Beam of Flight; Microwaving away some of space travel's long-lasting problems

San Diego Union-Tribune

February 12, 2003

By Scott LaFee

To slip the surly bonds of Earth has always been something of a trick, an extraordinary feat of engineering involving high technology, a little luck and lots of propellant.

At launch, for example, the three engines of a space shuttle can consume as much as 47,365 gallons of liquid hydrogen and 17,592 gallons of liquid oxygen per minute. Even then, they need help from a pair of disposable boosters, each packed with 1.1 millon pounds of solid rocket fuel.

But imagine if one could ride into space - or at least through it - on a beam of energy. Such a notion has long been the stuff of science fiction. In the 1980s, astrophysicist Robert Forward began publishing papers on the subject and wrote a novel, "Rocheworld," envisioning spacecraft pushed by an energy beam.

Now this novel notion is about to get a real-world test. Or more accurately, a test just above the real world.

Sometime this spring, a Russian submarine in the Barents Sea will launch a rocket carrying a 45-pound satellite called Cosmos 1. At roughly 500 miles above the Earth, the Cosmos 1 will unfurl into a 98-foot-wide solar sail. If all goes right, the sail, looking a bit like an eight-paneled umbrella, will use the pressure of sunlight to push it faster and higher through its temporary near-polar orbit around Earth.

The Cosmos 1 project is sponsored by the Pasadena-based Planetary Society and funded by private donors like Cosmos Studies, founded by the late writer and astronomer Carl Sagan, and the A&E Network. If successful, it will be the first solar sail flight, a critical early step in gathering vital data and experience about operating solar sails in space and, perhaps, about using them someday for interstellar travel.

Piggybacking on the Cosmos 1 test is another experiment, equally futuristic. At certain times during Cosmos 1's repeated orbits around Earth, a giant radio telescope dish in the Mojave Desert will transmit a beam of microwaves at the satellite. Watching researchers will then measure whether and by how much the microwaves increase the satellite's speed and altitude.

"It's our first step, too," said James Benford, president of Microwave Sciences Inc. and one of the project's primary investigators. "This is our demonstration of principle, our first attempt to prove you can use a beam of energy to effectively and efficiently send probes, and maybe manned spacecraft, to distant stars and places beyond our solar system."

Are we there yet? The problem with space is that, well there's just so much of it. Getting somewhere usually takes a lot of time.

Consider the space probe Voyager 1 - the most distant man-made object in the universe. Currently, Voyager 1 is about 85 times farther from the sun than the Earth is, or about 7.8 billion miles away. It is expected to reach the heliopause - the boundary between our solar system and the beginnings of interstellar space - sometime within the next few years. The journey, however has been long, even with the probe traveling at speeds of up to 1 million miles per day. Voyager 1 was launched more than a quarter - century ago, in 1977.

Practical exploration beyond the solar system requires spaceships to travel much farther and much faster. NASA has set a particular standard: reaching the Alpha Centauri star system (the closest to our sun at roughly 25 trillion miles away) within the working lifetime of a principal investigator, or about 40 years.

To do so means building a spacecraft that can travel at one-tenth the speed of light, or about 18,600 miles per second, said Benford.

"It takes a long time to get anything into the outer solar system with rockets," said Benford, "and you need much higher velocities to reach the stars or the galactic medium beyond the Oort Cloud (a theoretical shell surrounding the solar system that contains billions of comets)."

Current space transportation technologies are either inadequate or widly impractical, said Henry Harris, a senior scientist at the Jet Propulsion Laboratory and participant in the microwave experiment.

"Some people have talked about building a nuclear-powered manned spacecraft, but doing so would probably cost the country's entire gross national product for a year and require us to learn how to do things like mine other planets for fuel," said Harris. "It's just not going to happen anytime soon."

On the other hand, beamed energy propulsion may be just around the corner, relatively speaking. For example, laboratory lasers have been used to move a small object, but the technology remains prohibitively expensive for practical use.

"There's only one laser big enough to do the job." said Greg Benford, a UC Irvine physicist, science-fiction writer and twin brother of James. "It's operated by the Air Force and costs a million dollars a minute to use."

Closer to fruition is the use of microwaves as a power source for space travel. "The big advantage of microwaves is maturity of technology," said Harris. "We've been working with them for a long time. We don't have to wait for a new development or discovery."

The Cosmos 1 microwave experiment is, in general terms, relatively straightforward. The satellite will be carried into orbit by a Russian rocket that, ironically, once was tipped with a warhead targeted at Europe. The rocket, and many others, was decommissioned in the 1980s after President Reagan forced arms reduction negotiations by deploying similar Pershing 2 missiles in Europe.

