The Planets (20 page)

I wish I could tell you what happens next, how the interpretation of the
Huygens
data will play out, what
Cassini
will encounter as it sweeps by this or that Saturnian satellite—Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus—on the busy itinerary of its ongoing exploration. But what book can keep abreast of current events in an active field of study? If reading these pages has helped someone befriend the planets, recognizing in them the stalwarts of centuries of popular culture and the inspiration for much high-minded human endeavor, then I have accomplished what I set out to do.

For myself, I confess that none of the truly staggering facts I have been privileged to share here has altered the planets’ fundamental appeal to me as an assortment of magic beans or precious gems in a little private cabinet of wonder—portable, evocative, and swirled in beauty.

*
Later astronomers followed suit up through Pan, the eighteenth satellite of Saturn, discovered in 1990. The next twelve moons, including Mundilfari and Ymir, received names from broader cultural contexts. Since
Cassini
arrived at Saturn, at least sixteen additional satellites have been found.

Apogee
—the greatest distance from Earth reached by the Moon in its monthly orbit, or by an artificial satellite circling our planet.

Apparent Magnitude
—the brightness of a heavenly body as viewed from the vantage point of Earth, expressed as a number; the lower the number, the brighter the object appears. (The Sun, with an apparent magnitude of –27, is the brightest object in Earth’s sky, though if it were judged according to its
intrinsic
brightness, or absolute magnitude, it would pale in comparison to larger stars.)

Areographer
—one who makes maps of Mars (Ares).

Asteroid
—a minor planet, generally small and rocky, some one hundred thousand of which orbit the Sun in the wide gap between Mars and Jupiter.

Cartouche
—in cartography, a decorative emblem bearing text such as the title of the map, or the scale, and often including symbols of the regions represented.

Coma
—the fuzzy envelope surrounding the nucleus of a comet.

Comet
—a small icy body orbiting the Sun in a highly elliptical orbit, changing its appearance on close solar approach by emitting jets of gas and dust.

Coronae
—(singular: corona) sets of concentric rings surrounding features such as domes and depressions, unique to Venus, found where her surface crust is thinnest.

Duricrust
—loosely cemented dust seen on the surface of Mars, thought to be formed by the deposition and evaporation of water and carbon dioxide.

Eccentricity
—the degree to which a body’s orbit deviates from a circle. (The orbit of Pluto is highly eccentric—an exaggerated ellipse, while the orbits of Venus and Neptune appear virtually circular.)

Eclipse
—the disappearance of a part or all of one heavenly body behind another, or in the other’s shadow. (In a solar eclipse, the Moon blocks the Sun from view; in a lunar eclipse, Earth’s shadow falls on the Moon.)

Ecliptic
—the apparent path of the Sun, Moon, and planets as seen from Earth, so named for the eclipses that occur here; the plane of the Zodiac and of Earth’s orbit.

Electromagnetic Radiation
—light, in all its guises, ranging from high-energy gamma rays and X-rays through ultraviolet radiation, visible light, and infrared, to microwaves and radio waves.

Elongation
—the most favorable time to view Mercury or Venus, the planets interior to Earth, when they achieve their greatest apparent distance east or west of the Sun. The greatest possible elongation for Mercury is 28 degrees, and for Venus, 47 degrees.

Ephemeris
—a published table of predictions of heavenly bodies’ positions, especially those of planets and comets.

Escape velocity
—the speed a rocket (or any object) must attain to break free of the pull of gravity at a planet’s surface, and rise into space.

Extremophile
—any inhabitant of an extreme environment that would be toxic or otherwise unfit for all but properly adapted life forms.

Galaxy
—a collection of billions of stars, all gravitationally bound, as in the Solar System’s home galaxy, the Milky Way.

Igneous
—a term used to describe rocks formed from once-molten magma or lava.

Kuiper Belt
—a donut-shaped region beyond the orbit of Neptune, named for Gerard Kuiper, containing hundreds of thousands of icy planetoids. Some of these objects, when deflected by gravity or collisions into orbits that carry them close to the Sun, become the comets that return on regularly repeating schedules.

Magnetic Field
—the region around a magnet, throughout which the magnet affects charged particles or other magnets. Many planets, such as Jupiter and Earth, behave as giant magnets and generate their own magnetic fields.

Magnetosphere
—the invisible bubble of a planet’s magnetic field, defining the limits of the field’s sphere of influence.

Magnitude
—the brightness of a heavenly body, expressed as a number;
apparent
magnitude (the body’s relative brightness as seen from Earth) may differ significantly from its
absolute
magnitude, or intrinsic brightness.

