Satellites use special materials, lubricants, coatings and heaters because space has almost no air, harsh sunlight and swings from searing heat to deep cold every orbit.
Space looks empty and calm, but it is a tough place for machines. There is almost no air, sunlight is unfiltered, and a satellite in low orbit dashes from day to night and back again many times a day.
NASA engineers say spacecraft structures in sunlight can reach about 500 °F (260 °C) and drop to about −150 °F (−101 °C) in the shade. A spacecraft in low Earth orbit goes round in about 90 minutes, so it swings between day and night roughly 16 times a day.
Each swing makes materials expand and shrink. Repeated thousands of times, that can cause stress cracks. To stay safe, spacecraft use thin reflective films that bounce sunlight away or give off heat, and heaters that switch on when the inside gets too cold.
In a vacuum, many solid materials slowly release vapour. This is called outgassing. The vapour can condense on cold parts such as camera lenses and fog them. Ordinary oil lubricants also outgas, so spacecraft use powder-based lubricants that are tested to work in vacuum.
High above the thick air there is still a little oxygen. Ultraviolet sunlight splits oxygen molecules into single atoms, which are very reactive. This atomic oxygen eats into materials: in the Space Shuttle era, Kapton thermal blankets were eroded within days, which started serious study in the early 1980s. Unfiltered ultraviolet light also discolours paints and makes glass and blankets brittle, and charged particles can flip memory bits and slowly weaken solar cells.
Because these effects act together, ESA tests materials against several of them at once. Its laboratories handle everything from nanometre-scale wiring to a four-metre Spacelab structural ring; a standard satellite may carry 25 linked circuit boards, and every one has to survive. It also prepared 15 materials for the Euro Materials Ageing experiment, to be exposed outside the International Space Station and then examined again back on Earth.
Note the station's orbital period of about an hour and a half — each lap takes the outside of the station from full sunlight to deep shadow and back. Open the ISS tracker
Start with the temperatures. Converting with °C = (°F − 32) × 5/9: (500 − 32) × 5/9 = 260 °C, and (−150 − 32) × 5/9 ≈ −101 °C. The swing is about 361 °C.
Next, the cycle count. The period of a circular orbit is T = 2π √(r³/GM). For a height of 400 km, r = 6,378 + 400 = 6,778 km, so r³ ≈ 3.11 × 10¹¹ km³. Dividing by GM = 398,600 km³/s² gives about 7.81 × 10⁵ s², whose square root is about 884 s. Multiplying by 2π gives T ≈ 5,550 s ≈ 92.6 minutes.
One day is 1,440 minutes, so 1,440 / 92.6 ≈ 15.6 orbits — the “16 times a day” NASA's engineers quote. Over a year that is about 15.6 × 365 ≈ 5,700 thermal cycles; over a 10-year mission, about 57,000.
This is why ESA's laboratories test thermal cycling: repeated expansion and contraction can cause stress fractures, and testers can widen the temperature range or cycle faster to estimate how a part will behave over its whole life.
Outgassing is screened in vacuum chambers, and its products matter most for optics: vapour that condenses on a cold lens or detector can blur a camera for the rest of a mission. For crewed spacecraft, ESA also checks that outgassed products are not toxic or foul-smelling.
Outgassing is the slow release of vapour from materials in a vacuum. The vapour can condense on cold surfaces such as lenses and fog them.
Ultraviolet sunlight splits oxygen molecules in the thin upper atmosphere into single, highly reactive oxygen atoms that erode spacecraft materials.
Its orbit takes about 92.6 minutes, giving about 15.6 orbits a day, or roughly 5,700 cycles a year.