For the Escape From The Endless Vayage quest line, after fixing the grav drive of the ECS Constant it disappears from Porrima II orbit so the quest cannot be completed. I have seen some report that they come across it again randomly. However there are no less than 3 related tasks currently open on my missions screen:
ESCAPE FROM THE ENDLESS VOYAGE
Speak With Diana
ACTIVITIES
Location of the ECS Constant
Speak with Abe
All three of these take you back to Porrima II, but the ship is no longer there.