Editing Antimatter rocket

{{short description|Rockets using antimatter as their power source}}
{{More citations needed|date=April 2007}}

[[File:Antimatter Rocket.jpg|thumb|right|200px|A proposed antimatter rocket]]
{{Antimatter}}
An '''antimatter rocket''' is a proposed class of [[rocket]]s that use [[antimatter]] as their power source. There are several designs that attempt to accomplish this goal. The advantage to this class of rocket is that a large fraction of the [[rest mass]] of a matter/antimatter mixture may be converted to energy, allowing antimatter rockets to have a far higher [[energy density]] and [[specific impulse]] than any other proposed class of rocket.<ref>{{Cite book|last=Schmidt|first=George|title=62nd International Astronautical Congress 2011 : (IAC 2011) : Cape Town, South Africa, 3-7 October 2011.|publisher=International Astronautical Federation|others=International Astronautical Federation.|year=2012|isbn=978-1-61839-805-5|location=Paris|pages=6792–6812|chapter=Nuclear Systems for Space Power and Production|oclc=795367347}}</ref>

==Methods==
Antimatter rockets can be divided into three types of application: those that directly use the products of antimatter annihilation for propulsion, those that heat a working fluid or an intermediate material which is then used for propulsion, and those that heat a working fluid or an intermediate material to generate electricity for some form of [[Electrically powered spacecraft propulsion|electric spacecraft propulsion system]].
The propulsion concepts that employ these mechanisms generally fall into four categories: solid core, gaseous core, plasma core, and beamed core configurations. The alternatives to direct antimatter annihilation propulsion offer the possibility of feasible vehicles with, in some cases, vastly smaller amounts of antimatter but require a lot more matter propellant.<ref name=ADA446638>[http://apps.dtic.mil/dtic/tr/fulltext/u2/a446638.pdf ''Fusion Reactions and Matter-Antimatter Annihilation for Space Propulsion''] {{Webarchive|url=https://web.archive.org/web/20231004161223/https://apps.dtic.mil/dtic/tr/fulltext/u2/a446638.pdf |date=2023-10-04 }} Claude Deutsch, 13 July 2005</ref>
Then there are hybrid solutions using antimatter to catalyze fission/fusion reactions for propulsion.

===Pure antimatter rocket: direct use of reaction products===

[[Antiproton]] annihilation reactions produce charged and uncharged [[pion]]s, in addition to neutrinos and [[gamma ray]]s. The charged pions can be channelled by a [[magnetic nozzle]], producing thrust. This type of antimatter rocket is a '''pion rocket''' or '''beamed core''' configuration. It is not perfectly efficient; energy is lost as the rest mass of the charged (22.3%) and uncharged pions (14.38%), lost as the kinetic energy of the uncharged pions (which can't be deflected for thrust); and lost as neutrinos and gamma rays (see [[Antimatter#Fuel|antimatter as fuel]]).<ref name=AIAA-2003-4696>[http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38278/1/03-1942.pdf ''How to Build an Antimatter Rocket for Interstellar Missions: Systems level Considerations in Designing Advanced Propulsion Technology Vehicles''] {{webarchive|url=https://web.archive.org/web/20150502002952/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38278/1/03-1942.pdf |date=2015-05-02 }} Robert H. Frisbee, AIAA Paper 2003-4696, July 20–23, 2003,</ref>

[[Positron]] annihilation has also been proposed for rocketry. Annihilation of positrons produces only gamma rays. Early proposals for this type of rocket, such as those developed by [[Eugen Sänger]], assumed the use of some material that could reflect gamma rays, used as a [[light sail]] or [[parabolic reflector|parabolic shield]] to derive thrust from the annihilation reaction, but no known form of matter (consisting of atoms or ions) interacts with gamma rays in a manner that would enable specular reflection. The momentum of gamma rays can, however, be partially transferred to matter by [[Compton scattering]].<ref name=AIAA-2001-3231>[http://physicsx.pr.erau.edu/ExoticPropulsion/propulsion2.pdf ''The Antimatter Photon Drive: A Relativistic Propulsion System''] Darrel Smith, Jonathan Webby, AIAA Paper 2001-3231, 2001</ref><ref name=WebbTATRSBCAR>[http://physicsx.pr.erau.edu/ExoticPropulsion/APD/APD%20Word/Thermal.pdf ''Thermal Analysis of a Tungsten Radiation Shield for Beamed Core Antimatter Rocketry''] Jonathan A. Webb</ref>

