What is space, exactly?
When we look up at the night sky, we see stars, planets, and galaxies scattered across a seemingly infinite black canvas. But space is not just an empty backdrop; it is a physical, dynamic fabric that bends, stretches, and carries energy. It is governed by extreme conditions that defy human intuition—from utter silence and crushing vacuums to mind-bending temperature extremes. Understanding space requires us to unlearn the physics of Earth's atmosphere and embrace the bizarre reality of the cosmos.
A near-perfect vacuum
Unlike Earth, where the air is packed with billions of molecules per cubic centimetre, deep space is astonishingly empty. Interstellar space typically contains just a single hydrogen atom per cubic centimetre. This immense emptiness means there is virtually no atmospheric pressure, which is why spacecraft must be heavily pressurized to prevent the gases inside from violently expanding outward. However, space is never truly empty; it is filled with trace gases, cosmic dust, cosmic rays, and the pervasive glow of the cosmic microwave background.
The deafening silence
In Hollywood movies, spaceships roar past the camera and lasers make distinct 'pew-pew' noises. In reality, space is entirely silent. Sound requires a physical medium—like air, water, or solid metal—to propagate as pressure waves. Because the vacuum of space lacks enough particles to transmit these mechanical waves, any explosion, collision, or engine burn happens in absolute silence. The only 'sounds' we can record from space are radio emissions translated into audio frequencies by our instruments.
A realm of extreme temperatures
Temperature in space is a matter of direct sunlight and shadow. Because there is no atmosphere to trap, distribute, or diffuse heat, objects in space experience savage temperature swings. An object in direct sunlight near Earth's orbit can quickly heat up to over 120°C (250°F), hot enough to boil water. However, the moment that same object moves into the shadow of a planet or moon, the temperature plummets to a bone-chilling -150°C (-238°F) or lower. This thermal shock is one of the greatest engineering challenges in spacecraft design.
The Kármán line, the widely accepted boundary where space begins, is only 100 kilometres (62 miles) above sea level—a shorter distance than a typical daily commute for many people.