For small-bore puncture events, the Project should conduct research into making the skin self-sealing, with the disturbed soil-elements themselves helping to roughly plug the holes initially, and skin shrinkage from deflation helping to seal the holes more effectively later on.  With small-enough holes, slow-enough deflation, and relatively manageable internal chaos caused by the puncture, [[telebots]] and other internal robotics could be brought into emergency sealing operations.[^Although this may rely on uninterrupted communications, which is dependable after an impact.^] For larger holes, some automatic mechanism for rapidly sealing off affected sections of the exovivarium can serve to reduce total debris expulsion caused by outgassing, while helping to retain the value of the exovivarium.

* '''On-orbit propulsion''' - some de-orbiting technologies have orbit-maintenance propulsion value as well, particularly the [[electrodynamic tether]].[^"Terminator Tether(TM): A Spacecraft Deorbit Device", Robert L. Forward, Robert P. Hoyt, Chauncey W. Uphoff, ''Journal of Spacecraft and Rockets'', v. 37, No. 2, March-April 2000 http://dyna15.narod.ru/kts/lit/forward_hoyt_uphoff2000.pdf^] In particular (see below), such tethers might enable exovivaria to fly below most of the debris threat.

* '''Hypervelocity Earth-to-orbit propulsion''' - debris-impact studies require hypervelocity launchers like [[light gas guns]] and [[ram accelerators]]. Taken to larger scales, these tie in very closely with Earth-to-orbit [[projectile space launch]] research, and thus might have long-term value in reducing the cost of launching exovivarium materials and components.[^See e.g. "Ram Accelerators: Outstanding Issues and New Directions", A.J. Higgins, ''Journal of Propulsion and Power'', Vol. 22, No. 6, November-December 2006 https://people.mcgill.ca/files/andrew.higgins/AIAA-18209-632.pdf^]

* '''On-orbit lifecycle management''' - there is at least one commercial effort on a "space tug"[^See e.g., "Orbital Satellite Services AB", Business Week: Aerospace and Defense, http://investing.businessweek.com/research/stocks/private/snapshot.asp?privcapId=9872057^] and perhaps others coming.[^"Rocket Company Launches Stock Offering", Adam Williams, Tico Times, Sep 30, 2010 http://www.ticotimes.net/Business-Real-Estate/Rocket-Company-Launches-Stock-Offering_Friday-October-01-2010^] Exovivaria management might benefit by projects to move satellites at end-of-life to [[http://en.wikipedia.org/wiki/Graveyard_orbit | graveyard orbits]], or to orbital "salvage yards", instead of de-orbiting them. If exovivaria must have an end-of-life, perhaps they can be mostly recycled into new exovivaria.

* '''High surface-to-mass ratio''' - depending on the design, exovivaria might be mostly hollow. If so, they'll have more surface area exposed to debris impact than a typical satellite of the same mass. Natural micrometeorites and orbital debris ''will'' strike exovivaria during their lifetimes. A poor choice of surface or hull materials could make them greater sources of more dangerous debris than most objects of the same mass in LEO.  It is incumbent upon the Project to study how to reduce the threat.

* '''Soil''' - although an orbital aquarium has its attractions, water is heavy, therefore expensive to launch, just to provide a volume medium for living things.  Exovivaria ecosystems might be relatively [[xeric]] (dryland ecosystems) for this reason alone.  A puncture event would release dirt, and even if dirt clods were moist, any moisture in them would rapidly boil upon being exposed to vacuum, only tending to disperse the dirt particles even more. The problem might be serious enough that only soil-free [[http://en.wikipedia.org/wiki/Aeroponics#NASA_inflatable_aeroponics | aeroponics]] (which has been developed for [[CELSS]]) should be considered for botanical ecosystem components, and the animal life limited to creatures that don't need soil to burrow into.

* '''Pressurized inflatable''' - a single-puncture event (one in which initial impact byproducts are contained within the exovivarium, without causing subsidiary or pass-through punctures) would absorb a significant piece of debris. This could be considered a debris-mitigating effect, if anything. However, it could also cause expulsion of some of the contents of the exovivarium (especially soil) if air continues to escape through the puncture. With turbulence stirred up both by the puncture and the resulting leak, and with dry soil stirred up by the internal impacts (including ricochets), a new category of "rapid soil erosion" might be in order. Multi-puncture impact events would be even worse.

