Can satellites measure gravity on planets with thick atmospheres like Jupiter?

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

Let's understand it simply

Imagine you are playing cricket on a field. If you throw a ball up, gravity pulls it back down. Now, imagine you are on a giant football field covered by thick fog (like Jupiter’s thick atmosphere). Even if you can’t see the ground, gravity still pulls the ball down.

Satellites are like cricket balls thrown around a planet. By watching how they move—how fast they go, if they speed up or slow down—we can figure out how strong gravity is, even if there is a thick atmosphere in the way.

How it works

1. A satellite is sent to orbit (circle) the planet, just above the atmosphere or inside the upper layers.
2. The satellite’s path and speed are tracked very carefully from Earth using radio signals.
3. If gravity is stronger, the satellite moves faster or its path bends more.
4. Scientists use the satellite’s motion to calculate the planet’s gravity using the gravity formula.
5. Corrections are made for the atmosphere’s drag (air slowing the satellite), if the satellite is inside the atmosphere.
6. By repeating this at different places, we can even map where gravity is stronger or weaker on the planet.

The story behind it

The idea of using satellites to measure gravity comes from the time after humans launched the first artificial satellites, like Sputnik in 1957. Scientists noticed that satellites did not always move in perfect circles—they wobbled or sped up in certain places. This was because of changes in gravity from mountains, oceans, or even the planet’s shape. On Jupiter, the first detailed gravity measurements came from NASA’s Pioneer and Voyager missions in the 1970s, and later, the Juno spacecraft, which arrived in 2016, gave us the most accurate data.

One memorable detail: Juno flies very close to Jupiter’s cloud tops, braving strong radiation, just to measure tiny changes in gravity.

Where you see it in real life

• Juno spacecraft at Jupiter: Juno orbits Jupiter and measures gravity by tracking its speed and path changes.
• Chandrayaan-1 at the Moon: India’s satellite mapped the Moon’s gravity by noticing small wobbles in its orbit.
• GRACE satellites at Earth: Two satellites follow each other, and changes in distance show gravity changes due to water or mountains.
• Mars orbiters: Satellites around Mars have mapped gravity to find hidden volcanoes or underground ice.
• Weather satellites: Even Earth’s weather satellites must adjust for gravity changes to stay in the right orbit.
• GPS satellites: Their orbits are corrected for tiny gravity differences to give accurate locations.

Why it matters

• Space missions: Knowing gravity helps us send landers and rovers safely to planets.
• Planet structure: Gravity maps show what is inside a planet—like if it has a big core or hidden oceans.
• Weather and climate: On Earth, gravity changes tell us about melting ice or rising sea levels.
• Navigation: Satellites need gravity data to stay in the right path, or they might crash or drift away.

Worked example

Let’s see how a satellite can measure Jupiter’s gravity:

Given: Satellite orbits Jupiter at 70,000 km from center, speed = 42,000 m/s
Step 1: Use the rule for circular orbits: v² = G × M / r
Step 2: Rearrange to find M (mass of Jupiter): M = v² × r / G
Step 3: Put in the numbers:
  v = 42,000 m/s
  r = 70,000,000 m
  G = 6.67 × 10⁻¹¹ N·m²/kg²
  M = (42,000)² × 70,000,000 / (6.67 × 10⁻¹¹)
Step 4: Calculate:
  (42,000)² = 1,764,000,000
  1,764,000,000 × 70,000,000 = 1.2348 × 10¹⁷
  1.2348 × 10¹⁷ / 6.67 × 10⁻¹¹ = 1.85 × 10²⁷ kg
Step 5: Answer: Jupiter’s mass is about 1.85 × 10²⁷ kg (matches real value!)

Common mistakes

• Forgetting about atmosphere drag: Satellites inside thick atmospheres slow down; scientists must correct for this.
• Thinking satellites need to land: Satellites can measure gravity just by orbiting, not by landing.
• Mixing up gravity and weight: Gravity is the force; weight is how much gravity pulls on a mass.
• Assuming gravity is same everywhere: Gravity can change if the planet is not a perfect sphere or has mountains inside.

Remember it

Satellites measure gravity by watching how their path changes as they orbit—even thick atmospheres can’t hide gravity from careful tracking.