Mantle
—the middle substance of a planet, filling the space between the surface crust and the core of a terrestrial world, or the upper atmosphere and solid center of a gaseous one.

Meteor
—a “falling” or “shooting” star, i.e., the light from a
space rock or bit of comet dust descending through Earth’s atmosphere and becoming incandescent from the heat of friction.

Meteorite
—a landed piece of a meteoroid.

Meteoroid
—a space rock or chunk of a planet adrift in space.

Methane
—also known as marsh gas, the simplest compound of hydrogen and carbon.

Moon
—Earth’s natural satellite, and, by extension, a body in orbit around any planet or asteroid.

Nebula
—a blurry-looking heavenly object, such as the disk in which a star is born.

Oort Cloud
—a spherical region of the outer Solar System, beyond the Kuiper Belt, named for Dutch astronomer Jan Oort (1900–1992). Comets from the Oort Cloud follow extremely long-period orbits, and may leave the Solar System after one swing around the Sun.

Perigee
—that part of the Moon’s (or an artificial satellite’s) orbit that brings it closest to Earth, at which point it travels fastest.

Perihelion
—a planet’s or comet’s (or Sun-orbiting spacecraft’s) closest approach to the Sun, and therefore the time of its greatest orbital velocity.

Planet
—a heavenly body, generally but not necessarily expected to be larger than a thousand miles in diameter, and orbiting a star.

Planetesimal
—a chunk of material smaller than a planet, and which may join with other like pieces to become a planet or moon.

Regolith
—dusty and rocky debris coating the surface of a terrestrial planet or satellite, similar to soil but lacking any live components.

Roche zone
—the region close to a planet where tidal forces prohibit the build-up of planetesimals into satellites, named for French mathematician Edouard Roche (1820–1883), who first described it.

Satellite
—a natural satellite is a moon; an artificial satellite is a spacecraft in orbit around a planet.

Solstice
—either of the two days each year (in June and December) when the Sun reaches its farthest distance above or below the equator, resulting in the shortest or the longest day.

Star
—a ball of gas, mostly hydrogen and helium, massive enough to ignite thermonuclear fusion at its core, and shine by its own emitted light.

Syzygy
—the all-in-a-line arrangement of heavenly bodies, such as the Sun, Moon, and Earth during an eclipse, or the Sun, Venus, and Earth during a Transit of Venus.

Tessera (plural tesserae)
—extremely deformed and fault-scarred areas that constitute the second most common land form on Venus (after volcanic plains), from the Russian word for “tiled.”

Transit
—the passage of one heavenly body in front of another, as when Mercury or Venus is seen passing across the disk of the Sun. The satellites of Jupiter and Saturn can also be observed in transit across their parent planets.

Zodiac
—the circle of twelve constellations through which the Sun seems to pass as the Earth makes its annual journey. These constellations correspond to the astrological signs of the zodiac: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces.

4
. My own shoebox diorama of the Solar System fell far short of Lynette Cook’s rendering here, though I love the way she recaptured the experience. Even better is the view out the window, showing that the child at work on this project lives on some other planet—an exoplanet of another star.

21
. In this astronomical “Garden of Eden,” planets are taking shape in the topiary bushes, and the surface of the sundial is the face of our own Sun, sunspots and all. In the sky, the stages of partial and total solar eclipse parade behind the branches of a tree of life.

32
. Mercury, the god mentioned most frequently in the Greco-Roman myths, is captured on a Grecian urn that depicts some of the key events of his personal mythology. The urn divides the twilight landscape into the realms of life and death, while the planet Mercury hovers overhead.

67
. Traditional symbols of feminine beauty perch on the parched landscape of Venus, shown here in the fiery colors astronomers chose for rendering their radar images. The
Magellan
spacecraft buzzes by the cloud-swathed planet, depicted as the pull on the window shade. On the shade itself, Venus shows her phases above a temple dedicated to her, and also poses with the Moon.

83
. Many intellectual and actual voyages of discovery revealed the Earth to be a planet in orbit about the Sun and not, as had been thought for ages, the hub of the universe. Today the exploration of Earth continues out through the atmosphere to the place where space begins, and into the unseen core that drives the restless surface.

109
. Moon dust and the Moon illusion illuminate this earthly Moon garden, lighting a glimmer path on its water. On the dome of the temple honoring the
Apollo
astronauts is decorative ironwork in the shape of the lunar landing module, and the plaque at its base repeats their hopeful message, “We came in peace for all mankind.”