For non-puncture impact events, the ideal surface for an exovivarium would probably have a thin coating that resists [[http://en.wikipedia.org/wiki/Photodegradation | photodegradation]]. However, if chipped, scored or cratered by a debris strike or an accident, it would be better if the debris generated mostly photodegrades (except for a negligibly-thin photoresist layer on some particles). This surface might be maintainable [[telebots | telebotically]], so that scoring and cratering caused by strikes can be "patched" or filled in and resurfaced before photodegradation and other erosion processes eat much further into the skin.

Where there is relatively superficial debris-strike damage, ways might be devised to safely drill through the skin from the interior out to a stricken area, without loss of air pressure. Workers might then telebotically perform some effective "patch" repairs from the inside rather than using equipment outside in vacuum, without generating more debris. Failing that, some sort of telebot "[[EVA]]" might be possible. Even with a rotating exovivarium, the repair telebot might be able hover in the same relative location, close to the moving surface, and whenever the damage site rotated around to it, it could briefly spray patch materials (photodegradable filler, followed by [[http://en.wikipedia.org/wiki/Photoresist | photoresist]] coating) at the point of damage.[^Not as easy as it might sound, since it involves [[http://en.wikipedia.org/wiki/Satellite_formation_flying | satellite formation flying]] and each spray will cause an equal and opposite reaction.^] Similar EVA-style telebotic-sprayer maintenance might be necessary even if there were no need to repair the skin after debris strikes -- other erosion effects (e.g., [[http://en.wikipedia.org/wiki/Atomic_oxygen#Atomic_oxygen | atomic oxygen]] and [[http://en.wikipedia.org/wiki/Space_radiation | space radiation]]) will further contribute to its slow decay.

For small-bore puncture events, the Project should conduct research into making the skin self-sealing, with the disturbed soil-elements themselves helping to roughly plug the holes initially, and skin shrinkage from deflation helping to seal the holes more effectively later on.  With small-enough holes, slow-enough deflation, and relatively manageable internal chaos caused by the puncture, [[telebots]] and other internal robotics might be brought into emergency sealing operations.[^Alhough this might rely on uninterrupted communications, not something dependable after an impact.^] For larger holes, some automatic mechanism for rapidly sealing off affected sections of the exovivarium might serve to reduce total debris expulsion by outgassing, while helping to retain the value of the exovivarium.

All other things being equal, a lighter soil particle is a less dangerous piece of debris. Similarly for smaller particles and softer ones. The less dense particles will de-orbit themselves sooner.  Research into light soil minerals such as [[http://en.wikipedia.org/wiki/Zeolite#Agriculture | zeolite]], and horticultural uses of very fine dust made from them, might turn up substances in which plants can grow reasonably well, and which also degrade rapidly in low orbits. Plastic beads have been used in rooftop gardening to reduce roof loads; they might make excellent coarser-grain soil components.

Finally, although it depends on development projects that Project Persephone is not likely to ever be able to fund, electrodynamic tethers could help keep exovivaria from representing much of a debris threat.  These tethers should make it possible to sustain orbit long-term, without propellant, at significantly lower altitudes than hitherto possible.  There is far less debris at altitudes where atmospheric drag becomes significant, and any debris generated by a strike would be more likely to de-orbit soon simply because it originated at a higher-drag orbit.  Tethers admittedly have their own debris-generation issues. However, the fruits of Project effort to develop safer exovivarium cladding materials and techniques for maintaining cladding might turn out to be useful in extending the life of such tethers as well. If there's a major problem with any such technical synergy, it's that otherwise-optimal exovivaria are likely to be rotating toroids facing in the direction of the sun continuously, and the most familiar tether design is radial with respect to the Earth's center.  However, there are rotating ED tether designs, and an exovivarium rotating around the same axis (albeit at a different rate) might be reasonably compatible with the ED tether concept, in some design trade-off.

