In the famous 'pale blue dot' photo taken by Voyager 1 why are other planets/stars not obvious?

Question

This question is one of a few questions my (then) 8year old daughter asked me while I was showing her the Ultra Deep field images taken by Hubble and the image which Carl Sagan poetically described as the pale blue dot (home) so many years ago.

She asked some questions which stumped me and we attempted to contact NASA to see if we could get an answer, but never received a reply.


1) How did the Voyager take a photo from 4 billion miles away?

2) How do you measure four billion miles in space?

3) How close to the sun was the Voyager if the earth is shown in a sunbeam?

4) How was it determined that the 'pale blue dot' was earth and not a larger planet or star?

5) Why don't we see any other stars, planets, etc. in the photo from that distance?

Answer 1

A) In the famous 'pale blue dot' photo taken by Voyager 1 why are other planets/stars not obvious?

Answer: First of all, the planets are not all lined up to fit conveniently in the field of the picture. Some of them may be on the other side of the sun. The brightness of the sun was a real challenge because, even at that great distance, the camera had to be maneuvered not to look *directly* at the sun. Mercury and Venus would have been too close to the sun, trying for them would have zonked the delicate electronics. So if you want Earth, you position for Earth, where all the other planets are is a separate issue. At four billion miles you can "look around" the solar system the same way you can "look around" a football stadium from one of the high seats. But you wouldn't be able to photograph, from your seat, the ENTIRE stadium in ONE picture.

The stars do not show for a different reason. The camera has to be stopped down for bright conditions and so, it is not going to pick up faint objects. If you ever walk from the bright outside into the house with no lights you'll notice it seems pretty dark in there. After your pupils open you see more. The camera has the same problem but we tell it not to open its pupils because the sun is so bright in Earth's neighborhood. So the stars don't show.

Question. This question is one of a few questions my (then) 8year old daughter asked me while I was showing her the Ultra Deep field images taken by Hubble and the image which Carl Sagan poetically described as the pale blue dot (home) so many years ago.

Answer: This isn't your question but I'm answering the comment to emphasize that Hubble is a massive, huge telescope designed to work at distances "at infinity" whereas the much smaller telescope on Voyager was designed to work on planets at closer ranges. It is not a comparable instrument. There was a debate about the Pale Blue Dot shot--some engineers that a slight mistake (getting too much sunlight) would ruin the instrument.


1) How did the Voyager take a photo from 4 billion miles away?

Answer: It's a robot. It points and shoots where we tell it. Amazingly, it was still functioning, still transmitting, and still able to follow orders.

2) How do you measure four billion miles in space?

Answer: space robots can be "pinged." You send them a signal "Are you there?" which they receive and respond to. You measure the time from when you transmit "Are you there?" to the time you get the "I'm here!" and divide by two. Take that elapsed time and use the speed of light, you get the distance.

3) How close to the sun was the Voyager if the earth is shown in a sunbeam?

Well, it's not quite right, as someone said, that you just add 93 million miles to the 4 billion from Earth, because that distance assumes Voyager is in a "straight line" between Earth and the sun. Voyager was at an angle. I don't know the specifics but it could have been "on the other side" of the sun from us. The question of where Earth is positioned in its orbit from the sun adds about 2-4% uncertainty to the 4 billion miles number.

As for the sunbeam: be aware that the Earth was not "in a sunbeam" in that picture. Earth is flooded with light but is in inky blackness. What happened was that some light from the sun hit the satellite and reflected into the telescope's optics. This is called off-axis stray light. On Earth if you have a telescope your neighbor's porch light can be an annoying source of off-axis stray light and make you see rays in the eyepiece that aren't really in the sky. So telescope owners get crabby and drive out into the country side, and put up large shields on their telescopes, to control off-axis light.

In this case the off-axis stray light had a poetic effect even thought it undoubtedly degraded the image. An engineer probably would have edited it out but Carl Sagan, the poet, probably said "No! Leave that beam in there!" It is probably one of the few times that off axis stray light was ever welcomed by a telescope operator.

4) How was it determined that the 'pale blue dot' was earth and not a larger planet or star?

Well first off they had the position. Second, they have a good idea about Earth's LUMINOSITY, and can calculate how that luminosity would show up in their camera. Third, they have an excellent idea of the ANGULAR DIAMETER, that is, how many pixels on the camera the image of the Earth should take up.

