Lotus Milestone is a mysterious subject mentioned in a scan in Quarantine Monitor in the Phendrana Drifts in Metroid Prime. It is possible it could be a date, event or a deadline that the Space Pirates have set. The scan indicates that Thardus could be installed as a patrol guard in the Phazon Mines by Lotus Milestone. Whether this deadline passes during the events in Metroid Prime is unknown, although it would not matter regardless, as Samus fights and defeats Thardus upon entering the Quarantine Cave.
"The taming of Thardus continues. Barring unforeseen setbacks, it can be installed on Phazon mine patrol by Lotus Milestone."