You know the photo: Buzz Aldrin, standing on the moon and saluting the American flag.
Zoom in. What's really in the image? Why is the flag waving? Where are the stars? And how did those shadows get there?
Experts at NASA have reviewed its archives to explain the most notable details in the famed photo. Not everything, it seems, went as the space agency planned it.
The first moon landing.
Who's in the suit?
On July 20, 1969, about1140pm (EDT), the scene depicted in one of the most iconic photos ever taken unfolded.
Astronauts Neil Armstrong and Aldrin were more than 110 hours into the historic moon landing mission when they planted a US flag. Video of the event was broadcast to millions on Earth.
Aldrin stepped to the side to raise his hand in salute. Armstrong stepped back to photograph the moment.
"It's such an iconic image," said Jennifer Ross-Nazzal, a historian for the Johnson Space Center in Houston. ... You see this photo in textbooks."
The photo was taken during the Apollo 11 mission, the first manned moon landing. Aldrin and Armstrong landed in the Sea of Tranquility on the lunar surface as Command Module Pilot Michael Collins remained in lunar orbit.
About the flag: It fell over
Photos of the US flag planted on the moon have been topics of conversations concerning patriotism and conspiracies ever since the world first saw stars and stripes on the lunar surface.
However, the flag probably isn't still standing.
The working presumption at NASA is that the flag fell, said John Uri, manager of the Johnson Space Center History Office. Aldrin said he thought he saw the flag tip over from the exhaust when the lunar module lifted off, and the shadow of the flag is not visible in satellite images.
Uri said later Apollo missions placed flags farther from their lunar modules to prevent them from tipping over. It's likely that the colours have faded over the years from extreme exposure, Uri said.
Why isn't the flag drooping?
As expected, hoisting a flag presented NASA engineers with technical problems, Anne Platoff wrote in a paper in August 1993 for NASA on the history of the Apollo 11 flag.
Because the moon has no substantial atmosphere, NASA scientists led by Jack Kinzler designed a horizontal crossbar to support the flag and keep it from drooping down, Platoff wrote.
A hem was sewn across the top of the 90cm-150cm nylon flag so the bar could go through, then be lifted and locked into place at a 90-degree angle. The flagpole had a base that allowed it to more easily be driven into the moon's surface, and a red circle was painted at 45cm from the bottom to help judge how deep it needed to go.
The astronauts struggled to drive the flag's base beyond 15cm to 20cm deep into the surface, which probably contributed to the flag falling.
The flag travelled to space tacked onto the ladder on the lunar module, so the astronauts could access it when they walked on the moon. A protective shroud was built to protect the flag from the heat from the engines.
Packing the flag involved a 12-steps process and five people, Platoff wrote. The astronauts were able to access it by removing pins and Velcro it was attached to. (And no, NASA didn't invent Velcro during the Apollo missions, though it did popularise its use.)
What caused the ripples in the flag?
Many say the flag in the photo looks like it's flowing in the wind. Conspiracy theorists cite this as evidence the moon landing was filmed in a warehouse with airconditioning creating the ripple. That's false.
The horizontal crossbar was supposed to give a slight wavy effect as in a breeze, Platoff wrote, and Aldrin and Armstrong said they had trouble pulling the flag out all the way.
What you can't see: A golden olive branch and hidden messages
The flag was among various commemorative items that Aldrin and Armstrong left on the moon's surface.
A stainless steel plaque noting the feat of the first manned moon landing, a silicon disc with messages from world leaders in tiny text and a small pouch with an Apollo 1 patch, medals honouring two Soviet astronauts and a small gold olive branch were also left on the moon, according to NASA.
The patch commemorates Apollo 1 astronauts Gus Grissom, Ed White and Roger Chaffee, who died in a launchpad fire in 1967.
As for the Soviet medals, Aldrin and Armstrong honoured Vladimir Komarov, who died during the Soyuz 1 mission when his parachute failed, and Yuri Gagarin, the first human in outer space. NASA did not officially acknowledge at the time that the Soviet items were left on the moon.
Why the footprints are so distinct
When Aldrin and Armstrong stepped onto the lunar surface, they left well-defined footprints created by the outer boots of their spacesuits.
"If we went back there today, the footprints would still be there," Uri said. NASA's Lunar Reconnaissance Orbiter can show the disturbed topsoil in areas on the moon where lunar modules have landed, rovers have roamed and astronauts have walked, he said.
The Apollo 11 crew described the lunar regolith, or soil, as "almost a little bit like wet sand," Uri said. In reality, the regolith is made of rocks that have been ground up into a fine material over many years under the bombardment of micro-meteorites and solar wind radiation. Because the moon has no atmosphere, liquid water or wind erosion, the regolith probably remains well-preserved, Uri said.