These debris-mitigating measures are likely to be unique to Project Persephone for the foreseeable future, and most would probably require at least some specialist knowledge and equipment. However, some of the groundwork might require only modest funding. They should therefore be considered eventual Project targets for fund-raising efforts.

Orbital debris can include large objects such as dead satellites and spent upper stagers, but consists mostly of small fragments from satellite breakups and collisions, and fragments (especially paint flakes) from the upper stages of launchers.  [[http://en.wikipedia.org/wiki/Anti-satellite_weapon | ASAT]] testing and accidental collisions have contributed to the problem, while also dramatizing it. If current trends continue, the [[http://en.wikipedia.org/wiki/Kessler_syndrome | Kessler syndrome]] might set in - a slow chain reaction that destroys or disables much of what's operating in [[LEO]].[^See e.g., "Space junk: Hunting zombies in outer space", New Scientist, 15 September 2010  http://www.newscientist.com/article/mg20727772.300-space-junk-hunting-zombies-in-outer-space.html^]

Orbital debris has become such a concern that there is discussion of a treaty requiring that new satellites launched into lower orbits be equipped to de-orbit themselves when they reach end-of-life.[^See e.g., "An International Environmental Agreement for space debris mitigation among asymmetric nations", ''Acta Astronautica'', 68 (2011), pp.326-337 http://ipac.kacst.edu.sa/eDoc/2011/191431_1.pdf doi:10.1016/j.actaastro.2010.08.019 ^]

Orbital debris consists mostly of small fragments from satellite breakups and collisions, and fragments (especially paint flakes) from the upper stages of launchers.  [[http://en.wikipedia.org/wiki/Anti-satellite_weapon | ASAT]] testing and accidental collisions have contributed to the problem, while also dramatizing it. If current trends continue, the [[http://en.wikipedia.org/wiki/Kessler_syndrome | Kessler syndrome]] might set in - a slow chain reaction that destroys or disables much of what's operating in [[LEO]].[^See e.g., "Space junk: Hunting zombies in outer space", New Scientist, 15 September 2010  http://www.newscientist.com/article/mg20727772.300-space-junk-hunting-zombies-in-outer-space.html^]

Orbital debris has become such a concern that there is discussion of a treaty requiring that new satellites launched into lower orbits be equipped to de-orbit themselves when they reach end-of-life.[^See e.g., "An International Environmental Agreement for space debris mitigation among asymmetric nations", ''Acta Astronautica'', 68 (2011), pp.326–337 http://ipac.kacst.edu.sa/eDoc/2011/191431_1.pdf doi:10.1016/j.actaastro.2010.08.019 ^]

* '''On-orbit propulsion''' - some de-orbiting technologies have orbit-maintenance propulsion value as well, particularly the [[electrodynamic tether]].[^"Terminator Tether(TM): A Spacecraft Deorbit Device", Robert L. Forward, Robert P. Hoyt, Chauncey W. Uphoff, ''Journal of Spacecraft and Rockets'', v. 37, No. 2, March–April 2000 http://dyna15.narod.ru/kts/lit/forward_hoyt_uphoff2000.pdf^] In particular (see below), such tethers might enable exovivaria to fly below most of the debris threat.

* '''Hypervelocity Earth-to-orbit propulsion''' - debris-impact studies require hypervelocity launchers like [[light gas guns]] and [[ram accelerators]]. Taken to larger scales, these tie in very closely with Earth-to-orbit [[projectile space launch]] research, and thus might have long-term value in reducing the cost of launching exovivarium materials and components.[^See e.g. "Ram Accelerators: Outstanding Issues and New Directions", A.J. Higgins, ''Journal of Propulsion and Power'', Vol. 22, No. 6, November–December 2006 https://people.mcgill.ca/files/andrew.higgins/AIAA-18209-632.pdf^]

* '''Pressurized inflatable''' - a single-puncture event (one in which initial impact byproducts are contained within the exovivarium, without causing subsidiary or pass-through punctures) would absorb a significant piece of debris. This could be considered a debris-mitigating effect, if anything. However, it could also cause expulsion of some of the contents of the exovivarium (especially soil), if air under pressure continues to escape through the puncture. With turbulent air stirred up inside both by the puncture and the resulting leak, and with dry soil stirred up by the internal impacts (including ricochets), a new category of "rapid soil erosion" might need to be defined. Multi-puncture impact events would be even worse.

