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contemporaries and most of their followers up to the mid-1960s believed that Mercury maintained eternal “day” on one side and “night” on the other. But the Sun constrains the rotation and revolution of Mercury according to a different formula: The planet spins around its axis once every 58.6 days—a rate rhythmically related to its orbital period, so that Mercury completes three turns on its axis for every two journeys around the Sun.
    The 3:2 pattern affects observers on Earth by repeatedly offering them the same side of Mercury six or seven apparitions in a row. Schiaparelli and Antoniadi indeed beheld an unchanging face of Mercury throughout their studies, and must beforgiven for reaching the wrong conclusion about its rotation, since the planet’s behavior indulged them in their error.
    Throughout the twentieth and into the twenty-first century, Mercury has continued to be a difficult target. Even the Hubble Space Telescope, orbiting above the Earth’s atmosphere, avoided looking at Mercury, for fear of pointing its delicate optics so dangerously close to the Sun, and only one spacecraft has so far braved the hostile heat and radiation of the near-Mercury environment.
    Mariner 10,
Earth’s emissary to Mercury, flew by the planet twice in 1974 and once more in 1975. It relayed thousands of pictures and measurements of a landscape riddled with crater holes, from small bowls to giant basins. Light or dark trails of debris marked the places where newer assaults had overturned the rubble of the old. Lava that flowed among the impact scars had smoothed over some of the depressions, but overall poor battered Mercury preserved a clear record of the era, ended nearly four billion years ago, when leftover fragments of the Solar System’s creation menaced the fledgling planets.
    The most violent attack on Mercury inflicted a wound eight hundred miles wide, which has beennamed Caloris Basin (“the Basin of Heat”). The mile-high mountains on Caloris’s rim must have sprung up in response to the massive impact explosion that excavated the Basin, and all around the mountains, further signs of disturbance lay in ridges and rough ground rippling out for hundreds of miles. The collision at Caloris also sent shock waves clear through Mercury’s dense, metallic interior, to set off earthquakes that lifted the crust on the far side of this world and cut it to pieces.
    Mariner 10
photo montages, which captured less than half of Mercury’s surface, revealed a network of scarps and fault lines that indicate the whole planet must have shrunk to its present dimensions from some larger beginning. When Mercury’s interior contracted, the global crust readjusted itself to fit the suddenly smaller world—like some furtive trick of the god Mercury, disguising himself.
    After a thirty-year hiatus in exploration, a new mission called
MESSENGER
(an acronym for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is now en route to Mercury. Launched in August 2004, but unable to fly as quickly or directly as its namesake, the craft will not reach Mercury’s vicinity until January 2008. Atfirst sight of the planet,
MESSENGER
will start a detailed mapping effort requiring three flybys of Mercury over the following three years, while the spacecraft orbits the Sun, protected under a sunshade made of ceramic cloth. Then, in March 2011,
MESSENGER
will maneuver into orbit around Mercury itself, for a year-long odyssey (as measured in Earth time) to monitor the planet through two of its long days. Circling Mercury rapidly and repeatedly every twelve hours,
MESSENGER
will function as a new oracle, streaming answers to the questions posed by anxious truth-seekers on Earth.
     
    * The ancients recognized seven planets: Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn.
    * Gassendi quotes here from Ovid, referring to the Sun god Apollo by his other name, Phoebus.

BEAUTY
    For a breeze of morning moves,
    And the planet of Love is on