Several instruments have been used to write in outer space, including different types of pencils and pens. Some of them have been unmodified versions of conventional writing instruments; others have been invented specifically to counter the problems with writing in space conditions.
The wood pencil has been used for writing by NASA and Soviet space programs from the start. It is simple with no moving parts, except for the sharpener. But wood, graphite, and rubber (in the eraser) are all combustible and create dust. Graphite, in particular, burns to produce dust that conducts electricity.
The mechanical pencil has been used by NASA starting in the 1960s Gemini program. It can be made to be as wide as the width of astronauts' gloves, yet maintain its light weight. There are no wooden components which might catch fire and create dust. However, the pencil lead still creates graphite dust that conducts electricity.
Grease pencils on plastic slates was used by Soviet space program as an early substitute for wood pencils. It is simple with no moving parts. The paper shroud is peeled back when needed. The disadvantage is that the paper wrapper has to be disposed of. Writing done with the grease pencil is also not as durable as ink on paper.
Ballpoint pens have been used by Soviet and then Russian space programs as a substitute for grease pencils as well as NASA and ESA. The pens are cheap, use paper (which is easily available), and writing done using pen is more permanent than that done with graphite pencils and grease pencils, which makes the ball point pen more suitable for log books and scientific note books. However, the ink is indelible.
Felt-tip pens were used by NASA astronauts in the Apollo missions.
The Space Pen is a gas-charged ball point pen that is rugged and works in a wider variety of conditions, such as zero gravity, vacuum and extreme temperatures. However, it is slightly more expensive than the aforementioned alternatives. It has been used by NASA astronauts on Apollo, Shuttle, and ISS missions.