1. The short version / what to know first / where this sits in the series

• The "five enemies" differ in strength from place to place. Atomic oxygen and temperature cycling stand out in low Earth orbit, radiation in geostationary orbit, and the day-night temperature swing and dust on the Moon (our grouping).
• In numbers: the vacuum outside the ISS is 10⁻⁶ to 10⁻⁹ torr, and a low-orbit surface swings about 16 times a day, with a rule of thumb of −120 to +120 °C [Ref. 1]Sourced. About four years of atomic oxygen erodes polyimide (Kapton H) by roughly 0.25 mm [Ref. 2]Our calculation.
• Ground tests cannot reproduce all of space. Vacuum reproduces well, while atomic-oxygen facilities and debris impact speeds still differ from orbit [Ref. 1]Sourced.

2. Different environments in different places: three places, five enemies

Asked "how hot is space?" or "how much radiation is there in space?", we have no single answer. In low Earth orbit where the ISS flies, in the geostationary orbit of communications satellites, and on the Moon, the same enemy has completely different strength. The matrix gathers how the five enemies appear in three places. Numbers and conditions are checked with sources in sections 3 to 7.

3. Enemy 1, vacuum: heat cannot escape easily and materials give off gas

A thermos keeps its contents warm for hours because the space between its double walls is evacuated, cutting off heat carried by air (convection and conduction) (our commentary). Space is this "vacuum" spread all around. The heat that equipment itself produces also escapes with difficulty in vacuum.

(1) How good a vacuum

NASA's guide gives the hard vacuum outside the ISS as 10⁻⁶ to 10⁻⁹ torr [Ref. 1]Sourced. The torr is a unit of pressure; one atmosphere is 760 torr. Converted, 10⁻⁶ to 10⁻⁹ torr is about 1.3×10⁻⁴ to 1.3×10⁻⁷ Pa, roughly a billionth to a trillionth of atmospheric pressure Our calculation. The night-time surface pressure on the Moon is about 3×10⁻¹⁵ bar (2×10⁻¹² torr) in NASA's data [Ref. 6]Sourced. We could not confirm a pressure value for geostationary orbit.

(2) Effect 1: heat leaves only by radiation

On the ground, a hot object loses heat by three routes: conduction (passing through contact), convection (carried by moving air) and radiation (leaving as infrared). With no air in vacuum, there is no convection. Radiation is the only way out for heat (upper part of Fig. 3).

What, then, fixes the temperature of a plate left in the sun? The heat coming in is the share of sunlight the plate absorbs (absorptance α). The heat going out is what the plate radiates as infrared (emissivity ε). The temperature settles where the two balance, which gives T = (α·S / (ε·σ))^1/4. S is the solar irradiance (1361 W/m² in NASA's Moon data and in ECSS [Ref. 6, 3]) and σ is the Stefan-Boltzmann constant (5.670×10⁻⁸ W/m²K⁴ [Ref. 16]).

Check it with numbers. A plate with α/ε of 0.25 reaches about 5 °C, 0.5 about 58 °C, 1.0 about 120 °C, and 1.5 about 162 °C Our calculation. In the same sunshine, the surface material's properties (α and ε) shift the temperature by more than 100 °C. The 120 °C at α/ε of 1.0 is nearly the same as the rule-of-thumb +120 °C for low-orbit surfaces in section 5.

View original SVG, values and sources

Original SVG reference: Drawing with our calculation (vector drawing). In vacuum, heat leaves only by radiation. The upper part contrasts the ways heat can leave on the ground and in vacuum (concept). The lower part shows the equilibrium temperature of a plate facing the Sun squarely (our calculation): T = (α·S/(ε·σ))^(1/4), S = 1361 W/m², σ = 5.670×10⁻⁸. The back is insulated and Earth's heat is ignored, so this is idealized and not the temperature of any real vehicle.

(3) Effect 2: materials give off gas (outgassing)

In vacuum, volatile components inside a material escape. This is outgassing. NASA's guide notes that the released molecules deposit on line-of-sight surfaces, especially cold ones, and can affect optical surfaces and spacecraft performance [Ref. 1]Sourced. The ECSS environment standard lists what comes off: water, solvents, additives, uncured monomer, lubricants and contamination from ground processing and handling [Ref. 3]Sourced.

