In this article we examine the use of antimatter and fusion to propel future spacecraft to the stars. Along the way exploring the possibilities for both the production of antimatter, and also its use in conjunction with other technologies.

by Dave Dooling

Antimatter is one of the most attractive words in science fiction literature and nearly as good a topic at parties as black holes. It might also be the fuel that powers spaceships to the planets and perhaps the stars.

"Antimatter has tremendous energy density," said Dr. George Schmidt, chief of propulsion research and technology at NASA. Matter-antimatter annihilation - the complete conversion of matter into energy - releases the most energy per unit mass of any known reaction in physics.

The popular belief is that an antimatter particle coming in contact with its matter counterpart yields energy. That's true for electrons and positrons (anti-electrons). They'll produce gamma rays at 511,000 electron volts.

But heavier particles like protons and anti-protons are somewhat messier, making gamma rays and leaving a spray of secondary particles that eventually decay into neutrinos and low-energy gamma rays.

And that is partly what Schmidt and others want in an antimatter engine. The gamma rays from a perfect reaction would escape immediately, unless the ship had thick shielding, and serve no purpose. But the charged debris from a proton/anti-proton annihilation can push a ship.

"We want to get as close as possible to the initial annihilation event," Schmidt explained. What's important is intercepting some of the pions and other charged particles that are produced and using the energy to produce thrust."

This is not your father's starship

He's not going to use it the way that the Starship Enterprise did, creating a warp field to move the vessel across space faster than the speed of light. At its most basic level, an antimatter rocket is still a Newtonian rocket moving a space probe through action and reaction.

And what a reaction. Where the Space Shuttle Main Engine has a specific impulse, a measure of efficiency, of 455 seconds, and nuclear fission could reach 10,000 seconds, fusion could provide 60,000 to 100,000 seconds, and matter/antimatter annihilation up to 100,000 to 1,000,000 seconds.

But first: Where do you get it? And how do you store the nuclear equivalent of the universal solvent?

Antiprotons, explained Dr. Gerald Smith of Pennsylvania State University, can be obtained in modest quantities from high-energy accelerators slamming particles into solid targets. The antiprotons are then collected and held in a magnetic bottle.

While that's been done easily enough in small quantities, fueling a rocket will take much more.

"We're building a Penning trap," Smith said, "one that will be lightweight and robust." When completed, it will weigh about 100 kg (220 lbs), much of it liquid nitrogen and helium to keep about a trillion antiprotons - far less than a nanogram - quiescent in a zone about 1 mm (1/25th inch) across.

"How do you know that you have particles in the trap?" Smith asked. "They're odourless and colourless." However, they do have distinctive radio frequency signatures which Smith and his colleagues have been able to measure. They've also demonstrated that their trap design would hold a significant quantity for up to 5 days.

"Our aim is to get up to a microgram of antiprotons," Smith said. "There are some interesting propulsion technologies that work at that level. We think we can do it."

A trillion antiprotons is the maximum that can be stored under those conditions. More could be held if they were turned into anti-hydrogen, antiprotons plus positrons.

A lot of bang for the buck

Right now, antimatter is the most expensive substance on Earth, about $62.5 trillion a gram ($1.75 quadrillion an ounce). The production is, at best, 50 percent efficient because half of what's created are regular protons, and the equipment now used was not designed to fuel rockets. Harold Gerrish of NASA and others estimate that improvements in equipment to slow and trap the antiprotons could bring the price down to about $5,000 per microgram. A new injector at Fermilab outside Chicago will allow that facility to increase its production tenfold, from 1.5 to 15 nanograms a year.

Lab. for Energetic Particle Science A schematic of the heart of a Penning trap where a cloud of antiprotons (the fuzzy bluish spot) is kept cold and quiet by liquid nitrogen and helium and a stable magnetic field.

"Right now, a lot of antiprotons are produced, but most are wasted," Gerrish said.

CERN has now produced anti-hydrogen as part of the Athena fundamental physics program to determine if antimatter indeed is indistinguishable from matter. Using the same Ioffe-Pritchard trap developed at CERN, Dr. Steven Howe of Synergistic Technologies in Los Alamos, N.M, expects that large quantities of anti-hydrogen atoms could be stored safely for long periods. At low temperatures, the wavelength of the atom is several times that of the material making up the container walls, so the atoms are reflected with little effort.

"Our goal is to remove antimatter from the far-out realms of science fiction into the commercially exploitable realm for transportation and medical applications."

