Can NASA Launch a Rocket with a Laser?
Launching rockets into space takes a lot of fuel and costs a lot of money. Will a theoretical plan to use lasers to beam enough energy to launch things into space ever take off?
It's easy to forget that space is far away—low earth orbits don't even begin until you get about 100 miles from the surface of the earth, and a geostationary orbit is 22,000 miles straight up. Getting there is hard—to do it, the Space Shuttle's 37 million horsepower main engines accelerate it to 17,000 miles per hour. When it comes to satellites, the cheapest way to get them into space is to use the same towering rockets that were perfected for the Apollo missions. They combine liquid hydrogen with liquid oxygen, and blammo: A sustained explosion carries your cargo skyward.
These traditional chemical rockets have hit a wall: Currently, it costs about $10,000 a pound to get something into orbit. For a large communications satellite on the order of 5 tons, that's more than a $100 million in launch costs alone. That amount hasn't changed in decades. Companies like Elon Musk's SpaceX claim they'll be able to launch satellites for $1,000 a pound just by streamlining operations—basically, replacing government red tape with ruthless entrepreneurial efficiency—but there are no near-term technological breakthroughs that promise to fundamentally change how we get things away from the pull of Earth's gravity.
One radical solution to the problem is to leave most of the energy-containing stuff required to get a rocket into space on the ground. Ninety percent of the weight of a rocket on a launch pad is fuel, after all, leaving only a tiny sliver of usable mass left over for cargo. The notion, first proposed in 1972 by inventor Arthur Kantrowitz, is called beamed energy. The idea is simple: A massive power plant on the surface of the Earth sends energy to a rocket via an improbably huge laser or "maser"—which operates on the same principle as a laser, but involves microwaves.
A ground-based energy beam of sufficient power aimed at a rocket of the right composition could allow it to heat a supply of onboard hydrogen to a temperature of 2,500 degrees kelvin, hot enough to shoot out its nozzles with sufficient power to boost the vehicle into space. Alternately, the bottom of the rocket could consists of a chunk of metal that would be vaporized by a pulsed laser, creating a cloud of gas that is then super-heated by a second laser pulse, thrusting the rocket skyward.
A series of small-scale experiments with models over the past 20 years have demonstrate what rocket scientists like Alexander Bruccoleri of MIT already know: "There's nothing in the laws of physics that prohibit this from working."
The only real barrier is motivation on the part of NASA or some other deep-pocketed patron, a situation that is unlikely to change in the near future, according to historian of technology Jonathan Coopersmith: "The basic problem that the beamed energy people have is that [traditional] rocketry works."
Proponents of beamed energy argue that, despite our entrenched dependence on traditional rockets and the high costs of developing alternatives—one paper estimated that it would take billions of dollars to develop a successful beamed energy platform—once we have them, these rockets could reduce the cost of getting things into orbit by a factor of 100. The world launches about 20 satellites a year into space, but that's spread across multiple space agencies. Let's say the Russians, who are responsible for about half of all space launches (many of them manned) decide to invest in this technology; they might be able to recoup their investment within 10 to 20 years.
Ignoring the rocket itself, it turns out that most of the technology required to build the system already exists: Raytheon just demonstrated a giant, plane-killing death laser, which happens to be exactly the size of laser you would need (plus a few dozen more) to get a small rocket into orbit. If you prefer microwaves, Dmitriy Tseliakhovich, a graduate student at CalTech, estimates that you would only need a ground-based station as large as the Atacama Large Millimeter Array, the world's largest radio telescope, which consists of huge dishes meant to listen to space but which could also be used to transmit microwaves. (ALMA consists of 66 radio dishes of either 40 or 23 feet in diameter clustered in the Atacama desert in northern Chile, and it cost $1 billion to build.)
Bruccoleri, who worked on beamed energy propulsion for a decade before giving up on the whole enterprise, isn't convinced that it's a good alternative to traditional rockets even if you had all the money in the world. Factor-of-100 reductions in launch costs assume that beamed-energy rockets are fully re-usable, but that isn't likely to be the case.
The problem, he says, is that the "fuel" that the rocket would have to carry—pure hydrogen—is so light that to lift something the size of the Space Shuttle, the fuel tank attached to it would have to be five times as large (by volume) as the shuttle's current tank. Bruccoleri's own calculations show that using existing technology, something with a tank that big is ultimately as costly as the Space Shuttle itself. The reason is simple: Fuel costs are a negligible portion of launch costs. When going to space, what's so expensive is building and maintaining the incredibly well-tuned machinery that can burn all that fuel without blowing up—and then refurbishing it after every flight, since a craft like the Space Shuttle isn't so much re-used as rebuilt after each journey. A giant fuel tank filled with hydrogen is neither reusable nor recoverable (the shuttle's current fuel tank is jettisoned late in flight and burns up on re-entry).
If beamed energy rockets aren't all that reusable because they require giant (albeit light) fuel tanks, they might be only marginally more cost-effective than traditional rockets, and that's ignoring all the money you spent building a ground-based laser array big enough to burn your name on the surface of the moon.
"For now, to be honest, I think we are stuck with chemical rockets," says Bruccoleri.
That doesn't mean beamed energy will never find its niche. For example, if we're ever going to make space-based solar arrays a reality, it will be a must, because the alternative is an extension cord hundreds of miles long. The military also has an unflagging interest in building devices that can get thermal energy from point A to point B in the most expedient way possible: One version of the microwave array required to get a rocket into space has been used on civilians in Iraq since 2005. Designed to disperse protesters and other unruly groups who aren't worth, or don't deserve, a bullet, it painfully but mostly harmlessly heats the skin just like a microwave ovens heats a TV dinner. It's called the Active Denial System.