Phobos has always been a highly contested area. Skirmishes between the tireless Grineer and Corpus factions occur in orbit and on the Martian moon surface. Unanswered questions about this natural satellite still remain as all research is endlessly delayed by war.
Phobos is a moon of Mars where the Corpus are the controlling faction.
From Update 9.5 to Update: Specters of the Rail, Phobos was depicted as a round planetoid in-game, as opposed to being irregularly-shaped in real life. It is widely believed to be a captured asteroid and is speculated to either collide with Mars or disintegrate within the next 50 million years.
The nodes on Phobos are named after actual geological features of Phobos. Other than Monolith and Iliad, those in turn are named either after astronomers (e.g. Asaph Hall, who discovered Phobos) or after characters in Gulliver's Travels (as its author, Jonathan Swift, predicted the existence of Mars's moons).
The boss level, Iliad, is named after an ancient Greek epic poem, which tells of the battles and events during the Trojan War and of the quarrel between King Agamemnon and the Warrior Achilles, where Ares (father of Phobos) is a significant character.
Even after the change, the Lotus still refers to the final stage of The Law of Retribution, which used the Grineer Stellement Tileset, as Phobos, reflecting its previous environment.
This planet was unlocked as a reward for the community completing the Arid Fear Event. Much of the event involved the Grineer and the Corpus trying to hide the existence of Phobos from the Tenno.
Up until The Gradivus Dilemma, Phobos was the third planet in the game that was completely under the control of a given faction (besides the indigenous species), following the (then) Infested-controlled planets of Eris and Jupiter.
Phobos was the first planet to have an Assassination mission involving more than one entity, the second being the first phase of Lephantis.
Phobos is the only planet in which Research components do not drop.