In the 1950's Fisher began experimenting with the ball point pen refill. After much experimentation he perfected a refill using thixotropic ink-semisolid. It was dependable in freezing cold and desert heat. It could also write underwater and upside down. The Fisher cartridge did work in the weightlessness of outer space and the astronauts, beginning with the October, 1968 Apollo 7 mission began using the Fisher AG-7 Space Pen and cartridge.