Overshoes from the Last Man on the Moon

These are the last human-worn objects to touch another world. When the crew of Apollo 17, the last Apollo Moon landing, returned to Earth after their record-breaking mission in December 1972, commander Gene Cernan and lunar module pilot Harrison Schmitt brought back four items that had not been planned to return with them. Instead of tossing their lunar overshoes onto the surface of the Moon along with their personal life support systems, as had been done in the previous five missions, Cernan and Schmitt made the decision to bring the overshoes back with them. Pictured here and on display in our new Destination Moon exhibition are the pair that Gene Cernan used. They made the last human footprints on the lunar surface. These footprints remain 50 years later.

Gene Cernan's Apollo 17 lunar overshoes on display in the Destination Moon exhibition at the Museum in DC. (Smithsonian Institution)

Frequently, the Museum receives questions as to why the iconic lunar footprints that we see in NASA photographs do not match the soles of the spacesuit boots that one sees on Neil Armstrong's Apollo 11 spacesuit. The simple answer is that the boots that are integrated into the spacesuit provided adequate support and flexibility for launch and entry inside the spacecraft, but they were not adequate to provide traction and protection on the surface of another world. At the time of the Apollo spacesuit design, no one had a lunar sample to determine how human-made materials would interact with the Moon's surface. As a result, engineers and technicians had to make many assumptions and plan for extra safeguards when designing lunar galoshes for Moonwalks.

These lunar overshoes were the only part of the suit that would be in constant contact with the lunar surface. They would be in contact with an unknown surface that would experience temperatures up to 300 °F they had to be designed so that the astronauts could put them on and remove them with relative ease while wearing their spacesuits. Designers began their plans with blue silicone rubber that would make up the iconic soles of the boots. This silicone had a much higher melting temperature than the lunar surface. The horizontal ridges along the bottoms of the silicone soles were made to match two very different possibilities about the behavior of the lunar surface. At the time, we did not know if the lunar dust on the surface of the Moon would be thin and slippery or deep and sticky. Wide soles with lots of traction could address either variable.

Gene Cernan walks on the Moon during the Apollo 17 mission. (Image courtesy of NASA)

The silicone soles were not the only response to concerns about conducting lunar heat inside the spacesuit. The lunar overshoes had insoles made from felt consisting of a combination of fiberglass and Teflon, similar to the outer layer of the spacesuit. The innersoles were lined on the inside with the Teflon-coat Beta Cloth to create a fire-resistant encapsulation of the feet. Between the inner and outer layers of Beta Cloth were alternating layers of aluminized Mylar (think of Mylar balloons), Non-woven Dacron, and Beta Marquisette Kapton (a laminated film). These lightweight layers not only added to the thermal protection of the astronauts, but they also formed a penetration protection layer for the boots. Any fast-traveling particle that penetrated the outer layer of the boots would break apart, lose momentum, and come to rest among the 21 layers of the interior. This is a time-honored method of protecting astronauts and satellites that is still used today.

The outermost layer against penetration and abrasion protection took the form of that silvery-gray covering that you see on the tops of the lunar overshoes. Chromel-R is a high chromium content stainless steel fabric that is woven from steel threads. At the time this was the best penetration protection that money could buy. In 1968 dollars the fabric cost $2,500/yard. Today, that would be in excess of $20,000/ yard—a very expensive fabric. Designers had to make very judicious decisions about where they would place this very dear material. In the end, they decided that a piece would go at the base of the back of the spacesuits. You can see this on Neil Armstrong's suit. The Personal Life Support System (PLSS) backpack wasn't permanently fixed to the spacesuit, and it could rub and cause abrasions. The other place that you can see Chromel-R on Apollo suits is around the hands on the EVA gloves. Astronauts would be handling sharp rocks and using geological tools, so some of the best protection had to go there. And of course, astronaut feet had to have similar protection.

The back of Neil Armstrong's Apollo 11 spacesuit, which is on display in the Destination Moon exhibition at the Museum in DC. (Smithsonian Institution)

In the years after the Apollo 17 mission, Cernan maintained that it had been his decision to bring the boots back to Earth, despite precedent and the operational plan for his mission. His decision had not only historic but also scientific significance. Historically, we can now stand in awe of these boots on display. At the time, Cernan did not think that it would be over 50 years before the next humans stepped on the surface of the Moon—he had hoped that event would occur in his lifetime. Gene Cernan died in 2017. But he did bring back the material evidence that the footprints that we see in so many images from the Apollo program were indeed made by the boots that the astronauts wore.

The scientific significance of Cernan's decision has emerged over the last 50 years. If you look closely at Cernan's lunar overshoes, you will see that they have gray and black marks and look dirty. This is lunar dust. As a policy, the National Air and Space Museum does not clean objects of evidence of use. We periodically vacuum and brush off Earth-bound dirt from the surface of our objects to assist in their preservation, but we avoid more aggressive treatment that might damage the history of an object. The nature of lunar regolith, the coating of dust that covers the Moon's surface, has made this distinction between Earth dirt and lunar dirt very easy for our conservators. Lunar regolith is very aggressive. It doesn't merely sit on the surface of a material; it hooks on and embeds itself into it. The lunar dust that you see on the blue silicone could not be brushed off even if we tried.

Using a laboratory microscope, we can find it deep in the fibers of the Teflon-coated fiberglass fabric that covers the surface of the spacesuits. Even more remarkable, materials scientists have examined the Chromel-R fabric of Harrison Schmitt's boots with extremely high-resolution microscopy and seen that lunar dust has gone so far as to erode the stainless-steel fibers of the fabric. The uppers might have been able to withstand sharp rocks and geological tools, but not lunar dust.

It turns out that the long-term impact of Cernan's ad hoc decision to bring back his Apollo 17 overshoes will probably have its greatest impact on the design and materials choices for the next generation spacesuits for use on the Moon, asteroids, and Mars. Spacesuits are no longer custom-made for a single mission. They will have to be reusable, maintainable, and cleanable for long-term use. As a result, an important design factor will be to determine which materials are most resistant to the exploration environment. Contractors are working on that issue when planning for the Artemis missions that will carry the next humans to the Moon. And back in 2020, NASA's Perseverance rover to Mars carried with it samples of spacesuit materials as part of a study to see how they will respond to the Martian environment. Thank you, Captain Cernan. Your decision in 1972 will continue to have an impact for generations to come.