What Do We Mean by “Space”?
When we talk about space, we mean the vast region beyond Earth’s atmosphere where planets, stars, galaxies, and all other celestial objects exist. Space begins where Earth’s air becomes too thin to support aircraft or normal atmospheric flight.
Scientists commonly define the start of space at the Kármán Line, located about 100 kilometres (62 miles) above Earth’s surface. Beyond this point, aerodynamic lift no longer works, and spacecraft must rely on orbital motion instead of wings.
Is Space Really Empty?
Space looks empty because it contains far fewer particles than Earth’s atmosphere, but it is not completely empty.
Even in the emptiest regions of space, you’ll find:
Stray atoms (mostly hydrogen)
Cosmic dust
Electromagnetic radiation
Magnetic fields
Dark matter (which we can’t see directly)
On Earth, air contains billions of particles in every cubic centimetre. In deep space, that number may drop to just a few atoms — but “few” does not mean “none”.
Why Is Space Black?
Space appears black because there is no atmosphere to scatter sunlight. On Earth, our sky looks blue because air molecules scatter blue wavelengths of sunlight in all directions.
In space:
Light only travels in straight lines
There is nothing to scatter it
You only see light when it comes directly from a source (like the Sun or stars)
This is why astronauts can see bright sunlight and pitch-black shadows at the same time.
Gravity Still Exists in Space
A common myth is that space has no gravity. In reality, gravity exists everywhere.
Astronauts float on the International Space Station not because gravity disappears, but because they are constantly falling around Earth, creating a state called microgravity.
Key Takeaways
Space begins roughly 100 km above Earth
It is not completely empty
Gravity exists everywhere
Space looks black due to lack of atmosphere