PASADENA, Calif.—Apart from the sun, no celestial body has influenced life on Earth as much as Jupiter. The shape of the solar system is largely its doing. In our solar system’s youth, the gas giant’s footloose migration toward and away from our star flicked aside lesser worlds—a pruning that, as brutal as it was, opened up space for our planet. Jupiter may have seeded the early Earth with icy materials, while later shielding us from devastating comet collisions. And yet Jupiter itself is largely a cipher. Did it build up slowly, like the other planets, or was it born in a single gravitational whoosh, like a tiny star? If has moved around, where did it originally form?

We may soon find out. The Juno spacecraft, streaking toward its rendezvous with Jupiter, crossed the orbit of the outermost of that planet’s large satellites, Callisto, yesterday at 11 a.m. Pacific time. If a do-or-die rocket burn goes as planned on Monday evening, Juno will become the second spacecraft ever to orbit Jupiter, after the Galileo mission that arrived in 1995. Galileo toured the entire Jovian system, glimpsing many of its satellites up close before its mission ended in 2003. Juno will instead focus on the planet itself, systematically surveying it across up to 37 orbits that take it within 5,000 kilometers of the cloud tops. And, unlike Galileo, which used a nuclear power source, Juno’s electricity comes from the sun—a first for a spacecraft operating in the outer solar system.

celestial: (adj.) 天空的

footloose: (adj.)  自由自在

flick: (v.)  轻打

pruning: (adj.) 修剪用的

brutal: (adj.) 残忍的;野蛮的

shield: (v.) 保卫

devastate: (v.) 毁坏

comet collisions: 彗星碰撞

cipher: (n.)  密码

whoosh: (n.) 嘶嘶声;飞快的移动

streak: (v.) 飞跑

rendezvous: (n.) 约会

do-or-die: (adj.) 不屈不挠的

Jovian system: 木星系统

glimpse: (v.) 瞥见



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