Pale Blue Dot
The Pale Blue Dot is a photograph of planet Earth taken in 1990 by the Voyager 1 space probe from a record distance of about 6 billion kilometers (3.7 billion miles, 40 AU) from Earth, as part of the solar system Family Portrait series of images. In the photograph, Earth is shown as a fraction of a pixel (0.12 pixel in size) against the vastness of space. The Voyager 1 spacecraft, which had completed its primary mission and was leaving the Solar System, was commanded by NASA to turn its camera around and to take a photograph of Earth across a great expanse of space, at the request of Carl Sagan.
Subsequently, the title of the photograph was used by Sagan as the main title of his 1994 book, Pale Blue Dot: A Vision of the Human Future in Space.
The Voyager 1 spacecraft is a 722-kilogram (1,592 lb) robotic American space probe launched by NASA on September 5, 1977, to study the outer solar system and eventually interstellar space. Operating for 36 years, 11 months and 13 days as of today (20 August 2014), the spacecraft receives routine commands and transmits data back to the Deep Space Network. It is the first probe to leave the solar system and is the farthest man-made object from Earth.
The spacecraft is currently in extended mission, tasked with locating and studying the boundaries of the Solar system, including the Kuiper belt, the heliosphere and interstellar space. The primary mission ended on November 20, 1980, after encountering the Jovian system in 1979 and the Saturnian system in 1980. It was the first space probe to provide detailed images of the two largest planets and their major moons.
The Voyager 1 spacecraft was initially expected to work only through the Saturn encounter. When the spacecraft passed Saturn in 1981, Sagan promoted the idea of the spacecraft taking one last picture of Earth. He pointed out that this picture would not be mainly scientific, as the Earth would appear too small for the Voyager's cameras to make out any detail, but such a picture might be useful as a perspective on our place in the cosmos. Though many in NASA's Voyager program were supportive, most were of the opinion that taking a picture of Earth close to the Sun risked damaging the spacecraft's video system. By the end of 1989, instrument calibrations delayed the photographs further. The technicians who devised and transmitted the radio commands to Voyager 1 were also being laid off or transferred to other jobs. Finally, then-NASA Administrator Richard Truly interceded to ensure that the photograph was taken. The Pale Blue Dot is a narrow-angle photograph. NASA's Jet Propulsion Laboratory (JPL) has also published a composite image consisting of a portion of a wide-angle image containing the sun and the region of space where the Earth and Venus were at the time, inset with two narrow-angle pictures centered on each planet.
The Pale Blue Dot was taken when the Voyager 1 spacecraft reached the edge of the solar system, 12 years after its launch and travelling at 40,000 miles per hour (64,000 km/h) at a record distance of about 6 billion kilometers. On February 14, 1990, having completed its primary mission, the spacecraft was commanded by NASA to turn around and photograph the planets of the solar system. The design of the command sequence to be relayed to the spacecraft and the calculations for each photograph's exposure time were developed by space scientists Candy Hansen of JPL and Carolyn Porco of the University of Arizona. The NASA imaging team photographed the outer planets first because they were worried that pointing the camera near the sun would blind the spacecraft's cameras and prevent more pictures from being taken. Between February 14, 1990 and June 6, 1990, Voyager 1 returned 60 frames back to Earth, which were stored in an on-board tape recorder after being taken. One of the pictures returned was of Earth showing up as a pale blue dot in the grainy photograph.
The Pale Blue Dot was also shown as a portion of a wide-angle image containing the sun and the region of space where the Earth and Venus were at that time. The wide-angle image was inset with two narrow-angle pictures centered on Earth and Venus. It was taken with the camera's darkest filter (a methane absorption band), and the shortest possible exposure (5 milliseconds) to avoid saturating the camera's vidicon tube with scattered sunlight. The sun appears small in the sky as seen from Voyager's perspective at the edge of the solar system, but is still eight million times brighter than the brightest star in Earth's sky – Sirius. The image of the sun in the photograph is far larger than the actual dimension of the solar disk. The result of the brightness is a bright burned-out image with multiple reflections from the optics in the camera. The rays around the sun are a diffraction pattern of the calibration lamp which is mounted in front of the wide-angle lens. The image was composed of 640,000 individual pixels. In the photograph, Earth lies in the center of one of the scattered light rays resulting from taking the image with a small angle between the Sun and the Earth. Earth takes up less than a single pixel (only 0.12 pixel in size as referred to by NASA). The radio signal which carried the image back to Earth, travelling at the speed of light, took nearly 5 hours and 30 minutes to reach Earth. As the Deep Space Network was preoccupied with the Magellan and Galileo missions, the image reception was delayed. Detailed analysis of the Pale Blue Dot also suggested that Voyager 1 detected the Moon as well, but it is too faint to be seen without special processing.
