Voyager 1 - image of 'Pale Blue Dot'

[Tebor]

Main Question or Discussion Point

Please could someone explain why earth looks to bright when it is so far away? Also, why can't we see any other objects (other planets and stars) in the image?

Thank you.

 

[MullaTheMech]

We are bright because we are relatively big and we reflect and refract a good amount of light..
Space is pretty big so it's not hard to imagine that there was no other body in our solor system besides those asteroid belts in the shot.
I think the picture is probably littered with stars.

 

[MullaTheMech]

How do you add pictures on here? Do I need to upload it to imageshack or something and paste the url?

 

[Tebor]

MullaTheMech,

Thank you. I was confused because apparently by that point at had been past all of the planets... But they could have just turned the camera round to picture the others and then they're out of shot at the end - I hadn't thought of that! :)

 

[MullaTheMech]

Well it seems I was wrong. What looked like asteroid belts to me was just light from the sun (Wikipedia). Said that the streaks came from taking the picture too close to the sun, so probably just refracted light through the lens. Google search also told me that this is 3 different pictures put back together in 3 different color filters: violet blue and green. Which would make this picture totally filled with stars. It would be odd to see that many belts in one picture I think.

 

[phyzguy]

In terms of why there are no stars in the image, we can do an estimate. Based on the distance from Earth at the time the image was taken (about 40 AU, according to this Wikipedia article), I estimate the Earth would have been about magnitude +4. From the image in that Wiki article, we can calculate the image scale of the photograph, and the whole image is about 0.5 degree across, or 0.25 square degrees. This is about the size of the full moon. There are about 500 stars in the sky brighter than magnitude +4, and the entire sky is about 40,000 square degrees. So the odds of one of these m<4.0 stars landing in an image of 0.25 square degrees is
500*0.25/40000 = 0.3%. Voyager is currently in Ophiuchus, so assuming it was in Ophiuchus at the time the image was taken (I think this is a good assumption), then it was pointing roughly back at the constellation Gemini. Attached is a map of Gemini, and the small red box is the approximate size of the Pale Blue Dot image. You can see that the odds of a 4th magnitude star being in the image is quite small.

 

[MullaTheMech]

So I did more googling and found more stuff including another thread.
https://www.physicsforums.com/index.php?threads/475985/
It seems the internet is mixing up a photo from cassini and voyager one saying on some sites that the pale blue dot is earth with saturns rings in frame. Which is cassini's photo and not voyagers.

There were other photos taken by voyager, 60 of them called Family Portrait.
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-solarsystem.html

The photos of the 6 planets taken is here.

 

[MullaTheMech]

0.25^2 degrees(or is it 0.25 degrees^2)? Can you tell me about this unit in this context please. Is this used from a point of view like: the moon has a diameter of 0.5 degrees from a viewer on earth?

 

[phyzguy]

0.25^2 degrees(or is it 0.25 degrees^2)? Can you tell me about this unit in this context please. Is this used from a point of view like: the moon has a diameter of 0.5 degrees from a viewer on earth?

Yes, that's right. So the moon covers about 0.25 square degrees, which we would write as 0.25 degrees^2. Of course the moon is round, not square, but we're just making a rough estimate. The whole sky has an area of 4*π square radians, which is 4*π*(180/π)^2 = 41,253 square degrees.

 

[Tebor]

You guys are excellent. Thank you!! It's strange to think how small a space that picture represents, and how small we look inside that space. We are tiny.