Pale Blue Dot
The Pale Blue Dot is a photograph of planet 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, 40 AU), as part of the Family Portrait series of images of the Solar System. In the photograph, Earth's apparent size is less than a pixel; the planet appears as a tiny dot against the vastness of space, among bands of sunlight scattered by the camera's optics.
Voyager 1, which had completed its primary mission and was leaving the Solar System, was commanded by NASA to turn its camera around and take a photograph of Earth across a great expanse of space, at the request of astronomer and author Carl Sagan.
On September 5, 1977 the US' NASA launched Voyager 1, a 722-kilogram (1,592 lb) robotic spacecraft on a mission to study the outer Solar System and eventually the interstellar space.  After encountering the Jovian system in 1979 and the Saturnian system in 1980, the primary mission was declared complete on November, 20 of the same year. Voyager 1 was the first space probe to provide detailed images of the two largest planets and their major moons.
The spacecraft, the first one to leave the Solar System and the farthest man-made object from Earth, 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. Operating for 37 years, 3 months and 14 days as of today (21 December 2014), it receives routine commands and transmits data back to the Deep Space Network.
Voyager 1 was initially expected to work only through the Saturn encounter. When the spacecraft passed the planet in 1980, Sagan proposed the idea of the space probe taking one last picture of Earth. He pointed out that this picture would not have much scientific value, as the Earth would appear too small for Voyager 's cameras to make out any detail, but such a picture might be meaningful as a perspective on our place in the cosmos. Although 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, preventing more pictures from being taken. It was not until 1989 that Sagan's idea was put into practice, but then instrument calibrations delayed the process further, and the technicians who devised and transmitted the radio commands to Voyager 1 were also being laid off or transferred to other projects. Finally, then–NASA Administrator Richard Truly interceded to ensure that the photograph was taken.
The Pale Blue Dot was taken while Voyager 1 was travelling at 40,000 miles per hour (64,000 km/h). 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 NASA's Jet Propulsion Laboratory (JPL) and Carolyn Porco of the University of Arizona. The data from the camera was initially stored in an on-board tape recorder. Transmission to Earth was delayed also by the Magellan and Galileo missions being given priority over the use of the Deep Space Network. Then, between February 14 and June 6, 1990, Voyager 1 returned 60 frames back to Earth, with the radio signal travelling at the speed of light for nearly five and a half hours to cover the distance. One of the pictures returned was of Earth showing up as a pale blue dot in the grainy photograph.
The image is composed of 640,000 individual pixels, of which Earth takes up less than one (0.12 pixels, according to NASA). 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. Detailed analysis also suggested that Voyager 1 detected the Moon as well, although it is too faint to be visible without special processing.
The Pale Blue Dot photograph, which was taken by Voyager 1's narrow-angle camera, was also published as a part of a composite picture created from a wide-angle camera image showing 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: Pale Blue Dot and a similar photograph of Venus. The wide-angle photograph 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. Due to the intense brightness, the sun in the photograph appears far larger than the actual dimension of the solar disk. The result 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 pale blue color of the dot is the result of polarization and scattering of the light reflected from Earth. The polarization in turn depends on various factors such as cloud cover, exposed areas of oceans, forests, deserts, snow fields etc.
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 farther 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
Carl Sagan titled his 1994 book Pale Blue Dot: A Vision of the Human Future in Space after the photograph. In it, he expresses his thoughts on a deeper meaning of the image:
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
- The Blue Marble
- Family Portrait (MESSENGER)
- The Day the Earth Smiled
- Earth's location in the universe
- "From Earth to the Solar System-Pale Blue Dot". fettss.arc.nasa.gov. Retrieved 2011-07-27.
- "A Pale Blue Dot". The Planetary Society. Retrieved 2014-12-21.
- "Mission Overview". starbrite.jpl.nasa.gov. Retrieved 2011-07-27.
- "Voyager 1". nssdc.gsfc.nasa.gov. Retrieved 2011-07-27.
- Butrica, Andrew.J (1994). "Chapter 11". From Engineering Science To Big Science (1st ed.). New York: Random House. p. 251. ISBN 0-679-43841-6.
- "An Earthly View of Mars". space.com. Retrieved 2011-07-28.
- "It's our dot : For Carl Sagan, planet Earth is just a launch pad for human explorations of the outer universe". pqasb.pqarchiver.com. Retrieved 2011-07-28.
- "Voyager Celebrates 20-Year-Old Valentine to Solar System". NASA. Retrieved 2014-12-21.
- Sagan, Carl (September 9, 1990). "The Earth from the frontiers of the Solar system - The Pale, Blue Dot". PARADE Magazine. Retrieved 2011-07-28.
- Sagan, 1994, pp. 4–5
- "SOLAR SYSTEM PORTRAIT - EARTH AS 'PALE BLUE DOT". visibleearth.nasa.gov. Retrieved 2011-07-13.
- "An Alien View Of Earth". npr.org. Retrieved 2011-07-12.
- "PIA00452: Solar System Portrait - Earth as 'Pale Blue Dot'". photojournal.jpl.nasa.gov. Retrieved 2011-07-27.
- "PIA00450: Solar System Portrait - View of the Sun, Earth and Venus". photojournal.jpl.nasa.gov. Retrieved 2011-07-28.
- "Polarization of terrestrial planets and the ZIMPOL technique". planetquest1.jpl.nasa.gov. Retrieved 2011-07-27.
- Woolf, N.J.; Smith, P.S.; Traub, W.A.; Jucks, K.W. (March 28, 2002). "The Spectrum of Earthshine: A Pale Blue Dot Observed from the Ground". The Astrophysical Journal (The American Astronomical Society) 574 (1): 430. arXiv:astro-ph/0203465. Bibcode:2002ApJ...574..430W. doi:10.1086/340929. Retrieved 5 Dec 2014.
- "NASA's JPL Horizon System for calculating ephemerides for solar system bodies". ssd.jpl.nasa.gov. Retrieved 2011-07-13.
- "Nasa image shows it's a wonderful world". independent.co.uk. Retrieved 2011-07-28.
- "SPACECRAFT – Cassini Orbiter Instruments – ISS". saturn.jpl.nasa.gov. Retrieved 2011-07-27.
- "Planetary Data System - Instrument Information". starbrite.jpl.nasa.gov. Retrieved 2011-07-28.
- "Overview of Observations Made During the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Spacecraft's Flyby of Mercury". solarsystem.nasa.gov. Retrieved 2011-07-28.
- "Mercury – In Color!!". nasa.gov. Retrieved 2011-07-28.
- "Voyager 1 Narrow Angle Camera Description". pds-rings.seti.org. Retrieved 2011-07-27.
- Sagan, Carl (1994). Pale Blue Dot: A Vision of the Human Future in Space (1st ed.). New York: Random House. ISBN 0-679-43841-6. Text also available at http://x3m.us/library/read.php?book=Sagan-Pale_Blue_Dot Library x3m.us.
- Garfinkel, Simson.L (February 5, 1995). "Sagan looks to space for future salvation". The Daily Gazette. Retrieved July 28, 2011.
|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.
|Wikimedia Commons has media related to Pale Blue Dot.|
- Audio recording of Carl Sagan reading from Pale Blue Dot, from the US Library of Congress, Seth MacFarlane Collection of the Carl Sagan and Ann Druyan Archive
- Video produced for Pangea Day with Sagan reading from Pale Blue Dot
- Voyager 1 Narrow Angle Camera Description
- Sagan's rationale for human spaceflight – Article on The Space Review