For non-puncture impact events, the ideal surface for an exovivarium would probably have a thin coating that resists [[http://en.wikipedia.org/wiki/Photodegradation | photodegradation]]. However, if chipped, scored or cratered by a debris strike or an accident, it would be better if the debris generated mostly photodegrades (except for a negligibly-thin photoresist layer on some particles).  This surface might be maintainable [[telebots | telebotically]], so that scoring and cratering caused by strikes can be "patched" or filled in and resurfaced before photodegradation and other erosion processes eat much further into the skin.

Where there is relatively superficial debris-strike damage, ways might be devised to safely drill through the skin from the interior out to a stricken area, without loss of air pressure. Workers might then telebotically perform some effective "patch" repairs from the inside, rather than using equipment outside in vacuum, and without generating more debris. Failing that, some sort of telebot "[[EVA]]" might be possible. Even with a rotating exovivarium, the repair telebot might be able hover in the same relative location, close to the moving surface, and whenever the damage site rotated around to it, it could briefly spray patch materials (photodegradable filler, followed by [[http://en.wikipedia.org/wiki/Photoresist | photoresist]] coating) at the point of damage.[^Not as easy as it might sound, since it involves [[http://en.wikipedia.org/wiki/Satellite_formation_flying | satellite formation flying]] and each spray will cause an equal and opposite reaction.^] Similar EVA-style telebotic-sprayer maintenance might be necessary even if there were no need to repair the skin after debris strikes -- other erosion effects (e.g., [[http://en.wikipedia.org/wiki/Atomic_oxygen#Atomic_oxygen | atomic oxygen]] and [[http://en.wikipedia.org/wiki/Space_radiation | space radiation]]) will further contribute to its slow decay.

Finally, although it depends on development projects that Project Persephone is not likely to ever be able to fund, electrodynamic tethers could help keep exovivaria from representing much of a debris threat.  These tethers should make it possible to sustain orbit long-term, without propellant, at significantly lower altitudes than hitherto possible.  There is far less debris at altitudes where atmospheric drag becomes significant, and any debris generated by a strike would be more likely to de-orbit soon simply because it originated at a higher-drag orbit.  Tethers admittedly have their own debris-generation issues.  However, the fruits of Project effort to develop safer exovivarium cladding materials and techniques for maintaining them might turn out to be useful in extending the life of such tethers as well.  If there's a major problem with any such technical synergy, it's that otherwise-optimal exovivaria are likely to be rotating toroids facing in the direction of the sun continuously, and the most familiar tether design is radial with respect to the Earth's center.  However, there are rotating ED tether designs, and an exovivarium rotating around the same axis (albeit at a different rate) might be reasonably compatible with the ED tether concept, in some trade-off.

* '''Inflatable structures''' - the problem of cheap de-orbiting of satellites (and the large debris) is promoting research on [[inflatable space structures]]: the larger objects in the lower orbits could be literally dragged down by inflating a large, light "parachute" to increase upper-atmospheric drag.[^See e.g., "Development of a generic inflatable de-orbit device for CubeSats", D.C. Maessen, 30 May 2007, Delft University of Technology  http://lr.tudelft.nl/fileadmin/Faculteit/LR/Organisatie/Afdelingen_en_Leerstoelen/Afdeling_SpE/Space_Systems_Eng./Expertise_areas/Space_propulsion/Research/Thermal_thrusters/doc/Presentation.pdf^]

* "Meteoroids and Orbital Debris: Effects on Spacecraft", C.A. Belk, J.H. Robinson, M.B. Alexander, W.J. Cooke, and S.D. Pavelitz, NASA Reference Publication 1408, August 1997 http://www.awhir.com/design/nasa/Meteoroids%20and%20Orbital%20Debris--%20Effects%20on%20Spacecraft%20rp1408.pdf

All other things being equal, a lighter soil particle is a less dangerous piece of debris. Similarly for smaller particles and softer ones. They will also de-orbit themselves sooner than heavier/bigger particles.  Research into light soil minerals such as [[http://en.wikipedia.org/wiki/Zeolite#Agriculture | zeolite]], and horticultural uses of very fine dust made from them, might turn up substances in which plants can grow reasonably well, and which also degrade rapidly in low orbits. Plastic beads have been used in rooftop gardening to reduce roof loads; they might be excellent coarser-grain soil components.

