An ink cartridge with pressurized nitrogen (35 PSI).
The ink lasts three-times longer than normal ballpoint ink and the cartridge has a theoretical shelf-life of one-hundred years.
The first patent was filed fifty years ago, last year.
It was always a rumor that NASA spent a ton of money on research, but it looks like this is incorrect. They tried to develop the technology, but they hit their financial limits. It was privately funded by Paul Fisher with units purchased by NASA afterword. Russia bought one-hundred, NASA bought four-hundred, and the world turned round and round.
NASA originally used pencils during spaceflight, but, apparently, the broken tips and graphite dust presented a real risk to their electronics. They also had trouble with smeared (faded?) paperwork and did not want to use a flammable material (wood).
You can buy one for about $15 on Amazon, now. They are good-looking pens, actually.