There were even some fears that when a lunar module landed on the moon, the surface would be so thick with dust that it would sink, Uri said.
Is that a moon rock in the bottom right corner?
Yes. Though the moon's surface is mostly made up of the fine regolith, various rocks are scattered throughout.
The white specks seen in the background of the photo are probably lunar rocks reflecting light.
The astronauts found two main types of rocks at their landing: basalts and breccias, according to the Universities Space Research Association.
The basalts are 3.6 billion to 3.9 billion years old and solidified from molten lava. The breccias are fragments of older rocks broken up over the years by meteorites hitting the moon. Small rock fragments are sometimes fused under the intense heat and pressure of the impacts, forming breccias.
The sky is black in the photograph. Why are there no stars in the background?
When people think of what it would be like in space, they might conjure images of a star-filled expanse. Though the stars are definitely there, none is visible in Armstrong's photo.
Conspiracy theorists who say NASA fabricated the moon landing cite this as evidence; however, there is a scientific explanation.
"The only time the astronauts really saw the stars clearly was when they were orbiting the moon and in the shadow of the moon," Uri said.
Sunlight reflecting off the moon's surface washed out any of the light the stars emitted, Uri said, making them invisible in the photographs. Even the Earth's own shine could be bright enough to block out the stars, he said.
The cameras did not use long-enough exposure times to document the stars. Rather than focusing on space, the purpose of taking photos was to document the engineering feat of landing on the moon, Uri and Ross-Nazzal said.
Armstrong took most the photos the public associates with the landing. He used a 70mm Hasselblad, though NASA heavily modified the camera, so it would be easier to use while in a spacesuit. A variety of other cameras were also used during the mission.
According to the Universities Space Research Association, cameras operated using a trigger and were mounted on the front of the astronauts' spacesuits, which discredits a popular conspiracy theory about not being able to see a camera in the reflection in Aldrin's helmet in one photo Armstrong took. (We can thank NASA's later work for the creation of small-enough cameras for an essential device in our modern lives: the camera phone.)
What made the long dark shadow behind the flag?
The lunar module, the spacecraft Aldrin and Armstrong used to get from the main command and service module in lunar orbit to the moon's surface, is seen in the top left of the photograph.
There were no external lights brightening up the photos and video taken of the mission. The light comes from sunlight and reflections off the moon's surface, the astronauts' suits and the lunar module.
Buzz Aldrin's golden visor is shiny for a reason
Each piece of the astronauts' spacesuit is meticulously designed and serves a very specific purpose.
What's in the square pack on Aldrin's back? The Personal Life Support System, or PLSS, according to Cathleen Lewis, spacesuit curator at the Smithsonian's National Air and Space Museum.
"It provided oxygen, carbon dioxide cleaning, suit cooling, electronic communications to the astronaut," Lewis said.
Lewis said the suit was made from as many as 26 layers of material, including layers of mylar, dacron, beta marcasite and kapton. The suit's outer layer is teflon-coated beta cloth.
"Inside the suit, there are layers of high temperature nylon, fabric dipped in rubber, molded rubber, steel restraints, aluminum connectors, rubber gaskets, a double brass pressure zipper," Lewis said.
The gloves they wore were constructed from a fabric called Chromel-R, a woven stainless steel that insulated their hands. The fingertips of the gloves were made from silicone rubber for sensitivity, according to the Air and Space Museum.
To protect Aldrin's eyes from the intense sun exposure, his helmet had inner and outer visors.
"Worn on top of the clear bubble helmet, the visor had shades and an outer visor that was a plastic with a monomolecular layer of gold. The gold acted as a sunscreen, much in the way that sky goggles do," Lewis said.
They could have tripped on those wires in the bottom left
In the bottom left side of the photo, a series of cables appear to run along the moon's surface toward the lunar module.
It was those wires, powering a video camera, that allowed millions of people worldwide to watch Aldrin and Armstrong during their extra-vehicular activity, the official name for their moonwalk.
Because it was such a historic mission, NASA "wanted to share the experience with the rest of the world," Uri said.
Armstrong set up the camera so most of their activities would be visible to the millions back on Earth, according to the Universities Space Research Association. The set-up went smoothly, but the wiring created a tripping hazard for Armstrong and Aldrin.
According to Ross-Nazzal, the public affairs office at NASA pushed hard to get TV cameras on board spaceflights as early as the time of the Gemini missions.
The astronauts and engineers feared the added weight and operation of the cameras would create unnecessary complications, and even when the first live televised mission occurred, Apollo 7, resistance persisted within NASA, Uri and Ross-Nazzal said.
"Pubic affairs thought the public needed to see what was going on and get excited about what was happening," Ross-Nazzal said. "While the U.S. did it first, we really brought everyone else along."