Fisher Space Pens

In the 1960's, after much experimentation Paul Fisher perfected a pen refill using thixotropic ink, which stays semisolid until the shearing action of the rolling ball liquefies it so that only flows when needed. The cartridge was pressurized with nitrogen so that it didn't rely on gravity to make it work, so it was dependable in freezing cold and desert heat. It could also write underwater and upside down. The Fisher cartridge even works in the weightlessness of outer space and astronauts, beginning with the October, 1968 Apollo 7 mission began using the Fisher AG-7 Space Pen.