One method to reach relativistic velocities uses a matter-antimatter GeV gamma ray laser photon rocket made possible by a relativistic proton-antiproton pinch discharge, where the recoil from the laser beam is transmitted by the [[Mössbauer effect]] to the spacecraft.<ref name="AA-20120821">{{cite journal |last=Winterberg |first=F. |title=Matter–antimatter gigaelectron volt gamma ray laser rocket propulsion |date=21 August 2012 |journal=[[Acta Astronautica]] |volume=81 |issue=1 |pages=34–39 |bibcode = 2012AcAau..81...34W |doi = 10.1016/j.actaastro.2012.07.001 }}</ref>

A new annihilation process has purportedly been developed by researchers from the University of Gothenburg, Sweden. Several annihilation reactors have been constructed in the past years{{when|date=November 2024|reason=which past years?}} which attempted to convert [[hydrogen]] or [[deuterium]] into relativistic particles through laser annihilation. The technology was explored by research groups led by Prof. Leif Holmlid and Sindre Zeiner-Gundersen, and a third relativistic particle reactor is currently being built at the University of Iceland. In theory, emitted particles from hydrogen annihilation processes could reach 0.94c and can be used in space propulsion.<ref>{{cite journal |last1=Holmlid |first1=Leif |last2=Zeiner-Gundersen |first2=Sindre |title=Future interstellar rockets may use laser-induced annihilation reactions for relativistic drive |journal=Acta Astronautica |url=https://iopscience.iop.org/article/10.1088/1402-4896/ab1276/pdf  |date=1 October 2020 |volume=175 |pages=32–36 |doi=10.1016/j.actaastro.2020.05.034 |bibcode=2020AcAau.175...32H |doi-access=free |hdl=20.500.11815/2191 |hdl-access=free }}</ref> However the veracity of Holmlid's research is under dispute and no successful implementations have been peer reviewed or replicated.<ref>{{cite conference | title=Comment on 'Ultradense protium p(0) and deuterium D(0) and their relation to ordinary Rydberg matter: a review' 2019 Physica Scripta 94, 075005 | author=Klavs Hansen | year=2022| arxiv=2207.08133 }}</ref>

===Thermal antimatter rocket: heating of a propellant===

This type of antimatter rocket is termed a '''thermal antimatter rocket''' as the energy or heat from the annihilation is harnessed to create an exhaust from non-exotic material or propellant.

The '''solid core''' concept uses antiprotons to heat a solid, [[Atomic number|high-atomic weight]] ('''''Z'''''), refractory metal core. Propellant is pumped into the hot core and expanded through a nozzle to generate thrust. The performance of this concept is roughly equivalent to that of the [[nuclear thermal rocket]] (<math>I_{\text{sp}}</math> ~ 10<sup>3</sup> sec) due to temperature limitations of the solid. However, the antimatter energy conversion and heating efficiencies are typically high due to the short [[Mean free path|mean path]] between collisions with core atoms ([[efficiency]] <math>\eta_e</math> ~ 85%).<ref name=ADA446638/>
Several methods for the '''liquid-propellant thermal antimatter engine''' using the gamma rays produced by antiproton or positron annihilation have been proposed.<ref name=Vulpetti1987>{{cite journal |last1=Vulpetti |first1=G. |title=A further analysis about the liquid-propellant thermal antimatter engine design concept |journal=Acta Astronautica |date=August 1987 |volume=15 |issue=8 |pages=551–555 |doi=10.1016/0094-5765(87)90155-X |bibcode=1987AcAau..15..551V }}</ref><ref>{{cite web |title= Positron Propelled and Powered Space Transport Vehicle for Planetary Missions |url= http://www.niac.usra.edu/files/library/meetings/fellows/mar06/1147Smith.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.niac.usra.edu/files/library/meetings/fellows/mar06/1147Smith.pdf |archive-date=2022-10-09 |url-status=live |last= Smith |first= Gerald |author2= Metzger, John|author3= Meyer, Kirby|author4= Thode, Les |date=2006-03-07 |access-date=2010-04-21}}</ref> These methods resemble those proposed for [[nuclear thermal rocket]]s. One proposed method is to use positron annihilation gamma rays to heat a solid engine core. [[Hydrogen]] gas is ducted through this core, heated, and expelled from a [[nozzle|rocket nozzle]]. A second proposed engine type uses positron annihilation within a solid [[lead]] pellet or within compressed [[xenon]] gas to produce a cloud of hot gas, which heats a surrounding layer of gaseous hydrogen. Direct heating of the hydrogen by gamma rays was considered impractical, due to the difficulty of compressing enough of it within an engine of reasonable size to absorb the gamma rays. A third proposed engine type uses annihilation gamma rays to heat an ablative sail, with the ablated material providing thrust. As with nuclear thermal rockets, the [[specific impulse]] achievable by these methods is limited by materials considerations, typically being in the range of 1000–2000 seconds.<ref name=Vulpetti1989>{{cite journal |last1=Vulpetti |first1=Giovanni |last2=Pecchioli |first2=Mauro |title=Considerations about the specific impulse of an antimatter-based thermal engine |journal=Journal of Propulsion and Power |date=September 1989 |volume=5 |issue=5 |pages=591–595 |doi=10.2514/3.23194 }}</ref>

