ilan
09-07-2018, 12:06 PM
How far could you travel and still see Earth?
Deborah Byrd in ASTRONOMY ESSENTIALS | September 4, 2018
You couldn’t see Earth from another star. From even the closest stars, the sun’s glare makes Earth impossible to see. And inside our own solar system? Spacecraft photos tell the tale.
http://en.es-static.us/upl/2012/07/earth-from-mars.jpg
Earth and moon, as seen from Mars by NASA’s Curiosity rover on January 31, 2014
How far away from Earth can we be and see it still with our own eyes?
To find the answer, let’s take an imaginary trip through the solar system, to see how Earth looks from various other places, in our own neighborhood of space. Spacecraft exploring our solar system have given us marvelous views of Earth.
First, imagine blasting off and being about 200 miles (300 km) above Earth’s surface. That’s about the height of the orbit of the International Space Station (ISS). From the window of ISS, the surface of the Earth looms large. In the daytime, you can clearly see major landforms. At night, from Earth orbit, you see the lights of Earth’s cities.
Let’s get farther away, say, the distance of the orbit of the moon.
As we pass the moon – some quarter million miles (about 380,000 km) away – Earth looks like a bright ball in space.
It’s not terribly 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 Earth.
http://en.es-static.us/upl/2001/07/apollo_8_earthrise.jpeg
Earth seen from moon via Apollo 8 astronauts in 1968. Image via NASA.
In the decades since Voyager first began traveling outward, moon exploration has become more common. The robotic Kaguya spacecraft orbited around Earth’s moon in 2007. Launched by Japan, and officially named the Selenological and Engineering Explorer (SELENE), Kaguya studied the origin and evolution of the moon.
Speeding outward from the Earth and moon system, you pass the orbits of the planets Mars, Jupiter and Saturn. From all of these worlds, Earth looks like a star – which gets fainter as you get farther away. (See the Earth from Mars in the intro image.)
http://en.es-static.us/upl/2012/07/Pale_Blue_Dot.png
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 image above is from Saturn, the sixth planet outward in orbit around the sun. I’ve never seen any image of Earth from Uranus or Neptune or any other body beyond Saturn’s orbit. Only five spacecraft from Earth – the two Voyager spacecraft, the two Pioneers, and the New Horizons spacecraft, which passed Pluto in 2015 – have ever ventured that far. Those craft weren’t designed to look back at Earth, and, to my knowledge, they didn’t capture images of Earth from distances beyond Saturn.
But, speaking theoretically now, could Earth be seen from distances beyond Saturn?
Speaking only in terms of Earth’s brightness, the answer is yes. Our world doesn’t become too faint to see with the eye alone until far beyond Neptune’s orbit, at around 9 billion miles (14 billion km) from home. Now consider Pluto’s orbit. It’s highly elliptical, stretching from just 2.7 billion miles (4.4 billion km) to over 4.5 billion miles (7.3 billion km) from the sun. Pluto is within the limiting distance at which – if we just consider brightness alone, no other factors – we should be able to see Earth with the eye alone.
But there is another factor. As you go outward from Earth, our world appears closer and closer to the blazing sun. As you get farther away, the sun’s glare begins to overwhelm the view of Earth. From Pluto – even though Earth would be bright enough to see – you probably couldn’t see it in the sun’s glare.
So that is the answer. Although no one knows for sure because no one has tried it (and because human eyesight varies from person to person), the Earth would become impossible to see with the eye somewhere beyond Saturn’s orbit.
Deborah Byrd in ASTRONOMY ESSENTIALS | September 4, 2018
You couldn’t see Earth from another star. From even the closest stars, the sun’s glare makes Earth impossible to see. And inside our own solar system? Spacecraft photos tell the tale.
http://en.es-static.us/upl/2012/07/earth-from-mars.jpg
Earth and moon, as seen from Mars by NASA’s Curiosity rover on January 31, 2014
How far away from Earth can we be and see it still with our own eyes?
To find the answer, let’s take an imaginary trip through the solar system, to see how Earth looks from various other places, in our own neighborhood of space. Spacecraft exploring our solar system have given us marvelous views of Earth.
First, imagine blasting off and being about 200 miles (300 km) above Earth’s surface. That’s about the height of the orbit of the International Space Station (ISS). From the window of ISS, the surface of the Earth looms large. In the daytime, you can clearly see major landforms. At night, from Earth orbit, you see the lights of Earth’s cities.
Let’s get farther away, say, the distance of the orbit of the moon.
As we pass the moon – some quarter million miles (about 380,000 km) away – Earth looks like a bright ball in space.
It’s not terribly 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 Earth.
http://en.es-static.us/upl/2001/07/apollo_8_earthrise.jpeg
Earth seen from moon via Apollo 8 astronauts in 1968. Image via NASA.
In the decades since Voyager first began traveling outward, moon exploration has become more common. The robotic Kaguya spacecraft orbited around Earth’s moon in 2007. Launched by Japan, and officially named the Selenological and Engineering Explorer (SELENE), Kaguya studied the origin and evolution of the moon.
Speeding outward from the Earth and moon system, you pass the orbits of the planets Mars, Jupiter and Saturn. From all of these worlds, Earth looks like a star – which gets fainter as you get farther away. (See the Earth from Mars in the intro image.)
http://en.es-static.us/upl/2012/07/Pale_Blue_Dot.png
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 image above is from Saturn, the sixth planet outward in orbit around the sun. I’ve never seen any image of Earth from Uranus or Neptune or any other body beyond Saturn’s orbit. Only five spacecraft from Earth – the two Voyager spacecraft, the two Pioneers, and the New Horizons spacecraft, which passed Pluto in 2015 – have ever ventured that far. Those craft weren’t designed to look back at Earth, and, to my knowledge, they didn’t capture images of Earth from distances beyond Saturn.
But, speaking theoretically now, could Earth be seen from distances beyond Saturn?
Speaking only in terms of Earth’s brightness, the answer is yes. Our world doesn’t become too faint to see with the eye alone until far beyond Neptune’s orbit, at around 9 billion miles (14 billion km) from home. Now consider Pluto’s orbit. It’s highly elliptical, stretching from just 2.7 billion miles (4.4 billion km) to over 4.5 billion miles (7.3 billion km) from the sun. Pluto is within the limiting distance at which – if we just consider brightness alone, no other factors – we should be able to see Earth with the eye alone.
But there is another factor. As you go outward from Earth, our world appears closer and closer to the blazing sun. As you get farther away, the sun’s glare begins to overwhelm the view of Earth. From Pluto – even though Earth would be bright enough to see – you probably couldn’t see it in the sun’s glare.
So that is the answer. Although no one knows for sure because no one has tried it (and because human eyesight varies from person to person), the Earth would become impossible to see with the eye somewhere beyond Saturn’s orbit.