Black holes
What is a black hole?
A black hole is a place in space where gravity is so strong that nothing, not even light, can escape. Here's what that really means, in plain English.
The blog
All space explainers
Big space ideas, one at a time. Newest first.
Space basics
What is a light-year?
A light-year sounds like time, but it's actually a distance: how far light travels in one year. Here's how big that really is.
Planets
Why is Mars red?
Mars looks red because its dust is full of rusty iron. Here's how a whole planet turned the colour of rust, explained simply.