It’s all down to the details of orbital mechanics and rocket science, but in the end, how lucky that a NASA spacecraft will be entering orbit around Jupiter on the 4th of July.
Here’s The Verge’s excellent space writer Loren Grush:
After launching in August 2011, the vehicle has been traveling to the gas giant for the last five years and will finally reach its destination later this evening. Once it arrives, the spacecraft will turn on its main engine for 35 minutes to slow itself down by 1,200 miles per hour. If all goes well, the burn will put Juno into a 53-day orbit around the planet.
NASA won’t really be celebrating until that engine burn is successful though. Juno has only one opportunity to get into Jupiter’s orbit. If anything goes wrong during the engine firing and Juno blows past the planet, that’s it for the mission. Because of how crucial this moment is, the Juno mission team is being as cautious as possible. All of the spacecraft’s onboard instruments are powered down so that nothing interferes with orbit insertion.
Sometimes the general public loses sight of the fact that interplanetary missions are incredibly rare. We’ve only ever sent one spacecraft, Galileo, into Jupiter’s orbit. All the other missions to Jupiter just flew on past. Juno is set to be the second craft to orbit Jupiter, and it will orbit remarkably close to the planet, allowing us to peer inside and learn more about how it (and the solar system at large) came to be.