The '''gaseous core''' system substitutes the low-melting point solid with a high temperature gas (i.e. tungsten gas/plasma), thus permitting higher operational temperatures and performance (<math>I_{\text{sp}}</math> ~ 2 × 10<sup>3</sup> sec). However, the longer mean free path for thermalization and absorption results in much lower energy conversion efficiencies (<math>\eta_e</math> ~ 35%).<ref name=ADA446638/>

The '''plasma core''' allows the gas to ionize and operate at even higher effective temperatures. Heat loss is suppressed by magnetic confinement in the reaction chamber and nozzle. Although performance is extremely high (<math>I_{\text{sp}}</math> ~ 10<sup>4</sup>-10<sup>5</sup> sec), the long mean free path results in very low energy utilization (<math>\eta_e</math> ~ 10%)<ref name=ADA446638/>

===Antimatter power generation===

The idea of using antimatter to power an [[Electrically powered spacecraft propulsion|electric space drive]] has also been proposed. These proposed designs are typically similar to those suggested for [[nuclear electric rocket]]s. Antimatter annihilations are used to directly or indirectly heat a working fluid, as in a [[nuclear thermal rocket]], but the fluid is used to generate electricity, which is then used to power some form of electric space propulsion system. The resulting system shares many of the characteristics of other charged particle/electric propulsion proposals, that typically being high specific impulse and low thrust (see also [http://large.stanford.edu/courses/2017/ph240/payzer1/ antimatter power generation]).<ref name=Seitzman>[http://soliton.ae.gatech.edu/people/jseitzma/classes/ae4451/electricpropulsion2.pdf ''Electric Rocket Propulsion: A Background''] {{Webarchive|url=https://web.archive.org/web/20130805125119/http://soliton.ae.gatech.edu/people/jseitzma/classes/ae4451/electricpropulsion2.pdf |date=2013-08-05 }} Jerry M. Seitzman, 2003-2004</ref><ref name=US20140026535A1>[https://patents.google.com/patent/US20140026535 ''High Specific Impulse Superfluid and Nanotube Propulsion Device, System and Propulsion Method''] Michael Wallace, Joseph D. Nix, Christopher W. Smith, 2014</ref>

===Catalyzed fission/fusion or spiked fusion===

This is a hybrid approach in which antiprotons are used to [[Antimatter-catalyzed nuclear pulse propulsion|catalyze a fission/fusion reaction]] or to "spike" the propulsion of a [[fusion rocket]] or any similar applications.

The antiproton-driven [[Inertial confinement fusion]] (ICF) Rocket concept uses pellets for the [[Fusion power|D-T reaction]].  The pellet consists of a hemisphere of fissionable material such as [[Uranium|U<sup>235</sup>]] with a hole through which a pulse of antiprotons and positrons is injected. It is surrounded by a hemisphere of fusion fuel, for example deuterium-tritium, or lithium deuteride. Antiproton annihilation occurs at the surface of the hemisphere, which ionizes the fuel. These ions heat the core of the pellet to fusion temperatures.<ref name=NIAC98-02FR>{{cite report |first1=Terry |last1=Kammash |year=1998 |title=Antiproton Driven Magnetically Insulated Inertial Confinement Fusion (Micf) Propulsion System |url=http://www.niac.usra.edu/files/studies/final_report/361Kammash.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.niac.usra.edu/files/studies/final_report/361Kammash.pdf |archive-date=2022-10-09 |url-status=live |citeseerx=10.1.1.498.1830 }}</ref>

The antiproton-driven Magnetically Insulated Inertial Confinement Fusion Propulsion (MICF) concept relies on self-generated magnetic field which insulates the plasma from the metallic shell that contains it during the burn. The lifetime of the plasma was estimated to be two orders of magnitude greater than implosion inertial fusion, which corresponds to a longer burn time, and hence, greater gain.<ref name=NIAC98-02FR/>

