Fisher Space Pen
The Fisher Space Pen is a writing implement designed specifically for use in zero-gravity environments with variable atmospheric pressure. It is a gas-charged ball-point pen with replaceable ink cartridges.
Before this pen was developed, there were some issues with fine writing. Traditional pens depended somewhat on gravity, and even though ballpoint pens were self-contained, the ball could run dry while writing. Extreme temperatures, and even wet surfaces were also concerns. Pencils were the obvious choice, therefore, since they are durable, reliable and cheap. However, those didn't come without issue either. The graphite tip could break off, but even more problematic was the dust released into the air while writing. Without the affects of gravity, dust and debris poses a real threat to on-board equipment. Therefore, felt-tipped markers were often considered a safer option.
During the early days of NASA's space missions, Paul C. Fisher of the Fisher Pen Co. set about developing a better pen, which could deal with the known problems of writing in space. Using about $1 million of his own money, he developed a pen which used pressurized gas to push the ink out, independent of gravity. Fisher then offered these pens to NASA in 1965, but they did not immediately adopt it. However, they decided to do testing on the new pen, and finally in 1967, NASA accepted the product and chose to provide the Apollo astronauts with these pens. In all, the Apollo project was estimated to have bought about 400 space pens, for $6 each.
In February 1969, the Soviet Union decided to stop using their grease pencils in space, and purchased 100 of the Fisher pens as well, along with 1,000 of the pressurized ink cartridges. The Russians used these for their Soyuz space flights.
Since the design of the original space pen, few functional changes have been made. For example, the Astronaut Space Pen (which is still sold today) is very similar to the kind taken to the moon in 1969.