The exhaust velocity, though, is enormous—more than ten times that of a chemical rocket—and this makes ion propulsion extremely efficient.

但其排气速度却异常惊人，大约为化学燃料火箭的十倍—这使得离子推进器的效率非常高。

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Powered by ion propulsion instead of conventional rocket fuel, Dawn slid around Vesta on July 15 after a 1.7 billion-mile cruise.

以离子助推而非传统火箭燃料的“黎明号”在7月15日进入灶神星轨道之前已进行了一次17亿英里的巡航。

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