Having a writing implement handy when you need it is a good idea, but keeping a bulky pen in your shirt or your pants pocket can be a drag, especially when conventional pens leak. Your wallet is a natural place to keep a pen but just about anything available commercially is too big. So, make a Space Pen for your wallet. It's small, writes every time you need it, upside-down, and probably under water, too, although you'll have to test that yourself.
The "NASA Space Pen by Fisher" is a cool pen that retails in stores like Staples and Office Depot for about $22 in its basic, chrome-plated version, up to near $50 for the special, "signature" version. It is the famous pen the astronauts supposedly used in the space program. The pen uses a sealed, gas-pressurized ink cartridge, which delivers the ink at a perfect flow without clogging--important for a pocket pen. The problem is the NASA Space Pen body is very thick. It feels like a carrying around a big, chrome-plated bullet in your pocket.
It's way too thick to slip in your wallet, too. Overall not a good traveling pen, even at that price point. But, interestingly, the same outfits that sell the pens also sell the refills and they are a fraction of the cost: about $5.00. The one to look for is labeled, "Fisher Pressurized Space Pen Refill - SPR-F". As near as I can tell all the pens use the same, $5.00 refill.
(link: example of refill at online store)
With a bit of heat-shrinkable tubing, a pocket clip taken from a discarded mechanical pencil you can have a cool pen that keeps in your wallet nicely. And since it uses the pressurized, Fisher Space Pen refill, it isn't likely to leak in your pocket.
And, it writes upside down. So, let's get to work. This one will take about five minutes.
Step 1: Adding the heat-shrinkable tubing
The tubing makes the pen cartridge a bit more holdable then just the bare metal. I used black tubing for mine, but it can be obtained in various colors. Radio Shack sells a very reasonably-priced assortment in various colors. The black tubing used in this project was ordered in bulk from Mouser Electronics. For this, use 3/8 inch tubing. Cut the tubing with an X-ACTO knife, leaving about 1/4" past the first ridge in the point, or about 3/4" from the tip.
Step 2: Top View
For a really finished look on the top, leave a millimeter or two of excess before shrinking it. It will wrap over the top, giving it a sealed, "factory" look.
Notice the clip, here, is already in place.
Step 3: Shrinking the tubing using a small pocket heatgun
I used a small pocket heat-gun to shrink the tubing. A butane lighter can also be used. When using either, be sure to keep the flame moving across the surface, never stopping at any given point. This ensures that the tubing won't burn and that the ink cartridge does not get too hot.
Step 4: The Pocket Clip
I used a pocket clip from a discarded mechanical pencil. Slip it off and, using a pair of needle-nose pliers, crimp the band of the clip slightly to hold it snugly against the body of the pen.
Check your pencil-cups around the house for pens or pencils with clips to scavenge. It's amazing how many things have pocket clips.
Step 5: The clip should fit snugly but not bind
You may require a bit of adjustment with the pliers to get a good, snug, but not binding fit.
Step 6: Keeping it in your wallet
Since the pen is small in diameter and length, it is easy to slip it into your wallet, where it is handy for taking notes in the field or signing a check at the store.
Either way for about $5.00 you can have a Space Pen in your wallet.