The antimatter-driven [[aneutronic fusion|P-B<sup>11</sup>]] concept uses antiprotons to ignite the P-B<sup>11</sup> reactions in an MICF scheme. Excessive radiation losses are a major obstacle to ignition and require modifying the particle density, and plasma temperature to increase the gain. It was concluded that it is entirely feasible that this system could achieve I<sub>sp</sub>~10<sup>5</sup>s.<ref name=NASA7347634540>{{cite journal |last1=Kammash |first1=Terry |last2=Martin |first2=James |last3=Godfroy |first3=Thomas |title=Antimatter Driven P-B11 Fusion Propulsion System |journal=AIP Conference Proceedings |date=17 January 2003 |volume=654 |issue=1 |pages=497–501 |doi=10.1063/1.1541331 |bibcode=2003AIPC..654..497K |hdl=2027.42/87345 |hdl-access=free }}</ref>

A different approach was envisioned for [[AIMStar]] in which small fusion fuel droplets would be injected into a cloud of antiprotons confined in a very small volume within a reaction [[Penning trap]]. Annihilation takes place on the surface of the antiproton cloud, peeling back 0.5% of the cloud. The power density released is roughly comparable to a 1 kJ, 1 ns laser depositing its energy over a 200&nbsp;μm ICF target.<ref name=AIAA-99-2700>{{cite journal |last1=Lewis |first1=Raymond |last2=Meyer |first2=Kirby |last3=Smith |first3=Gerald |last4=Howe |first4=Steven |title=AIMStar - Antimatter Initiated Microfusion for pre-cursor interstellar missions |journal=35th Joint Propulsion Conference and Exhibit |year=1999 |doi=10.2514/6.1999-2700 |citeseerx=10.1.1.577.1826 }}</ref>

The [[ICAN-II]] project employs the antiproton catalyzed microfission (ACMF) concept which uses pellets with a molar ratio of 9:1 of D-T:U<sup>235</sup> for [[nuclear pulse propulsion]].<ref name=AIAA-1998-3589>[https://www.engr.psu.edu/antimatter/Papers/ICAN.pdf  "Antiproton-Catalyzed Microfission/Fusion Propulsion Systems for Exploration of the Outer Solar System and Beyond"] {{webarchive |url=https://web.archive.org/web/20140805152150/https://www.engr.psu.edu/antimatter/Papers/ICAN.pdf |date=August 5, 2014 }} G. Gaidos, R.A. Lewis, G.A. Smith, B. Dundore and S. Chakrabarti, AIAA Paper 1998-3589, July 1998</ref>

==Difficulties with antimatter rockets==
The chief practical difficulties with antimatter rockets are the problems of creating antimatter and storing it. Creating antimatter requires input of vast amounts of energy, at least equivalent to the rest energy of the created particle/antiparticle pairs, and typically (for antiproton production) tens of thousands to millions of times more.<ref>{{cite web|title= Laser Pulse Produces Positrons |url= http://www.photonics.com/Article.aspx?AID=35682 |date=2008-11-18|publisher= Photonics Media |access-date=2008-11-18<!--DASHBot-->}}</ref><ref>{{cite journal|doi=10.1103/PhysRevLett.102.105001|pmid=19392120|title=Relativistic Positron Creation Using Ultraintense Short Pulse Lasers|date=2009|author=Chen, Hui|journal=Physical Review Letters|volume=102|pages=105001–105004|last2=Wilks |first2=Scott C. |last3=Bonlie |first3=James D. |last4=Liang |first4=Edison P. |last5=Myatt |first5=Jason Myatt
|last6=Price |first6=Dwight F.  |last7=D. Meyerhofer |first7=David D. |last8=Beiersdorfer |first8=Peter  |issue=10|bibcode=2009PhRvL.102j5001C|url=https://zenodo.org/record/1233811}}</ref> Most storage schemes proposed for interstellar craft require the production of frozen pellets of antihydrogen. This requires cooling of antiprotons, binding to positrons, and capture of the resulting antihydrogen atoms - tasks which have, {{As of|2010|lc=on}}, been performed only for small numbers of individual atoms. Storage of antimatter is typically done by trapping electrically charged frozen antihydrogen pellets in [[Penning trap|Penning]] or [[Quadrupole ion trap|Paul trap]]s. There is no theoretical barrier to these tasks being performed on the scale required to fuel an antimatter rocket. However, they are expected to be extremely (and perhaps prohibitively) expensive due to current production abilities being only able to produce small numbers of atoms, a scale approximately 10<sup>23</sup> times smaller than needed for a 10-gram trip to Mars.

