Nov 12, 1997

When Isaac met Albert (Antimatter may propel future spacecraft to Mars)




When Isaac Met Albert


electron-positron pair creation from a photon
November 12, 1997: Antimatter is one of the most recognized and attractive words in science fiction. It's the stuff that drives fictional starships from one side of the universe to the other. Now NASA is giving it serious consideration as a rocket propellant to get around the solar system. A gram of antimatter would carry as much potential energy as 1,000 Space Shuttle external tanks carry.

Don't expect to go zipping away at warp factor anything, though. While antimatter may become the fuel of tomorrow, the rockets will employ the ages old action-reaction principle in an interesting meeting of Albert Einstein (E=mc2) and Isaac Newton (F=ma).

The image above, right shows the creation of an electron - positron pair from a photon in a liquid hydrogen bubble chamber in a magnetic field.1

On Tuesday, Russian scientists were informally invited to join NASA in this venture to produce, store, and use antimatter for medical and construction purposes as well a for space propulsion.

The Russians are attending the week-long Science and Technology Advisory Council meeting, a review of scientific research conducted by Russian scientists with NASA funding as part of America's participation in the Mir space station program.

While most of the sessions involve Russians presenting their results, on Tuesday Harold P. Gerrish Jr. of Marshall Space Flight Center's Propulsion Laboratory discussed American work in antimatter propulsion.

"Future spacecraft won't be designed around the propellant tank," Gerrish predicted, since it would take only a few billionths of a gram of antimatter to propel a 400-ton spacecraft to Mars and back in four months - including a one-month stay on the surface. Instead, the spacecraft will be designed around massive systems to "burn" antimatter with matter, and to protect the crew from radiation.


But first, you have to make the antimatter.

As most science fiction fans know, every particle in the universe has an antiparticle, a mirror image that acts pretty much the same except its charge is reversed. When the two meet, they convert themselves into pure energy.

Well, almost. Electrons and positrons (antielectrons) do a neat job of it, but protons and antiprotons are messy. They yield three types of pions that 1) decay to produce gamma radiation 2) decay to produce muons and neutrinos plus electrons and positrons that make more gamma rays.Electrons are lightweight and tough to store in a magnetic model, so scientists have been working on antiprotons whose greater mass makes them easier to handle. (Even anti-atoms that might be stored at near absolute zero are a possibility.) Billionths of a gram of antiprotons are created each year in high-energy particle accelerators high in the Alps. A billion antiprotons are produced every 10 minutes, but only 1,000 of those are captured and stored.

"Even though the efficiency isn't that high" Gerrish said, European scientists "have demonstrated the physics to slow, cool, and store them."


mars spacecraft concept
ICAN-II engine

Artist's concepts show a manned Mars spaceship using the Ion Compressed Antimatter Nuclear (ICAN-II) engine concept under study at Pennsylvania State University (top). It could take astronauts to Mars and back in 120 days, including a 1-month stay, or to Jupiter and back in 18 months with a 3-month stay. Concepts also are being studied to propel an unmanned observatory to deep space - 550 times the distance from Earth to sun - and use the sun as a gravitational lens to make images of the core of our galaxy.

Dr. Gerald Smith of Pennsylvania State University is developing a portable penning trap, a special magnetic bottle, that will store up to 10 billion cold antiprotons for transit. A High Performance Antimatter Trap is being designed that will carry up to 1 trillion anti-protons, enough for a few dozen experiments in each bottle.

"If it meets our design goal," Gerrish said, "then we would bring some antiprotons to Marshall" for propulsion research experiments in conjunction with Penn State, the Jet Propulsion Laboratory, and the U.S. Air Force Philips Laboratory.

The kind of physics that lets science fiction writers warp space isn't available yet, so this is where Einstein - who wrote that matter and energy can be converted into each other - meets Newton - who said that every action has an opposite and equal reaction.

After all the neat high energy physics and technology to make the antimatter, it will just be used as a heat source to propel a high-energy gas jet through a rocket nozzle.

The simplest, Gerrish explained, is a variation on the solid-core fission reactor designed in the late 1960s. Antimatter reactions would heat a tungsten core which, in turn, heats hydrogen as it flows through and then out the nozzle. Its specific impulse (sort of a miles-per-gallon measure) is 800 to 1,000 seconds, more than twice that of the Space Shuttle Main Engine.

The most elegant engine is the beam core in which magnetic coils would contain and direct the particles produced by proton-anti-proton annihilation. Its efficiency is 10 million seconds, but thrust would be much lower.

High-speed manned trips to the planets, or unmanned trips to Pluto and beyond, might be accomplished with the Ion Compressed Antimatter Nuclear (ICAN-II) engine design being developed at Penn State. The design would use antiprotons to implode pellets with nuclear fusion targets at their cores. massive shock absorbers would cushion the ship as a series of small blasts propelled it through space.

It would not get us anywhere near the speed of light, at least not for a few decades, but it could lets us zip through the solar system when compared to the schemes now on the table.

Other potential uses for antiprotons include beaming them through large metal structures to detect flaws and even to cure flaws internally, and bombardment of deep cancer tumors that resist other treatment.