The signal took 48 minutes to travel 534 million miles. At mission control in Pasadena, the tone indicating Juno’s main engine had started its burn arrived at 8:18 p.m. Pacific Time. The engine, a Leros-1b, had to fire for precisely 35 minutes. If it burned too short or too long, the $1.1 billion spacecraft would sail past Jupiter into oblivion. All instrumentation was powered down; the probe was a spinning, blind bullet relying on pre-programmed commands. At 8:53 p.m., the engine cut off. Cheers erupted. Juno had successfully inserted itself into a 53.5-day polar orbit around a world with twice the mass of all other planets combined.
Juno was built not merely to orbit, but to see through. Its mission was to map Jupiter’s gravitational and magnetic fields, measure the composition of its atmosphere, and observe its polar magnetosphere. The planet is a vault holding the recipe for the early solar system. To survive the intense radiation belts, Juno’s sensitive electronics were housed in a 400-pound titanium vault. Its three solar array wings, each 30 feet long, provided power despite sunlight being 25 times weaker than at Earth.
The spacecraft followed a long, looping path to minimize radiation damage during its 37 science orbits. Each perijove, or closest approach, brought it within 2,600 miles of the cloud tops. Its JunoCam instrument returned the first clear images of Jupiter’s poles, revealing chaotic storms unlike the organized bands of the equator. The data revealed a fuzzy core, deep atmospheric weather layers, and cyclones arranged in geometric patterns.
Juno completed its primary mission in July 2021. NASA extended its operations, and the spacecraft continues to return data, along with studying Jupiter’s moons. Its final task, scheduled for 2025, is a deliberate deorbit into Jupiter’s atmosphere, ensuring no contamination of the potentially habitable moon Europa. It will be destroyed by the planet it traveled so far to understand.