Space is closer than you might think -- about 62 miles up, only a little farther away from you than San Jose is from San Francisco. Heck, you can get halfway to space in a balloon.
The hardest part about space, it turns out, isn't so much getting there as staying there. That's where the idea of orbiting comes into play. Once you accomplish the hard work of getting a spacecraft into orbit, you can get years of use out of it as it loops more or less effortlessly around the planet on its own invisible track.
Orbits are "roadways in space," said Ajmal Yousuff, a Drexel University professor who studies aerospace vehicles. "You place a vehicle in space, and it stays there."
Scientists figured out how orbits work centuries before humans could launch spacecraft, but there's lots for the rest of us to learn about these looping tracks above the Earth -- and good reason to learn it. With new government and private sector projects, space stands to become even more important than it was during the 1960s at the start of the Space Age.
Among other efforts, several companies are filling the heavens with internet-beaming satellites, new SpaceX rockets have begun sending astronauts to the International Space Station, the US military has founded its new Space Force, and NASA is planning missions to the moon and Mars.
"It's the new Space Age -- and the new space race," said Ben Lamm, chief executive of software company Hypergiant. His company is working with the US Air Force on its Chameleon spacecraft, designed to be more adaptable, more independent and smarter than traditional spacecraft.
Let's start with Isaac Newton
If you want to understand orbits, a great place to start is Isaac Newton, whose research paved the way to modern science with explanations of motion, light and gravity. Newton's Treatise of the System of the World from 1685 elegantly encapsulates how orbits work with a thought experiment that requires no calculus whatsoever.
The idea, sometimes called Newton's cannonball, goes like this. Imagine shooting a stone horizontally from a tall mountain, gradually increasing the speed at which it's shot.
"The greater the velocity is with which it is projected, the farther it goes before it falls to Earth," Newton said. With increasing horizontal velocity, "it would describe an arc of 1, 2, 5, 10, 100, 1,000 miles before it arrived at the Earth, till at last exceeding the limits of the Earth, it should pass quite by without touching it."
In other words, the stone would fall at exactly the same rate that the Earth's surface receded because of the Earth's curvature. In Newton's experiment, a stone shot with the right speed would circle the Earth and smack right back into the mountain.
In the real world, friction with the Earth's atmosphere would slow the projectile long before it could circle the Earth and return to the mountain. But a few miles up into space, where air is scarce, that projectile would keep on orbiting with almost nothing to stop it.
Traveling fast sideways, not up
That brings us to the main difficulty of putting a satellite into orbit: getting enough horizontal velocity.
Whether you're watching enormous Saturn V rockets carrying humans to the moon or slender candlesticks launching smaller spacecraft, the rockets you see produce immense amounts of thrust. The vast majority of rocket fuel, though, propels the spacecraft laterally, not up. When you watch a rocket launch, the tilt toward the horizontal begins almost immediately after the craft leaves the launchpad.
How fast are those spacecraft going? The first artificial satellite, the Sputnik-1 that Russia launched in 1957, orbited at about 18,000 miles per hour over the surface of the Earth, or about 8 kilometers per second. The International Space Station whizzes by at a speed of 7.7 kmps, or about 17,000 mph.
In comparison, the supersonic Concorde passenger jet dawdled along only at about 1,500 mph.
It takes a lot more power for SpaceX to carry NASA astronauts to the ISS than it does for Blue Origin, the rocketry startup funded by Amazon Chief Executive Jeff Bezos, to pop its New Shepard rockets up and down without entering orbit.
The lower a spacecraft orbits, the faster it goes. That's why the Hubble Space Telescope, about 340 miles up (547km), circles the Earth every 95 minutes, but Global Positioning System satellites for navigation services, at 12,550 miles (20,200 km) up, take 12 hours for each orbit.
Getting a launch boost from Earth
The Earth's rotation gives rockets a healthy eastward fling, and the closer to the equator a launch is, the bigger the fling.
That's in part why US launch sites are located toward the southern parts of the country and why European spacecraft sometimes are launched from the Guiana Space Center in South America, just 5 degrees of latitude away from the equator. NASA considered launching moon missions from an equatorial site -- though the fling factor was secondary to fuel considerations matching the moon's orbit.
