Why things stay up

Gravity, speed and falling around the Earth — the physics every satellite obeys.

1. What Is a Satellite? — A satellite is any object that orbits a larger object in space.
2. What does gravity do to a satellite? — Gravity pulls a satellite towards Earth's centre at every moment, bending what would be a straight-line path into a curve around the planet.
3. Why don't satellites fall down? — Satellites are falling all the time, but they move sideways so fast that Earth's curved surface drops away beneath them as quickly as they fall.
4. How fast do satellites go? — It depends on height: the ISS at about 400 km moves at roughly 7.7 km per second, while a geostationary satellite 36,000 km up moves at about 3.1 km per second.
5. Why do higher satellites move more slowly? — Gravity is weaker farther from Earth, so a higher satellite needs less sideways speed to keep curving around the planet — and it also has a longer loop to travel.
6. What is an orbital period? — An orbital period is the time a satellite takes to go once around its orbit — about 92 minutes for the ISS and almost exactly one day for a geostationary satellite.
7. What is escape velocity? — Escape velocity is the speed an unpowered object needs to break free of a planet's gravity for good — about 11.2 km per second from Earth's surface.
8. Why do astronauts float in space? — Astronauts float because they and their spacecraft are falling around Earth together, not because gravity is missing — at the ISS, gravity is still about 90 percent as strong as on the ground.
9. What are Kepler's laws? — Kepler's three laws say orbits are ellipses, an orbiting body moves faster when closer and slower when farther, and the square of the period grows with the cube of the orbit's size.
10. What are latitude and longitude? — Latitude says how far north or south of the equator a place is, and longitude says how far east or west of the Greenwich meridian it is, both measured in degrees.
11. What is the sub-satellite point? — The sub-satellite point is the spot on Earth's surface directly beneath a satellite, where a line from the satellite to Earth's centre meets the ground.
12. What is a satellite's ground track? — A ground track is the line on a map traced by the point directly beneath a satellite as it orbits and Earth turns underneath it.
13. Why does the ISS cross the equator somewhere new on each orbit? — During the roughly 92 minutes the ISS takes for one orbit, Earth turns about 23 degrees eastward, so the next equator crossing lands that much farther west.
14. What are UTC and IST, and why does satellite data use UTC? — UTC is the world's reference time, built from atomic clocks; Indian Standard Time is UTC plus 5 hours 30 minutes.