What are satellites? How do we use them? And why are there so many of them hanging out in Earth's orbit? Everyday Einstein answers these and many other questions about satellites.
Satellites in low earth orbit are so close (300-1,250 miles up) that they have to move very quickly (more than 15,000 miles per hour) to balance the strong force of gravity. They can make one lap around the Earth in two hours and are usually tracked by antennae on the ground. Both the Hubble Space Telescope and the International Space Station are in low Earth orbits as are many weather-tracking satellite.
Satellites in geostationary orbits (or GEO satellites) appear fixed over one location on the Earth and can typically see around 1/3 of the Earth’s surface at once. In order to match the speed of the Earth’s rotation, GEO satellites are typically placed around 22,000 miles above the Earth and move close to 7,000 miles per hour at the equator.
The Geostationary Operational Environmental Satellites or GOES are examples of geostationary satellites that look for changes in weather patterns that can lead to storms as well as track ice and sea levels.
GPS (Global Positioning System) satellites are not usually geostationary which is why you sometimes have trouble getting a signal when you first start using a new GPS or when you take your GPS to a new location. Since GEO satellites have to be near the equator to stay fixed over a rotating Earth, they do not provide good coverage for locations farther north or south.
To provide coverage closer to the Earth’s poles, some satellites are placed in highly elliptical orbits that take the shape of a very stretched circle, or ellipse. These satellites move more slowly when they are farther from the Earth and so appear relatively stationary during that period of time from the Earth’s perspective. These satellites thus mimic geostationary orbits and can be placed at large angles askew from the equator so that they cover northern and southern areas. The use of highly elliptical orbits was pioneered by the Russians since they are so far north.
Once a satellite is no longer functional, it becomes orbital debris. NASA estimates that there are about 20,000 pieces of orbital debris larger than the size of a softball and about 500,000 pieces larger than a marble. Any orbital debris that is large enough to be tracked is dubbed “space junk.” NASA and the Department of Defense together track over 21,000 pieces of space junk.
However, the largest threat to astronauts and working satellites is actually the stuff that’s too small to track. Even something as small as a fleck of paint can cause major damage when traveling at speeds of 17,000 miles per hour.
The crowding of space with such debris is becoming a real issue (and not just a science fiction scenario like the one that endangered George Clooney and Sandra Bullock in the movie Gravity). In 2007, a Chinese mission to destroy a defunct satellite resulted in an estimated 3,000 additional pieces of orbital debris.
In 2009, a U.S. Iridium communication satellite (used for satellite phones) collided with a Russian satellite adding thousands more independent chunks to the orbital highway. A major effort to clean up space is needed to avoid endangering the working satellites we rely on, as well as future manned space missions.
Thanks to the large numbers of working satellites in addition to all this space junk, with a little patience and a wide view of the sky, you should be able to see as many as 10-20 satellites per hour. In a comfortable, reclined position, scan the sky with a set of good binoculars just after sunset or just before sunrise, so that the Sun is no longer bright in the sky but is still high enough that its light reflects off passing satellites.
Look for bright points of light that streak across the sky and quickly fade out. You can easily tell them apart from planes which move much more slowly and steadily and are often accompanied by blinking lights.
One of the most searched for satellites by amateurs on the ground is the International Space Station (ISS). Launched in 1998 in a joint effort between the U.S. and Russia, the ISS is the largest artificial satellite weighing in at just under 1 million pounds. Orbiting at around 250 miles from Earth, ISS provides a microgravity environment good for studying biology, physics, and astronomy, and has been continuously occupied since November of 2000.
ISS is so large that you can actually see it with your naked eye. You just need to know where and when to look! Check out NASA’s Spot the Station web site for details.
If you have a question that you’d like to see on a future episode, send me an email at firstname.lastname@example.org or post it on my Facebook page.
Until next time, this is Dr. Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science..