An astronaut pen is a specially designed ballpoint pen that can write in zero gravity, upside down, underwater and in a wide range of different temperatures. The microgravity encountered in space rendered traditional ballpoint pens useless, so both the United States and Soviet Union began to study ways to create a writing instrument for use in space. Both countries adopted the design created by the Fisher Space Pen Company.
The main innovation of a space pen is the pressurized ink cartridge. Unlike traditional ballpoints, which rely on gravity to feed the ink to the writing tip, the increased pressure inside this cartridge ensures a steady flow of ink no matter which way it is pointing. The internal pressure also allows the ink to flow underwater and makes writing possible on a greater variety of surfaces. The ink inside a space pen is designed to remain liquid at temperatures as low as minus 30 degrees Fahrenheit and to survive temperatures as high as 250 degrees Fahrenheit.
Both the United States and the Soviet Union attempted to develop their own space pens, but the costs soon inflated out of control. Before the Fisher company debuted their unique design, NASA used mechanical pencils in space, while the Russians adopted plastic slates and grease pencils for note-taking in zero gravity.