You can see the planet Jupiter on a clear night, but Jupiter is humongous. Mars and Mercury are smaller than that, and you can see them, too. Wait a minute, our moon is even smaller, and you can see that. Could anything smaller than the moon be visible without a telescope?

 

Actually, yes. Though it takes a lot of patience and excellent sky conditions, it is possible to see a tiny object called Vesta. Although it had a molten interior and volcanoes, Vesta isn’t a planet or a moon: it’s an asteroid. Asteroids are straychunks of rocky or metallic materials that orbit the sun but were never drawn into any of the planets.

 

Vesta is quite far away. Like many asteroids, it has an orbit out between the orbits of Mars and Jupiter. Astronomers first spotted it when looking for a new planet they thought should exist between these two.

 

As I said, Vesta is quite small–about three hundred and twenty-five miles across. So if it’s small and far away, how come we can see it from here? Vesta’s surface has a coating of pyroxine and olivine, two minerals that are actually also spewed up by volcanoes here on earth. They give it a whitish exterior that reflects well, making it just possible to see Vesta with your eyes, even when the only two larger asteroids, Ceres and Pallas, are completely invisible.

 

 

在晴朗的夜空下我们能够看到木星，那是因为木星够大；尽管火星和水星比木星要小，但我们依然能看到。等等，月球更要小一些，我们还是能看到。那么，在不使用望远镜的情况下，我们还能看到比月亮更小的天体吗？

 

事实上，答案是肯定的。只要我们足够有耐心，天公也作美，我们还有机会看到一个小天体——它就是被命名为灶神星的小天体。尽管它以溶化的金属为核心且地表遍布火山，灶神星却既不是行星也不是月球：它仅仅是一颗小行星。小行星是由岩石或是金属块构成的小天体，他们绕太阳运动但绝不会卷入其他行星的运行轨道中。

 

灶神星距地球的距离很远。像其它许多小行星一样，它在火星与木星之间有自己的运行轨道。天文学家们首次发现它时是在寻找介于这两大行星间的另一个行星。

 

正如我所说，灶神星是很小的——它的直径也就只有325英里而已。那问题就是，它又小，距离我们又远，我们怎么能从地球上靠肉眼就看到它呢？在灶神星的表面覆盖了厚厚的一层玄武岩和橄榄石，这两种矿物质都是由此处的火上喷发而出的。这两种矿物质使得灶神星的表面呈白色，且发光的效果相当好。如此一来，我们便仅凭肉眼就可以看到灶神星了。甚至在比它还大的另外两个小行星——谷神星和智神星都无法看到的情况下，我们还是能看到它。

 

 

Note: spew up  喷出