How do satellites orbit the earth?

Satellites are, to some degree, “mysterious” objects. They travel in space, which feels like an exotic place because most of us have never been there. They are so far away that we cannot see them. They usually cost millions or billions of dollars, which means none of us will ever own one personally. And so on…Orbital mechanics can also be mysterious because there is no easy way for us to experience orbital mechanics personally. However, with a little imagination, you can understand the basic idea behind orbital mechanics very easily. It turns out that we play with orbital mechanics all the time!

Think about what happens when you throw a ball. Imagine that you are standing in a big field and throw a baseball as hard as you can — like a pitcher. The ball might go 100 feet (30 meters) and then hit the ground. You put the ball in orbit — It’s just that a ball’s orbit is very short!

Now imagine that you shot a rifle straight and level instead of throwing a ball. The bullet might travel a mile (1.6 km) before succumbing to gravity and hitting the ground.

Now imagine that you shoot a very large cannon that is able to give its shell an extremely high initial velocity. Also imagine that our world is completely covered in water to remove any worries about hills, and that the cannon is shot straight and level. Its path might look like this:

One thing that gums these examples up is air resistance, so imagine that you took this cannon to the moon and mounted it on top of the highest mountain. The moon has no atmosphere and is completely surrounded by the vacuum of space. If you adjusted the speed of the shell just right and shot the cannon, the shell would follow the curve of the moon perfectly. It would fall at exactly the same rate that the curve of the moon falls away from it, so it would never hit the ground. Eventually it would curve all the way around the moon and ram right into the back of the cannon! On the moon you could actually have satellites in extremely low orbits like that — just a mile or two off the ground to avoid the mountains. And satellites could conceivably be launched from cannons.

On earth, it’s not so easy because satellites have to get up above the atmosphere and into the vacuum of space to orbit for any length of time. 200 miles (320 km) up is about the minimum to avoid atmospheric interference. The Hubble space telescope orbits at an altitude of 380 miles (600 km) or so. But the principle is exactly the same. The speed of the satellite is adjusted so that it falls to earth at the same rate that the curve of the earth falls away from the satellite. The satellite is perpetually falling, but it never hits the ground!

What is AMSAT?

AMSAT is a non-profit organization of ham radio operators worldwide that uses its own membership-supported satellites. The official name for AMSAT is the Radio Amateur Satellite Corporation. Hams that belong to AMSAT participate in:

• The actual development and assembly of over 40 satellites to date
• Ground control after the satellite is in orbit
• Conversations using the satellite and listening to others using the satellite as a radio relay link

Photo courtesy AMSAT Electronic modules in equipment bay of the Phase 3D AMSAT satellite: At top left is camera equipment; along the bottom row are various receivers. (Launch date: late July 2000, on Ariane 507)

AMSAT satellites can often be heard by use of a short-wave receiver or a radio scanner. Ham operators make use of the satellites during natural disasters when terrestrial links and cell phone systems may be down or overloaded.

The AMSAT-built satellites “hitch” a rocket launch on a “payload-space-available” basis. The first AMSAT satellite orbited in 1961 and was called OSCAR (Orbiting Satellite Carrying Amateur Radio). Tracking software is available for personal computers. Various AMSAT satellites have a combination of data, image and voice capabilities.

How Much Do Satellites Cost?

Satellite launches don’t always go well, as shown by this story on failed launches in 1999. There is a great deal at stake. For example, this hurricane-watch satellite mission cost $290 million. This missile-warning satellite cost $682 million.Another important factor with satellites is the cost of the launch. According to this report, a satellite launch can cost anywhere between $50 million and $400 million. A shuttle mission pushes toward half a billion dollars (a shuttle mission could easily carry several satellites into orbit). You can see that building a satellite, getting it into orbit and then maintaining it from the ground control facility is a major financial endeavor!

