Free Packing For Mars by Mary Roach Page A

grains coalesced. These grains would have spent eternity moving through space, ignoring each other, had gravitational attraction not brought them together. Gravitation is the lust of the cosmos. As more particles joined the orgy, these celestial blobs grew in size. The bigger they became, the bigger the pull they exerted. Soon (in a thousands-of-centuries sort of way) they could lure larger and more distant particles into the tar pit of their gravitational influence. Eventually stars were born, objects big enough to pull passing planets and asteroids into orbit. Hello, solar system.
    Gravity is the prime reason there’s life on Earth. Yes, you need water for life, but without gravity, water wouldn’t hang around. Nor would air. It is Earth’s gravity that holds the gas molecules of our atmosphere—which we need not only to breathe but to be protected from solar radiation—in place around the planet. Without gravity, the molecules would fly off into space along with the water in the oceans and the cars on the roads and you and me and Larry King and the dumpster in the In-N-Out Burger parking lot.
    The term “zero gravity” is misleading when applied to most rocket flights. Astronauts orbiting Earth remain well within the pull of the planet’s gravitational field. Spacecraft like the International Space Station orbit at an altitude of around 250 miles, where the Earth’s gravitational pull is only 10 percent weaker than it is on the planet’s surface. Here’s why they’re floating: When you launch something into orbit, whether it’s a spacecraft or a communications satellite or Timothy Leary’s remains, you have launched it, via rocket thrust, so powerfully fast and high and far that when gravity’s pull finally slows the object’s forward progress enough that it starts to fall back down, it misses the Earth. It keeps on falling around the Earth rather than to it. As it falls, the Earth’s gravity keeps up its tug, so it’s both constantly falling and constantly being pulled earthward. The resulting path is a repeating loop around the planet. (It is not endlessly repeating, though. In low Earth orbit, where spacecraft roam, there’s still a trace of atmosphere, enough air molecules to create a teeny amount of drag and—after a couple years—slow a spacecraft* down enough that without a rocket engine blast it falls out of orbit.) In order to escape the Earth’s gravitational pull completely, an object must be hurtling at Earth’s escape velocity: 25,000 miles per hour. The more massive a celestial entity, the harder it is to break its hold. To escape the monstrous gravity of a black hole (a huge collapsed star), you’d need to travel faster than the speed of light (about 670 million miles per hour). In other words, even light can’t escape a black hole. That’s why it’s black.
    Getting back to weightlessness. Weight is a bit of a mind-bender. I had always thought of my weight, on any given day, as a constant, a physical trait like my height or my eye color. It’s not. I weigh 127 pounds on Earth, but on the much smaller moon, whose gravitational pull is one sixth of Earth’s, I weigh about as much as a beagle. Neither weight is my real weight. There is no such thing as a real weight, only real mass. Weight is determined by gravity. It’s a measure of how fast you’ll accelerate if you happen to be dropping through the air like Newton’s apple. (Here on Earth, were there no atmospheric drag to slow you down, gravity would accelerate you at the rate of 22 miles per hour faster for each second that you fall.) If you’re standing on the ground, you obviously don’t speed up, but the pull is still there. You’re not falling, just pressing. The acceleration reads as weight on a bathroom scale. When there’s nothing to press against, as in the free fall of orbit, then you are weightless. The “zero gravity” that astronauts experience aboard an orbiting spacecraft is simply a continuous state of