One countermeasure is a bake-out: heating in vacuum beforehand to drive off volatiles. NASA's guide says a material known to outgas should be vacuum-baked for at least 24 hours at a temperature above that expected in orbit (100 °C if that is unknown) [Ref. 1]Sourced. For the ISS, limits on external contamination are set in a document (NASA SSP 30426) and the list of materials used is reviewed in advance [Ref. 1]Sourced.

4. Enemy 2, radiation: belts, dose and shielding

Space radiation is not only light-like rays (such as X-rays) but fast particles (electrons and protons). Fast particles break bonds in materials and create charge inside electronic parts. NASA's guide lists three natural sources of charged-particle radiation: galactic cosmic rays (high-energy particles from far away), solar proton events (particles that surge after solar explosions) and the trapped radiation belts [Ref. 1]Sourced.

(1) The radiation belts and where the orbits sit

Earth's magnetic field captures charged particles and holds them in belts: the radiation (Van Allen) belts. The ECSS environment standard says the belts extend from 100 to 65,000 km in altitude, with electrons up to a few MeV and protons up to several hundred MeV [Ref. 3]Sourced. The South Atlantic Anomaly is where the inner edge of the belts dips to low altitude [Ref. 3]Sourced. The ISS receives most of its particle dose crossing this region (mostly electrons; the proton dose is also significant for some materials and parts) [Ref. 1]Sourced.

(2) How large is the dose

The amount of radiation a material or part receives is the absorbed dose, in Gy (gray) or rad. 1 Gy = 100 rad, and 1 Gy is 1 J of energy absorbed per kilogram of material. For electronic parts, dose is usually the amount absorbed by silicon, written rad(Si) or Gy(Si). Even in the same orbit, dose varies greatly with the thickness of metal around the part (shielding).

A NASA technical report (NASA/TM-20220011775) estimates dose for one-year design reference missions, assuming 5 mm of aluminum shielding and using SHIELDOSE-2 in SPENVIS (a European space-environment computation system) [Ref. 4]. Its annual total absorbed doses are 136 rad for low-inclination LEO (avoiding the SAA), 333 rad for polar LEO, 430 rad for an ISS-like orbit (500 km, 51° inclination), 2,150 rad for a lunar cruise and 5,930 rad for geostationary orbit Sourced.

And on the lunar surface? The dosimeter (Lunar Lander Neutrons and Dosimetry, LND) of China's Chang'E-4 lander made the first direct measurements of radiation on the Moon after landing on 3 January 2019 in the von Kármán crater on the far side. The average absorbed dose rate in silicon was 13.2 ± 1 µGy/hour, with average shielding around the instrument of about 3.5 mm aluminum equivalent [Ref. 8]Sourced. No solar particle event was observed in that period, and solar activity was approaching its minimum. The paper states that such events can raise the dose by orders of magnitude behind thin shielding [Ref. 8].

13.2 µGy/hour is about 0.32 mGy a day, or about 0.12 Gy (about 12 rad) a year Our calculation. This simply extends the observation period's average to a year and excludes solar particle events. Because shielding and method differ from the table above (5 mm aluminum, calculated), we do not compare them as larger or smaller.

(3) Shielding does not simply reduce dose in proportion to thickness

The NASA report states that with 2 mm of aluminum the electron contribution becomes negligible, at 5 mm the proton shielding effectiveness bottoms out, and beyond 5 mm even 20 mm of aluminum reduces the dose by only about another factor of two [Ref. 4]Sourced. As particles pass through metal, the spectrum shifts toward higher energies (beam hardening), because particles easily stopped by thin shielding are removed first. That is why shielding is designed by layering several materials.

(4) Effects on materials and parts

In polymers, particle radiation either links molecular chains (cross-linking) or cuts them (chain scission), leaving the material hard and brittle [Ref. 1]Sourced. NASA's guide says that for most ISS materials the effects of atomic oxygen and UV can overshadow those of particle radiation [Ref. 1]. The greater effect is on avionics: single-event upsets (one particle flips a bit), bit errors and latch-up (overcurrent that stops operation) [Ref. 1]Sourced. Mitigations are choosing radiation-hard parts, shielding, error-correcting circuits and triple redundancy (majority vote) [Ref. 1].

