The little metal sphere hadn’t escaped gravity at all. In fact, it needed gravity to be there, to make sure that it stayed in orbit and didn’t just carry on and leave the Earth behind. As it zoomed along at this fantastic speed, the Earth was pulling it downward with almost as much gravity as there is on the ground.? But because Sputnik had such a huge sideways speed, by the time it had fallen a little way down toward the Earth, it had gone so far forward that the Earth had curved away beneath it. And as it kept falling, so the Earth’s surface kept curving away. This is the beautiful balance of being in orbit. You’re going sideways so quickly that you fall toward the ground and miss. And because there’s almost no air resistance, you can just keep falling and missing, as you go around and around.
To get into orbit, you have to be going sideways fast enough to make this balance work. And Kazakhstan already has a pretty significant sideways speed because it’s whizzing around the Earth’s axis once a day. The farther you are from the spin axis, the faster your sideways speed is. So by launching from somewhere close to the equator, you’re giving yourself a pretty significant head start. A sideways speed of about 5 miles per second is needed to make low-Earth orbit work. Kazakhstan is whizzing sideways at about 437 yards per second (894 mph). So if you launch eastward, with the spin of the Earth, just starting in Kazakhstan instead of at the North Pole means that 5 percent of the work is done for you.
In the spin dryer, the outside of the drum pushes the clothes inward, so they can’t escape. In the velodrome, it was the alarmingly sloped track that was pushing me inward. And for Sputnik, the tiny peeping herald of humanity’s first adventure into space, gravity was doing that job. Everything that spins needs something to be pulling or pushing it toward the center of the spin all the time. For the clothes in the spin dryer just as for Sputnik, if that force had vanished, they would have kept going in a straight line.
So gravity definitely still matters a few hundred miles above our heads. But surely the whole point of being in space is that you get to be weightless. What about all those astronauts drifting about in zero gravity, trying desperately not to spill anything because it’ll float around for days? Today, the International Space Station is orbiting above our heads. The astronauts who live on board this huge scientific facility proudly state that they are flying on particular missions, and I don’t begrudge them that. It sounds a lot less exciting to say that you’re going to spend six months falling. But they’re not flying and they are falling. Just as Sputnik was falling toward the ground and missing, so are the astronauts and the space station.
While you’re in free fall, you can’t feel gravity because there’s nothing pushing back. Since the astronauts can’t feel anything pushing back, they can’t tell that gravity is there. It’s just like the moment when an elevator starts to descend, and you briefly feel lighter—the floor isn’t pushing back on you as hard as it was. If the elevator were to fall as fast as it could, through a very deep elevator shaft, you’d feel weightless, too. In orbit, you haven’t escaped gravity. You’ve just found a way to ignore it. But while you can’t feel it, it’s still there, its inward pull keeping you spinning around our planet.
Rotation is useful in all sorts of ways, but there are times when it’s just a nuisance. For example, why is it that toast falls butter-side down? You’ve just whisked the hot toast out of the toaster and applied a layer of butter which is now starting to melt. All it takes is a moment of distraction while you reach for your tea, and you knock the toast toward the edge of the table. It teeters on the edge, and the next thing you know it’s on the ground, face down. The lovely melting butter is now decorating your floor. It’s a nuisance to clean up, made worse by the feeling that this must be the universe getting back at you for something. Why does it have to happen in the messiest possible way? Why does it turn over like that?
This is a real phenomenon. Various people have conducted experiments where they have patiently pushed toast off tables many times, and it really does fall butter-side down far more often than it falls butter-side up. It depends a bit on how the fall starts, but generally, this is the way the world works and we’re stuck with it. And it’s got nothing to do with the additional weight of the butter. Most of the butter soaks into the middle of the toast, and even if it didn’t, it’s only adding a tiny amount to the total mass of the bread.
The first question is, why does it turn over at all? It all happens so quickly that it’s hard to see (and anyway, if you’d been looking at the toast, you probably wouldn’t have knocked it off the table to start with). You can watch it happening if you’re happy to sacrifice a bit of toast# or even a placemat or book that’s about the same size. Put your sacrificial slice of toast flat on the table right next to the edge and nudge it toward the precipice. Just as the halfway point of the toast is over the edge of the table, two things happen. One is that the toast starts to pivot around the table edge like a seesaw. The other is that the slice starts to slide outward without anymore nudging. The toast will now take care of itself. Slip, spin, splat.
So the rotation starts when the toast is just over halfway off the table. The key to it all is that at this moment, for the first time, less of the toast is supported by the table than is hanging off the edge. Gravity is pulling down on all of the toast. The table pushes back up, but the air can’t. It’s all about balance, just like a seesaw. The halfway point is the time when the gravity pulling down on the overhanging side is just barely enough to lift up the toast that’s still on the table. Physicists call the position of that halfway point the “center of mass,” which just means that a seesaw pivoted at that point would be perfectly balanced.
The moment you realize the toast is falling is the moment it’s too late to do anything about it. Once the toast has slipped off the table, it’s going to take a fixed amount of time to fall. If your table is about 30 inches high, it will take the toast just less than half a second to hit the floor. But once the rotation has started there’s no reason for it to stop, and the toast keeps turning as it falls.** Since gravity is always the same, and tables are pretty much always the same height, the toast always has the same spinning speed. In 0.4 seconds, it will turn 180 degrees. Since the butter started on the top, it ends up on the bottom. The physics is pretty much the same every time, so the outcome is pretty much the same every time. Toast falls butter-side down.