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=                           Pale_Blue_Dot                            =
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                            Introduction
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'Pale Blue Dot' is a photograph of Earth taken on February 14, 1990,
by the 'Voyager 1' space probe from an unprecedented distance of over
kilometers ( miles, 40.5 AU), as part of that day's '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 reflected by the camera. Commissioned by NASA and
resulting from the advocacy of astronomer and author Carl Sagan, the
photograph was interpreted in Sagan's 1994 book, 'Pale Blue Dot', as
representing humanity's minuscule and ephemeral place amidst the
cosmos.

'Voyager 1' was launched on September 5, 1977, with the initial
purpose of studying the outer Solar System. After fulfilling its
primary mission and as it ventured out of the Solar System, the
decision to turn its camera around and capture one last image of Earth
emerged, in part due to Sagan's proposition.


Over the years, the photograph has been revisited and celebrated on
multiple occasions, with NASA acknowledging its anniversaries and
presenting updated versions, enhancing its clarity and detail.


                             Background
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'Voyager 1' is a 722 kg robotic spacecraft on a mission to study the
outer Solar System and eventually interstellar space. After
encountering the Jovian system in 1979 and the Saturnian system in
1980, the primary mission was declared complete in November 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, still traveling at 40000 mph, is the most distant
human-made object from Earth and the first to leave the Solar System.
Its mission has been extended and continues to this day, with the aim
of investigating the boundaries of the Solar System, including the
Kuiper belt, the heliosphere and interstellar space. Since its launch,
it receives routine commands and transmits data back to the Deep Space
Network.

'Voyager 1' was expected to work only through the Saturn encounter.
When the spacecraft passed the planet in 1980, astronomer and author
Carl Sagan proposed the idea of the space probe taking one last
picture of Earth. He acknowledged that such a picture would not have
had much scientific value, as the Earth would appear too small for
'Voyager's' cameras to make out any detail, but it would be meaningful
as a perspective on humanity's place in the universe.

Although many in NASA's Voyager program were supportive of the idea,
there were concerns that taking a picture of Earth so close to the Sun
risked damaging the spacecraft's imaging system irreparably. It was
not until 1989 that Sagan's idea was put in motion, but then
instrument calibrations delayed the operation further, and the
personnel who devised and transmitted the radio commands to 'Voyager
1' were also being laid off or transferred to other projects. Richard
Truly, the then NASA Administrator, interceded to ensure that the
photograph was taken. A proposal to continue to photograph Earth as it
orbited the Sun was rejected.


                               Camera
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'Voyager 1's' Imaging Science Subsystem consists of two cameras: a
low-resolution wide-angle camera used for spatially extended imaging,
and a high-resolution narrow-angle camera intended for detailed
imaging of specific targets. Both cameras are  slow-scan vidicon tubes
with a selenium sulphur storage surface, and are fitted with eight
colored filters mounted on a filter wheel in front of the tube. 'Pale
Blue Dot' was taken with the narrow-angle camera, a 1500 mm f/8.5
catadioptric  cassegrain telescope  whose design was based on the 1973
Mariner mission.

The challenge was that, as the mission progressed, the objects to be
photographed would increasingly be farther away and would appear
fainter, requiring longer exposures and slewing (panning) of the
cameras to achieve acceptable quality. The telecommunication
capability also diminished with distance, limiting the number of data
modes that could be used by the imaging system.

After taking the 'Family Portrait' series of images, which included
'Pale Blue Dot', NASA mission managers commanded 'Voyager 1' to power
its cameras down, as the spacecraft was not going to fly near anything
else of significance for the rest of its mission, while other
instruments that were still collecting data needed power for the long
journey to interstellar space.


                             Photograph
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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 and
Carolyn Porco of the University of Arizona. The command sequence was
then compiled and sent to 'Voyager 1', with the images taken at 04:48
GMT on February 14, 1990. At that time, the distance between the
spacecraft and Earth was 40.47 astronomical units (6,055 million
kilometers, 3,762 million miles).

The data from the camera was stored initially in an on-board tape
recorder. Transmission to Earth was also delayed by the 'Magellan' and
'Galileo' missions being given priority use of the Deep Space Network.
Then, between March and May 1990, 'Voyager 1' returned 60 frames back
to Earth, with the radio signal traveling at the speed of light for
nearly five and a half hours to cover the distance.

