What is a space pen?

A "space pen" is a pressurized ink pen adapted for use in zero gravity or underwater.

Details

The "space pen" is a writing instrument that was designed to overcome one of the limitations of using inkpens in space. Without gravity, the ink would not flow to the ball point. This was solved by adding a slightly over-pressurized chamber behind the ink in the actual pen cartridge. This would force the ink to flow.

The copyrighted design by the Fisher company, also known as the Zero Gravity Pen, appeared in 1965. Because of its design, it could also write on wet or greasy surfaces with better results than ordinary ballpoint pens. NASA also used grease pens, mechanical pencils, and felt-tip markers such as the Marks-a-Lot or Sharpie design.

The Russian space program famously used pencils instead. The problem was that graphite and wood posed a combustion hazard, and that stray graphite (being a conductor) could conceivably short out electronic circuits in a spacecraft.