotes:

If you want to know more about this, Lunar Orbiter photography was the topic of my master’s thesis, which can be found in the about section. ↩︎
There’s a famous story of how scientists feared the landing vehicle would sink into the soil, an idea that did come from a fairly well-known scientist. But many geologists at the time were pretty dismissive of his claims. There were other potential issues though, including ignorance of the electrostatic properties of the lunar material, which could have led to severe dust build-up on equipment. Bottom line: not a lot was known for sure about the nature of the surface. This was an issue if you wanted to land there. ↩︎
For a description of lunar mapping efforts around this time, see Kopal and Carder, Mapping of the Moon. The difficulty of interpreting the photographs can be seen in a variety of scientific papers from the time. Examples can be found in Interpretation of Lunar Probe Data, ed. Jack Green, 1966. ↩︎
Boeing was the primary contractor on Lunar Orbiter. Images and methods can be found in their contractor reports for NASA. ↩︎
Detailed comments the relative priority of science on the Apollo Mission can be seen in A Review of Space Research, the document that came out of the 1962 Iowa Summer Study. ↩︎

Inverting Vision

Tag: lunar orbiter

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