I read this book in the 80's.
In the near future, humans found an hangar full of small alien space ships in the Moon (~3 passengers). Ships have no controls other than START/STOP.
You can't select a destination. The ship takes you to a random (but valuable) location far away. Trip duration is unknown. An indicator light goes on when half of the trip is completed. The trip can be too long and you will die of starvation/lack of oxygen.
The ship returns automatically to his Moon base.
The story is about one of those expeditions aboard an alien ship.