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Antimatter Propulsion for Space Exploration

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Antimatter Propulsion for Space Exploration Journal of the British Interplanetary Society, Vol. 39, pp. 391-409, 1986. ANTIMATTER PROPULSION FOR SPACE EXPLORATION GIOVANNI VULPETTI ·-..Telespazio SpA per le Comunicazioni Spaziali, Via A. Bergamini 50, 00159 Rome, Italy. This is chiefly a review paper with additional considerations and suggestions about aspects of matter-antimatter propulsion for space exploration. The paper is divided into six main sections. In the first section the fundamentals of the low-energy antiproton-nucleon annihilation physics are presented. In the second section geocentric, interplanetary and interstellar flights possible by means of antimatter engines are reviewed. In the third section the big problems related to the antimatter handling (production, storage, control) are discussed. In the fourth section proposed matter-antimatter thruster design concepts for Earth-space, interplanetary and out-of-Solar-System missions are examined and some results from computer simulations are discussed. In the fifth section a comparative cost between antimatter-based engines and conventional engines is made. Finally, aspects of a world-wide cooperation to try to make antimatter propulsion a reality are pointed out. ''·" 1. INTRODUCTION In this paper the author is concerned with both a critical review annihilation being a secondary process from a propulsion view­ of the past research and some suggestions for future research point. about the so-called Antimatter Propulsion. This is not a math­ Our paper is then structured as follows: the next section ematical paper; nevertheless we will also discuss results of is devoted to a general survey of the antiproton-proton and computer simulations of complex mathematics. antiproton-neutron annihilation-at-rest reactions. Successively, Let us begin our dissertation by explaining the concept of we will switch to the discussion of the requirements of missions antimatter propulsion. Any propulsion system utilises some in the space of the Earth, Solar System and nearby stars. From source of energy to be controlled, in particular to be directed. this basis we will go back to the problems of antimatter The ultimate release of energy lies in microstructures such as production, storage and annihilation c~ntrol. We will then molecules, atoms, atomic nuclei; subnuclear particles. These describe some possible antimatter-matter engine design con­ last ones display two states: particle and antiparticle; in the cepts. Comparative cost estimates between antimatter propul­ known Universe it is believed that matter overwhelms anti­ sion systems and conventional thruster systems will be men­ matter. When a particle and its corresponding antiparticle tioned on a parametric basis. Finally, we will stress a need of "collide" at a sufficiently low energy, the annihilation proba­ an international cooperation in the near future for pursuing bility is high and their interactions results in a high conversion of effective research on antimatter propulsion. their rest masses into kinetic energy of other massive particles, The interested· reader .cean find a number of references to photons and neutrinos, the remaining fraction of the initial introduce himself more. deeply into the fields dealt with here mass being found in the form of the produced particle's rest (notice in particular th~t Ref. 57 rec.. eived approval for public mass (the types of products depend upon the type of inter·a-ction release in September 1985; therefore Ref. 57 and this paper responsible for the annihilation). Only the first two energies represent two quite independent efforts about antimatter pro­ (i.e. kinetic energy and photons) could be utilised by means of pulsion). some control process or device. A matter-antimatter annihila­ tion propulsion system is conceived as a device which would exploit those energies. 2. THE ANTIPROTON-NUCLEON ANNIHILATION Only really stable or long-life particles-antiparticles could, AT REST in principle, be considered for a technological system. This entails that electron-positron and nucleon-antinucleon annihil­ In Refs. 1-18 the reader can find a lot of details about the ations are the only elements of our set of admissible annihil­ antiproton-nucleon annihilation (pN) at rest. Because we are ations for potential space propulsion application. As is well­ chiefly interested in the "global" properties of this annihilation known, the former pair annihilates only into two gamma pho­ process for a potential space application, we make a synthesis tons which cannot be effectively controlled and directed as of Ref. 19 whic.h updated old experimental data. We first such. The latter pair annihilates by strong interaction and describe the antiproton-proton (pp) annihilation and, successi­ produces a lot of particles, the great part of which are massive, vely, the antiproton-neutron (pn) annihilation. Before