Beyond the Enterprise - Fusion Power
A step back from antimatter is fusion, the power source of the future for the last five decades. Controlled fusion - joining two lightweight nuclei to get a slightly heavier nucleus and a lot of energy - has been challenging. In their quest to exceed Q=1, the break-even point, scientists have moved from low energy yields of Q=0.0000000000001 in the late 1950s to Q=0.3 today, and developed a large body of engineering and scientific knowledge showing that it can be made practical.

"From the NASA perspective, the challenge is to adapt fusion for space propulsion," said Dr. Francis Thio, a principal research scientist in the Propulsion Research Centre. "Magnetised Target Fusion is one of the major approaches that we are studying." NASA/Marshall is working with Los Alamos National Laboratory and the Air Force Research Laboratory to adapt MTF for propulsion.

"MTF tries to operate in an intermediate regime between the conventional magnetic fusion, and inertial confinement using a laser," Thio explained. The problem with conventional magnetic confinement is it operates at very low density. To achieve sufficient power, the fusion reactor must be large, which translates to a high cost.

On the other hand, inertial confinement fusion uses a tiny plasma, 1,000 trillion times denser than in a magnetic confinement scheme. But that requires a driver - usually banks of intense, short-pulse lasers - that heat and compress the target in a short time. That also drives the cost up.

"MTF tries to operate at not too low or too high a density," Thio explained, "and achieve a reasonable rate of fusion activity with a density 10,000 to 100,000 times higher than magnetic confinement, and 10,000 to 100,000 times lower than laser fusion."

It's more economical and uses pulse-power drivers - powerful capacitor banks that drive electromagnetic implosion - that are available today at low cost. It does not have the implosion speed generated by a laser beam, but a magnetic field confines the target plasma and insulates the inertial wall that implodes to cause the fusion.

Can I get the compact model?
Even if fusion is achieved, current methods are too cumbersome to use in rockets.

"The mass is quite prohibitive," said Professor T. Kammash of the University of Michigan. "We want to make the physics work without using very large magnets." The mirror magnets for a fusion rocket would weigh about 401 tonnes (metric tons), about 16 times a single Space Shuttle payload. The heat radiators would add 240 tonnes.

Kammash's students are experimenting with a droplet radiator design that, using liquid lithium as a coolant, could reduce the radiator mass to 57 tonnes.

A rotating magnetic field could induce a magnetic field and electrical currents, "a clever way of fooling the plasma" into behaving as if it was in a conventional magnetic mirror system.

In turn, the mass of the spacecraft would come down from 720 to 230 tonnes, and the 44-metre (144-ft) long engine would have a specific impulse of 130,000 seconds.

"It's quite impressive," Kammash said.

One of the most intriguing possibilities raised actually dates back to the 1950s and a concept developed by Philo Farnsworth, who pioneered most of the fundamental technologies for television in the 1920s and '30s.

"This is a really neat concept, something you can literally put your hands around," said Dr. Jon Nadler of NPL Associates, who is working with the University of Illinois Urbana-Champaign to develop the idea that Farnsworth had in 1950: fusion in a small bottle.

"You can use the power [it would generate] to power electric propulsion, or use the plasma for thrust," Nadler explained.

A star in a bottle
The technique is called inertial electrostatic confinement (IEC), a technique that avoids the use of massive magnets and laser systems used in other fusion-power techniques. Instead, the IEC device uses a hollow cathode, and the natural charges of electrons and ions, to form virtual electrodes that confine ions in a spherical region at the centre of the 61 cm (2 ft) diameter IEC vacuum chamber.

While true antimatter and true fusion propulsion will remain the "rockets of the future" for some time, a hybrid of the two might work in the near term.

"It's a good short cut," Schmidt said of antimatter-catalyzed fusion. In this approach, a small quantity of antiprotons is beamed into a fusion target. The resulting matter-antimatter annihilation heats a target enough to cause thermonuclear fusion.

Because of the energies and expense involved in producing antimatter, this method is not practical for power production on Earth. Overall, it is a net energy loser. Like all other forms of rocket propulsion, it's a sort of battery in which energy is expended to provide a large quantity in a tiny space, available on demand.

But, it could yield a rocket with a specific impulse of 13,500 to 67,000 seconds (30-147 times better than the Shuttle Main Engine), depending on the scheme used.

"Fusion missions would need just micrograms to reach the Oort cloud," the deep freeze of comets beyond the orbit of Pluto, Gerrish said. The antimatter load would cost about $60 million. But reaching the stars would require metric tons.

So a journey to the stars using antimatter alone may remain the stuff of books for just a little while to come.

For more information visit this site which is dedicated to Antimatter or else visit the website of the Advanced Space Transportation Programs at NASA.