When you hug your daughter and she buries her face in your blouse your own ANGULAR DIAMETER appears huge to her. Probably 150 degrees. As you walk away from her, she sees your ANGULAR DIAMETER as smaller. You shrink to one degree, then parts of a degree (arc minutes) and then parts of an arc minute (arc seconds). If she goes way high up on a mountain and looks down, she probably sees you as thirty arc seconds big. The tiny dot is you and she sees the rest of the world around you. The relationship between known size and how big it looks to you, and the distance, is very well established. If I take your picture in a typical portrait you cover thousands of pixels in my camera. If I take your picture from high up on a mountain you will cover only a few pixels in the camera and make a real tiny image. All these things are very reliable indicators of size and distance.

So, if the planet was too big the image would cover more pixels on the camera and they would say "oops!" As for stars, they have STAR ATLASES. (These are very cool, I have several). So you figure out where you are pointing the telescope, and look in the atlas to see if there are any bright stars in the immediate neighborhood. This is going to be a very teeny neighborhood and the number of stars that are bright enough to show up close to the sun is very small and we know them quite well. The other stars are going to lack the LUMINOSITY to show up in the picture: they know because they know the EXPOSURE TIME (the length of time the camera will collect) and the f/stop (how much light the camera will open up to accept) they are going to set the camera for.

5) Why don't we see any other stars, planets, etc. in the photo from that distance?

Well, the people who don't believe in the Moon landings have a hard time with this one, but the answer is in the nature of photography: It is hard to photograph stars when it is bright. Look at it this way. If you go to Wal Mart at night and look up from the parking lot, you will see a very few stars in the sky. But if I try to photograph you and the stars over your head I have a huge problem. I need light to photograph you, but if I take in enough light to get the stars, my picture will be a washed out mess from all the extra light that lets me take a picture of you in the same parking lot. As a practical matter I can't do the stars. So the sky will show as black or it will show the arc lights, but it won't show stars.

Earth is just bright enough, compared to the stars, that it could show up even though the stars were too dim to be photographed. As you can see from the picture, it was "marginal," if the Earth had been much dimmer, it wouldn't have been possible.

Hope that helps,

GN

Answer 2

Your daughter sounds very precocious!

1) The earth is pretty bright. A well-lit earth from 4 billion miles would look brighter than a well-lit Neptune from your back yard; and you can see Neptune with your naked eye (if the sky is very dark).

2) The scientists know exactly where Voyager is in space at all times (see: http://www.heavens-above.com/solar-escape.asp). They can make good predictions of its position because it moves according to well-known laws of gravity; they can tell how fast it's receding using the doppler effect, and by measuring the time it takes to respond to a signal, they can tell how far away it is.

3) That's kind of an optical illusion. That sunbeam is not really passing through the earth; it's sort of the same thing you get when you aim your camera too close to the sun: you see "beams" of light in the picture, that are really just caused by light reflecting within the glass of your camera lens. Still, the sun would have appeared pretty close in the frame. If you were riding along with Voyager, you'd be able to blot out both the sun and the earth with your thumb held at arm's length.

4) The scientists are able to aim the cameras very precisely. They purposely aimed the camera to center exactly on the earth.

5) They used a very small field of view. Also, the picture was taken with green and violet filters, to highlight the earth's colors and dim out other light sources.

Answer 3

1) It pointed its camera towards (not at) the Sun, and Earth is barely visible over the dust in the solar system and other stars are not.

2) The easiest way is with radar, or using the time the spacecraft takes to respond to a radio command.

3) A little over 4 billion miles.

4) They new where Earth would be in the picture from the precision at which they can direct the camera.

5) The Earth was much brighter than any stars in the background. You could find some stars in the raw image (not the JPEG) if you used some simple transforms.

Answer 4

Well, it's actually a false-image photo; the image of the planets are highlighted, while the background stars are reduced.

1. It still had functioning cameras; it was easy to take the picture & transmit it.
2. Generally by how long the signal takes to get to/from the spacecraft. But, they know it's speed, and how long it's been out there so - speed = velocity X time.
3. um... Voyager was about 4 billion miles out there; 93 million miles (the distance from earth to sun) wouldn't matter much.
4. They know the location of the Earth, and determined that the spot seen WAS Earth.
5. If it's the same photo I'm thinking of - you do... Except for Pluto, if I recall correctly - but it's not a planet anymore anyway.

Answer 5

well, there are thousands of stars in the picture, they are just not as bright nor as large as the Earth. The exposure is set to show the Earth, and not the stars. If you take a picture of the moon, you will probably not see any stars, since the moon is too bright in comparison.

There are no other planets because they are not in the frame. It is just a lack of coincidence.

:-D Wow it is interesting how many guys answered the questions of a pretty lady! Sorry, I am busy now.