{{Anchor|effecient2016-01-30}}Generally, the energy from antiproton annihilation is deposited over such a large region that it cannot efficiently drive nuclear capsules.  Antiproton-induced fission and self-generated magnetic fields may greatly enhance energy localization and efficient use of annihilation energy.<ref>{{cite journal|last=Solem|first=J. C.|year=1991|title=Prospects for Efficient Use of Annihilation Energy|journal=Fusion Technology|volume=20|issue=4P2 |pages=1040–1045|doi=10.13182/FST91-A11946978 |bibcode=1991FuTec..20.1040S |osti=6628569}}</ref><ref>{{cite journal|last1=Augenstein |first1=B. W.|last2=Solem|first2=J. C.|year=1990|title=Antiproton initiated fusion for spacecraft propulsion |journal=Report ND-3555-SDI (The RAND Corporation, Santa Monica, CA)}}</ref>

A secondary problem is the extraction of useful energy or momentum from the products of antimatter annihilation, which are primarily in the form of extremely energetic [[ionizing radiation]]. The antimatter mechanisms proposed to date have for the most part provided plausible mechanisms for harnessing energy from these annihilation products. The classic [[Tsiolkovsky rocket equation|rocket equation]] with its "wet" mass (<math>M_0</math>)(with [[propellant mass fraction]]) to "dry" mass (<math>M_1</math>)(with [[Payload fraction|payload]]) [[Mass ratio|fraction]] (<math>\frac {M_0}{M_1}</math>), the velocity change (<math>\Delta v </math>) and specific impulse (<math>I_{\text{sp}}</math>) no longer holds due to the mass losses occurring in antimatter annihilation.<ref name=AIAA-2003-4696/>

Another general problem with high powered propulsion is excess heat or [[waste heat]], and as with antimatter-matter annihilation also includes extreme radiation. A proton-antiproton annihilation propulsion system transforms 39% of the propellant mass into an intense high-energy flux of gamma radiation. The gamma rays and the high-energy charged pions will cause heating and radiation damage if they are not shielded against. Unlike neutrons, they will not cause the exposed material to become radioactive by transmutation of the nuclei. The components needing shielding are the crew, the electronics, the cryogenic tankage, and the magnetic coils for magnetically assisted rockets. Two types of shielding are needed: [[radiation protection]] and [[cold shield|thermal protection]] (different from [[Heat shield]] or [[thermal insulation]]).<ref name=AIAA-2003-4696/><ref name=Forward1985>[http://arc.aiaa.org/doi/abs/10.2514/3.22811 ''Antiproton Annihilation Propulsion''] R. L. Forward, September 1985</ref>

Finally, relativistic considerations have to be taken into account. As the by products of annihilation move at [[Special relativity|relativistic velocities]] the [[Mass in special relativity|rest mass changes]] according to [[mass–energy equivalence|relativistic mass–energy]]. For example, the total mass–energy content of the neutral pion is converted into gammas, not just its rest mass.  It is necessary to use a [[Relativistic rocket#Relativistic rocket equation|relativistic rocket equation]] that takes into account the relativistic effects of both the vehicle and [[Working mass|propellant exhaust]] (charged pions) moving near the speed of light. These two modifications to the two rocket equations result in a mass ratio (<math>\frac {M_0}{M_1}</math>) for a given (<math>\Delta v </math>) and (<math>I_{\text{sp}}</math>) that is much higher for a relativistic antimatter rocket than for either a classical or relativistic "conventional" rocket.<ref name=AIAA-2003-4696/>

===Modified relativistic rocket equation===
The loss of mass specific to antimatter annihilation requires a modification of the relativistic rocket equation given as<ref name=AIAA-98-3403>[http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/20238/1/98-1132.pdf ''Evaluation of Propulsion Options for Interstellar Missions''] {{webarchive|url=https://web.archive.org/web/20140508061651/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/20238/1/98-1132.pdf |date=2014-05-08 }} Robert H. Frisbee, Stephanie D. Leifer, AIAA Paper 98-3403, July 13–15, 1998.</ref>

{{NumBlk|:|<math>\frac {M_0}{M_1} = 
\left(\frac{1+ \frac{\Delta v}{c}}{1- \frac{\Delta v}{c}}\right)^{\frac{c}{2 I_{\text{sp}}}}
</math>|{{EquationRef|I}}}}

where <math>c</math> is the speed of light, and <math>I_{\text{sp}}</math> is the specific impulse (i.e. <math>I_{\text{sp}}</math>=0.69<math>c</math>).