5. Enemy 3, temperature swings: 16 cycles a day, and the lunar day and night

The temperature problem is not only "how hot at the maximum". How many times it swings back and forth decides how a material tires.

(1) Low Earth orbit: about 16 times a day

The ISS circles the Earth in about 92 minutes and moves between sunlight and shadow about 16 times a day [Ref. 1]Sourced. NASA's guide gives −120 °C to +120 °C as a rule of thumb for this swing, though a surface with high solar absorptance and low infrared emissivity swings further. The actual temperature depends on the surface properties, its view of the Sun, the Earth and other parts of the vehicle, the time in sunlight and shadow, and thermal mass and equipment heat [Ref. 1]Sourced.

Sixteen cycles a day is about 5,840 a year; over an assumed 15 years it would be about 88,000 Our calculation (15 years is a period we assume as an example). What matters here is a mismatch in the coefficient of thermal expansion (CTE). If a coating and its substrate expand differently, only one of them stretches and shrinks more at every swing, causing cracks, peeling, spalling and pinholes. Once there is a hole, atomic oxygen attacks the material beneath [Ref. 1]Sourced. NASA's guide shows a case in MISSE-4 (a materials exposure experiment outside the ISS) in which thermal cycling and atomic-oxygen erosion together destroyed two thin-film coatings [Ref. 1].

(2) Geostationary orbit: mostly sunlit, with two "shadow seasons" a year

In geostationary orbit a satellite is usually in sunlight almost all the time. NOAA explains that the GOES weather satellites pass through two periods a year in Earth's shadow, during which they run on batteries up to 72 minutes each day; the shadow periods run roughly from late February to mid-April and from late August to mid-October [Ref. 5]Sourced. Each season lasts about 45 to 49 days, so there are about 90 days a year with one shadow a day Our calculation (a rough estimate from NOAA's dates).

(3) The lunar surface: day and night of about two weeks each

A lunar day (from one noon to the next) is about 29.53 days [Ref. 6]Sourced. In NASA's Moon data the equatorial surface temperature swings widely, from 95 K to 390 K (about −178 °C to +117 °C) [Ref. 6]Sourced. In permanently shadowed craters at the poles, the Diviner instrument (a thermal mapper) on NASA's Lunar Reconnaissance Orbiter recorded places on crater floors as cold as −248 °C [Ref. 7]Sourced. The cycles are few, about 12 a year (365.25 days ÷ 29.53 days) Our calculation.

6. Enemy 4, atomic oxygen and ultraviolet light: surfaces erode and discolor

For materials in low Earth orbit, the toughest enemy is atomic oxygen (AO). NASA's guide calls AO the most significant cause of material degradation at ISS altitude [Ref. 1]Sourced.

(1) What atomic oxygen is

The oxygen in ground-level air is a molecule of two joined atoms (O₂). High up, short-wavelength ultraviolet light from the Sun splits O₂ into single oxygen atoms: atomic oxygen. In low Earth orbit, below about 1,000 km, it is the most abundant species. At 400 km an atom travels on the order of 10⁴ m before meeting another, so it rarely recombines and stays atomic [Ref. 2]Sourced.

A spacecraft "plows" into these atoms at about 7.7 km/s. The flux of atoms on surfaces facing the direction of travel (ram surfaces) is reported as about 5.23×10¹³ atoms/cm²·s at 400 km, averaged over the 11-year solar cycle, with an impact energy of about 4.5 eV per atom [Ref. 2]Sourced. NASA's guide gives about 5.2 eV, so sources differ [Ref. 1]. The amount of AO varies by an order of magnitude with solar activity, and on the opposite (wake) side it is about an order of magnitude lower [Ref. 1]Sourced.

Check it. Multiply 5.23×10¹³ atoms/cm²·s by the 3.156×10⁷ seconds in a year and you get about 1.65×10²¹ atoms/cm² a year Our calculation. The PEACE samples of MISSE 2 (a materials exposure experiment outside the ISS) were exposed for 3.95 years and the dose, determined from the mass loss of polyimide reference samples, was 8.43×10²¹ atoms/cm² (about 2.1×10²¹ a year) [Ref. 2]Sourced. The two agree to the same order of magnitude.

