Why are astronauts in space said to be "weightless," but not "massless"?

Imagine tossing a ball in the air. It goes up, slows down, and falls back to Earth. Now think about an astronaut in a spacecraft orbiting around Earth. Something similar is happening to the spacecraft and its inhabitants. They are constantly falling towards Earth but with an important twist—the spacecraft is also moving forward so fast that, as it falls, it keeps missing the Earth. This is what we call 'free fall in orbit.'

This continuous free fall creates the condition of 'weightlessness' or microgravity. It's not that gravity isn't there; after all, it's gravity that keeps the spacecraft orbiting Earth. It’s just that everything in the spacecraft is falling at the same rate, including the astronauts. This means there’s no solid surface pushing back against them to create a force that we perceive as weight.

• Free fall affects all objects on the spacecraft in the same way.
• Interestingly, aboard the International Space Station (ISS), astronauts feel weightless even though they are only 400 km above Earth, where gravity is still about 90% as strong as on the surface.

This phenomenon is what enables astronauts to float inside the spacecraft, and also why it's tricky to drink water or handle ordinary objects without them floating away. This experience is a cornerstone in understanding the difference between the concepts of mass and weight.

Let's unpack the difference between mass and weight, as it's often a source of confusion. Mass is a measure of how much matter an object contains and is typically measured in kilograms or grams. It remains constant regardless of where you are—in space, on Earth, or on the Moon.

• It is an intrinsic property and doesn't change due to location or movement.

Weight, on the other hand, is the force exerted by gravity on the mass of an object. It's measured in newtons or pounds-force, depending on the system of measurement you're using. Unlike mass, weight can change with location because it is dependent on the strength of the gravitational pull in that location.

So when astronauts are in space, they have not lost any of their mass—they are 'mass-full,' so to speak. But because they are in a state of free fall in an orbiting spacecraft, the normal force (the supportive force from a surface) that gives us our sense of weight is absent. Hence, they feel 'weightless.' This distinction is crucial for many aspects of physics and is especially important in understanding how to operate in different gravitational environments, like space.

Microgravity is the term we use to describe the conditions in space that seem like very low gravity. It's not that gravity is missing, but its effects are greatly reduced when in free fall. This has some interesting implications for objects in space:

• Physical behaviors we take for granted on Earth, like pouring a glass of water or writing with a pen, don't work the same way.
• Fluids tend to form spheres and cling to surfaces due to surface tension.
• Flames burn differently, assuming a spherical shape rather than the teardrop shape we see on Earth due to convection currents.
• Bones and muscles can weaken over time without regular exercise, as they're not supporting or resisting weight as they do on Earth.

These effects impact how astronauts live and work. For example, exercise in space is vital to maintain muscle mass and bone density. Similarly, the way astronauts handle tools and materials must adjust for the lack of resistance and weight. Research conducted in this unique environment provides valuable data that extends our understanding of physics, biology, and material science.