Solar conjunction occurs when a planet or other solar system object is on the opposite side of the sun from the Earth. From an Earth reference, the object will pass between the sun and the Earth. Communication with any spacecraft in solar conjunction will be severely limited due to the sun's interference on radio transmissions from the spacecraft. There is also a risk that an antenna equipped with auto-tracking will begin following the suns movements instead of the satellite once they are no longer inline with each other. This is because the Sun acts as a large electromagnetic noise generator which creates a signal much stronger than the satellite's tracking signal. When in reference to Satellite Communications, solar conjunction occurs when the sun is directly in line with an orbiting Satellite and the terrestrial receiving station.
The term can also refer to the passage of the line of sight to an interior planet (Mercury or Venus) being very close to the solar disk. If the planet passes directly in front of the sun, a solar transit occurs.