Space Chronicles: Facing the Ultimate Frontier (23 page)

H
ere’s a safer idea for the return trip: Why not put a filling station in Earth orbit? When it’s time for the shuttle to come home, you attach a new set of tanks and fire them at full throttle, backward. The shuttle slows to a crawl, drops into Earth’s atmosphere, and just flies home like an airplane. No friction. No shock waves. No heat shields.

But how much fuel would that take? Exactly as much fuel as it took to get the thing up there to begin with. And how might all that fuel reach the orbiting filling station that could service the shuttle’s needs? Presumably it would be launched there, atop some other skyscraper-high rocket.

Think about it. If you wanted to drive from New York to California and back again, and there were no gas stations along the way, you’d have to tug a truck-size fuel tank. But then you’d need an engine strong enough to pull a truck, so you’d need to buy a much bigger engine. Then you’d need even more fuel to drive the car. Tsiolkovsky’s rocket equation eats your lunch every time.

In any case, slowing down or landing isn’t only about returning to Earth. It’s also about exploration. Instead of just passing the far-flung planets in fleeting “flybys,” a mode that characterized an entire generation of NASA space probes, the craft ought to spend some time getting to know those distant worlds. But it takes extra fuel to slow down and pull into orbit. Voyager 2, for instance—launched in August 1977—has spent its entire life coasting. After gravity assists, first from Jupiter and then from Saturn (the gravity assist is the poor man’s propulsion mechanism), Voyager 2 flew past Uranus in January 1986 and past Neptune in August 1989. For a spacecraft to spend a dozen years reaching a planet and then spend only a few hours there collecting data is like waiting two days in line to see a rock concert that lasts six seconds. Flybys are better than nothing, but they fall far short of what a scientist really wants to do.

O
n Earth, a fill-up at the local gas station has become a pricey activity. Plenty of smart scientists have spent plenty of years inventing and developing alternative fuels that might one day see widespread use. And plenty of other smart scientists are doing the same for propulsion.

The most common forms of fuel for spacecraft are chemical substances: ethanol, hydrogen, oxygen, monomethyl hydrazine, powdered aluminum. But unlike airplanes, which burn fuel by drawing oxygen through their engines, spacecraft have no such luxury; they must bring the whole chemical equation along with them. So they carry not only the fuel but an oxidizer as well, kept separate until valves bring them together. The ignited, high-temperature mixture then creates high-pressure exhaust, all in the service of Newton’s third law of motion.

Bummer. Even ignoring the free “lift” a plane gets from air rushing over its specially shaped wings, pound for pound any craft whose agenda is to leave the atmosphere must carry a much heavier fuel load than an airplane does. The V-2’s fuel was ethanol and water; the Saturn V’s fuel was kerosene for the first stage and liquid hydrogen for the second stage. Both rockets used liquid oxygen as the oxidizer. The space shuttle’s main engine, which had to work above the atmosphere, used liquid hydrogen and liquid oxygen.

Wouldn’t it be nice if the fuel itself carried more punch than it does? If you weigh 150 pounds and you want to launch yourself into space, you’ll need 150 pounds of thrust under your feet (or spewed forth from a jet pack) just to weigh nothing. To actually launch yourself, anything more than 150 pounds of thrust will do, depending on your tolerance for acceleration. But wait. You’ll need even more thrust than that to account for the weight of the unburned fuel you’re carrying. Add more thrust than that, and you’ll accelerate skyward.

The space mavens’ perennial goal is to find a fuel source that packs astronomical levels of energy into the smallest possible volumes. Because chemical fuels use chemical energy, there’s a limit to how much thrust they can provide, and that limit comes from the stored binding energies within molecules. Even given those limitations, there are several innovative options. After a vehicle rises beyond Earth’s atmosphere, propulsion need not come from burning vast quantities of chemical fuel. In deep space, the propellant can be small amounts of ionized xenon gas, accelerated to enormous speeds within a new kind of engine. A vehicle equipped with a reflective sail can be pushed along by the gentle pressure of the Sun’s rays, or even by a laser stationed on Earth or on an orbiting platform. And within a decade or so, a perfected, safe nuclear reactor will make nuclear propulsion possible—the rocket designer’s dream engine. The energy it generates will be orders of magnitude more than chemical fuels can produce.

While we’re getting carried away with making the impossible possible, what we really want is the antimatter rocket. Better yet, we’d like to arrive at a new understanding of the universe, to enable journeys that exploit wormhole shortcuts in the fabric of space and time. When that happens, the sky will no longer be the limit.

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CHAPTER TWENTY-TWO

THE LAST DAYS OF THE SPACE SHUTTLE

May 16, 2011: The Final Launch of Endeavour

June 1, 2011: The Final Return of Endeavour

July 8–21, 2011: The Final Journey of Atlantis & the End of the Shuttle Era