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Orbital Debris

Shuttle window damage

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. ASAT testing and accidental collisions have contributed to the problem, while also dramatizing it. If current trends continue, the Kessler syndrome sets in, if it hasn't already in - a slow chain reaction that, in some narratives, destroys or disables much of what's operating in LEO.¹

Whatever the real risks, 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.²

Implications for Project Persephone

Exovivaria would be satellites too - biosatellites. They would therefore be both exposed to, and possible sources of, orbital debris. However, there is a silver lining even as the debris clouds darken, both for long-term prospects for space access and development, and for Project Persephone in particular. Researchers are studying de-orbiting techniques³ and the nature of the impacts.⁴ This growing emphasis will spur R&D in the following areas of potential benefit to the Project.

Tethered satellite

On-orbit propulsion - some de-orbiting technologies have orbit-maintenance propulsion value as well, particularly the electrodynamic tether ?.⁵ In particular (see below), such tethers should enable exovivaria to fly below most of the debris threat.
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.⁶

Hypervelocity impact

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 would have long-term value in reducing the cost of launching exovivarium materials and components.⁷
On-orbit lifecycle management - there is at least one commercial effort on a "space tug"⁸ and perhaps others coming.⁹ Exovivaria management is likely to benefit by projects to move satellites at end-of-life to 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.

Goals

In meeting the SPEC, the Project aims to improve natural 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 Earth observation satellites in LEO, and the Project should naturally avoid endangering those resources by becoming a net debris-generator. Unfortunately, exovivaria have some features that make them problematic.

NASA inflatable station prototype (1961)

High surface-to-mass ratio - depending on the design, exovivaria will 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 may 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 is likely to be serious enough that only soil-free 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" may 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 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 should be maintainable 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.

Airglow - atomic oxygen

Where there is relatively superficial debris-strike damage, ways should probably be devised to safely drill through the skin from the interior out to a stricken area, without loss of air pressure. Workers operating in VR telepresence could 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" may be possible. Even with a rotating exovivarium, the repair telebot should 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 photoresist coating) at the point of damage.¹⁰ Similar EVA-style telebotic-sprayer maintenance may be necessary even if there were no need to repair the skin after debris strikes -- other erosion effects (e.g., atomic oxygen and 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 could be brought into emergency sealing operations.¹¹ 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.

Polystyrene foam pellets before expansion

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 zeolite, and horticultural uses of very fine dust made from them, may 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.

Advanced Tether Experiment¹²

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 could 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) may turn out to 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 would require only modest funding. They should therefore be considered eventual Project targets for fund-raising efforts.

Notes

¹ 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 ⇑

² 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 ⇑

³ See e.g., "A Systems Study on How to Dispose of Fleets of Small Satellites", Jason M. Andringa and Daniel E. Hastings, SERC #2-01, Space Systems Laboratory, MIT, 2001. ⇑

⁴ See e.g., ""Hypervelocity Launcher for Laboratory Testing of Orbital Debris and Micrometeoroid Impact", A. Higgins, Mc Gill ? University. ⇑

⁵ "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 ⇑

⁶ 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 ⇑

⁷ 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 ⇑

⁸ See e.g., "Orbital Satellite Services AB", Business Week: Aerospace and Defense, http://investing.businessweek.com/research/stocks/private/snapshot.asp?privcapId=9872057 ⇑

⁹ "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 ⇑

¹⁰ Not as easy as it sounds, since it involves satellite formation flying and each spray will cause an equal and opposite reaction. ⇑

¹¹ Although this may rely on uninterrupted communications, which wouldn't exactly be dependable after an impact. ⇑

¹² "AT Ex ?", NASA, 8 Jul 2011 http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1998-055C ⇑

Project Persephone

Orbital Debris

Implications for Project Persephone

Goals

Notes

Further reading