* '''On-orbit lifecycle management''' - there is at least one commercial effort on a "space tug".[^See e.g., "Orbital Satellite Services AB", Business Week: Aerospace and Defense, http://investing.businessweek.com/research/stocks/private/snapshot.asp?privcapId=9872057^] Exovivaria management might benefit by projects to move satellites at end-of-life to [[http://en.wikipedia.org/wiki/Graveyard_orbit | graveyard orbits]], or to orbital "salvage yards", instead of de-orbiting them. If exovivaria must have an end-of-life, perhaps they can be mostly recycled into new exovivaria.

For non-puncture impact events, the ideal surface for an exovivarium would probably have a thin coating that resists [[http://en.wikipedia.org/wiki/Photodegradation | photodegradation]]. However, if chipped, scored or cratered by a debris strike or an accident, it would be better if the debris generated mostly photodegrades (except for its very thin photoresist layer).  This surface might be maintainable [[telebots | telebotically]], so that scoring and cratering caused by strikes can be "patched" or filled in and resurfaced before photodegradation and other erosion processes eat much further into the skin.

Where there is relatively superficial debris-strike damage, ways might be devised to safely drill through the skin from the interior out to a stricken area, without loss of air pressure. Workers might then telebotically perform some effective "patch" repairs from the inside, rather than using equipment outside in vacuum, and without generating more debris. Failing that, some sort of telebot "[[EVA]]" might be possible. Even with a rotating exovivarium, the repair telebot might be able hover in the same relative location, close to the moving surface, and whenever the damage site rotated back around to it, it could spray patch materials (photodegradable filler, followed by [[http://en.wikipedia.org/wiki/Photoresist | photoresist]] coating) at the point of damage. Similar EVA-style telebotic-sprayer maintenance might be necessary even if there were no need to repair the skin after debris strikes -- other erosion effects (e.g., [[http://en.wikipedia.org/wiki/Atomic_oxygen#Atomic_oxygen | atomic oxygen]] and [[http://en.wikipedia.org/wiki/Space_radiation | space radiation]]) will further contribute to its slow decay.

* '''Earth-to-orbit Propulsion''' - debris-impact studies require hypervelocity launchers like [[light gas guns]] and [[ram accelerators]]. Taken to larger scales, these tie in very closely with Earth-to-orbit [[projectile space launch]] research, and thus might have long-term value in reducing the cost of launching exovivarium materials and components.[^See e.g. "Ram Accelerators: Outstanding Issues and New Directions", A.J. Higgins, ''Journal of Propulsion and Power'', Vol. 22, No. 6, November–December 2006 https://people.mcgill.ca/files/andrew.higgins/AIAA-18209-632.pdf^]

* '''Orbital logistics''' - there is at least one commercial effort on a "space tug".[^See e.g., "Orbital Satellite Services AB", Business Week: Aerospace and Defense, http://investing.businessweek.com/research/stocks/private/snapshot.asp?privcapId=9872057^] Exovivaria management might benefit by projects to move satellites at end-of-life to [[http://en.wikipedia.org/wiki/Graveyard_orbit | graveyard orbits]], or to orbital "salvage yards", instead of de-orbiting them. If exovivaria must have an end-of-life, perhaps they can be mostly recycled into new exovivaria.