(2) How much is eroded: erosion yield

The amount of material removed by AO is expressed as the erosion yield (volume lost per incident AO atom, in cm³/atom). The eroded thickness is "erosion yield × AO dose (fluence)". The erosion yield of polyimide (Kapton H) is 3.0×10⁻²⁴ cm³/atom and it is also used as the reference for measuring AO dose [Ref. 2]Sourced. At a fluence of 8.43×10²¹ atoms/cm², 3.0×10⁻²⁴ × 8.43×10²¹ ≈ 0.0253 cm, or about 253 µm, is removed Our calculation.

NASA's guide says AO oxidizes many metals (especially silver, copper and osmium) and reacts strongly with materials containing carbon, nitrogen, sulfur or hydrogen bonds, so many polymers erode [Ref. 1]Sourced. Even a protective coating can fail: AO can undercut and erode the material beneath at defects in the coating [Ref. 1]Sourced.

(3) Ultraviolet light: discoloration and change of the material

The intensity of sunlight at the mean Earth-Sun distance is 1361 W/m² [Ref. 3, 6]Sourced. Earth's atmosphere absorbs most harmful ultraviolet light, but in space materials take it directly [Ref. 1]. NASA's guide says AO may bleach materials while UV generally darkens them (especially in the presence of contamination) and damages polymers by cross-linking (hardening) or chain scission (weakening). UV in high vacuum can also create oxygen vacancies in oxides, leading to large color changes [Ref. 1]Sourced.

7. Enemy 5, microgravity and debris: free fall and high-speed impacts

(1) Microgravity

Microgravity is not the absence of gravity: it is the state of free fall, in which gravity is not felt (NASA expresses it as about 10⁻⁶ g [Ref. 12]). NASA's guide lists phenomena changed by microgravity: surface wetting and interfacial tension, multiphase flow and heat transfer, solidification, and combustion [Ref. 1]Sourced. The sinking, rising and convective mixing taken for granted in making materials on the ground are suppressed in space. Making materials with this property is at the research and demonstration stage; the article on in-space manufacturing and microgravity covers it.

(2) Debris and micrometeoroids: small but fast

NASA's guide says every area of a spacecraft can be struck by micrometeoroids traveling as fast as 60 km/s, and that surfaces facing the direction of travel are more likely to meet debris traveling at an average of about 10 km/s [Ref. 1]Sourced. ESA's FAQ relates size to damage: a collision with a 10 cm object would cause catastrophic fragmentation of a typical satellite, a 1 cm object will most likely disable a spacecraft and penetrate the ISS shields, and a 1 mm object could destroy subsystems [Ref. 10]Sourced.

Why are small fragments dangerous? Kinetic energy is ½mv², proportional to the square of speed. An aluminum sphere 1 cm in diameter (mass about 1.4 g) hitting at 10 km/s carries ½ × 1.4×10⁻³ kg × (10⁴ m/s)² ≈ 7×10⁴ J (about 70 kJ). That equals the kinetic energy of a 1,500 kg car traveling at about 35 km/h Our calculation.

The ISS crew modules are protected by two metal sheets about 10 cm apart. The outer "bumper" shatters the debris and the inner wall absorbs the resulting spray of fragments; fabric with the same function as a bulletproof vest sits between them. This design can defeat debris up to about 1 cm [Ref. 10]Sourced. The orbits of large, tracked fragments can be predicted, and when the risk exceeds a threshold the ISS performs an avoidance maneuver (more than 26 by the end of 2020) [Ref. 10]Sourced. In ESA's 2026 report there are estimated to be 68,450 objects larger than 10 cm (including about 11,300 active payloads), about 1.5 million pieces of debris of 1 to 10 cm, and about 230 million of 1 mm to 1 cm [Ref. 11]Sourced.

The impact marks on returned hardware were mostly small craters of 0.5 mm diameter or less. MISSE-1 and MISSE-2 averaged less than two impacts per square foot (about 0.09 m²) per year, under conditions where the incomplete ISS structure gave little shielding [Ref. 1]Sourced. On the other hand, NASA's guide says the deliberate destruction of a Chinese satellite in 2007 and the 2009 collision of Iridium 33 and Cosmos 2251 increased trackable debris by 60 percent [Ref. 1]Sourced.

