Going Interstellar (collection)

We next assume a twenty percent energy conversion efficiency for the solar cells coating Mercury’s surface. The electrical energy input into the hypothetical antimatter factory constructed on this hot, small planet, is therefore about 3 X 10¹⁶ watts.

If our Mercury antimatter factory works continuously and 4 X10^-5 of the electrical energy input is converted into matter/antimatter pairs (as in the Tevatron), about 5 X 10¹⁸ Joules of energy is converted into antimatter each year. Every year, this antimatter factory will convert about 4 X 10¹⁹ Joules of energy into antiprotons.

Optimistically, we assume that all of these can be collected, decelerated, perhaps neutralized with positrons and safely stored until ready for use in the engines of a starship. The total antiproton annual production mass from this hypothetical antimatter factory can be calculated from a variation of Einstein’s famous equation (E = 2Mc²), where the factor 2 accounts for the fact that half the energy (E, in Joules) is converted into protons, M is the antimatter mass in kilograms and c is the speed of light in vacuum (three hundred million meters per second).

Even then, our hypothetical Mercury-based antimatter factory can produce only about five hundred kilograms of anti-hydrogen atoms. If the factory works continuously for a century, about fifty thousand kilograms of antimatter will be produced. This may be hardly enough for Eugen Sanger’s photon rocket, which requires equal amounts of matter and antimatter. But, as we shall see in the section on antimatter rockets below, an operational spacecraft propelled by antimatter/matter-annihilation may function quite well if antimatter is a very small fraction of the total fuel mass.

It should also be mentioned that it is not necessary that our antimatter factory or factories be located on a planet’s surface. Another location would be free space. Here, a huge parabolic, micron-thin reflector might be used to concentrate and focus solar energy on a bank of efficient, hyper-thin and low mass solar photovoltaic cells. Robert Kennedy, Ken Roy and David Fields have suggested that humans may ultimately construct approximately one thousand-kilometer solar-sail sunshades in space to slightly reduce the amount of sunlight striking the Earth and thereby alleviate global warming. Such in-space devices could also be used to concentrate solar energy on Mars. There is no inherent reason why these sunshades or solar concentrators could not serve a dual function and direct sunlight towards in-space antimatter factories.

Also, as Forward speculates, the antiproton conversion efficiency he quotes for the Tevatron may not be the ultimate. There is plenty of room for improvement if some of humanity’s brightest minds turn their attention to the problems of antimatter production and storage.

How Do We Store Antimatter??—VERY, VERY CAREFULLY!!!

No matter where the antimatter is produced, the next challenge is the safe storage of the stuff until we are ready to use it in a starship engine. This is especially difficult since antimatter is the most volatile material in the universe and will disappear in a puff of radiation if brought into contact with normal matter.

As it turns out, there are a number of options. But none of these is especially easy. This section describes some candidate antimatter storage systems.

One possibility is magnetic storage rings. Using combinations of electric and magnetic fields, antiprotons would be spun continuously around one ring at constant velocity, positrons (if necessary) around another. When reaction with normal matter in the starship’s combustion chamber is required, an appropriate mass of antiparticles could be magnetically diverted towards the target without touching chamber walls. Antiparticles have been stored in such a manner after deceleration in existing antimatter factories. But we wonder what the limits are on antiparticle density in the ring. And is it possible to reliably alter field strength in parts of the storage ring as the ship changes its acceleration rate?

Many of the potential solutions to antimatter storage have been reviewed in a paper by the American physicists Steven Howe and Gerald Smith. They describe a version of the Penning trap they constructed at Pennsylvania State University. This device might be able to store one hundred billion antiprotons per cubic centimeter. That sounds like a lot of antiprotons, but a Penning trap at least a kilometer across would be required to store a kilogram of antiprotons!