Lower it to crush one marine. Raise the sphere again. Now go into the remaining tube to fix the gravity ring. Step on the plates to rotate the highlighted segments into a single line. Watch out for asteroids.
Some plates move more than one segment.
Head back to receive Dr. Escher's logbook. Read it for the medical security code. The Marine near the airlock has the information you need to get in the airlock in his logbook. Go read your identity from the medical monitor in the operating room, using the security code to open up the classified data. Go to the hangar.