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posted: 8/7/2012 11:37 AM

Experts say gravity in space causes satellites, other space jump to fall

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  • This artist rendering provided by the U.S. Air Force, shows the AEHF-1 satellite in orbit above the earth.

    This artist rendering provided by the U.S. Air Force, shows the AEHF-1 satellite in orbit above the earth.
    Associated Press


"How do satellites and stuff like that fall out of space if there isn't any gravity in space?" asked a student in Gregg Thompson's sixth-grade social studies class at Woodland Middle School in Gurnee.

Geza Gyuk, Adler Planetarium director of astronomy, says gravity is everywhere.

"There isn't any place in the universe where there isn't at least some gravity," Gyuk said.

The force of the Earth's gravity on an object can be calculated using the distance that object is from the center of the Earth.

"Here on the surface, we are about 4,000 miles from the center. The force of gravity is 1g," Gyuk said. "If we were in a spacecraft 4,000 miles above the surface, then we would be twice the normal distance from the Earth's center. The force of gravity would be 2x2=4 times weaker."

"The International Space Station orbits at an altitude of 230 miles above the ground, so it is 4,230 miles from the center of the earth -- 1.06 times further than we are. That means gravity only is about 10 percent weaker at the ISS," he said.

How can people swim around the inside of the shuttle cabin if the effects of gravity are only 10 percent less than on Earth? It's because the shuttle has been launched into an orbit and, once there, the space vehicle is constantly falling at a very fast speed in a trajectory that follows the Earth's shape.

The same goes for satellites and other space objects, even natural ones like meteors. As they travel through space, they are constantly "falling" around some other object, such as the Earth, the sun, or even the center of the Milky Way Galaxy, Gyuk said.

Satellites and space craft are launched into space using rockets. Typically, they are placed into "low Earth orbit" somewhere above 100 miles from the earth's surface. The thrust of the rockets drive these devices some 17,000 mph, or even faster.

Satellites have a life span of 5 to 20 years, depending on the use. Once the mission is over, some satellites are moved into the higher "graveyard" orbit to lessen the chance of a collision with other satellites. In some cases, the orbit is intentionally lowered to destroy the satellite. At this final stage, the satellite or satellite parts begin to descend into Earth's atmosphere.

"When in lower orbits, where the atmosphere is thicker, it may be only a few years until the speed is reduced substantially. At a lower speed, gravity can bend the path enough during one trip around the Earth so that the object dips even lower in the atmosphere and gets slowed down even more," Gyuk said.

The U.S. Space Surveillance Network and the Inter-Agency Space Debris Coordination Committee monitor the estimated 22,000 floating space items that are larger than a softball.

The debris consists of parts from old spacecraft, abandoned launch vehicles, mission-related debris and hunks of old satellites. They can collide with other space flotsam and can be a real danger to ongoing space missions, some of which have experienced broken windows from out-of-control space objects that might be as small as a fleck of paint.

About one piece of space junk lands on Earth every day.

"The whole process of satellites returning to the Earth can be made faster when the sun is strongly active. The activity of the sun puffs up the outer atmosphere of the Earth, which means satellites slow down faster," Gyuk said.

August brings one of nature's more famous displays of space junk -- the Perseid Meteor Shower. The Adler Planetarium celebrates with a party -- the Perseid Meteor Shower Party -- at Cantigny Park in Wheaton Sunday, Aug. 12, from 7:30 to 11 p.m. Event details are at

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