It provides a Damage Resistance of 1 and has a very high item health (10 million HP). Radiation suits and advanced radiation suits can be used to repair it. Although a spacesuit is designed to block out harmful interstellar radiation, this suit will not protect you from radiation sources.
When you complete the quest Not of This World, you will unfreeze people from different time periods. One of them, Colonel Hartigan, died from the thawing, allowing you to loot his spacesuit. It will be located in the pod to the far right.
When the player is doing their spacewalk, you can hear respiration, despite the lack of visible air supply on the suit.
During the spacewalk, the player is unable to use their Pip-Boy, as it is inside the spacesuit's sleeve. However, the Pip-Boy (and fingerless glove) are still visible in third person view.
If the depressurization is activated without the suit on, the player's head explodes; the game will warn the player if they do not have the spacesuit equipped prior to the depressurization.
If you take off the spacesuit during the spacewalk, your suicide will result in positive or negative Karma based on your own Karma level (positive for evil characters, negative for good). It is as if you are killing yourself and gain/lose Karma accordingly.
After returning to the Wasteland, the spacesuit can be given to a companion. Despite the extremely low damage resistance, non-player characters like Charon and Star Paladin Cross will wear the spacesuit instead of their default armor or other pieces of armor, even power armor. This may be due to its extremely high condition.
You may find the display case for a spacesuit in the Museum of Technology near the final terminal in the Jiggs' Loot unmarked quest. Whether or not this is foreshadowing is unknown.
Although the suit has its own oxygen tanks for spacewalks, if it is equipped before swimming you will still have the normal amount of oxygen, as well as the possibility of drowning.
After completion of the Mothership Zeta quest line, this suit is no longer a quest item, and weighs 5 pounds. However, if the player takes the suit from Hartigan before he falls down and technically dies, the suit will remain weightless indefinitely.
Behind the scenes
The suit is based on either the Project Mercury or Project Gemini spacesuits from the early 1960s. Early previews specifically christened the suit a "Gemini-era spacesuit".
At one point during development the suit was to consist of two separate pieces: a bodysuit and a helmet. Though not included in the final release, the helmet remains in the add-on's resources and can be obtained using console commands.
End of information based on Fallout 3 cut content.
The suit cannot be hotkeyed. [verified]
If the Pip-Boy light was active before starting the spacewalk, it will remain active during that sequence, but it cannot be activated by the player during the sequence if it was initially inactive. It can, however, be deactivated. [verified]
When highlighted in the Pip-Boy, "Equip" isn't lit, though the player can still equip it, most likely because the suit is marked as a quest item; until you finish the space walk then it becomes a normal item. [verified]