Conjunction

Conjunction is the astronomical condition in which two objects have the same ecliptic longitude from the point of view of an observer body, usually Earth. The objects will generally form a straight line or Syzygy with Earth. The most common form of conjunction occurs when the Sun is directly between the Earth and another object. This is known as a superior conjunction. An inferior conjunction occurs when the object is between the sun and the Earth, such as with Mercury or Venus. This occurs at the time when Earth would be in Opposition with the planet. An @Opposition is the inverse of a Conjunction.
Superior Conjunction occurs when Earth and the other planet or object are on opposite ends of their orbital period and at the furthers distance from each other. During Conjunction (assuming roughly coplanar orbits) the object will not be visible from Earth, as it will appear to have passed behind the Sun. This means that radio or light communication and visual observation will not be possible without an intermediary relay at an oblique angle to the two positions. Sensitive communications are not advised within a few degrees of Conjunction, as Solar interference could produce unpredictable latency issues.
Leading up to conjunction, the time it takes light to travel to and from Earth will be at its longest. Starship travel distances will also be at their longest, requiring navigational vectors to take the Sun's gravity well into account. Travel during and leading up to a conjunction is ill advised barring an emergency.
In terms of Mars, Conjunction occurs roughly halfway between two Opposition events. During this time, TASA Marine Corps forces on Mars will be unable to communicate with Earth and cannot request support or reinforcements.