Three of the frames received showed the Earth as a tiny point of light
in empty space. Each frame had been taken using a different color
filter: blue, green and violet, with exposure times of 0.72, 0.48 and
0.72 seconds respectively. The three frames were then recombined to
produce the image that became 'Pale Blue Dot'.

Of the 640,000 individual pixels that compose each frame, Earth takes
up less than one (0.12 of a pixel, according to NASA). The light bands
across the photograph are an artifact, the result of sunlight
reflecting off parts of the camera and its sunshade, due to the
relative proximity between the Sun and the Earth. 'Voyager's' point of
view was approximately 32° above the ecliptic. Detailed analysis
suggested that the camera also detected the Moon, although it is too
faint to be visible without special processing.

'Pale Blue Dot', which was taken with the narrow-angle camera, was
also published as part of a composite picture created from a
wide-angle camera photograph showing the Sun and the region of space
containing the Earth and Venus. 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 darkest filter (a
methane absorption band) and the shortest possible exposure (5
milliseconds), to avoid saturating the camera's vidicon tube with
scattered sunlight. Even so, the result was a bright burned-out image
with multiple reflections from the optics in the camera and the Sun
that appears far larger than the actual dimension of the solar disk.
The rays around the Sun are a diffraction pattern of the calibration
lamp which is mounted in front of the wide-angle lens.


                          Pale blue color
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Earth appears as a blue dot in the photograph primarily because of
Rayleigh scattering of sunlight in its atmosphere. In Earth's air,
short-wavelength visible light such as blue light is scattered to a
greater extent than longer wavelength light such as red light, which
is the reason why the sky appears blue from Earth. (The ocean also
contributes to Earth's blueness, but to a lesser degree than
scattering.) Earth is a 'pale' blue dot, rather than dark blue,
because white light reflected by clouds combines with the scattered
blue light.

Earth's reflectance spectrum from the far-ultraviolet to the
near-infrared is unlike that of any other observed planet and is
partially due to the presence of life on Earth. Rayleigh scattering,
which causes Earth's blueness, is enhanced in an atmosphere that does
not substantially absorb visible light, unlike, for example, the
orange-brown color of Titan, where organic haze particles absorb
strongly at blue visible wavelengths. Earth's plentiful atmospheric
oxygen, which is produced by photosynthetic life forms, oxidizes
organics in the atmosphere and converts them to water and carbon
dioxide, causing the atmosphere to be transparent to visible light and
allowing for substantial Rayleigh scattering and hence stronger
reflectance of blue light.


                            Reflections
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In his 1994 book, 'Pale Blue Dot', Carl Sagan comments on what he sees
as the greater significance of the photograph, writing:


                           Anniversaries
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In 2015, NASA acknowledged the 25th anniversary of the photograph. Ed
Stone, Voyager project scientist, commented: "Twenty-five years ago,
'Voyager 1' looked back toward Earth and saw a "pale blue dot", an
image that continues to inspire wonderment about the spot we call
home."

In 2020, for the image's 30th anniversary, NASA published a new
version of the original 'Voyager' photo: 'Pale Blue Dot Revisited',
obtained using modern image processing techniques "while attempting to
respect the original data and intent of those who planned the images."
Brightness levels and colors were rebalanced to enhance the area
containing the Earth, and the image was enlarged, appearing brighter
and less grainy than the original. The direction of the Sun is toward
the bottom, where the image is brightest.

To celebrate the same occasion, the Carl Sagan Institute released a
video with several noted astronomers reciting Sagan's "Pale Blue Dot"
speech.


                              See also
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* List of photographs considered the most important
* 2010 'Family Portrait'
* Bluedot Festival
* Early Earth, or "pale orange dot"
* 'Earthrise'
* Overview effect
* Space selfie
* 'The Blue Marble'
* The Day the Earth Smiled
* Timeline of first images of Earth from space


                           External links
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* [https://www.loc.gov/item/cosmos000110/ Audio recording] of Carl
Sagan reading from 'Pale Blue Dot' (US Library of Congress)
* [http://www.thespacereview.com/article/261/1 Sagan's rationale for
human spaceflight] - Article in 'The Space Review'
*
[https://web.archive.org/web/20150302110651/http://www.pangeaday.org/filmDetail.php?id=75
Video produced for Pangea Day] with Sagan reading from 'Pale Blue Dot'
*
*
*
[https://www.esa.int/ESA_Multimedia/Images/2013/07/Cassini_s_Pale_Blue_Dot
Cassini's Pale Blue Dot] - European Space Agency


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