# Do Astronauts Have Less Density on the Moon?

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Space exploration is a topic that captures people's imaginations and challenges them to think about exactly what might happen once they leave Earth’s protective bubble. For one, the microgravity of space or lower gravity on the moon means that astronauts’ bodies are no longer tethered to the ground in the same way. Laws and definitions studied in physics allow you to determine how this affects their density.

## Mass Vs. Weight

First, it's necessary to understand the difference between mass and weight. Mass is a measure of how much matter is in an object -- in this case, an astronaut. It is a tally of the quantity of atoms present, and it is the same whether a person is on Earth or in space. Weight, on the other hand, measures the effect of gravity on the mass of an object. That means your weight on Earth is a combination of how much mass is in your body and how hard the Earth is pulling you down toward the ground. On the moon, there is only about one-sixth the gravity of Earth, and so the astronaut weighs much less.

## Defining Density

Density and mass are related concepts. Density is the amount of matter per unit of volume. For example, an astronaut might have a volume of 65 liters and a mass of 68 kilograms. If you divide her mass into her volume, you reach a density of 1.05 kilograms per liter. Not so coincidentally, this happens to be very close to the density of water, which is 1.00 kilograms per liter. You have probably heard that humans are more than half water, so it makes sense that they have about the same density.