The question of how much you would weigh on another planet is common, driven by the knowledge that your body feels different when you travel. The answer is less about any physical change to your body and entirely about the environment in which you are standing. Your personal composition of matter remains constant, but the force pulling that matter downward changes dramatically across the solar system. This difference lies in a fundamental distinction between two commonly confused concepts in physics: mass and weight.
Understanding Mass and Weight
The terms mass and weight are often used interchangeably, but they represent two distinct physical properties. Mass is a measure of the total amount of matter that makes up your body. This property is intrinsic and remains the same regardless of your location, whether you are standing on Earth, the Moon, or floating in space.
Weight, by contrast, is a measure of the force of gravity acting upon that mass. Because it is a force, weight is variable and directly dependent on the strength of the local gravitational field. This is why a standard bathroom scale, which measures the downward force you exert, would give you a different reading on Mars than it does on Earth.
How Gravity Determines Planetary Weight
The mechanism that determines your weight on any celestial body is its surface gravity, represented by the letter ‘\(g\)‘. Surface gravity is the acceleration an object experiences due to the gravitational pull of that world. Your weight is calculated by multiplying your constant mass (\(m\)) by the local value of surface gravity (\(W = m \times g\)).
The strength of a planet’s surface gravity is determined by two primary physical characteristics: its total mass and its radius. A body with greater mass has a stronger gravitational pull, which increases the value of \(g\). However, the gravitational force weakens rapidly with distance from the center of the mass. Therefore, a planet with a very large radius places the surface farther from the gravitational center, resulting in a weaker surface gravity.
This relationship explains why some massive planets can have a surprisingly low surface gravity. Jupiter, for example, is far more massive than Earth, but its immense radius means you would be far from its core, balancing out some of its gravitational advantage. Conversely, a smaller but extremely dense planet can exhibit a relatively high surface gravity because its mass is packed into a compact radius.
Your Interstellar Weigh-In
To put these physical principles into a practical context, consider a person who weighs 150 pounds on Earth, where surface gravity is defined as \(1g\). Moving this mass to other celestial bodies immediately changes its weight based on the local \(g\) value.
Here is how that 150-pound weight would change across the solar system:
- Moon: With a surface gravity of approximately \(0.166g\), the person would weigh only about 25 pounds. This dramatic decrease in weight allowed Apollo astronauts to bound across the surface.
- Mars: A common target for future exploration, Mars has a surface gravity of \(0.38g\). The 150-pound person would weigh a modest 57 pounds there.
- Mercury: Despite being much smaller than Mars, Mercury has a nearly identical surface gravity of \(0.38g\) due to its high density and large iron core. The person would weigh about 57 pounds.
- Venus: Often called Earth’s twin, Venus has a surface gravity of \(0.91g\). This results in a weight of 136.5 pounds, very close to their Earth weight, reflecting its similar size and mass.
- Jupiter: The gas giant presents a stark contrast, possessing a surface gravity of \(2.53g\) at the cloud tops. The 150-pound person would weigh a crushing 379.5 pounds, making movement extremely difficult.
- Pluto: At the extreme low end of the scale, the distant dwarf planet has a tiny surface gravity of approximately \(0.063g\). Stepping onto Pluto would reduce the person’s weight to a mere 9.45 pounds.