Paul Fisher, a pen maker and inventer, created a pressurised ballpoint refill that would work in space. It will also write well on damp and dusty paper, greasy paper and in excessively warm or cold temperatures. The Bullet Pen design was created in 1948 (20 years before the moon landing!) and is still as appealing today.
With the cap on, the pen is small and smooth, and fits well in a pocket or in a small bag, and measures only 95mm. When it's posted for writing, it extends to 134mm making it a comfortable fit in the hand.