Skip to main content
No. 1831:
Many Moons

Moon fragmentToday, we find many moons. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

This August of 2003, Mars looms over our house like a lantern — the closest it's been to Earth in sixty thousand years. Some people look at it like some astral omen; and even I feel a vestigial crease of worry that it might fall through my roof.

But, near as it is, your telescope won't pick up its moons, for they are very small. Yet there are two of them. As we move outward from the Sun, we begin with moon-poor planets. Mercury and Venus have none. We have only one (but, I must say, ours is large, lush, and glorious to see.)

Mars' moon Phobos is probably a captured asteroid. It's oblong, and about six by nine miles in size. Deimos is even smaller, and about four miles in diameter. Both are so small that they weren't discovered until 1877.

The last planet, tiny Pluto, is three billion miles out from the Sun, and it has only one moon — Charon, named after the boatman on the River Styx. Charon is half as large as Pluto itself. Indeed, Pluto might be thought of as a double planet orbiting itself.

However, we know of 124 moons that orbit Jupiter, Saturn, Uranus, and Neptune. We've found most of those only since 1997, using new digital cameras that can track very faint objects.

Galileo discovered the four largest of Jupiter's 61 moons in 1610. They're all comparable to our moon in size. It took three centuries to find the next largest one, because the rest of them range from a hundred miles in diameter down to less than one mile. Now the whole matter of what a moon is has shifted under our feet.

Our solar system is littered with small objects. Some orbit planets, but most orbit the sun. And here we need the concept of a Hill Sphere. That's an imaginary sphere surrounding a planet. Within it, a moon will orbit stably. Outside it, the gravitational pull of the planet is too small, and that of the Sun is too large. Then the Sun will destabilize the moon's motion and take it away from the planet. Jupiter is so immense, and so far from the Sun, that it has a huge Hill Sphere.

While that large sphere is the reason that Jupiter and Saturn hold on to so many moons, it doesn't tell us how those moons got there in the first place. If a fast-moving satellite or comet passes a planet, the planet needs some means for grasping it, and slowing it down to just the right orbital speed.

The New York Times now reports an idea that's just taking shape. It is that those big planets must've once had far larger atmospheres — enough to exert drag, and haul passing bodies into orbit. But how they first created, and then lost, those atmospheres, remains under debate.

And so I look over my house at red Mars with its tiny unseen moons, and I realize how much mystery still shrouds space — even so close as our own solar system, even right here in my own back yard.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

H. Fountain, Newfound Moons Tell Secrets of Solar System. New York Times, Science Times, Tuesday, August 12, 2003, pp. D1-D4

More on moons see:

This page shows a large presence of objects orbiting orbit the Sun along with Jupiter:

Luna Regia, Roman goddess of the moon

Luna Regia, Roman goddess of the moon