Effects of polarization and scattering of light
The Earth appears as a pale blue dot because of the effects of polarization and scattering of the light reflected from Earth. The polarization effect of planet Earth depends on various factors such as cloud cover, exposed areas of oceans, forests, deserts and snow fields, etc. The contribution of each surface type is a combination of Rayleigh scattering above the surface and reflection at the surface. The integrated effect causing Earth to appear as a pale blue dot depends on wavelength, scattering angle and cloud cover. The degree of polarization is particularly strong in the blue region of the visible spectrum due to Rayleigh scattering in the atmosphere. The degree of polarization for Earth to appear as a pale blue dot at 443 nanometers and 90° scattering angle, as calculated using polarization observations of Earth made from the POLDER satellite radiometer, is 23% for 55% (average) cloud cover and up to 40% for 10% (minimal) cloud cover.
NASA's Jet Propulsion Laboratory's HORIZONS web interface system provides a web-based limited interface to its HORIZONS system which can be used to generate ephemerides for Solar-System bodies. According to this tool, the distances between Voyager and the Earth on February 14, 1990 and June 9, 1990 were as follows: a
The picture was taken by Voyager 1 using the Voyager imaging science subsystem's narrow-angle camera. Narrow-angle cameras (1500 mm focal length), as opposed to wide-angle cameras, are equipped to photograph specific details in an area of interest. The Voyager imaging science subsystem (ISS) is a modified version of the slow scan vidicon camera designs that were used in the earlier Mariner flights. Unlike the other on-board instruments, operation of the camera is not autonomous, but is controlled by an imaging parameter table residing in one of the spacecraft computers – the Flight Data Subsystem (FDS). As the Voyager mission progressed, the objects being photographed were getting further away from the spacecraft and so were appearing fainter, even-though longer exposures were used. As the Voyager 1 spacecraft's distance from the Earth increased, the telecommunication capability decreased. The reduced telecommunication capability limited the number of data modes that can be used by the imaging system. In addition, the camera was slewed (a slight panning or tracking movement to compensate for the brief time exposure) in order to avoid smeared imaging. The picture was taken at 32° above the ecliptic and it was created using blue, green, and violet filters with exposure times for each filter being 0.72, 0.48 and 0.72 seconds respectively. In the photograph, the light band over Earth is an artifact: sunlight is scattering off parts of the camera and its sunshade.
After taking the Family Portrait images including the Pale Blue Dot, NASA mission managers commanded Voyager 1 to power its camera down as the spacecraft was not going to fly near anything else of significance in the near future, and other instruments that were still collecting data needed power for the long journey to interstellar space.
Reflections by Sagan
In his book Pale Blue Dot: A Vision of the Human Future in Space, astronomer Carl Sagan related his thoughts on a deeper meaning of the photograph:
From this distant vantage point, the Earth might not seem of any particular interest. But for us, it's different. Consider again that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every "superstar," every "supreme leader," every saint and sinner in the history of our species lived there – on a mote of dust suspended in a sunbeam.
The Earth is a very small stage in a vast cosmic arena. Think of the rivers of blood spilled by all those generals and emperors so that in glory and triumph they could become the momentary masters of a fraction of a dot. Think of the endless cruelties visited by the inhabitants of one corner of this pixel on the scarcely distinguishable inhabitants of some other corner. How frequent their misunderstandings, how eager they are to kill one another, how fervent their hatreds. Our posturings, our imagined self-importance, the delusion that we have some privileged position in the universe, are challenged by this point of pale light. Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity – in all this vastness – there is no hint that help will come from elsewhere to save us from ourselves.
The Earth is the only world known, so far, to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment, the Earth is where we make our stand. It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another and to preserve and cherish the pale blue dot, the only home we've ever known.—Carl Sagan, Pale Blue Dot: A Vision of the Human Future in Space, 1997 reprint, pp. xv–xvi
|Wikimedia Commons has media related to Pale Blue Dot.|
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- ^ See the HORIZONS web-interface Tutorial to learn how to generate ephemerides and hence calculate the distance between Earth and Voyager 1 at the specified dates.
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|Library resources about
Pale Blue Dot
- Sagan, Carl; Head, Tom (2006). Conversations with Carl Sagan (1st ed.). United States of America: The University Press of Mississippi. ISBN 1-57806-736-7.
- Sagan, Carl; Freeman J., Dyson; Jerome, Agel (2000). Carl Sagan's Cosmic Connection: An Extraterrestrial Perspective. Cambridge University Press. pp. XV,302. ISBN 0-521-78303-8.