So to talk about where meteorites come from, we need to talk about comets and asteroids.
那么讲到流星体从何而来，我们就需要讨论下彗星和小行星。
Rubin suggests that asteroids' heat could have derived from collisions between asteroids.
鲁宾认为，小行星的热量可能来自小行星之间的碰撞。
Earth's crust contains very little of this element, but most asteroids contain a lot more.
地壳中含有的这种元素很少，但大多数小行星上的要多得多。
A recent discovery that asteroids are highly porous makes Rubin's hypothesis more plausible.
最近的一项发现表明小行星非常多孔，这使得鲁宾的假设更加可信。
I'm not going to go into any depth on comets and asteroids now, but we'll come back later and do that.
我现在不打算深入讨论彗星和小行星了，但是我们稍后会再回到这个点上，讨论这方面的内容。
The solar system also has two analogous classes of objects, smaller than planets—namely, asteroids and comets.
太阳系也有两类类似的物体，比行星还小——即小行星和彗星。
These asteroids are like the terrestrial planets, in that they're composed mostly of rocky material and metals.
这些小行星像类地行星一样，主要由岩石物质和金属构成。
The Sun is the hub of a huge rotating system consisting of nine planets, their satellites, and numerous small bodies, including asteroids, comets, and meteoroids.
太阳是一个巨大的旋转系统的中心，这个旋转系统由九大行星、它们的卫星和无数的小天体组成，包括小行星、彗星和流星体。
Skeptics have argued that a single impact would raise an asteroid's overall temperature very little and that asteroids would cool too quickly between impacts to accumulate much heat.
怀疑派认为，一次撞击只会使小行星的整体温度上升很小，而且小行星在两次撞击之间会迅速冷却，从而无法积聚大量热量。
Dr Bottke has been tracking the orbits of asteroids.
伯克(Bottke)博士一直在追踪小行星的运行轨迹。
If there were no asteroids would there be no extinctions?
如果没有小行星，就不会有灭绝了吗？
Astronomers know these condense into planets and asteroids.
而天文学家知道这些云状物最终将凝结成大小行星。
It's most likely the result of a recent collision between two asteroids.
这很可能是两个小行星之间相撞产生的结果。
Just as most orbiting planets spin on their axes, so asteroids also rotate.
正如其它沿轨道运行的行星自转一样，这些小行星也有自转。
Stars would stay fixed, but objects like comets, asteroids, and planets moved.
通过这种方法可以观察到，恒星是固定不动的，而像彗星、小行星和行星这样的物体都在移动。
Scale models of asteroids fill the windowsill of his office at JPL in Pasadena.
在帕萨迪纳喷气推进实验室，他办公室的窗台上摆满了小行星按比例制作的模型。
This goes on until the particles get to the size of boulders or small asteroids.
这现象一直持续，直到微粒达到巨石或者是微型小行星的大小。
Apollo asteroids pose a threat to our planet as they routinely cross Earth's orbit.
阿波罗型小行星因定期地穿越地球轨道而构对地球的威胁。
They have also looked for interesting asteroids, and kept an eye out for solar storms.
业余天文学家们也已参与寻找有趣的小行星和观测太阳风暴。
And a new study shows that asteroids weren't just passive couriers of organic material.
并且新的研究显示，小行星并不只是被动地携带着有机物。
It turns out that NASA is taking on that challenge, but not necessarily to chase asteroids.
结果是美国国家航空航天管理局接受了那项挑战，但目的并不是一定要去追赶什么小行星。
Astronomers group asteroids into different classes based on how the rocks reflect light in space.
天文学家通过在太空中反射光的方式将小行星分门别类。
Rock Farm: Most asteroids lie in the main belt [white] between Mars and Jupiter and pose no threat.
太空岩石场：大部分小行星位于火星和木星之间的小行星带（图中白色部分），对地球没有威胁。
The researchers observed 95 asteroids in the vicinity of our planet, called near-Earth asteroids (NEAs).
研究人员们观测了95颗地球附近的小行星，它们被称为近地小行星(NEAs)。
Many asteroids are weakly bound piles of rubble, held together by their own minute gravitational attraction.
大多数的小行星都是由碎石组成，而这些碎石靠着自身之间微弱的万有引力结合在一起。
A fragment from the same family of asteroids is thought to have wiped out the dinosaurs 65 million years ago.
据认为，同一小行星家族的一个碎片曾经在六千五百万年前灭绝了恐龙。
Hill and his fellow NEO hunters hope to detect large asteroids sooner, preferably years or decades in advance.
希尔和他的同事希望能够更早地发现较大的小行星，最好提前几年或几十年。
That's according to a survey of the sky that NASA is calling the most accurate census yet of near-Earth asteroids.
这是根据美国宇航局(NASA)称之为“最准确的”一次近地小行星普查的结论。
That's according to a survey of the sky that NASA is calling the most accurate census yet of near-Earth asteroids.
这是根据美国宇航局(NASA)称之为“最准确的”一次近地小行星普查的结论。
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