An astronaut's mass on the Moon is the same as their mass on Earth.

Here's why:

* Mass is the amount of matter in an object. It's a fundamental property and doesn't change based on location.

* Weight, on the other hand, is the force of gravity acting on an object's mass. It does change depending on the gravitational pull of the celestial body.

The Moon's gravity is about 1/6th that of Earth's. This means an astronaut on the Moon would weigh 1/6th of their weight on Earth, but their mass would remain unchanged.