Much like the Logbook and Weapon the Helmet is a meta item. It cannot be used to build with. Attempting to use it in advanced build mode makes the game generate a small error message. However using it while in Astronaut mode makes the Dave model place the helmet on his head. If dropped the helmet shows up in space near you as an item and can be picked back up. This item cannot be sold to a shop. This item's appearance can be changed by editing the skin file.
See helmet in use
To see the animation of placing the helmet you can zoom out your camera by holding shift and using your pointing device's scroll down function. On a standard computer mouse you would roll the scroll wheel downward but on laptop track pads and other pointing devices it varies. When you use the item with the view zoomed out you can watch Dave put on the helmet and then remove it again if repeated.
Replace lost helmet
To replace a lost helmet: