Antimatter-Rocket Plan Fuels Hope for "Star Trek" Tech
by Mark Anderson
for National Geographic News
May 4, 2006
 
Warp drives may be the stuff of science fiction, but another Star Trek staple appears to be edging toward science fact.

The energy source that enables the starship Enterprise to boldly go where no one has gone before has, according to one controversial new claim, moved much closer to reality.

A New Mexico company has just completed its initial studies of an antimatter-powered rocket that it hopes will someday take astronauts to Mars in 90 days or less. (Related interview: Mars expert on how to get there.)

As with Trek when it first aired in the 1960s, many critics doubt the ambitious new program will ever get off the ground.

The existence of antimatter was first predicted in 1928. It's said to be a mirror image of matter.

(See "Scientists Ponder Universe's Missing Antimatter.")

If a particle makes contact with its antiparticle, the two substances annihilate—they both vanish in a flurry of high-energy radiation known as gamma rays.

The electron, carrier of electricity, has an antimatter twin called the positron, or antielectron, which was discovered in 1932.

Sci-fi authors and screenwriters have since cashed in on the reflective, perplexing, and overpowering possibilities of this mystery substance.

Examples include the "Anti-Matter Man" episode of the 1960s TV series Lost in Space; the "positronic brains" of the cyborgs in Isaac Asimov's book I, Robot; and R.L. Forward's novel Martian Rainbow, in which antimatter rockets boost both a Mars mission and world domination.

But Forward was not the only visionary who saw antimatter as the ultimate form of rocket propulsion.

In the 1950s Austrian engineer Eugen Sänger first suggested using positron-electron annihilations to power spacecraft. But one of the chief problems that dogged his efforts was storage.

Positrons cannot be brought into contact with matter, which rules out any storage medium other than a vacuum filled with a magnetic field that contains the particles.

Moreover, positrons carry electric charge and naturally repel each other.

So storing just 0.0001 percent of the positrons needed to propel a spaceship to Mars would require containing a million tons of repelling electric force pushing on the walls of the fuel tank.

Radical Innovations

Several recent discoveries since the late 1990s have altered the picture radically, says Gerald Smith.

Smith is a retired professor of physics at Pennsylvania State University in University Park. He now heads a company called Positronics Research, LLC, in Santa Fe, New Mexico.

Last week Smith and his colleagues sent NASA a final report on Positronics' initial investigation into the feasibility of positron-fueled rocketry, which the space agency had partially funded. Positronics Research also receives funding from the United States Department of Defense.

Positronics' researchers base their novel antimatter rocket design on published studies that an electrically neutral positron atom could be artificially held together for at least several years.

The atoms, called positronium, consist of an electron and positron orbiting each other. Normally positronium can exist for only fractions of a millisecond before the two mirror particles annihilate each other.

However, in a series of papers published in the journals Physical Review Letters and Physical Review in 1997 and 1998, a team of German and U.S. theorists calculated that the right combination of electric and magnetic fields would stretch out the positronium like a barbell and greatly reduce the probability of the electrons and positrons annihilating.

"We've done the calculations," Smith said. "And it's not uncommon to find that the lifetime [of the enhanced positronium] is [practically] infinite."

Smith says that Positronics has begun experiments to verify their calculations, although he says the data from the experiments is not yet for public consumption.

Roger H. Miller is a professor emeritus of physics at Stanford University in Palo Alto, California. He has seen Smith present his publicly released data and is skeptical.

"The details are not available, so we don't know how many positronium atoms were stored, what their lifetime was, and how these quantities were measured," Miller said via email.

One undisputed fact is that, if a storage tank could be created that keeps the positronium in a stable state, an astonishingly small amount of positronium would power a spaceship.

Ten milligrams (0.0004 ounce)—the mass of a grain of rice—would be sufficient to take a manned spaceship to Mars. Mere grams would be enough to fuel a hundred-year expedition to the nearby star Alpha Centauri.

Creating those milligrams of antimatter would be very difficult and expensive, however. Smith, of Positronics, estimates that a dedicated 1.5-billion-U.S.-dollar facility could churn out the positronium needed to go to Mars in three years.

Stanford's Miller, on the other hand, counters that "the most powerful positron source ever built [so far] would take about 300 years to produce enough positrons."

From Power to Propulsion

Positronium is ultimately just a very compact power source. Converting that power into propulsion is another crucial hurdle.

Under their more public NASA grant, Positronics Research has released specifications for a rocket that consists of a surface coated with silicon carbide—also known as Moissanite, a diamond-like substance that glistens atop rings and pendants on late-night television shopping networks.

The positronium in this proposed rocket would be shuttled from the fuel tank to the engine core, where it annihilates, producing two gamma rays that evaporate silicon carbide from the nearby surface. The resulting silicon carbide gas, then, becomes the exhaust that propels the spacecraft forward.

Miller remains dubious.

"While I can't say that the concept of space travel powered by positrons is impossible, I am sure that it is a very difficult development," he said.

"I think it is worthwhile to point out that in the mid-1950s, knowledgeable people thought useful controlled [nuclear] fusion power generation [which, in contrast to nuclear fission—commonly used in power plants—generates energy by combining hydrogen atoms to create helium] would be achieved in about 25 years. Today it is still believed to be about 25 years away."

SOURCE: National Geographic