Orbital Velocity and Altitude

A rocket must accelerate to at least 25,039 mph (40,320 kph) to completely escape Earth’s gravity and fly off into space (for more on escape velocity, visit this article at kidsplanet.com and this one at Northwestern University).Earth’s escape velocity is much greater than what’s required to place an Earth satellite in orbit. With satellites, the object is not to escape Earth’s gravity, but to balance it. Orbital velocity is the velocity needed to achieve balance between gravity’s pull on the satellite and the inertia of the satellite’s motion — the satellite’s tendency to keep going. This is approximately 17,000 mph (27,359 kph) at an altitude of 150 miles (242 km). Without gravity, the satellite’s inertia would carry it off into space. Even with gravity, if the intended satellite goes too fast, it will eventually fly away. On the other hand, if the satellite goes too slowly, gravity will pull it back to Earth. At the correct orbital velocity, gravity exactly balances the satellite’s inertia, pulling down toward Earth’s center just enough to keep the path of the satellite curving like Earth’s curved surface, rather than flying off in a straight line (read this page for details on orbits).

The orbital velocity of the satellite depends on its altitude above Earth. The nearer Earth, the faster the required orbital velocity. At an altitude of 124 miles (200 kilometers), the required orbital velocity is just over 17,000 mph (about 27,400 kph). To maintain an orbit that is 22,223 miles (35,786 km) above Earth, the satellite must orbit at a speed of about 7,000 mph (11,300 kph). That orbital speed and distance permits the satellite to make one revolution in 24 hours. Since Earth also rotates once in 24 hours, a satellite at 22,223 miles altitude stays in a fixed position relative to a point on Earth’s surface. Because the satellite stays right over the same spot all the time, this kind of orbit is called “geostationary.” Geostationary orbits are ideal for weather satellites and communications satellites.

The moon has an altitude of about 240,000 miles (384,400 km), a velocity of about 2,300 mph (3,700 kph) and its orbit takes 27.322 days. (Note that the moon’s orbital velocity is slower because it is farther from Earth than artificial satellites.)

• To get a better feel for orbital velocities at different altitudes, check out NASA’s orbital velocity calculator.
• To learn more about orbits and other topics in space flight, check out JPL’s Basics of Space Flight Learners’ Workbook.
• A detailed technical treatment of orbital mechanics can be found at this site.

In general, the higher the orbit, the longer the satellite can stay in orbit. At lower altitudes, a satellite runs into traces of Earth’s atmosphere, which creates drag. The drag causes the orbit to decay until the satellite falls back into the atmosphere and burns up. At higher altitudes, where the vacuum of space is nearly complete, there is almost no drag and a satellite can stay in orbit for centuries (take the moon as an example).

Satellites usually start out in an orbit that is elliptical. The ground control station controls small onboard rocket motors to provide correction. The goal is to get the orbit as circular as possible. By firing a rocket when the orbit is at the apogee of its orbit (its most distant point from Earth), and applying thrust in the direction of the flight path, the perigee (lowest point from Earth) moves further out. The result is a more circular orbit.

How Satellites Work

Not so long ago, satellites were exotic, top-secret devices. They were used primarily in a military capacity, for activities such as navigation and espionage. Now they are an essential part of our daily lives. We see and recognize their use in weather reports, television transmission by DIRECTV and the DISH Network, and everyday telephone calls. In many other instances, satellites play a background role that escapes our notice:

• Some newspapers and magazines are more timely because they transmit their text and images to multiple printing sites via satellite to speed local distribution.
• Before sending signals down the wire into our houses, cable television depends on satellites to distribute its transmissions.
• The most reliable taxi and limousine drivers are sometimes using the satellite-based Global Positioning System (GPS) to take us to the proper destination.

Satellite Image Gallery

Photo courtesy NASA NAVSTAR GPS satellite.  See more satellite pictures.

• The goods we buy often reach distributors and retailers more efficiently and safely because trucking firms track the progress of their vehicles with the same GPS. Sometimes firms will even tell their drivers that they are driving too fast.
• Emergency radio beacons from downed aircraft and distressed ships may reach search-and-rescue teams when satellites relay the signal (read this page for details).

In this article, we will show you how satellites operate and what they do. You’ll get to see what’s inside a satellite, explore the different kinds of orbits and find out why the intended use of the satellite affects the choice of orbit. We’ll even tell you how to see and track a satellite yourself!