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 stray chunks 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.