Earth Orbit

 

Before the 1950s, the only objects orbiting Earth were the moon and a few asteroids. Then in 1957 the Soviet Union launched the first artificial satellite to successfully orbit the Earth, Sputnik 1. In the decades that followed, we decided to fill our skies. Today there are thousands of artificial satellites orbiting our planet including communications, television, global positioning, weather and military satellites, and scientific probes and telescopes like the Hubble Space Telescope. There now also people constantly orbiting the Earth in the International Space Station.

If you go outside on a clear night within a few hours after sunset and patiently watch the sky you will eventually see a faint light moving among the stars, too slow and distant to be an airplane. Assuming you are not about to be abducted by extraterrestrials, this is an artificial satellite lit by reflected sunlight. You will not see any satellites in the middle of the night because the Earth is blocking any sunlight from reflecting off satellites in the sky above you.

There are many computer programs and websites that will predict visible satellites for your location. The best satellites to see are the International Space Station and Iridium flares, which are bright bursts of light caused by sunlight bouncing off the highly reflective aluminum antenna on an Iridium satellite.

Heavens Above is a great site for satellite pass predictions and Iridium flares. For example, here are current ISS orbit data from this site.

 

Artificial Satellites

The Clarke Belt

All geostationary satellites orbit the Earth equatorially (in an east-west circle) at a height of approximately 22,237 miles. At this height they orbit the Earth at the same speed as its rotation, so they appear stationary to a ground observer. This orbit is called the Clarke Belt and is named after science fiction writer Arthur C. Clarke who first published the concept of artificial geostationary satellites in 1945. Communications, television, and weather satellites like the Geostationary Operational Environmental Satellites (GOES) are found in the Clarke belt. A major advantage of geostationary satellites is that the communications dish on the ground need not move to track the satellite. A disadvantage is that geostationary satellites are subject the same period of night as the ground beneath them, making them a very poor choice for imaging satellites.

Low Earth Orbit

Other satellite orbits are equatorial (east-west circle), polar (north-south circle) and elliptical, the latter having an orbit that changes drastically in altitude during a single orbit. Besides those in elliptical orbits, non-geostationary satellites orbit at a much lower altitude than the Clarke Belt, typically between 100 and 12,000 miles. For example, the International Space Station orbits the Earth at an altitude of about 240 miles. The Hubble Space Telescope orbits at about 375 miles. When the Hubble Space Telescope was deployed in 1990 (shuttle mission STS-31R) the shuttle reached a record altitude of 385 miles.

Military imaging satellites orbit at altitudes ranging from 100 to 300 miles. This lower altitude means that they can take sharper, more detailed pictures of the ground. Other spy satellites that do electronic surveillance orbit from 600 to 1,200 miles. Navigation satellites like the U.S. GPS and the equivalent Russian GLONASS systems operate from 6,000 to 12,000 miles altitude. Satellites in elliptical orbits swing from as close as 250 miles out to as far as 60,000 miles. Intelligence agency communications satellites and early-warning satellites to detect the launch of nuclear ICBMs have elliptical orbits.


Solar Synchronous Satellite Orbit

Many polar orbits are calculated to be solar synchronous, meaning that the plane of the orbit remains at a constant angle to the light of the sun. The satellite will therefore always pass over a ground point at the same local time, and these points on the ground will always be illuminated by the sun.

 

Orbital Decay

This graph is from Heavens Above.
"This plot shows the orbital height of the ISS over the last year. Clearly visible are the re-boosts which suddenly increase the height, and the gradual decay in between. The height is averaged over one orbit, and the gradual decrease is caused by atmospheric drag. As can be seen from the plot, the rate of descent is not constant and this variation is caused by changes in the density of the tenuous outer atmosphere due mainly to solar activity."

 

The Lagrangian Points

The Earth-Moon Lagrangian Points

These are the five Lagrangian Points where a gravitational equilibrium between the Earth and the Moon and a smaller third body is achieved. The third body will remain stationary in that spot relative to the two larger bodies without needing any thrust corrections to maintain the orbit. Well, this is mostly true. L4 and L5 in the picture are truly stable; that is to say if the third body at this point is bumped out of position, it will drift back to its original position. The L1, L2, and L3 points are only stable for a time. If the third body at any of the other three points is bumped, it will drift toward one of the larger bodies unless its orbit is corrected. Potential bodies to place in the Lagrangian Points are a space stations, telescopes, or other any other artificial satellite or probe.

Any system with two large bodies and a smaller third body will have five Lagrangian Points. So, there are five different Lagrangian Points in the Sun-Earth system. In this system NASA has a spacecraft called SOHO, the Solar and Heliospheric Observatory, sitting at L1, the point between the two larger bodies. Apparently it can orbit there for 23 days before needing a course correction. If it fails to make a course correction it will either fall toward the Earth of the Sun depending on which direction it happened to drift away from the L1 point.

The L2 point on the night side of the Earth in the Sun-Earth system has been chosen by NASA for the future site of a large infrared observatory, the James Webb Space Telescope (JWST) due to launch in 2010. In order for the telescope to effectively detect faint infrared signals from very distant objects it must be kept very cold. At the L2 point, the Sun, Earth, and Moon will all be in the same about the same direction from the telescope. The JWST will take advantage of this convenient fact by blocking all light from these bodies, which would otherwise heat up the telescope, with a large shield. Unfortunately at this position it will be out of reach of the space shuttle and NASA will not have the luxury of being able to make repairs as was done with the Hubble Space Telescope. NASA currently has a probe called Wilkinson Microwave Anisotropy Probe (WMAP) sitting at the L2 point in the Sun-Earth system. It was launched in 2001. The L3 point on the far side of the Sun is of no interest to NASA since at this point the gravitational pull of the other planets has more effect than that of Earth. Also, the sun would effectively block radio communication between L3 and Earth.

Another interesting bit of trivia is that the L4 and L5 points in the Sun-Jupiter system are occupied by several hundred Trojan asteroids, all named after heroes from the Iliad.

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Please contact Adam if you have questions or comments about this page. Research and image sources are provided when possible.

Sources:

Images:
Images are shown here for noncommercial educational purposes.
The cute sketch of satellite orbits is from a How Satellites Work site.
The image of a solar synchronous orbit is from a European Space Agency site.
The graphic of the Lagrangian points is from a New Scientist article on next generation space stations.