The space environment

Space is not empty, and the ways in which it is not empty are what damage hardware. This page covers the non-ionising environment: vacuum, neutral species, plasma, particulates and heat. Ionising radiation is large enough to warrant its own page, Space radiation.

Vacuum

Three distinct failure mechanisms follow.

Outgassing

Volatiles trapped in polymers, adhesives and composites diffuse out in vacuum, then recondense on the coldest nearby surface. That surface is usually an optic, a radiator or a solar cell, and contamination there degrades exactly the performance the mission depends on.

Material screening is standardised under ASTM E595, which imposes two limits measured after 24 hours at 125 °C and $10^{-3}$ Pa:

TML

Total mass loss, must be at most 1.00%

CVCM

Collected volatile condensable material, must be at most 0.10%

Practical effect

Rules out most commercial adhesives, tapes, foams and unqualified potting compounds

Mitigation

Vacuum bakeout before integration, and venting paths that direct outgassed species away from sensitive surfaces

Cold welding

In air, every metal surface carries an oxide layer a few nanometres thick that prevents true metal-to-metal contact. In vacuum that layer is not replenished, and where it is abraded away by micromotion, clean metal meets clean metal and diffusion bonds them.

The consequence is that mechanisms which worked perfectly in atmosphere can seize in orbit. Deployment hinges, latches, bearings and separation devices are the usual victims. Mitigations are dissimilar metal pairs, hard coatings, solid lubricants such as MoS₂, and avoiding sustained contact loads on clean surfaces.

Liquid lubricants have their own problem: they evaporate and creep. Space-rated oils are low-vapour-pressure formulations, usually perfluoropolyether, and are retained by labyrinth seals or porous reservoirs.

Electrical breakdown

Breakdown voltage in a gas depends on the product of pressure and gap distance, and that curve has a minimum. For air the minimum is near 327 V at a pressure-distance product of about 0.76 Pa·m, which for centimetre-scale gaps corresponds to a pressure band running from roughly 10 Pa to a few kPa. A vehicle ascending through that band, or a spacecraft venting a tank, passes through conditions where a few hundred volts will arc across a gap that is entirely safe at sea level and entirely safe in hard vacuum.

High-voltage systems must therefore be either sealed and pressurised, fully potted, or kept unpowered until the ambient pressure is below the danger band. The related phenomenon in RF hardware is multipaction, a resonant secondary-electron avalanche in waveguides and filters that sets a hard power ceiling on transmit chains.

The residual atmosphere and atomic oxygen

Between about 180 and 650 km the dominant neutral species is atomic oxygen, produced by photodissociation of O₂ by solar ultraviolet. A spacecraft in low Earth orbit sweeps through it at 7.7 km/s, giving each impact a collision energy near 5 eV, which is enough to break most organic bonds.

The result is progressive erosion of the ram-facing surface. The damage is quantified by an erosion yield in cm³ of material removed per incident oxygen atom:

Fluence at 400 km runs from roughly $10^{20}$ atoms/cm² per year at solar minimum to over $3 \times 10^{21}$ at solar maximum, driven by how much the thermosphere has expanded. At the upper end, unprotected Kapton loses of order 100 µm per year on the ram face, which is why multilayer insulation is finished with a germanium or silicon oxide coating and why bare silver interconnects are never flown exposed.

The same neutral density produces aerodynamic drag, the only perturbation that removes orbital energy irreversibly. Density at a given altitude varies by more than an order of magnitude over a solar cycle, which is why re-entry date predictions carry such wide error bars.

Plasma and spacecraft charging

The ambient plasma charges spacecraft surfaces, and differential charging between surfaces produces discharges that couple into electronics.

Surface charging dominates in geostationary orbit and in the auroral zones. During a geomagnetic substorm, keV electrons injected into the outer magnetosphere can drive a shadowed spacecraft surface to several kilovolts negative relative to the plasma. Sunlit surfaces meanwhile stay near zero because photoemission supplies a compensating positive current, so the potential difference appears between surfaces on the same vehicle.

Internal, or deep dielectric, charging is a distinct mechanism. Electrons above about 1 MeV penetrate the structure and deposit inside cable insulation and circuit boards. Charge accumulates over days until the local field exceeds the dielectric strength, then discharges directly into the harness.

