On 4 July, NASA intends to finish a job that started with the agency’s Galileo mission 21 years ago. At 8:18 p.m. Pacific time, the Juno spacecraft will ignite its main engine for 35 minutes and nudge itself into orbit around Jupiter. If all goes well, it will eventually slip into an even tighter path that whizzes as close as 4,200 kilometres above the planet’s roiling cloud-tops — while dodging as much of the lethal radiation in the planet’s belts as possible.