When SpaceX launches a rocket, it reserves some fuel to return the first stage of the rocket to Earth after its job getting a spacecraft into orbit is done. For launches from Cape Canaveral in Florida, the rocket stage lands on a drone ship floating on the Atlantic hundreds of miles to the east.
Low Earth orbit: Join the party
Space starts about 62 miles (100km) above us, though the boundary is somewhat arbitrary. A bit higher than that, reaching up to about 1,243 miles (2,000 km) above the Earth's surface, is the most popular part of space, called low Earth orbit, or LEO.
This is where you'll find the International Space Station along with satellites for weather forecasting, spying, television, imaging and, increasingly, satellite-based broadband. Every human who's been in space, aside from a few who made it to the moon's vicinity during NASA's Apollo missions, have hugged the earth in LEO.
The SpaceX Starlink service, now in beta testing, is nearing 1,000 satellites in its constellation, on its way to more than 2,200. Amazon's Project Kuiper plans 3,200 satellites. OneWeb envisions a whopping 48,000 satellites, though its near-term plans ran into a bankruptcy problem this year. Companies based in Canada, Russia and China plan more.
It's easier than ever to get to LEO, and that's triggered "a golden age of LEO innovation," said HawkEye 360 Chief Executive John Serafini, whose company helps government and military customers track radio signals to spot subjects like smugglers or lost boats.
"It would have been almost impossible for HawkEye 360 to build out a constellation of satellites 10 years ago," but SpaceX's reusable rockets and other improvements have lowered launch costs. "There are more opportunities to catch rides to orbit than ever before," he said.
Because LEO is relatively accessible, though, it's also where most of the Earth's space junk orbits. Friction with the upper fringes of the atmosphere drags a fraction of the detritus out of the way. Satellites must reckon with atmospheric friction, too, often nudging themselves to maintain proper orbit with gentle but conveniently solar-powered ion thrusters.
Heading higher to geosynchronous orbit
Medium Earth orbit, which reaches up to about 22,233 miles (35,780 km) above Earth, is a desert compared with LEO. But there are some notable denizens of this zone, in particular navigation satellite constellations.
The big sat-nav constellations, each with roughly 24 satellites, are the United States' GPS, Europe's Galileo, Russia's Glonass and China's BeiDou. GPS is handy for smartphone navigation, but military use is also a top justification for the expense of launching and maintaining these satellites.
Just above the upper boundary of MEO is geosynchronous orbit, a sweet spot where the orbital period matches the Earth's rotation. A satellite in geosynchronous orbit above the equator, called geostationary orbit, appears in the exact same spot in the sky as viewed from Earth.
That's particularly useful for communications because you can point a fixed ground station antenna directly at the satellite. However, radio transmission delays and signal strength are worse than with spacecraft in lower orbits.
Not all parking places in geosynchronous are created equal. Variations in the Earth's density nudge some satellites out of their spot, requiring occasional propulsion to keep them in line, Drexel's Yousuff said.
Circles and ellipses
Although many orbits are circular, some are elongated into more elliptical shapes that can slow a satellite's speed when it's farther away from the Earth.
Ellipses also are handy for changing orbits. NASA's Apollo missions began by launching the spacecraft into Earth's orbit, then a new rocket burn launched them into an elliptical orbit that stretched toward the moon, letting the astronauts coast most of the way. Another rocket burn inserted the spacecraft into lunar orbit.
One of Yousuff's favorite orbit types is elliptical. Most of Russia is well north of the equator, which limits geostationary satellites' usefulness. So the Russians came up with an alternative called the Molniya orbit.
With the Molniya orbit, a satellite whips over Australia at its closest point in orbit, called perigee, then naturally slows as it reaches its highest point above Moscow, called apogee. That way it spends much of its orbiting time usefully accessible.
There are plenty of other orbit types, too, like polar orbits that cross over both of the Earth's poles. And spacecraft that reach Earth's escape velocity can orbit the sun instead. The orbit of SpaceX's Starman just carried Elon Musk's publicity stunt close to Mars, for example. If today's commercial activity in low Earth orbit keeps lowering rocket launch costs, perhaps actual humans will follow him.