Space is the layers of defined block volumes above where air exists. There is no barrier to reaching it other than the necessity of building a way to get there or making and using a jetpack. Space is also extremely cold and has no visible precipitation or obvious separation from the air layers.
Any time a blockhead's head is in space, its Air Bar will appear and begin decreasing. Eating kelp will partially restore the air bar, but it will continue to reduce. There is also a chance of the blockhead taking additional damage much as it would in cold weather. Wearing a Fur Coat or the North Pole Hat Of Warmth will reduce the chance of damage.
What most people call "space" is the rarefied layers or vacuum above the Earth's breathable atmosphere. While it can't be breathed, it isn't intrinsically cold because vacuum doesn't conduct heat. Instead, the lack of air pressure can cause liquids to boil at lower temperatures, which may draw heat from objects they're touching. Yet objects in space can overheat more easily than they can freeze.