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EarthSky // Astronomy Essentials, Science Wire, Space Release Date: Sep 07, 2015

How Earth looks from outer space

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

Here is Earth from 900 million miles away, from the vantage point of the rings of Saturn.  Image via the Cassini spacecraft, which has been orbiting Saturn since 2004.

Here is Earth from 900 million miles away, from the vantage point of the rings of Saturn. Image via the Cassini spacecraft, which has been orbiting Saturn since 2004. This image was acquired by Cassini on July 19, 2013.

How far away from Earth can we be, to see it still with our own eyes?

To answer this question, you have to take into account how brightly Earth reflects sunlight. And the sun itself is an important factor. As seen from any great distance, Earth appears right next to the sun; from a great distance, the glare of our local star would make Earth difficult or impossible to see. But spacecraft exploring our solar system have given us marvelous views of Earth. 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. In the daytime, you can clearly see major landforms. At night, you see the lights of Earth’s cities.

Earth in daylight, from ISS in 2012. The U.S. Great Lakes shine in the sun.  Read more about this image.

Earth in daylight, from ISS in 2012. The U.S. Great Lakes shine in the sun. Read more about this image.

ng at an altitude of about 240 miles over the eastern North Atlantic, the Expedition 30 crew aboard the International Space Station photographed this nighttime scene. This view looks northeastward. Center point coordinates are 46.8 degrees north latitude and 14.3 degrees west longitude. The night lights of the cities of Ireland, in the foreground, and the United Kingdom, in the back and to the right, are contrasted by the bright sunrise in the background. The greens and purples of the Aurora Borealis are seen along the rest of the horizon. This image was taken on March 28, 2012.

Earth at night, from ISS in 2012. Ireland in the foreground, and the United Kingdom in the back and to the right. A bright sunrise is in the background. Greens and purples shows an aurora borealis along the rest of the horizon.

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. The first images of the Earth from the moon came from the Apollo mission. Apollo 8 in 1968 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.

Earth seen from moon via Apollo 8 astronauts in 1968. Image via NASA

Then came the mind-blowing moment of seeing both the Earth and moon together in space. The next picture shows a crescent-shaped Earth and moon – the first of its kind ever taken by a spacecraft – on September 18, 1977. NASA’s Voyager 1 spacecraft was 7.25 million miles (11.66 million kilometers) from Earth at the time.

The moon (top) and Earth as viewed by Voyager in 1977. Image via NASA

Now moon exploration has become more common, though still amazing. This mosaic below shows images of Earth and the moon acquired by the multispectral imager on the Near Earth Asteroid Rendezvous Spacecraft (NEAR) on January 23, 1998, 19 hours after the spacecraft swung by Earth on its way to the asteroid 433 Eros. The images of both were taken from a range of 250,000 miles (400,000 kilometers), approximately the same as the distance between the two bodies.

This mosaic shows images of Earth and the moon acquired by the multispectral imager on the Near Earth Asteroid Rendezvous Spacecraft (NEAR) on January 23, 1998, 19 hours after the spacecraft swung by Earth on its way to the asteroid 433 Eros. The images of both were taken from a range of 250,000 miles (400,000 kilometers), approximately the same as the distance between the two bodies.  Read more about this image.

Earth and moon seen by NEAR spacecraft in 1998. Read more about this image.

The robotic Kaguya spacecraft orbited around Earth’s moon in 2007. Launched by Jpan, and officially named the Selenological and Engineering Explorer (SELENE), studied the origin and evolution of the moon. The frame below is from Kaguya’s onboard HDTV camera.

The robotic Kaguya spacecraft orbited around Earth's moon in 2007. Japan launched this scientific mission of the Selenological and Engineering Explorer (SELENE), nicknamed Kaguya, in order to study the origin and evolution of the moon. This frame is from Kaguya's onboard HDTV camera.

Earth viewed from the moon by Kaguya in 2007. Image via SELENE Team, JAXA, NHK

Another image from ___, which captured the Japanese craft got footage and stills of Earth setting.  Remember that, if you were on the moon, you would not see Earth rise or set.  But spacecraft in orbit around the moon do experience this scene.

Another image from Kaguya, which got footage and stills of Earth setting. Remember that, if you were on the moon, you would not see Earth rise or set. But spacecraft in orbit around the moon do experience this scene.

Speeding outward from the Earth and moon system, 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. From the world next door, though, Mars, a human observer with normal vision could easily see Earth and the moon as two distinct, bright evening or morning “stars.”

Earth and moon, as seen from Mars by the Curiosity rover on January 31, 2014.  Read more about this image.

Earth and moon, as seen from Mars by the Curiosity rover on January 31, 2014. Read more about this image.

This is the famous image known as Pale Blue Dot.  It's a photograph of Earth taken on February 14, 1990, by the Voyager 1 space probe from a record distance of about 6 billion kilometers (3.7 billion miles).

This is the famous image known as Pale Blue Dot. It’s a photograph of Earth taken on February 14, 1990, by the Voyager 1 space probe from a record distance of about 6 billion kilometers (3.7 billion miles). Earth is the blueish-white speck approximately halfway down the brown band to the right.

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. Of course, if an astronaut or 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 – about 9 billion miles – from home.

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