doing charged, relativistic and short-lived. Before they decay, such this, we must outline. some considerations common to the two particles and their energy can in principle be controlled. As we types of processes with a special emphasis on the annihilation shall hint in the sequel, even the gammas produced in some environment. - · decay stage of this annihilation process may be partially con­ For possible space propulsion applications the pN annihila­ trolled in energy, namely, if converted. The other advantages tion must occur at rest. The initial p's momentum (relative to of the nucleon-antinucleon annihilation at-rest with respect to N assumed at rest in the ship frame) should be, say, below 5 the electron-positron annihilation is the much bigger amount MeV /c, essentially for transportation problems on-board (see of energy released, as it is apparent. Because we ultimately Section 4.2). Anyway, the antiprotons must' pass through a need neutral antimatter in order to be better handled, positrons certain amount of matter, 'generally lose·· their energy and are to be produced in the same number of antiprotons. Free thereafter annihilate. The last step is not a simple thing. antineutrons cannot be controlled. Most of the present-day understanding of the antiproton­ Therefore the high-energy process of interest consists of the nucleon dynamics comes from analyses of photographs from antiproton-nucleon annihilation at-rest, the electron-positron bubble chambers filled with liquid hydrogen or deuterium. 391 / G. Vulpetti E ( KeV) rrl1 2 3 4 29 annihilation at rest (in liquid hydrogen) can be quantitatively written as follows [19] (a tilde above a particle symbol denotes 30fm-- : = ==··== ..·- the corresponding antip~rticle t 4 ii 3 IC -my p + p => 1.527 ?r+ + 1.527 'Ir- + 1.96 11"0 + (1) 0.012 K+ + 0.012K- + 0.013 KO+ 0.013 KO 3.2 2 II A N N Therefore the pp annihilation generates pions and kaons, the I H latter particles being about 1 per cent of the former ones. I l This quantity can be neglected from the jet power viewpoint; A T however, it is not negligible from other points relevant to an I antimatter-matter engine design [ 19]. 9.4-t.E,s 0 N The average energy spectra of the pions and kaons in Eq. (1) are the following [ 19]: L Hz E(?r+) = E(?r-) = 374 MeV E(?rO) = 358.5 MeV - (2) E(K+) = E(K-) = 633 MeV E(KO) = E(KO) = 633 MeV t2.5 Equation (2) regard the pion and kaon total energy. This can 1· -- be evaluated from measurements either directly or through a , complex procedure of estimate of partial-rate tables of extensive Fig. 1. Energy levels of the protonium (p·p atom). In a liquid molecular experiments. However, for evaluating the performance of a target such as the bubble chamber H2, the collisional Stark mixing potential annihilation engine, we must consider the following effect populates the S-levels with high principal quantum number. mass-energy quantities: These levels decay chiefly via annihilation. P-wave annihilations are very rare. When an antiproton is stopped there, an H atom captures it M the rest mass of massive particles by its Coulomb field. Stopping and capturing means that the KE the kinetic energy of massive particles p has been slowed-down to a few eVs in energy and its impact GE the energy of gamma rays parameter with respect to the H-atom's proton is about the classical radius of the K~shell. The system (p,e-,p) is unstable NE the energy of neutrinos and partially de-excites by emitting the electron. The remnant system is called antiprotonic hydrogen or protonium. The Neutrinos are considered zero-mass particles here. They do not protonium's energy levels correspond to the ordinary H levels contribute to thrust in any case. with the Rydberg constant scaled up by about 918 (Fig. 1). Equation (1), which represents the first step after the annihil­ The anti proton capture occurs in high-n (n = principal quantum ation (apart from the resonances), is characterised by the number) orbitals, typically n = 30. In states lower than that of following values of the above set: capture, the nuclear strong potential adds to the Coulomb potential and causes a shift and broadening of the protonium ME= 715.6 MeV KE= 1161 MeV GE= 0 NE= 0(3) energy levels. The excited protonium decays from a high-n. In vacuum this entails a high angular momentum by electric Such values correspond to the energy sharing just after the eta­ dipole transition; in real environments the Stark effect mixes boson decay [19]. Unfortunately, these parameters undergo a the degenerate states and, in particular, provides the S-state for time evolution after 100 attoseconds since the annihilation time. a non-negligible probability to be populated. All this ultimately We describe and plot such variations because they are of prime results in a large population in the S-states before annihilation importance for space propulsion. Details can be found in Ref. - lSO and 3Sl - for which the principal quantum number is 19. still . rather high.
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