Once Cosmos 1 has unfurled its solar sail, it will begin to circle the Earth once every 100 minutes. The first month in space will be devoted solely to learning how to remotely operate the sail and boost its orbit.

"Even at 500 miles high, there's a lot of wind friction," said Greg Benford. "It will be crucial to elevate the sail's orbit. Otherwise, the sail will start to descend and, after two months, start to tumble. Once that happens, there' no way to control it anymore."

Successful elevation of the sail, however, would open the door for the Benfords' project. During some of Cosmos 1's orbits, researchers at the huge Goldstone Apple Valley radio telescope in the Mojave Desert will transmit a beam of microwave energy, then look for changes in the satelite's speed and trajectory.

The changes will be minuscule, and a challenge to detect, in part because the solar sail's movement will be influenced by other factors, such as bumps in the Earth's gravitational field caused by the planet's non-uniform density.

But scientists are reasonably certain they will see an effect. For one thing, the basic physics are well-known and documented. Microwaves, like other parts of the electromagnetic spectrum, have no mass. But they do have momentum. When a microwave or a photon (a particle of light) strikes something, it can impart all or part of that momentum to what it's striking.

In groundbreaking laboratory tests in 2000, James Benford and colleagues were able to lift a small sail against gravity using a beam of microwave energy. One significant breakthrough was the development of a new sail material by San Diego-based Energy Sciences Laboratories. Composed of microscopic, three-dimensional carbon fibers, the sail is extremely strong, rigid, light and able to withstand temperatures well above 3,000 degrees Fahrenheit - a critical necessity since high speed generates heat.

The Cosmos 1 microwave experiment (which will be using an older Russian - made sail composed of aluminized reinforced Mylar roughly one-quarter the thickness of a plastic trash bag) will likely employ two different modes of microwave transmission.

The first mode involves exposing the satellite to microwaves as soon as it appeaers over the horizon, within line of sight of the Goldstone radio telescope. The telescope's 229-foot dish can follow the sail until it reaches 23 degrees, after which Cosmos 1 will be moving faster than the dish can follow.

The second mode involves sending a beam of microwaves almost straight up into space, then waiting for the solar sail to pass overhead and through the beam.

The second mode, called the impulse mode, maximizes the intensity of the microwave beam because the solar sail is passing through the beam at the shortest distance from the dish.

The drawback is time of exposure, just 0.07 seconds.

"The tracking mode is preferable," said James Benford. "Beam intensity is considerably less because the energy has to travel through more to the Earth's atmosphere, but the force is applied a lot longer, in the hundreds of seconds. With impulse, you would get a higher momentary acceleration, but with tracking, you get more time to measure the effect."

Whatever the effect, it will be invisible without extremely sensitive monitoring. While it doesn't require huge amounts of energy to, in theory at least, propel a spacecraft to high velocity, said Benford, it does require massive power for a brief amount of time.

The Goldstone Apple Valley radio telescope was not built for this purpose. Rather, it is part of NASA's Deep Space Network, an international network of antennas used to maintain continuous observation and communication with spacecraft, and for radio and radar astronomical studies. There are three DSN telescopes: Goldstone in the Mojave Desert; in Madrid, Spain; and Canberra, Australia.

Using beamed energy to actually lift a craft, even a small probe, into space is not possible, at least in the foreseeable future. The act simply requires more power and acceleration than existing lasers or microwaves can produce. "We're stuck with rockets or something like the space plane," said Greg Benford.

Beamed energy propulsion becomes feasible once the object is in space. Even then, say supporters, it will require a new source of energy, something with orders of magnitude more power.

Nothing like this currently exists on Earth, but the solution may eventually be found in space. NASA's ongoing Space Solar Power program envisions the possibility of building gargantuan power plants in orbit around the planet, each bedecked with square miles of solar panels that collect the sun's energy, then transmit it to Earth-based receiving stations in the form of microwaves.

"If a Space Solar Power system were ever built," said James Benford, "it could provide all of the electricity the world needed, and the excess could be used to propel space probes."

Specifically, energy beams could boost space probes to extremely high velocities, perhaps even to the Holy Grail of one-tenth the speed of light. In time, say forward-thinking advocates, the technology could be used for manned spaceflight.

"There are issues like how would you slow such a spacecraft down once you reached your destination, but people have thought about that too and there are reasonable ideas out there," said Harris.

The journey begins with Cosmos 1, whose stay in space will be visible to the naked eye. "When it's passing overhead, it will be the brightest light in the sky," said James Benford.

A bright idea whose time seems to have come and, with luck, will eventually go - very, very fast.