The Pale Blue Dot is a photograph of Earth taken on February 14, 1990, by the Voyager 1 space probe from a distance of more than 6 billion kilometers (3.7 billion miles, 40.5 AU) as part of the Family Portrait series of images of the Solar System.
In the photograph, Earth appears as a tiny dot, smaller than a single pixel, against the vastness of space and the bright sunlight reflected by the camera. The image was created by NASA and inspired by the work of astronomer and author Carl Sagan. In his 1994 book, Pale Blue Dot, Sagan described the image as a representation of humanity’s small and temporary place in the universe.
Voyager 1 was launched on September 5, 1977, to study the outer Solar System. After completing its main mission and leaving the Solar System, the probe turned its camera to take one final image of Earth. This decision was influenced by Sagan’s suggestion.
Over the years, the photograph has been revisited and celebrated on many occasions. NASA has marked its anniversaries and released updated versions of the image to improve its clarity and detail.
Background
Voyager 1 is a robotic spacecraft that weighs 722 kilograms (1,592 pounds). It was sent into space to study the outer parts of our Solar System and eventually explore space beyond it. After visiting the Jupiter system in 1979 and the Saturn system in 1980, its main mission was completed in November of that year. Voyager 1 was the first spacecraft to send back clear images of Jupiter and Saturn, along with their largest moons.
The spacecraft continues to travel at a speed of 64,000 kilometers per hour (40,000 miles per hour). It is the farthest human-made object from Earth and the first to leave the Solar System. Its mission has been extended to study the edges of the Solar System, including the Kuiper belt, the heliosphere, and space beyond the Solar System. Since its launch, it regularly receives instructions and sends data back to the Deep Space Network.
Voyager 1 was expected to communicate only during its encounter with Saturn. In 1980, when the spacecraft passed Saturn, scientist Carl Sagan suggested taking one final picture of Earth. He said the image might not help scientists learn much, as Earth would appear too small for the spacecraft’s cameras to see details. However, the image could show how small Earth is in the universe.
Many people working on NASA’s Voyager program supported Sagan’s idea. Around the same time, Carolyn Porco, a scientist from the University of Arizona, also wanted to take a picture of Earth after joining the program in 1983. She had been asking about the possibility since 1985. However, some worried that taking a photo of Earth near the Sun could damage the spacecraft’s camera. The plan was approved in 1989, but delays occurred due to camera testing and staff changes. NASA Administrator Richard Truly helped ensure the photo was taken. A later plan to take more photos of Earth as it orbits the Sun was not approved.
Camera
The Imaging Science Subsystem on Voyager 1 includes two cameras: one with low resolution for capturing large areas, and another with high resolution for detailed images of specific objects. Both cameras use slow-scan vidicon tubes with a selenium sulfur storage surface and have eight colored filters attached to a filter wheel in front of the tubes. The famous "Pale Blue Dot" image was taken using the high-resolution narrow-angle camera, which is part of a 1500 mm f/8.5 catadioptric Cassegrain telescope designed based on the 1973 Mariner mission.
As Voyager 1 traveled farther from Earth, the objects it photographed became dimmer and harder to see. This required longer camera exposures and adjusting the camera angles to maintain image quality. The spacecraft’s ability to send data back to Earth also weakened over time, limiting the types of images the cameras could capture.
After taking the "Family Portrait" series of images, which included "Pale Blue Dot," NASA turned off Voyager 1’s cameras. This was because the spacecraft would not pass near any significant objects for the rest of its mission. Other instruments needed power to continue collecting data during the long journey into interstellar space.
Photograph
The plan for sending commands to the spacecraft and calculating the time each photo needed to be exposed to light was created by Porco and space scientist Candy Hansen from NASA's Jet Propulsion Laboratory. These commands were then collected and sent to Voyager 1, and the images were taken at 04:48 GMT on February 14, 1990. At that time, Voyager 1 was 40.47 astronomical units (6,055 million kilometers, 3,762 million miles) away from Earth.
The camera’s data was first stored on a tape recorder on the spacecraft. Sending the data to Earth was delayed because other missions, Magellan and Galileo, used the Deep Space Network first. Between March and May 1990, Voyager 1 sent 60 images back to Earth. The radio signal traveled at the speed of light for nearly five and a half hours to reach Earth.
Three of the received images showed Earth as a small dot of light in space. Each image used a different color filter: blue, green, and violet. The exposure times for these images were 0.72, 0.48, and 0.72 seconds, respectively. These three images were combined to create the famous picture called "Pale Blue Dot."
Each image consists of 640,000 pixels, and Earth appears in less than one pixel (0.12 of a pixel, according to NASA). The light bands in the photo are an artifact caused by sunlight reflecting off parts of the camera and its sunshade, due to the close distance between the Sun and Earth. Voyager 1’s position was about 32° above the ecliptic. Analysis suggested the camera also detected the Moon, though it is too faint to see without special processing.
The "Pale Blue Dot" image was taken with the narrow-angle camera and was included in a larger picture made from a wide-angle camera image showing the Sun, Earth, and Venus. The wide-angle image included two narrow-angle pictures: "Pale Blue Dot" and a similar image of Venus. The wide-angle photo used the darkest filter (a methane absorption band) and the shortest exposure time (5 milliseconds) to avoid overloading the camera’s sensor with sunlight. Despite this, the image was bright and had reflections from the camera’s optics and the Sun. The rays around the Sun are a pattern caused by a calibration lamp placed in front of the wide-angle lens.
Pale blue color
Earth looks like a blue dot in space mainly because sunlight is scattered by air molecules in its atmosphere. In Earth's air, light with shorter wavelengths, such as blue light, is scattered more than light with longer wavelengths, like red light. This is why the sky appears blue from Earth. (The ocean also helps make Earth look blue, but not as much as the scattering of light.) Earth appears as a pale blue dot, not a dark blue one, because white light from clouds mixes with the scattered blue light.
Earth's ability to reflect light from far-ultraviolet to near-infrared wavelengths is different from all other planets studied, and this is partly because life exists on Earth. Rayleigh scattering, which causes Earth's blue color, happens more in an atmosphere that does not absorb visible light. For example, Titan, a moon of Saturn, has an orange-brown color because organic haze in its atmosphere strongly absorbs blue light. Earth has a lot of oxygen in its atmosphere, which is produced by living things that use photosynthesis. This oxygen reacts with organic materials in the air, changing them into water and carbon dioxide. This process makes Earth's atmosphere clear to visible light, allowing Rayleigh scattering to happen strongly and increasing the reflection of blue light.
Reflections
In his 1994 book, Pale Blue Dot, Carl Sagan talks about the important meaning of the photograph, writing:
Anniversaries
In 2015, NASA recognized the 25th anniversary of the photograph. Ed Stone, a scientist working on the Voyager project, said: "Twenty-five years ago, Voyager 1 looked back at Earth and saw a 'pale blue dot,' a picture that still makes people feel amazed about our home planet."
In 2020, to mark the 30th anniversary of the image, NASA shared an updated version of the original Voyager photo called Pale Blue Dot Revisited. This new version used modern methods to improve the picture while keeping the original data and goals in mind. The brightness and colors were adjusted to make Earth more visible, and the image was enlarged to look brighter and clearer. The Sun appears at the bottom of the picture, where the light is strongest.
To celebrate the same event, the Carl Sagan Institute created a video featuring several well-known astronomers reading Carl Sagan's "Pale Blue Dot" speech.