Both mechanisms are addressed by the same design principle: every conductor on the vehicle must have a defined path to structure ground, and every exterior surface must be at least weakly conductive. Anomalies traced to charging include a substantial fraction of historical GEO satellite failures.

Micrometeoroids and orbital debris

The particulate environment has two populations. Micrometeoroids are natural, arrive from all directions at 10 to 70 km/s, and are dominated by sub-millimetre grains. Orbital debris is anthropogenic, concentrated in specific altitude bands and inclinations, and collides at an average of about 10 km/s in LEO, reaching 15 km/s for a head-on encounter.

At those speeds, impact physics is not mechanical. The projectile and a portion of the target both vaporise and partly ionise on contact. Stopping a hypervelocity particle by making the wall thicker is enormously inefficient; the effective answer is to make it disintegrate before it reaches the wall.

That is the Whipple shield: a thin sacrificial bumper standing off from the pressure wall by 10 to 50 cm. The bumper shatters and vaporises the projectile, and the resulting expanding cloud of debris spreads its momentum over a large area of the rear wall. A stuffed variant adds Nextel and Kevlar layers between bumper and wall, and is what protects the crewed modules of the ISS.

Risk is assessed with flux models, not with a single number:

NASA ORDEM

Orbital Debris Engineering Model, the US source of record for debris flux

ESA MASTER

Meteoroid and Space Debris Terrestrial Environment Reference

Output

Flux against particle size, for a given orbit, epoch and surface orientation

Design metric

Probability of no penetration over the mission, typically specified at 0.99 or better for crewed elements

Particles below about 1 mm are handled by shielding. Objects above about 10 cm are tracked and avoided by manoeuvre. Between those bounds lies a population that is too small to track and too large to shield against, which is the central difficulty discussed in Space debris and traffic management.

The thermal environment

With no atmosphere there is no convection, and conduction only occurs within the structure. Radiation is the only path for heat to leave a spacecraft, and that single fact drives the entire thermal subsystem.

The external loads are three:

Equilibrium temperature

For a surface absorbing solar flux $S$ with absorptivity $\alpha$ and radiating with emissivity $\varepsilon$, balance gives

with $\sigma = 5.670 \times 10^{-8}$ W m⁻² K⁻⁴.

For a flat plate facing the Sun with $\alpha = \varepsilon$, absorbing and radiating over the same area, $T = 394$ K, or 121 °C. For a sphere, which absorbs over $\pi R^2$ but radiates over $4\pi R^2$, the same calculation gives $T = 278$ K, or 5 °C. That factor of $4^{1/4}$ between the two is why spin stabilisation was such an effective passive thermal strategy on early spacecraft.

The design lever in that expression is the ratio $\alpha/\varepsilon$, and it can be selected over a wide range purely by surface finish:

Because a low-Earth orbit passes in and out of eclipse every 90 minutes, exposed surfaces cycle between roughly +120 °C and −150 °C many thousands of times over a mission. The resulting thermal fatigue, and the differential expansion between dissimilar materials, is a structural design driver in its own right. Full treatment is in Thermal control.

Microgravity

Orbit is free fall, not absence of gravity. At ISS altitude the local gravitational acceleration is still about 89% of its surface value; the station and everything in it are simply falling together.

What remains is a small residual acceleration field:

For materials science and fluid physics experiments the quasi-steady component is the figure that matters, and the transient component is why sensitive payloads sit on vibration isolation mounts. For everything else, the practical consequences are that convection does not occur, so heat transfer must be forced; that fluids are governed by surface tension, so tanks need propellant management devices; and that anything not restrained will drift into something that matters.

References

• Tribble, A. C. The Space Environment: Implications for Spacecraft Design, revised ed., Princeton University Press, 2003.
• Pisacane, V. L. (ed.) The Space Environment and Its Effects on Space Systems, 2nd ed., AIAA, 2016.
• NASA-HDBK-4002, Mitigating In-Space Charging Effects.
• ASTM E595, Standard Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment.
• Banks, B. A. et al. “Atomic Oxygen Erosion Yield Prediction for Spacecraft Polymers”, NASA TM-2009-215812.
• Christiansen, E. L. “Meteoroid/Debris Shielding”, NASA TP-2003-210788.