Jacopo Prisco , CNN Written by

NASA marked its 60th anniversary last year, and celebrates the 50th anniversary of the moon landing on July 20.

One thing that ties together these decades of scientific discovery and intrepid missions is the amazing photographs the agency has distributed -- for free and in abundance -- as part of its mission.

Some 400 of the best, including a selection of lesser-known images, have been collected in the book " The NASA Archives: 60 Years in Space ," a visual celebration of NASA from its inception to its near future.

The Mercury Control Center at Cape Canaveral, from which seven human spaceflights were supervised between May 1961 and March 1965. Credit: NASA

But NASA didn't initially intend to keep such a detailed and vast visual record, according to the book's author and editor, Piers Bizony.

"Photography was not actually one of the early priorities," he said in a phone interview. "There was even an early proposal for the first human-carrying capsule, the Mercury, to not have a window at all. It was the astronauts who started taking Hasselblad cameras with them to see what they would get.

"NASA hadn't really planned for this, but they realized that pictures were a very important part of the message," he added, "and that people back on Earth wanted to see what was going on up there."

Jupiter's moon Io as seen by the Cassini spacecraft, launched in 1997. Credit: NASA

Cameras made by Swedish manufacturer Hasselblad were used in several historic NASA missions, including Apollo 11, the moon landing. Peculiarly, most of the photographs from that mission -- including the most iconic one -- didn't feature the first man on the moon, Neil Armstrong, but the second, Buzz Aldrin.

"It just happened to be Armstrong holding the camera most of the time," said Bizony. "So although Buzz Aldrin didn't have the satisfaction of being the first man on the moon, it's his image that represents that great achievement."

Buzz Aldrin's foot on the moon. It's likely that the footprint is still intact, because the moon has no atmosphere or rain to erode it. Credit: NASA

The Hubble Space Telescope has been another source of astonishing images, although when it first launched in 1990, the initial photographs sent back were all blurry due to a fault with the main mirror (it was fixed after three years following a special 11-day mission ).

Hubble's successor, the James Webb Space Telescope, is scheduled to launch in 2021, after several delays.

"James Webb is going to operate at a very great distance from the Earth, and it's not going to be easy -- if indeed possible at all -- for astronauts to service it if something goes wrong," said Bizony. "That's why everybody's being so cautious about getting it in good order before it launches."

Astronaut Dave Scott looks at planet Earth from the hatch of Apollo 9 in March 1969. Credit: NASA

James Webb will be much more powerful than Hubble (and its primary mirror much larger), representing the technological advancements of nearly three decades. Over time, the imaging technology used on NASA probes and telescopes has improved vastly, although the nature of space exploration means the agency often relies on outdated equipment.

"Even the most high-tech NASA space probe is using technology from 10 years ago, because it takes 10 years to develop and build a complex space program," said Bizony.

Astronaut Ed White, photographed by commander Jim McDivitt, in June 1965 aboard Gemini 4, NASA's first spacewalk mission. Credit: NASA

"But having said that, we've gone from the earliest days of black and white pictures, with few pixels of resolution, to the ultra-high-resolution images we're getting from the surface of Mars.

"You can see the progress that's been made, and it's amazing just how clear the images are. It's almost as though Mars is just around the corner."

NASA hasn't launched a program capable of manned spaceflight since the retirement of its Space Shuttle fleet in 2011. Trips to and from the International Space Station have been operated by Russia using the country's Soyuz spacecraft.

The rover Curiosity in a self-portrait taken on Aug. 5, 2015 on Mars. Credit: NASA

"NASA is uncertain about its future direction," said Bizony. "They've been trying to talk about going to Mars and putting humans on Mars for a long time. But the truth is: That's just not possible in the current generation, because we don't really know how to solve a lot of the problems that come with that.

"Going back to the moon would be much more realistic, but whether or not there's the money to put humans back on the moon -- I'm not sure."

Apollo 11 Command Module Pilot Michael Collins inspects NASA's Lunar Receiving Laboratory at the Manned Spacecraft Center in Houston. Credit: NASA

Many NASA photographs have made history, but there's one that -- perhaps more than any other -- has given us Earthlings some cosmic perspective.

"Everybody on Earth said 'A-ha! -- if I was flying around the moon, that's what I would see.'" said Bizony. "And so we shared that perspective,"

View of the Earth as seen by the Apollo 17 crew traveling toward the moon, known as "Blue Marble." Credit: nasa

"This image of the Earth floating on an absolute infinite blackness, as seen by human beings, was incredibly important."

Top image: Mothership "Balls Three" overflies an X-15 in 1961.