(3) A different enemy on the Moon: dust

On the Moon, fine lunar dust is troublesome. A NASA report (Gaier, 2005) examined the records of the six Apollo landings and sorted dust effects into nine categories: vision obscuration, false instrument readings, dust coating and contamination, loss of traction, clogging of mechanisms, abrasion, thermal control problems (reduced heat rejection), seal failures, and inhalation and irritation [Ref. 9]Sourced. Some problems (clogging, abrasion, reduced heat rejection) were not solved by simple measures, and the report says ground tests consistently underestimated the severity [Ref. 9]Sourced.

8. The enemies combine: real examples

On real spacecraft the five enemies act at the same time and for long periods. Here are three examples.

SourcedNASA technical reports and guide. The LDEF results are reported to serve as a benchmark for materials design data for low Earth orbit.

9. How far ground tests reproduce space

Tests that reproduce space's enemies on the ground are tools for choosing materials. But how well they reproduce differs by enemy. Based on NASA's guide, the picture is as follows.

For example, vacuum corresponds well to orbit in ground facilities at 10⁻⁶ to 10⁻⁹ torr [Ref. 1]Sourced. For AO, plasma ashers mix in high-energy ultraviolet (Lyman-alpha) and ions, and erosion came out higher than in space for all 40 polymers compared [Ref. 1]Sourced. AO beam facilities come within about ±10% of orbit for polymers without fluorine, while fluoropolymers show higher reactivity because of the UV generated at the same time [Ref. 1]Sourced. To tie ground results to orbit, materials of known reactivity such as Kapton H or HN are tested alongside.

10. How to read the yardsticks and standards

The yardsticks of the space environment divide into quantities that count the environment itself and quantities that measure a material's reaction. Environmental quantities include vacuum level (torr, Pa), absorbed dose (rad, Gy), atomic-oxygen fluence (atoms/cm²), the number of thermal cycles and impact flux. Reaction quantities include outgassing amount, erosion yield (cm³/atom), change in optical properties and loss of strength. Environmental quantities change with orbit, altitude, period and shielding, so comparing bare numbers is meaningless. Concrete test procedures and judgments are covered in the next article on reading evaluations of space materials (checked 17 October 2026).

Our calculationThe table is organized by us. Standard numbers, titles, editions and status were checked on the ISO pages (checked 17 October 2026), the ECSS page and the JAXA list of common technical documents. Standard texts are paid or unchecked, so we give only titles and scope, not requirements.

Cautions in reading (for materials engineers)

• Ground tests differ from in-orbit results: vacuum reproduces well, but atomic oxygen, UV and debris leave gaps (Fig. 9). In-orbit track records (MISSE, LDEF, Hubble) are numbers tied to orbit, period and orientation
• Check whether a standard is a recommendation or a requirement: whether an ECSS or JAXA standard applies depends on the contract or project. ISO standards underlie national ones but are not law. When "compliant" is claimed, ask which standard and which edition
• Qualification and acceptance testing differ: qualification, which shows a new material's environmental resistance, and lot-by-lot acceptance tests are separate stages (details in the next article)
• Plans and achievements are different: lunar standards are still being developed and may be updated. The status of standards here is as of the checked date

11. Limits, and what could not be confirmed

• We could not confirm a pressure value for geostationary orbit: neither NASA's guide nor the ECSS text we read gave one.
• Radiation doses are orders of magnitude only: the table and text of NASA/TM-20220011775 disagree about geostationary orbit, and other documents give figures that differ by orders of magnitude. The measured lunar value (Chang'E-4) differs in shielding and method and is not compared.
• We give no figure for the share of solar energy that is UV: documents describe it differently and we could not reconcile them from primary sources, so no percentage is used.
• "Atomic oxygen is not dominant in geostationary orbit" and "there is no Earth-orbit atomic oxygen on the Moon" are our commentary drawn from NASA's statement that AO is the main species below about 1,000 km. We did not check AO values for geostationary orbit or the Moon.
• JAXA standards were confirmed by title only: JERG-2-141, 143, 144, 145 and JMR-010 and so on were checked by title in JAXA's list; text, edition and requirements were not checked.
• The in-orbit track record is a set of examples: MISSE, LDEF and Hubble are representative examples, not all materials or orbits.

