Juno is a NASA spacecraft designed to study Jupiter from close range while flying in a steep polar orbit. It launched in 2011, arrived at Jupiter in 2016, and became one of the farthest solar-powered spacecraft ever operated. Its mission matters because Jupiter is the largest planet and preserves clues about how the solar system formed.
By measuring Jupiter’s gravity, magnetic field, atmosphere, and auroras, Juno helps scientists test models of giant planets.
Juno follows an elongated orbit that carries it close over Jupiter’s cloud tops and then far away again, reducing long exposure to intense radiation. Its three large solar arrays provide electrical power even though sunlight at Jupiter is only about 1/25 as strong as at Earth. Instruments such as a magnetometer, microwave radiometer, gravity science system, and particle detectors work together to probe different layers of the planet.
Each close pass, called a perijove, gives Juno a narrow but detailed slice of Jupiter from pole to pole.
Key Facts
- Juno launched on August 5, 2011 and entered orbit around Jupiter on July 4, 2016.
- Jupiter is about 5.2 AU from the Sun, so sunlight there is about 1/(5.2)^2 = 0.037 times the sunlight at Earth.
- Solar intensity follows the inverse square law: I = I0/r^2.
- Orbital speed near a planet can be estimated with v = sqrt(GM/r) for a circular orbit.
- Juno uses a polar orbit, so it passes over Jupiter’s north and south polar regions instead of staying near the equator.
- Juno studies Jupiter’s deep atmosphere, magnetic field, gravity field, water abundance, auroras, and possible core structure.
Vocabulary
- Perijove
- Perijove is the point in Juno’s orbit where the spacecraft is closest to Jupiter.
- Polar orbit
- A polar orbit is an orbit that passes over or near a planet’s north and south poles.
- Magnetometer
- A magnetometer is an instrument that measures the strength and direction of a magnetic field.
- Solar array
- A solar array is a set of solar panels that converts sunlight into electrical energy for a spacecraft.
- Gravity assist
- A gravity assist is a maneuver that uses a planet’s motion and gravity to change a spacecraft’s speed or direction.
Common Mistakes to Avoid
- Assuming Juno uses nuclear power is wrong because Juno is solar-powered and relies on three large solar arrays to operate at Jupiter.
- Drawing Juno in a circular low orbit is wrong because its mission orbit is highly elongated, with brief close passes and long distant arcs.
- Treating Jupiter like a solid surface planet is wrong because Jupiter is a gas giant with cloud layers, deep atmosphere, and no solid surface like Earth’s crust.
- Forgetting the inverse square law for sunlight is wrong because solar power drops rapidly with distance, making sunlight at Jupiter much weaker than at Earth.
Practice Questions
- 1 At Jupiter’s distance of 5.2 AU, what fraction of Earth’s sunlight reaches Juno? Use I/I0 = 1/r^2.
- 2 If one of Juno’s solar arrays produces 4000 W near Earth in the same orientation, estimate its power at 5.2 AU using the inverse square law.
- 3 Explain why a polar, highly elongated orbit helps Juno study Jupiter while reducing the danger from Jupiter’s intense radiation belts.