The derivative form of the equation is<ref name=AIAA-2003-4696/>

{{NumBlk|:|<math>\frac {dM_{\text{ship}}}{M_{\text{ship}}}=
\frac {-dv ( 1 - I_{\text{sp}} \frac {v}{c^2})}
{(1 - \frac {v^2}{c^2})(-\frac {I_{\text{sp}} v^2}{c^2} +  (1 - a) v + a I_{\text{sp}})}
</math>|{{EquationRef|II}}}}

where <math>M_{\text{ship}}</math> is the non-relativistic (rest) mass of the rocket ship, and <math>a</math> is the fraction of the original (on board) propellant mass (non-relativistic) remaining after annihilation (i.e., <math>a</math>=0.22 for the charged pions).

{{EquationNote|Eq.II}} is difficult to integrate analytically. If it is assumed that <math>v \sim I_{\text{sp}}</math>, such that <math>(1 - \frac {I_{\text{sp}} v}{c^2}) \sim (1 - \frac {v^2}{c^2})</math> then the resulting equation is

{{NumBlk|:|<math>\frac {dM_{\text{ship}}}{M_{\text{ship}}}=
\frac {-dv}{(-\frac {I_{\text{sp}} v^2}{c^2} + (1 - a) v + a I_{\text{sp}})}
</math>|{{EquationRef|III}}}}

{{EquationNote|Eq.III}} can be integrated and the integral evaluated for <math>M_0</math> and <math>M_1</math>, and initial and final velocities (<math>v_i = 0</math> and <math>v_f = \Delta v</math>).
The resulting relativistic rocket equation with loss of propellant is<ref name=AIAA-2003-4696/><ref name=AIAA-98-3403/>

{{NumBlk|:|<math>\frac{M_0}{M_1}=\left(\frac{(-2I_{\text{sp}}\Delta v/c^2+1-a-\sqrt{(1-a)^2+4aI_{\text{sp}}^2/c^2})(1-a+\sqrt{(1-a)^2+4aI_{\text{sp}}^2/c^2})}{(-2I_{\text{sp}}\Delta v/c^2+1-a+\sqrt{(1-a)^2+4aI_{\text{sp}}^2/c^2})(1-a-\sqrt{(1-a)^2+4aI_{\text{sp}}^2/c^2})}\right)^{\frac{1}{\sqrt{(1-a)^2+4aI_{\text{sp}}^2/c^2}}}
</math>|{{EquationRef|IV}}}}

===Other general issues===

The cosmic background [[hard radiation]] will ionize the rocket's hull over time and poses a [[Health threat from cosmic rays|health threat]]. Also, gas plasma interactions may cause [[space charge]]. The major interaction of concern is differential charging of various parts of a spacecraft, leading to high electric fields and arcing between spacecraft components. This can be resolved with well placed [[plasma contactor]]. However, there is no solution yet for when plasma contactors are turned off to allow maintenance work on the hull. Long term space flight at interstellar velocities causes erosion of the rocket's hull due to collision with particles, [[Interstellar medium|gas]], [[Cosmic dust|dust]] and [[micrometeorite]]s. At 0.2<math>c</math> for a 6 light year distance, erosion is estimated to be in the order of about 30&nbsp;kg/m<sup>2</sup> or about 1&nbsp;cm of aluminum shielding.<ref name=NASA20110406>[https://web.archive.org/web/20110621012238/http://science1.nasa.gov/science-news/science-at-nasa/2001/ast13nov_1sidebar/ ''Space Charge''] NASA science news, April 6, 2011</ref><ref name=HGarrett>[http://www.kiss.caltech.edu/workshops/systems2012/presentations/garrett.pdf ''There and Back Again: A Layman's Guide to Ultra-Reliability for Interstellar Missions''] {{webarchive|url=https://web.archive.org/web/20140508062130/http://www.kiss.caltech.edu/workshops/systems2012/presentations/garrett.pdf |date=2014-05-08 }} Henry Garrett, 30 July 2012</ref>

==See also==
*[[Nuclear photonic rocket]]

==References==
{{reflist|30em}}

[[Category:Antimatter]]
[[Category:Rocket propulsion]]