12. Glossary

13. References

1. NASA ISS Research Integration Office (Finckenor M. M., de Groh K. K.) "A Researcher's Guide to: Space Environmental Effects" (March 2015, revised September 2020) — www.nasa.gov/wp-content/uploads/2020/10/researchers-guide-space-environment-effects_tagged.pdf
2. de Groh K. K. et al. "MISSE PEACE Polymers Atomic Oxygen Erosion Results", NASA/TM—2006-214482 — ntrs.nasa.gov/citations/20070002707
3. ECSS ECSS-E-ST-10-04C Rev.1 "Space environment" (15 June 2020) — ecss.nl/standard/ecss-e-st-10-04c-rev-1-space-environment-15-june-2020/
4. NASA "Mission Radiation Environment Modeling and Analysis", NASA/TM-20220011775 — ntrs.nasa.gov/citations/20220011775
5. NOAA NESDIS "GOES Eclipse Schedule" — www.nesdis.noaa.gov/our-satellites/currently-flying/goes-east-west/goes-eclipse-schedule
6. NASA NSSDC "Moon Fact Sheet" — nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html
7. NASA "10 Cool Things NASA's Lunar Reconnaissance Orbiter is Teaching Us About the Moon" — science.nasa.gov/solar-system/moon/10-cool-things-nasas-lunar-reconnaissance-orbiter-is-teaching-us-about-the-moon/
8. Wimmer-Schweingruber R. F. et al. "First measurements of the radiation dose on the lunar surface", Science Advances 6, eaaz1334 (2020), DOI 10.1126/sciadv.aaz1334 — pmc.ncbi.nlm.nih.gov/articles/PMC7518862/
9. Gaier J. R. "The Effects of Lunar Dust on EVA Systems During the Apollo Missions", NASA/TM—2005-213610 — ntrs.nasa.gov/citations/20050160460
10. ESA Space Debris Office "Space debris: frequently asked questions" (updated April 2021) — www.esa.int/Space_Safety/Space_Debris/FAQ_Frequently_asked_questions
11. ESA Space Debris Office "ESA's Annual Space Environment Report", Issue 10.1 (8 September 2026) — www.sdo.esoc.esa.int/publications/Space_Environment_Report_I10R1_20260908.pdf
12. NASA "What is Microgravity?" — www.nasa.gov/missions/station/iss-research/what-is-microgravity/
13. NASA Glenn Research Center "Environmental Exposure Conditions for Teflon FEP on the Hubble Space Telescope Investigated" — ntrs.nasa.gov/citations/20050192348
14. NASA Glenn Research Center "The Effect of Heating on the Degradation of Ground Laboratory and Space Irradiated Teflon FEP" — ntrs.nasa.gov/citations/20020070610
15. NASA "An interim overview of LDEF materials findings" — ntrs.nasa.gov/citations/19930009140
16. NIST "Stefan-Boltzmann constant" (CODATA value) — physics.nist.gov/cgi-bin/cuu/Value?sigma
17. ISO ISO 21348:2007 "Space environment (natural and artificial) — Process for determining solar irradiances" — www.iso.org/standard/39911.html
18. ISO ISO 15390:2004 "Space environment (natural and artificial) — Galactic cosmic ray model" — www.iso.org/standard/37095.html
19. ISO ISO 14222:2022 "Space environment (natural and artificial) — Earth's atmosphere from ground level upward" — www.iso.org/standard/77492.html
20. ISO ISO 14200:2021 "Space environment (natural and artificial) — Process-based implementation of meteoroid and debris environment models (orbital altitudes below GEO + 2 000 km)" — www.iso.org/standard/77129.html
21. JAXA JAXA Safety and Mission Assurance Department, list of JAXA common technical documents (JERG and JMR) — sma.jaxa.jp/techdoc.html

14. Claim-to-source audit

Last updated: 17 October 2026. Sources are limited to official materials from NASA (guide, technical reports, data), ECSS, NOAA, ESA, ISO, JAXA and peer-reviewed papers. Worked examples (equilibrium temperature, cycle counts, eroded thickness, debris energy and so on) are our own illustrative calculations, not values of real vehicles. The pressure in geostationary orbit, firm radiation doses, the share of solar energy in the UV, and the text of JAXA standards could not be checked and are not stated.