How many solar system members can you see with the naked eye? With a good basic education, you’ll rattle off seven--the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn. Throw in some meteors, the odd passing comet, and you’ve successfully matched the knowledge of the average caveman.
Those were all the ones known until the era of the telescope. But two additional bodies do show themselves to our unaided eyes. You’d think in centuries of sky gazing somebody would have spotted them, but astoundingly, no one did. By chance, both these baffling bodies are visible this month, lying near each other in the midnight sky. The first is Uranus; the other is the brightest asteroid--Vesta.
Asteroids have a curious history that began on the first night of the nineteenth century. That’s when the Italian astronomer Giuseppe Piazzi found a new planet, which came to be called Ceres. Its status was soon clouded by the bewildering discovery of a second object, Pallas, in nearly the same orbit. Then came Juno in 1804, and, in 1807, Vesta. Pallas and Vesta were both found by Heinrich Olbers, of Olbers’s paradox fame. (He reasoned that the night sky in a limitless universe filled with an infinite number of stars should not be dark, because every line of sight would ultimately intercept a star. The solution eluded us until recently: night is black largely because the universe is too young for distant starlight to have had time to reach us.)
No further asteroids surfaced for another 38 years, correctly implying that the 500,000 or so visible to today’s instruments are mostly small chunks of rocky rubbish, much dimmer than the Big Four. They chiefly occupy the gap between Mars and Jupiter. Some astronomers once thought the asteroids were the remains of a planet rent by Jupiter’s gravity. Most now believe this debris, ranging from Ceres, with its 600-mile diameter, down to stones the size of meatballs, never formed a planet to begin with. Nor do we need to invent such cataclysmic tales for asteroidal excitement: some nerve-racking mavericks brush past our world too closely for comfort. Toutatis (4,000 have names; does anybody know them all?) will skim just four Earth-moon distances from us in 2004, even surpassing the close call we had with it this past December 8.
Vesta, which is about the size of Georgia, orbits safely in the main asteroid belt. What makes it notable, though, is its exotic vanilla- colored surface--it reflects so much sunlight that it reaches naked-eye visibility when Earth passes by for a closer look, as we will in late August and early September.
Okay, it’s still not easy. Vesta can just be glimpsed to the lower right of the star Delta Aquarii. If you don’t have a chart, look halfway between Saturn and Fomalhaut, which are the only two bright midnight objects low in the south. Look again the next night and if the star you thought was Vesta has moved, it was.
If you’re really going whole hog with maps and field glasses, swing the glasses two arm’s lengths to the right of Vesta, to the rare grouping of Uranus and Neptune. Under moonless country skies, put down the binoculars and Uranus remains plainly visible. You’ll be the first on your block to claim six planets and an asteroid with the naked eye. In an earlier era this would have brought you fame and fortune. Today it’s more likely to brand you a fruitcake, especially if you start raving about a vanilla asteroid. Tell no one.