The orbits comets take sometimes end with them colliding with planets and their moons.
一些在轨彗星有时会以与行星或其卫星发生碰撞的形式走向终结。
Do we have an ethical duty to respect and preserve the natural environments of neighboring planets and their moons?
我们有一个道德义务，尊重并维护邻近行星及其卫星的自然环境吗？
This family of planets, with their moons, is known as the solar system.
这个行星家庭以及它们的卫星，就是我们所知道的太阳系。
In time, we will want to dispatch spacecraft that will go into orbit around these alien planets, land robots on their moons, and even return rock and soil samples back to Earth.
迟早，我们将发送飞船进入环绕那些陌生行星运转的轨道，让机器人在它们的卫星上降落，甚至将岩石和土壤样品送回地球。
It is an easily provable fact that with only chemical rockets, our ability to explore the outer planets and their moons is hindered.
如果仅仅使用化学燃料火箭，我们探索外层空间行星及其卫星的能力将受到限制，这是不难证明的事实。
Eventually, we will want to send humans to their moons, on at least a couple of which liquid water - the fundamental requirement for life as we know it - is believed to be abundant.
最终，我们要将人类送到这些行星的卫星上去，至少送到据信有大量液态水存在的几个卫星上去——我们知道，液态水是生命的基本需要。
Alternatively, a craft could fly to one of their moons to collect rock samples and also accumulate hydrogen, by separating water obtained from melted ice, for the trip back to Earth.
另外，飞船可以飞到行星的一个卫星上采集岩石标本，还可以通过离析冰水以积累氢，为返回地球补充燃料。
Alternatively, a craft could fly to one of their moons to collect rock samples and also accumulate hydrogen, by separating water obtained from melted ice, for the trip back to Earth.
另外，飞船可以飞到行星的一个卫星上采集岩石标本，还可以通过离析冰水以积累氢，为返回地球补充燃料。
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