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posted: 12/20/2013 12:19 PM

Light gravitational pull allows astronauts to walk on moon

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  • In this July 20, 1969, file photo, Astronaut Edwin E. "Buzz" Aldrin, Jr., lunar module pilot, descends the steps of the Lunar Module ladder as he prepares to walk on the moon.

    In this July 20, 1969, file photo, Astronaut Edwin E. "Buzz" Aldrin, Jr., lunar module pilot, descends the steps of the Lunar Module ladder as he prepares to walk on the moon.
    Courtesy AP/NASA,


You wanted to know

Nancy Sullivan's sixth-graders from Frederick Nerge Elementary in Roselle asked, "Why do objects float in space, but you can still walk on the moon?"

Nearly 45 years ago, astronaut Neil Armstrong sunk his oversized space boot into the moon's fluffy topsoil. He was the very first person to make that step.

His partner, Buzz Aldrin, the second person to place a footprint on the moon's surface, said the experience was like walking on a giant trampoline.

"The feeling of reduced gravity and the limitations of the space suit resulted in a slow-motion movement. Perhaps not too far from a trampoline, but without the springiness and instability," Aldrin said in a 1998 interview with Scholastic.

Gravity is a phenomenon experienced throughout the universe that gives weight to all objects. Weight is a measure of gravitational pull. Since the moon is one-fourth the size of the Earth and about 1/80th the mass, things on the moon weigh only 1/6th as much as on Earth. A 70-pound dog would look exactly the same if it played catch on the moon, but it would weigh a very slim 11.6 pounds.

Dr. Geza Gyuk, director of astronomy at the Adler Planetarium, described gravity: "Gravity is a 'together' force. It pulls objects together. Normally we don't notice this because the force is so very small between regular-sized objects. But if one object is very large, like a planet, then the force is noticeable."

Gravity holds us to the Earth's surface because the Earth's pull is equal to our weight, Gyuk explained.

"The moon isn't nearly as big as the Earth, but it is still very large and so objects near it will be pulled toward it. The astronauts walking on the moon felt the tug of gravity toward the moon.

"Since the moon isn't as massive, the force pulling them toward the moon's surface was only about 1/6th as strong as the force that holds us on the ground here on Earth."

The lighter gravitational pull is enough to keep astronauts walking on the moon's surface, even if steps were more like leaps or large hops.

If there is only a little gravity on the moon, does it mean that there is no gravity in space? Is that why astronauts and objects inside space vehicles are weightless?

"The reason things fly in space is that the astronauts inside the vehicle are falling," Gyuk responded. "They don't feel the force of gravity because they are falling 'around' the Earth. Space craft in orbit have a very, very large sideways motion. By the time they would have fallen to the ground, they have already swung right around the Earth. They keep falling and missing the ground."

Want to learn more about space? Chicago's Adler Planetarium has experts on hand every Monday through Friday from 2 to 3 p.m. On Saturdays, talk to scientists from the University of Chicago's Kavali Institute of Cosmological Physics, available each week from 1-3 p.m.

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