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The famed Astronaut Pen is manufactured by the Fisher Space Pen Company, and was invented independent of the space program. The pen works because it relies on gas pressure to provide ink to the ball point rather than gravity. Because of its clever design, the pen writes at all angles in extreme cold and heat, underwater, and in zero gravity. The closed design of the pen also gives it a estimated shelf-life of 100 years, but just in case, they provide refills in ten different colors.

The ink cartridge is pressurized with nitrogen gas hermetically sealed at nearly 50 PSI, features a tungsten carbide ball point in a stainless steel socket, and is filled with a thixotropic ink which is liquified by the shearing action of the ball point. The astronauts from the Mercury and Gemini projects had been using pencils because a suitable pen had not yet been invented. After experimenting with the Fisher pen, NASA chose the design for use in the space program. Beginning with the Apollo 7 mission, Fisher Astronaut Pens have been on every manned U.S. space mission, and were also used by cosmonauts on Mir, while it was still functioning.

This dispels another piece of Internet humor:

During the heat of the space race in the 1960s, the U.S. National Aeronautics and Space Administration decided it needed a ball point pen to write in the zero gravity confines of its space capsules. After considerable research and development, the Astronaut Pen was developed at a cost of approximately $1 million U.S. The pen worked and also enjoyed some modest success as a novelty item back here on earth.

The Soviet Union, faced with the same problem, used a pencil.

The Fisher Space Pen was designed by Paul C. Fisher for the Apollo Program. Due to the hostile space environment, there were strict design requirements for the pen; it should be able to write in a vacuum, in the absence of gravity and at temperatures varying from -120 °C to 150 °C. Fisher had worked on a pen design with a pressurized ink cartridge. This design was perfected in 1966, and in October 1968 the first Space Pen was used in the Apollo 7 mission.

The ink of the Space Pen is fed to the ball point by pressurized nitrogen, and thus can be used to write upside down and in a vacuum. Due to the thixotropic behavior of the ink it remains highly viscous and won't leak when the pen is not in use. When the ink is put in motion by the ball point, its viscosity will drop so that it starts flowing. The composition of the ink allows for writing on a wide array of surfaces, under water, and also at extreme temperatures.

There is a great urban legend regarding the design of the space pen:

When NASA started the space program in the 1960's, they were faced with the problem of writing in space. NASA spared costs nor efforts design a pen that would perform in a vacuum, and in the absence of gravity. Fortunately, the resulting Space Pen was a true marvel of engineering that proved its worth in the early space missions. The enthusiastic NASA engineers showed off the Space Pen to a group of Soviet-engineers working for the Russian space program, and asked them how they had solved the problem of writing in space.

"We use pencils", was their reply.

Although there is a valid lesson to be learned from this story (Keep It Simple, Stupid!), it is merely an urban legend. Prior to the Apollo 7 mission, both Russian Cosmonauts and American Astronauts were relying on pencils for writing in space. However, sometimes the pencil leads would break and go on a free flight in the space capsule. The lead tips could float into an eye or nose, or even into electrical devices causing a short circuit. Furthermore, the wood and lead of the pencils could burn rapidly in a pure oxygen environment. Because of a fire in Apollo 1 (resulting in the death of three astronauts), NASA required a writing instrument that would not burn in a 100% oxygen atmosphere, and would operate in space conditions.

(urban legend)

In fact there is a followup to this. Not only did the pen allegedly get the astronauts back from the moon, but the development costs for the pen have been recouped many times over (mainly by selling to enthusiasts on the earth), in fact according to Pedro Duque, the Russians use very ordinary ballpoint pens- they work just fine thanks! ;-)

The return from the moon story, which neatly cannot be corroborated, is that Neil and Buzz went for their parambulation on the moon and:

"When about to leave the moon, and the astronauts were climbing back into the Lunar Module, the life support backpack on one of the astronauts brushed against the plastic arming switch and broke it. The switch was to have activated the LM's engines for the module's rendezvous with the mother spacecraft."

The urban legend is that they undid a Fisher space pen and used it to activate the broken switch.

However, Buzz states that the pen he used was not a ball-point pen, but rather a felt-tip pen (see: http://www.hq.nasa.gov/alsj/a11/a11.posteva.html).

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