Can satellites measure gravity on other planets or the Moon?

In one line

The rule

Gravity is a natural force that attracts two bodies towards each other. It is responsible for keeping planets in orbit around the sun and objects on the ground.

The formula for gravitational force is: F = G × (m₁ × m₂) / r² where:

• F = force of gravity (in newtons, N)
• G = universal gravitational constant
• m₁ and m₂ = masses of the two objects (in kg)
• r = distance between their centers (in meters)

Let's understand it simply

Imagine you are riding a cycle on a bumpy road. If the road goes up and down, you feel it in your body. Now, think of a satellite as a cycle flying above a planet. The planet’s gravity is like the bumps and dips. If gravity is stronger in one place, the satellite is pulled a bit more; if weaker, it is pulled less. By watching how the satellite speeds up, slows down, or changes its path, scientists can "feel" the gravity, just like you feel the road.

Satellites have very sensitive instruments—like weighing scales and rulers—that measure tiny changes in their movement. These changes tell us about the gravity below them, even if no one is standing there with a weighing machine!

How it works

1. A satellite is sent to orbit (go around) a planet or the Moon.
2. The satellite carries special tools to measure its position and speed very accurately.
3. As the satellite moves, gravity from the planet or Moon pulls on it. If gravity is stronger in one area, the satellite speeds up; if weaker, it slows down.
4. Scientists track these small changes using radio signals sent between the satellite and Earth.
5. By studying these changes, they create a gravity map showing where gravity is strong or weak on that planet or the Moon.

The story behind it

The idea of measuring gravity from space started with Earth. Scientists wanted to know why the sea level is not the same everywhere and why some places have higher mountains. The first big mission was GRACE (Gravity Recovery and Climate Experiment), launched by NASA and Germany in 2002. It used two satellites to measure Earth's gravity very precisely. Later, similar methods were used for the Moon (GRAIL mission) and other planets.

One interesting detail: the GRAIL satellites were called "Ebb" and "Flow," named by school children!

Where you see it in real life

• Earth’s gravity mapping: Satellites like GRACE measure how water moves underground or how ice melts in Antarctica by tracking gravity changes.
• Moon’s gravity: NASA’s GRAIL satellites orbited the Moon and made a detailed gravity map, showing hidden mountains and craters under the surface.
• Mars gravity: Orbiters around Mars measure gravity to find out where there might be underground water or lava tunnels.
• Jupiter’s moons: Satellites measure gravity to guess if there are oceans under the ice of moons like Europa.
• Finding oil or minerals: On Earth, gravity maps made by satellites help find places where oil or minerals might be hidden underground.
• Space missions: Knowing gravity helps plan safe landings for spacecraft on the Moon or Mars.

Why it matters

• Safe landings: Spacecraft need to know gravity to land safely on other planets or the Moon.
• Finding resources: Gravity maps help find water, oil, or minerals, both on Earth and other planets.
• Understanding planets: Gravity shows what is inside a planet or moon—like mountains, valleys, or even hidden oceans.
• Climate studies: On Earth, gravity changes show how much ice is melting or how water moves, helping us understand climate change.

Worked example

Let’s say a satellite is orbiting the Moon. Scientists notice the satellite speeds up when it passes over a certain spot. They measure:

Given:  m₁ = mass of satellite = 1,000 kg
        m₂ = mass of Moon = 7.35 × 10²² kg
        r = distance from Moon’s center = 1,740,000 m (Moon’s radius)
        G = 6.67 × 10⁻¹¹ N·m²/kg²
Step 1: Write the rule        F = G × (m₁ × m₂) / r²
Step 2: Put in the numbers    F = 6.67 × 10⁻¹¹ × (1,000 × 7.35 × 10²²) / (1,740,000)²
Step 3: Calculate numerator   1,000 × 7.35 × 10²² = 7.35 × 10²⁵
Step 4: Calculate denominator (1,740,000)² = 3.0276 × 10¹²
Step 5: F = 6.67 × 10⁻¹¹ × 7.35 × 10²⁵ / 3.0276 × 10¹²
Step 6: F ≈ 1.62 × 10³ N

This is the gravitational force pulling the satellite towards the Moon at that height. If the force changes as the satellite moves, it means gravity is different in different places.

Common mistakes

• Thinking satellites need to land to measure gravity: They can measure it while flying above the surface.
• Believing gravity is the same everywhere: Gravity changes if the planet is not a perfect ball or has mountains, craters, or different materials inside.
• Confusing weight and gravity: Weight is the force you feel due to gravity; satellites measure gravity, not weight directly.
• Forgetting about distance: Gravity gets weaker the farther you are from the planet or Moon’s center.

Remember it

Satellites "feel" gravity by how their path changes—like a cycle feeling bumps on a road.