The Space Pen (also known as the Zero Gravity Pen), marketed by Fisher Space Pen Company, is a pen that uses pressurized ink cartridges and is claimed to write in zero gravity, upside down, underwater, over wet and greasy paper, at any angle, and in extreme temperature ranges.
The Fisher Space Pen was invented by American industrialist and pen manufacturer Paul C. Fisher and is manufactured in Boulder City, Nevada, USA. Paul C. Fisher first patented the AG7 "anti gravity" pen in 1965. Pens claiming some or all of the same abilities have also appeared on the market from other manufacturers.
There are two prominent styles of the pen: the AG7 "Astronaut pen", a long thin retractable pen shaped like a common ballpoint, and the "Bullet pen" which is non-retractable, shorter than standard ballpoints when capped, but fullsize when the cap is posted on the rear for writing. One such bullet-style Space Pen is on permanent display at the New York Museum of Modern Art (MoMA).
Several of the Fisher Space Pen models (the "Millennium" is one) are claimed to write for a lifetime of "average" use; however the product literature states that the pen will write exactly 30.7 miles (approximately 48.15 kilometers).
Standard Space Pen refills can be used in any pen able to take a standard Parker Pen Company ballpoint refill, using the small plastic adaptor that is supplied with each refill. Fisher also makes a Space Pen-type refill that fits Cross pens, one that fits 1950s-style Papermate pens (or any pen that uses that type of refill), and a "universal" refill that fits some other ballpoint pens.
The ballpoint is made from tungsten carbide and is precisely fitted in order to avoid leaks. A sliding float separates the ink from the pressurized gas. The thixotropic ink in the hermetically sealed and pressurized reservoir is claimed to write for three times longer than a standard ballpoint pen. The pen can write at altitudes up to 12,500 feet (3810 m). The ink is forced out by compressed air at a pressure of nearly 35 pounds per square inch (240 kPa). Operating temperatures range from -30 to 250 degrees Fahrenheit (-35 to 120 degrees Celsius). The pen has an estimated shelf life of 100 years.
There exists a common urban legend claiming that the Americans spent millions of dollars developing the Space Pen, and the Russians used a pencil. In fact, NASA programs have used pencils (for example a 1965 order of mechanical pencils) but because of the danger that a broken-off pencil tip poses in zero gravity and the flammable nature of the wood present in pencils a better solution was needed.
NASA never approached Paul Fisher to develop a pen, nor did Fisher receive any government funding for the pen's development. Fisher invented it independently, and then asked NASA to try it. After the introduction of the AG7 Space Pen, both the American and Soviet (later Russian) space agencies adopted it. Previously both the Russian and American astronauts used grease pencils and plastic slates.
Another rumor has it that the Apollo 11 astronauts accidentally snapped off a switch which was necessary to permit them to fire the engine to return to Earth, and that a Fisher Space Pen was used to press this button. While the incident did occur, Buzz Aldrin has stated that, in fact, he used a felt-tip pen for this.