There is a famous story about pens. Picture the scene. Russian space agency and NASA or having a chat at some space conference.Lets call the Russian Igor and the American Buddy.

Buddy: We have come up with a great way of solving the writting in space problem. You know how pens don’t write upside down as gravity it not pushing the ink down thus you can’t write in space.

Igor : Yes indeed.

Buddy: Well  we have spent millions on this space pen.It uses a pressured capsule to force the ink down thus it will work in zero gravity amazing ya.

Igor: Oh my god that is so cool and some piece of engineering. We use pencils.

The idea that the simple solution is often is appealing alas it is not true. There is two things wrong with the story. Firstly Both NASA and the Americans used pencils in the first place. But found them unsuitable. Firstly because if they break the lead would float about the cabin which is not a good thing as it could get into someones eye or the controls. Also wood and graphite are both combustible. So pencils were abandoned.

The second thing wrong with that story is that contrary to belief pens work in space. This is a popular mis-conception indeed Pedro Duque of the European Space Agency (which Ireland is a member of) wrote of his surprise when he used his ballpoint pen in space. So how come you can use a pen in space with no gravity but not write upside down.

We had a quick think about this in the office this afternoon and what we came up with is as follows. It is not gravity that pushes the ink down but the pressure. The cabin in space is pressurized so this helps push the ink down. Coupled together with its viscosity and the basic “lumpiness” in the narrow tube. I.e it does not really want to move out of the tube trhe ink is kept in making it useful. Think about it even though most pens are kept side down the ink does not pour out of the tube. However upside down on Earth the force of gravity is overcoming this force and forcing the ink down just enough that it clears the ball point hence not working.

That may or may not be the answer to why pens work in space. But it is the best I could come up with.