In order for a rocket to work, it needs two things: reaction mass and energy.

There are several different space drives that need little or no reaction mass to function.

And we don't have enough reaction mass left to slow down.

We know now how much reaction mass we'll need for the trip.

Plenty of reaction mass, even if it didn't fall as rain.

The other worker saw what had happened long before that, but he didn't have the reaction mass to go out and back.

"We'll top off our reaction mass here and take on local produce," he continued.

Water for reaction mass because it's easy to store and pump.

Exactly how much reaction mass they would need at the other end to remain inside their safety margins.

It depends on how much reaction mass they have left.