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| Space on Jul 08, 2012

How far away in space can you still see Earth?

If you were looking with the eye alone, how far away in space would our planet Earth still be visible?

“How far away from Earth do we have to go to not see it with our own eyes?”

To answer this question, you have to take into account how brightly Earth reflects sunlight. And the sun itself is another important factor. As seen from any great distance, Earth appears right next to the sun – and, from a great distance, the glare of our local star would make Earth difficult or impossible to see.

Earth seen from moon via Apollo 8 astronauts in 1968. Apollo 8 was the first human spaceflight to leave Earth orbit. It was the first earthly spacecraft to be captured by and escape from the gravitational field of another celestial body, in this case the moon. It was the first voyage in which humans visited another world and returned to return to Earth. Image Credit: NASA

This picture of a crescent-shaped Earth and Moon -- the first of its kind ever taken by a spacecraft -- was recorded Sept. 18, 1977, by NASA's Voyager 1 when it was 7.25 million miles (11.66 million kilometers) from Earth. The Moon is at the top of the picture and beyond the Earth as viewed by Voyager. Image Credit: NASA

So imagine blasting off and being about 300 kilometers – about 200 miles – above Earth’s surface. That’s the height at which the International Space Station (ISS) orbits. The surface of the Earth looms large in the window of ISS. You can clearly see major landforms, and the lights of cities.

As you pass the moon – about 380,000 kilometers away – or a quarter million miles – Earth looks like a bright ball in space – not very different from the way the moon looks to us.

Speeding outward, you pass the orbits of the planets Mars, Jupiter, Saturn, Uranus and Neptune. From all of these worlds, Earth looks like a star – which gets fainter as you get farther away.

The light from Earth finally becomes too faint to see with the eye alone at around 14 billion kilometers – about 9 billion miles – from home – around the outer limit of our solar system – nowhere near as far as even the next-nearest star.

By the way, the Voyager 1 spacecraft is now farther away than that. In February 2012, it was about 18 billion kilometers (11 billion miles) from our sun.

If an astronaut/alien had a telescope, he or she could definitely see Earth further away than that.

Bottom line: How far away in space can you view Earth with the eye alone? About as far away as the outer reaches of our own solar system at about 14 billion kilometers, or 11 billions miles.

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