Kinds of orbits

Low, medium, geostationary, polar, transfer and beyond — and why each satellite flies where it does.

1. What Is an Orbit? — An orbit is the curved path one object takes around another under the pull of gravity.
2. What are LEO, MEO and GEO? — They are the three main height bands for Earth satellites: low Earth orbit up to 2,000 km, geostationary orbit at 35,786 km, and medium Earth orbit in between.
3. What is a geostationary orbit? — A geostationary orbit is a circular orbit 35,786 km above the equator where a satellite circles once per Earth rotation, so it seems to hang still over one spot.
4. What is the difference between geostationary and geosynchronous orbits? — Any orbit taking one sidereal day is geosynchronous; geostationary is the special geosynchronous orbit that is circular and over the equator, so it stays still overhead.
5. What is a polar orbit? — A polar orbit passes over or near both poles, so as Earth turns underneath, the satellite eventually flies over almost every part of the planet.
6. What is a sun-synchronous orbit? — A sun-synchronous orbit is a near-polar orbit whose plane slowly turns with the Sun, so the satellite passes over each place at about the same local time every day.
7. What is a geosynchronous transfer orbit (GTO)? — A GTO is a long, stretched orbit with its low point a few hundred kilometres up and its high point near geostationary height, used to carry satellites towards GEO.
8. How do satellites raise their orbits? — A satellite climbs by firing its engine forward: burning at the low point lifts the high point, and burning at the high point lifts the low point.
9. What are apogee and perigee? — Perigee is the point in an orbit closest to Earth and apogee is the farthest; a satellite moves fastest at perigee and slowest at apogee.
10. What is orbital inclination? — Inclination is the angle between a satellite's orbit and Earth's equator: 0° follows the equator, 90° crosses the poles, and above 90° goes against Earth's spin.
11. What is orbital eccentricity? — Eccentricity is a number from 0 to just under 1 that tells how stretched an orbit is: 0 is a perfect circle, and values near 1 are long, thin ellipses.
12. Why do orbits slowly turn? — Earth bulges at the equator, and that uneven pull slowly swings a satellite's orbit plane around and rotates its ellipse — a drift called precession.
13. What is a graveyard orbit? — A graveyard orbit is a parking zone a few hundred kilometres above geostationary height, where old GEO satellites are moved at the end of their lives.
14. Why do satellites in low orbit fall back to Earth? — Thin air at low altitude drags on a satellite, slowly lowering its orbit until it re-enters the atmosphere and burns up or falls into the ocean.
15. What are Lagrange points? — Lagrange points are five places near two large bodies, like the Sun and Earth, where a small spacecraft can keep the same position relative to both.
16. What is a halo orbit? — A halo orbit is a large looping path around a Lagrange point, letting a spacecraft stay near L1 or L2 without sitting exactly on the point.
17. How do spacecraft get to the Moon? — A spacecraft first circles Earth and raises its orbit step by step, then a final burn sends it to the Moon, where braking burns let the Moon's gravity capture it.
18. How do spacecraft get to Mars? — A spacecraft leaves Earth fast enough to orbit the Sun on its own, coasts for months on a path that meets Mars, then brakes so Mars can capture it.