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The hard thing here is that you're fighting fundamental physics. The cheapest orbit available is LEO. People often think that "orbit" means there's no gravity. This is incorrect. In fact, the International Space Station (which is in LEO right now) is usually around 250 miles high and experiences about 90% of the gravity you experience on Earth. So why do the astronauts float around like there's no gravity? Because they are going really, really, really fast. About 5 miles per second. Although they are pulled toward the Earth all the time, they always "miss" it.

Think of it like this: Imagine you fire a cannonball from the top of a tower. If you fire it softly, the ball will go a little ways then fall to the ground. If you fire it incredibly fast, it will just fly off into space. But between falling right down and going off into space, there are a lot of intermediate regimes. For a given height, there is some speed that is slow enough that it can't leave Earth, but fast enough that you'll never plop to the ground. If you were riding that cannonball, you'd be falling, because gravity is tugging you down. At the same time, because you're going so fast, you'd be able to see Earth's curve. As you move from a point on the globe in a straight line, Earth curves down and away from you, increasing your distance from the surface. At this particular speed, you have two balanced effects: Gravity wants you down low, but your speed keeps you up high. So you just keep going around and around and around.

You "orbit."

Even though LEO is the cheapest orbit to achieve, it's still pretty expensive to get there. Getting a big hunk of metal to 5 miles per second is not an easy task. If we ever want spaceships that look like the ones in movies instead of giant tin cans wrapped in foil, we're going to need a cheaper way.

Method 1: Reusable Rockets

Reusable rockets are the best bet for cheaper spaceflight in the short term. They are traditional rockets, but rather than falling into the ocean as they do now, they fall to Earth and land after they finish the mission. This doesn't fix the problem that the rocket only holds 4% cargo, but it potentially drives the cost way down.

There are a few difficulties with this approach, though. You have to keep extra propellant onboard for the landing phase, which lowers efficiency. You want to carry the smallest amount of extra propellant possible, but this makes the landing phase very hard.

A very serious issue is that nobody yet knows what it'll cost to refurbish a used rocket. This thing has gone to space, man. You can't just put a spit shine on it and put it back on the launchpad.

The U.S. Space Shuttle, which was designed to be a reusable launch vehicle, ended up being more costly than a regular rocket precisely because refurbishing was so expensive. There's an ongoing argument over whose fault this was—the engineers, Congress, the Air Force, a risk-averse public, and more—but the bottom line is that the program was largely done in by the cost of getting the Shuttle launch ready again after a flight. This is why, when lots of people were sad about the Shuttle retiring, a lot of space nerds were glad to see it go.

But there is reason to hope that a better reusable launch vehicle can be created. As we were writing this chapter, SpaceX became the first company to successfully put cargo into space, then land part of its rocket.

If it really can bring the price down, this may prove to be the biggest development in space travel in a generation. As we were watching a launch, a reader of ours tweeted that although he had witnessed the moon landing as a young boy, he found the reusable rocket even more exciting. It sounds crazy, but he's got a point—the moon landing was certainly the greater technical feat, but it was done at a cost that more or less guaranteed it couldn't become commonplace. Exactly how much the cost can be dropped is a matter of debate. Elon Musk apparently claimed he could eventually get the cost down by a factor of 100. In the more near term, SpaceX's president Gwynne Shotwell said their current Falcon 9 should be able to offer a 30% discount.

But even if reusable rockets only mean a small price drop now, they may yet represent a path to greater future savings. The road to Mars may be paved with small discounts.


Method 2: Air-Breathing Rockets and Spaceplanes

Airplanes already go really high. Can't we just have them go a bit higher so they get to space?

No. Why would you even ask that? Jesus.

If you want to put a satellite in orbit, the hard part is not going really high.

The hard part is going really fast. That takes a lot of propellant. But using a spaceplane might allow a serious reduction. To understand why, you have to understand what propellant is.

If you refer to propellant as "fuel," a NASA engineer will beat you with a TI-83. Propellant is actually a combination of two things: fuel and oxidizer. When you want a combustion reaction, you need three things: fuel, oxidizer, and energy. For example, when you light a campfire, the fuel is wood, the oxidizer is (you guessed it) oxygen, and the energy is a lit match.

In a rocket, you carry both fuel and oxidizer inside the ship. The actual ratio of oxidizer to fuel varies by rocket and mission, but generally speaking the majority of the propellant's mass is oxidizer. The oxidizer is often just liquid oxygen.

This excerpt ends on page 20 of the hardcover edition.

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