The Environment That Breaks Anything Built for Earth
Module 1 drew the line: astronautics means building things that survive space and keeping them working. This module is about exactly what "space" demands of anything you send there — and the orbital mechanics that decide where it ends up once it's there.
Three Things Space Does to Hardware
None of these exist as a problem on Earth's surface. All three are the default the moment you leave it.
Vacuum
No air means no medium for sound, no convective cooling, and no pressure holding liquids in their normal state — unprotected fluids can boil at body temperature. Everything that keeps a human or a component alive has to be sealed and carried along.
Radiation
Trapped particles in the Van Allen belts, solar wind and flares from the Sun, and a constant low-level background of galactic cosmic rays from outside the solar system. Electronics need hardening; crews need shielding and mission-length limits.
Thermal Extremes
With no atmosphere to moderate temperature, a sunlit surface can exceed 120°C while a shadowed one drops below -100°C — sometimes on the same object, minutes apart. Spacecraft carry radiators, insulation, and heaters just to manage the swing.
"Zero Gravity" Is a Myth
"Zero Gravity"
The Myth
The common assumption: astronauts float because they've left gravity behind — as if gravity switches off a few hundred kilometers up.
- Implies gravity has an edge
- Wrong even at ISS altitude
- Not the term scientists use
Microgravity
The Reality
At ISS altitude (~400 km), Earth's gravity is still about 90% of what it is at the surface. Astronauts float because they and the station are both in continuous free-fall together — not because gravity is absent.
- Gravity is doing almost full work
- "Micro" = small residual forces remain
- The term used in mission documentation
If a station and everything in it are falling at the same rate, nothing presses against anything else — that reads as weightlessness. It's the same principle behind the next section: an orbit isn't the absence of falling. It is falling, arranged so it never lands.
Why Orbits Don't Fall
Newton's cannonball, explained in one diagram: launch speed decides whether something crashes, orbits, or leaves entirely.
- Too slow — the ball follows a short curve and lands back on the surface. This is any sub-orbital hop, including most fireworks and ballistic missiles.
- Orbital velocity — fast enough that the ball falls at the same rate the surface curves away beneath it. It never lands: a circular orbit. At sea-level altitude this is roughly 7.8 km/s, ignoring air resistance.
- Faster still — the path stretches into an ellipse: closer to Earth at one point (perigee), farther at the opposite point (apogee). Most real orbits are elliptical, not circular.
- Escape velocity — past about 11.2 km/s from Earth's surface, the object never comes back at all. It leaves Earth's gravity well entirely.
Conceptual diagram — trajectory shapes are illustrative, not to physical scale.
Where Things Actually Orbit
Altitude decides what an orbit is good for. Three bands cover almost everything humans have ever put in space.
- LEO — Low Earth Orbit (~160–2,000 km). Orbital period ~90 minutes. Home to the ISS (~400 km) and most Earth-observation and constellation satellites (Starlink included).
- MEO — Medium Earth Orbit (~2,000–35,786 km). GPS and other GNSS constellations sit around 20,200 km, with orbital periods of several hours.
- GEO — Geostationary Orbit (exactly ~35,786 km, above the equator). Orbital period matches Earth's 24-hour rotation, so the satellite appears fixed in the sky — used for communications and weather satellites with stationary ground antennas, like Japan's Himawari weather satellite series.
Bands are illustrative, not to scale — GEO is proportionally far larger relative to LEO than shown here.
Between the Two Bodies
Sits between Earth and the Sun. Good for continuous solar observation — the SOHO spacecraft has operated there since the 1990s.
Beyond Earth, Away From the Sun
A stable, cold spot with an unobstructed view outward. Home to the James Webb Space Telescope.
Opposite Earth, Behind the Sun
Always hidden from Earth by the Sun. Rarely used in practice — hard to observe and even harder to communicate with.
60° Ahead in Earth's Orbit
Gravitationally stable — forms an equilateral triangle with Earth and the Sun. Where Jupiter's Trojan asteroids naturally collect at that planet's L4.
60° Behind in Earth's Orbit
The mirror of L4, equally stable. Proposed more than once as a site for a future space colony or observation platform — none built yet.
Key Terms
- Vacuum
- The absence of matter, including air. No medium for sound, no convective cooling, and nothing holding liquids in their normal state.
- Microgravity
- The small residual forces experienced during continuous free-fall — the accurate term for what's casually called "zero gravity." Gravity is still doing almost all of its normal work.
- Free-Fall
- Falling under gravity alone, with nothing else acting on you. What every orbit actually is — a continuous fall that never lands.
- Van Allen Belts
- Two donut-shaped zones of charged particles trapped by Earth's magnetic field — part of the radiation environment spacecraft and crews have to plan around.
- Orbital Velocity
- The speed at which a falling object matches the rate a planet's surface curves away beneath it, producing a stable circular orbit rather than a crash — roughly 7.8 km/s at Earth's surface.
- Escape Velocity
- The speed needed to leave a body's gravity well entirely rather than orbiting it — roughly 11.2 km/s from Earth's surface.
- LEO / MEO / GEO
- Low, Medium, and Geostationary Earth Orbit — the three altitude bands covering most satellites, defined by altitude, orbital period, and typical use.
- Lagrange Point
- One of five positions in a two-body system (like Earth and the Sun) where gravitational and centrifugal forces balance, letting a spacecraft hold a stable position relative to both bodies.
For This Module
NASA Glenn Research Center orbital mechanics primers, NASA's microgravity and Van Allen belt educational materials, and NASA/ESA reference material on geostationary and Lagrange-point missions (SOHO, JWST).