There is three different sizes of the Nebula Navigator.
The sizes range from 20 to 100.
''After using this item, you will need to dispatch a fleet for the Mysterious Nebula. The same fleet will explore the Mysterious Nebula 20 to 100 times. Any ships lost during the runs be made up from the ships on your planet, until you use up the 20 to 100 runs. If you run out of ships on the planet of hit the limit SAFE Mysterious Nebula EXPLORATIONS, the item will stop working. The items' effects to all planets.
There is special events that you can buy the 50 runs or even the 100.
Some times you might win them in one of the Events.