Orbital debris consists mostly of small fragments from satellite breakups and collisions, and fragments (especially paint-flakes) from the upper stages of launchers.  [[http://en.wikipedia.org/wiki/Anti-satellite_weapon | ASAT]] testing and accidental collisions have contributed to the problem, while also dramatizing it. If current trends continue, the [[http://en.wikipedia.org/wiki/Kessler_syndrome | Kessler syndrome]] might set in - a slow chain reaction that destroys or disables much of what's operating in [[LEO]].[^See e.g., "Space junk: Hunting zombies in outer space", New Scientist, 15 September 2010  http://www.newscientist.com/article/mg20727772.300-space-junk-hunting-zombies-in-outer-space.html^]

In accordance with [[meeting the SPEC]], [[Project Persephone]] aims to improve environmental conditions wherever it can -- or, at the very least, to avoid net degradation.  This imperative extends to the orbits in which [[exovivaria]] would be operated.  Global environmental management is dependent on [[http://en.wikipedia.org/wiki/Earth_observation_satellite | Earth observation satellites]] in [[LEO]], and the Project should naturally avoid endangering those resources by becoming a debris-generator.  Unfortunately, exovivaria have some features that make them problematic.

* '''Orbital logistics''' - there is at least one commercial effort on a "space tug".[^See e.g., "Orbital Satellite Services AB", Business Week: Aerospace and Defense, http://investing.businessweek.com/research/stocks/private/snapshot.asp?privcapId=9872057^] Exovivaria management might benefit by projects to move satellites at end-of-life to higher orbits, or to orbital "salvage yards", instead of de-orbiting them. If exovivaria must have an end-of-life, perhaps they can be recycled into new ones.

%rframe% http://upload.wikimedia.org/wikipedia/commons/1/16/Atex.jpg | Advanced Tether Experiment (ATEx)

Finally, although it depends on development projects that Project Persephone is not likely to ever be able to fund, electrodynamic tethers could help keep exovivaria from representing much of a debris threat.  These tethers should make it possible to sustain orbit long-term, without propellant, at significantly lower altitudes than hitherto possible.  There is far less debris at altitudes where atmospheric drag becomes significant, and any debris generated by a strike would be more likely to de-orbit soon simply because it originated at a higher-drag orbit.  Tethers admittedly have their own debris-generation issues.  However, the fruits of Project effort to develop safer cladding materials and techniques for maintaining them might turn out to be useful in extending the life of such tethers as well.  If there's a major problem with any such technical synergy, it's that otherwise-optimal exovivaria are likely to be rotating toroids facing in the direction of the sun continuously, and the most familiar tether design is radial with respect to the Earth's center.  However, there are rotating ED tether designs, and an exovivarium rotating around the same axis (albeit at a different rate) might be reasonably compatible the ED tether concept, in some trade-off.

These debris-mitigating measures are likely to be unique to Project Persephone for the foreseeable future, and most would probably require at least some specialist knowledge and equipment.  However, some of the groundwork might require only modest funding.  They should therefore be considered eventual Project targets for fund-raising efforts.

Where there is relatively superficial debris-strike damage, ways might be devised to safely drill through the skin from the interior "[[down]]" to a stricken area, without loss of air pressure. Workers might then telebotically perform some effective "patch" repairs from the inside, without using equipment outside in vacuum, and without generating added debris. Failing that, some sort of telebot "[[EVA]]" might be possible. Even with a rotating exovivarium, the repair telebot might be able hover in the same relative location, close to the moving surface, and whenever the damage site rotated back around to it, it could spray patch materials (photodegradable filler, followed by [[http://en.wikipedia.org/wiki/Photoresist | photoresist]] coating) at the point of damage. Similar EVA-style telebotic maintenance might be necessary even if there were no need to repair the skin after debris strikes -- other erosion effects (e.g., [[http://en.wikipedia.org/wiki/Atomic_oxygen#Atomic_oxygen | atomic oxygen]] and [[http://en.wikipedia.org/wiki/Space_radiation | space radiation]]) will further contribute to its slow decay.

For small-bore puncture events, there should be research into making the skin self-sealing, with the disturbed soil itself helping to roughly plug the holes initially, and skin shrinkage from deflation helping to seal the holes more effectively later on.  With small-enough holes, slow-enough deflation, and relatively manageable internal chaos caused by the puncture, [[telebots]] and other internal robotics might be brought into emergency sealing operations.[^Alhough this might rely on uninterrupted communications, not something dependable after an impact.^] For larger holes, some automatic mechanism for rapidly sealing off affected sections of the exovivarium might serve to reduce total debris expulsion by outgassing, while helping to retain the value of the exovivarium.