© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

17.3.5  Total Conversion Drives

The basic idea of using antimatter to power starships has been discussed in the technical and fictional literature for many decades. The most common system is the "photon drive"*: Simply bring together equal quantities of matter and antimatter, allow annihilation or "total conversion" to take place, and then convert the products to useful thrust. Gamma rays as well as high energy electrons and positrons are thrown off, but about half of the energy liberated is in the form of neutrinos which escape isotropically and are wasted.

The usual antimatter propulsion scheme thus amounts to no more than "partial conversion" with an efficiency well below 50%. Better results may perhaps be obtained by using cold or "frozen" positronium gas as fuel. Positronium consists of pseudo-atoms in which a negatively charged electron orbits a positively charged positron (the electron’s antiparticle) -- this has been observed experimentally. As the fuel is warmed, electron and positron annihilate, producing a pure beam of gamma radiation. Eugene Sänger proposes using an electron gas reflector to focus and direct the photonic jet.²⁸⁴⁰ Alan Bond estimates that a million-ton starship with a mass ratio (fueled/unfueled, by weight) of 7.4 and an acceleration of one gee could reach 60%c.¹¹⁵⁹

Dr. D.D. Papailiou at Jet Propulsion Laboratory in Pasadena claims that for missions to nearby stars optic exhaust velocities are not necessary. A far more efficient technique is to use a small amount of antimatter to energize a large amount of ordinary matter. Preliminary calculations by Papailiou show that a mass ratio of 5.0 and a 2% charge of antimatter (by weight) is optimal to achieve a probe coast velocity of 33%c.²⁷⁵⁷

So if we wanted to launch a 10-ton starship on an encounter-capture mission, we must built a top stage of 51 tons -- the 10-ton probe, 40 tons of ordinary matter, and 1 ton of antimatter -- to obtain a mass ratio of 5. This stage serves to decelerate the probe at its destination. To get it there, we will need a 205-ton bottom stage -- 200 tons of ordinary matter and 5 tons of antimatter -- in order to accelerate the top stage up to 33%c. Delivery of a 10-ton space probe with a coasting speed of 33%c to another star system thus requires a total ship mass of about 256 tons, of which 6 tons are antimatter.

A related scheme is the Antimatter Ramjet, which gathers normal matter in the forward scoop primarily as reaction mass. In a manner similar to Papailiou’s antimatter drive outlined above, interstellar matter would be commingled with bits of antimatter stored onboard. In this case the probe need only carry the requisite 6 tons of antimatter fuel when it leaves the planet of origin (perhaps stored in the form of frozen antihydrogen maintained a few degrees above absolute zero), and can pick up the remaining 240 tons of ordinary matter en route to its destination.

Many physicists object to the feasibility of total conversion drives such as those mentioned above because of the difficulty today of generating macroscopic quantities of antimatter. But the problems of creation and control should not prove insuperable. Writes Dr. Forward:

The present methods for producing antimatter involve the use of large accelerators which can produce a proton beam of 10¹⁵ protons per second. When such a beam collides with a target, antiprotons are produced as part of the debris. The antiproton yield of present machines is very low. However, the presently used methods are not designed for antimatter production but rather for studies in the physics of elementary particles. Rough calculations assuming special purpose high amperage colliding beam accelerators indicate that the generation of kilograms of antimatter per year is not out of the question. The containment and control of the antimatter, once made, should not be too difficult since we have a number of ways of applying forces to the antimatter without touching it. Electric fields, magnetic fields, rf fields and laser beams are all used in present day technology to levitate and control small amounts of regular matter that we do not want to contaminate. These would all be equally effective on antimatter.⁷¹⁸

Another considerably more speculative total conversion system involves so-called Hawking Black Holes (HBHs). According to Dr. Stephen H. Hawking at the University of Cambridge, all black holes (if any exist) radiate energy due to quantum mechanical "tunneling" effects. This is equivalent to mass loss, so eventually the entire corpus of the black hole "evaporates." A stellar-mass BH is very "cold" -- in fact, close to absolute zero -- but a low-mass HBH is extremely "hot" and prone to explosive evaporation.²⁰²¹ For instance, a million-ton HBH should radiate about 10¹⁸ watts at a temperature of about 10¹⁵ K, and will take about one year to finish evaporating.

The HBH becomes a total conversion engine when we start "feeding" it. Evaporation can be indefinitely postponed simply by shoveling in raw matter -- any matter -- at an appropriate rate (Table 17.4). Such a device could be used to construct an extremely high-efficiency photon drive propulsive system. The main problem is how to construct an HBH in the first place. Dr. John A. Wheeler estimates that a black hole of mass 10⁹ kilograms might be generated artificially by the controlled thermonuclear fusion implosion of approximately 5 x 10¹³ kg of deuterium.²⁰²² This involves the handling of some 5 x 10²⁸ joules of energy, which looks like a job for an ambitious mature Type II civilization or an early Type III galactic society.

* Acceleration A (m/sec²) of a perfect photon rocket of mass M (kg) and power output P (watts) is given by: A = 2P/Mc, where c is the speed of light (3 x 10⁸ m/sec).

Last updated on 6 December 2008