The achievement is unlocked when you dock at the Stellar Bay landing pad or arrive, on foot, at the Town Gates having landed on the planet Monarch. It is missable because it is possible to get to the point of no return without going through Monarch. There are several ways to get a Stellar Bay Navkey. The two main ones are by completing the Passage to Anywhere main story quest if you elect to follow the Phineas path or the Long Distance main story quest if you choose to follow the Halcyon path.
When you land on Groundbreaker, your ship is impounded. You must talk to Ambassador Udom Bedford to have it released. You are presented with a choice—whether or not to turn Phineas in to the Board. If you elect not to, your next stop is Monarch. You play through the main story quest Passage to Anywhere. At its conclusion, you receive a Stellar Bay Navkey from Gladys. When you use it, the achievement pops as you land.
You may, elect, however, to tell Udom Bedford you have information about Phineas. You play through the main story quest Balance Due. Your next stop is not Stellar Bay on Monarch. Instead, Balance Due rewards you with the Navkey for Adjudant Akande's Landing Pad on Terra 2.
You still can acquire the Stellar Bay Navkey. You play through the main story quests The Puppet Masters, The Demolished Woman, and Long Distance. At the conclusion of Long Distance, talk to Percival who will give you the Stellar Bay Navkey. Select Monarch as your destination and the achievement will pop upon docking at the Stellar Bay landing station.
CAUTION: You can go directly from Byzantium where you acquire the dimethyl sulfoxide to Phineas’s lab. Once you give Phineas the chemicals, you can go to the Hope and--after skipping it--to Tartarus. When you go to Tartarus, you reach the point of no return without ever going to Monarch and, thus, missing the achievement.
An alternative is to go to the Planet Monarch without the Stellar Bay Navkey. The Cascadia Landing Pad is available although you are presented with the admonition, Warning! Dangerous!. After docking, run and/or fight your way to Stellar Bay. Once you enter town on foot, the achievement will pop. It may seem inelegant but it provides the earliest opportunity to unlock the achievement. The enemies around Cascadia are quite powerful so, if you’re playing on supernova, it’s quite a challenge to deal with them before you’ve properly leveled up.