Geocentric orbit facts for kids
A geocentric orbit (also called an Earth orbit) is when any object, like the Moon or a satellite, travels around Earth. Think of it like a tiny car driving around a giant roundabout!
In 1997, NASA said there were about 2,465 working artificial satellites orbiting Earth. They also tracked 6,216 pieces of space debris, which are like space junk. Over 16,291 objects that were launched before have fallen out of orbit and burned up in Earth's atmosphere.
For a spacecraft to stay in orbit, it needs to move very fast. In a low Earth orbit, it travels around 7.8 km/s (28,100 km/h; 17,400 mph) (that's about 28,000 kilometers per hour!). To give you an idea, the fastest crewed airplane ever, the North American X-15, only went about 2.2 km/s (7,900 km/h; 4,900 mph) in 1967. Getting into orbit needs a lot of energy. It takes six times more energy to reach orbit than just to climb to the same height!
Spacecraft that orbit below about 2,000 km (1,200 mi) are affected by Earth's thin atmosphere. This causes them to slow down and slowly lose altitude. How fast they fall depends on their size, weight, and how dense the air is. Below 300 km (190 mi), satellites fall much faster, sometimes lasting only days. If a satellite drops to 180 km (110 mi), it only has a few hours left before it burns up in the atmosphere. To completely escape Earth's gravity and travel into deep space, an object needs to reach an escape velocity of about 11.2 km/s (40,300 km/h; 25,100 mph).
Contents
Understanding Orbit Terms
When we talk about orbits, some words are very important to know.
What is Altitude?
Altitude means how high an object is above the average level of the Earth's oceans. So, if a satellite is at a high altitude, it's very far from Earth.
What is Apogee and Perigee?
- Apogee is the point in an orbit where a satellite is farthest from Earth. When a satellite is at its apogee, it moves the slowest.
- Perigee is the point in an orbit where a satellite is closest to Earth. When a satellite is at its perigee, it moves the fastest.
What is Eccentricity?
Eccentricity tells us how much an orbit is shaped like a stretched-out oval instead of a perfect circle. A perfect circle has an eccentricity of zero. The more stretched out an orbit is, the higher its eccentricity.
What is Inclination?
Inclination is the angle between a satellite's orbit and Earth's equatorial plane (an imaginary flat surface extending from Earth's equator). For example, a satellite orbiting right over the equator has an inclination of 0 degrees.
What is Orbital Period?
The orbital period is the time it takes for a satellite to complete one full trip around the Earth.
What is Velocity?
Velocity is an object's speed in a specific direction. It's not just how fast something is going, but also where it's headed.
Types of Earth Orbits
There are many different ways objects can orbit Earth. Here are some common types.
Orbits by Height
Satellites can orbit at different heights above Earth.
- Low Earth Orbit (LEO): These orbits are relatively close to Earth, usually between 160 km (99 mi) and 2,000 km (1,200 mi) high. Many communication satellites and the International Space Station use LEO.
- Medium Earth Orbit (MEO): These orbits are higher than LEO, typically between 2,000 km (1,200 mi) and 35,786 km (22,236 mi). GPS satellites are in MEO.
- Geosynchronous Orbit (GEO): This is a very special orbit at about 35,786 km (22,236 mi) above the equator. Satellites here take exactly one sidereal day (about 23 hours, 56 minutes) to orbit Earth. This means they stay above the same spot on Earth.
Orbits by Angle
The angle of an orbit compared to Earth's equator is called its inclination.
- Polar orbit: A satellite in a polar orbit passes over or very close to both the North and South Poles on each trip around Earth. Its inclination is almost 90 degrees.
- Polar Sun synchronous orbit: This is a special type of polar orbit. The satellite passes over the equator at the same local time each day. This is great for satellites that take pictures of Earth because the shadows will always look the same, making it easier to compare images over time.
Orbits by Shape
Orbits can be circular or more stretched out (elliptical).
- Hohmann transfer orbit: This is a fuel-efficient way to move a spacecraft from one circular orbit to another. It uses two short engine burns.
- Highly elliptical orbit (HEO): These orbits are very stretched out. They have a very high apogee (farthest point from Earth) and a low perigee (closest point). Satellites in HEO spend a long time near their apogee, which is useful for observing a specific area of Earth for extended periods.
- Molniya orbit: A type of HEO that takes about 12 hours to complete one orbit. It's designed so the satellite spends most of its time over a specific region, often used for communication in high-latitude areas.
- Tundra orbit: Another type of HEO that takes about 24 hours to complete one orbit. Similar to Molniya, it allows a satellite to stay over a particular area for a long time.
Special Geosynchronous Orbits
- Geostationary orbit (GSO): This is a type of geosynchronous orbit that is exactly above the equator (inclination of zero). To someone on the ground, a satellite in GSO looks like it's staying in the same spot in the sky. This is why TV and weather satellites use GSO.
- Graveyard orbit: When satellites in geosynchronous orbit run out of fuel or reach the end of their life, they are often moved to a slightly higher orbit, called a graveyard orbit. This helps prevent them from crashing into other active satellites.
See also
- Earth's orbit
- List of orbits
- Orbital mechanics
- Satellite
- Space station
