Are artificial satellites such as communication or weather satellites completely sealed or are they open to space? (In either case, why? How are their electronics (un)affected? Presumably they can be shielded.)
For the most part, satellites are opened loosely to space. The components are designed to work in a vacuum, and in fact, are tested to work in a vacuum on the ground. However, there's a few things of some note along with this:
- There are typically a few components which must be sealed in. The most common is propulsion tanks, also, some Reaction Wheels are designed to work in atmosphere, and there are probably some others that I don't know about as well. They have to be hermetically sealed.
- The main box of the satellite is not air tight, but usually is completely closed in. This allows for better thermal management, as well as preventing EMI hitting the antenna/receivers. Typically this results in a loose sealing, which will take some time to get to complete vacuum (On the order of hours typically). Note that the hours is for a near complete vacuum, a rough vacuum is obtained quite quickly.
Generally open to space. Maintaining a perfect seal a long time is hard. If your equipment needs non-vacuum to operate, and you expect it to last, then it is pretty likely it will leak over time, so might as well design to operate in a vacuum.
One benefit of an atmosphere inside would be better cooling. But since there is no difference between hot/cold air in weight, due to micro-gravity, you do not get natural convection anyway. Now you have to maintain a fan for active maintenance. Another moving part, another hard thing to keep going long term with no humans around.
Each benefit and trade-off, usually lead towards just designing to operate in a vacuum.
Early Russian probes were pressurized. The reason was to give the equipment similar conditions which could be tested on Earth, and to simplify the thermal environment (making conduction/convection possible, not just radiation as in vakuum). They used nitrogen at 113 hectopascal, this source says: Andrew J. LePage at the space review http://thespacereview.com/article/2477/1
Satellites are not sealed. A satellite is exposed to rapidly changing pressure during lift-off and has to be designed to release air trapped within the main frame (the bus) in a controlled manner without damage. An example, due to changing temperatures in space, they are wrapped in a thermal blanket, which has to have vent holes as it could rip during lift-off and potentially obstruct the payload (eg the antenna).