A pencil didn't really work that well. It had all combustible parts: wood, graphite and rubber. It also created dust.

Russians used greased pencils on plastic. Didn't work that well either.

How about a mechanical pencil. Maybe, but graphite is still there, and guess what? It conducts electricity. If you are an astronaut in outer space you don't want dust that conducts electricity in your cabin.

Then what? you might ask. A ballpoint pen. OK. But remember that ordinary ballpoint pens rely on gravity for the ink to flow. Thus, we need a different kind of ballpoint pen. We want a pen that can write regardless of environmental conditions and gravity.

Pressurized cartridges are born

Paul Fisher designed a nitrogen pressurized cartridge in 1965. Just the perfect timing since the space race was in its apogee. The AG7 space pen and cartridge were used by the Apollo 7 mission in 1968.

The components

Fisher space pen cartridge use nitrogen gas and thixotropic ink, a kind of semisolid ink that becomes fluid when in contact with an ultra-hard tungsten carbide rolling ball. That is the reason why it can write in zero gravity and that is why you can use it anytime anywhere without worrying it will stain your pants if you like to carry your pen in your pockets.

Refill characteristics

Space pen refills can write at high altitudes (12,500 feet) and work perfectly in a temperature range from −30 to 250 °F. They have a shelf life of 100 years.

Space Pen refill types

All Fisher pens use the PR or pressurized cartridge. The Fisher pen company also manufactures the SU or universal cartridge which can fit in different pens. Cross pens have their own peculiar cartridge: the SC pressurized model.

The PR cartridge is fatter and smaller compared to the SU cartridge and can also be used by Parker pens by using the adapter that comes with the cartridge. The SU refill carries less ink and can write around 12000 feet compared to the PR which will write more than 12000 feet.

Pros and Cons

The pressurized cartridge can write almost on any surface, no matter how dirty it is, regardless of the force of gravity to allow the ink to flow. However, as time goes by, the tip of the cartridge might start to collect a bit of an ink blob that would have to be cleaned every now and then before you start to write.

In spite of this little disadvantage, space pen refills will never decrease their performance and will continue to write until they run out of ink.

In conclusion, the space pen cartridge is a remarkable technological achievement. It has been around for more than forty years showing excellent results. It has been the perfect choice of police officers, contractors, sailors, pilots, astronauts and obsessive writers.