Astronomy

What is this pale blue dot in a daytime photo of a Sun halo?

What is this pale blue dot in a daytime photo of a Sun halo?


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This photo of the sun behind a cloud was taken on the south coast of England in early May 2021 using an iPhone 11. A halo is visible which I think is the 22-degree halo. However, the photo also shows a small, pale blue dot near the middle of the photo, within the photo, left and above the Sun. Please could somebody suggest what the blue dot is?


Yes, I believe this is the internal lens reflection. Lens reflections have hexagonal shape, and if you look closely, it somewhat resembles this shape. Also, see other answers in the comments.

Internal lens reflection happens because the lenses don't have 100% transmission. This often happens when taking bright objects, like Sun in your example.


What is this pale blue dot in a daytime photo of a Sun halo? - Astronomy

On October 13, 1994, the famous astronomer Carl Sagan was delivering a public lecture at his own university of Cornell. During that lecture, he presented this photo:

The photo above was taken by Voyager 1 in 1990 as it sailed away from Earth, more than 4 billion miles in the distance. Having completed its primary mission, Voyager at that time was on its way out of the Solar System, on a trajectory of approximately 32 degrees above the plane of the Solar System. Ground Control issued a command that directed the distant space craft to turn around and, looking back, take photos of each of the planets it had visited. From Voyager's vast distance, the Earth was captured as a infinitesimal point of light (between the two white tick marks in the image above), actually smaller than a single pixel of the photo. The image was taken with a narrow angle camera lens, with the Sun quite close to the field of view. Quite by accident, the Earth was captured in one of the scattered light rays caused by taking the image at an angle so close to the Sun. Dr. Sagan was quite moved by this image of our tiny world. Here is an enlargement of the area around our Pale Blue Dot and an excerpt from the late Dr. Sagan's talk:

"We succeeded in taking that picture [from deep space], and, if you look at it, you see a dot. That's here. That's home. That's us. On it, everyone you ever heard of, every human being who ever lived, lived out their lives. The aggregate of all our joys and sufferings, thousands of confident religions, ideologies and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilizations, every king and peasant, every young couple in love, every hopeful child, every mother and father, every 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 in 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 the dot on scarcely distinguishable inhabitants of some other corner of the dot. 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. It is up to us. It's been said that astronomy is a humbling, and I might add, a character-building experience. To my mind, 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 and compassionately with one another and to preserve and cherish that pale blue dot, the only home we've ever known."


NASA's iconic 'Pale Blue Dot' photo of Earth from space just got a 21st-century makeover

On Feb. 14, 1990, NASA's Voyager 1 spacecraft captured one of the most iconic photographs of the space age to commemorate the moment's 30th anniversary, NASA has digitally dusted off the image.

Known as the "Pale Blue Dot" photo, the original image showed Earth as a tiny speck within a band of brightness caused by sunlight striking the spacecraft's instrument. The photograph was the result of a campaign led by scientist Carl Sagan to convince NASA to turn Voyager 1 around and take a photo of the planet where its story began. Just 34 minutes later, according to NASA, the spacecraft's camera shut down so the probe could save power.

During the same imaging sequence, Voyager 1 also photographed five other planets and the sun, in a total of 60 images that NASA nicknamed "The Family Portrait of the Solar System."

For the anniversary, Kevin Gill, an engineer at NASA's Jet Propulsion Laboratory in California and an imaging processor, gave the photo a new look. During the process, he got input from Candy Hansen and William Kosmann, who both helped plan the original photograph.

"This updated version uses modern image-processing software and techniques to revisit the well-known Voyager view while attempting to respect the original data and intent of those who planned the images," according to a NASA statement.

An artist's illustration showing where Voyager 1 and the planets were when the spacecraft took the iconic "Pale Blue Dot" image.

The "Family Portrait of the Solar System" series of images taken by Voyager 1 before its camera shut down.

The original photo was a compilation of images taken using three different color filters. In the new image, those channels have been rebalanced to make the image look sharper, and the beam of sunlight surrounding Earth was adjusted to look white, as sunlight does to our eyes.

The twin Voyager probes haven't produced images in decades they don't fly close enough to any objects worth photographing. But the two venerable spacecraft are still going strong. Both have crossed the bubble that marks the extent of space shaped by the sun. Now, their instruments are telling scientists about interstellar space.


Today is the 25th anniversary of the Pale Blue Dot photograph

25 years ago today, Voyager I turned around to take a photo of Earth on its way out of the Solar System. You are looking at it. Our planet—6 billion kilometers (3.7 billion miles) away from the spacecraft—is that tiny pale blue dot, "a mote of dustsuspended in a sunbeam." It is one of the most important photos ever.

In green: Approximate location of the Voyager I at the time it took the photograph.

The photo was taken at the request of Carl Sagan, who convinced NASA that the photo was worth the cost even if it had no scientific value. This picture, he argued, will show us "our place in the universe." Many opposed this idea because pointing back at the Sun may damage the imager in the interplanetary probe. At the end, and thanks to the tenacity of many and with the help of NASA Administrator Richard Truly, Voyager I took this photo:

Carl Sagan wrote the following about it in The Pale Blue Dot, a must-read book that I can't recommend highly enough.

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.


Voyager 1’s Iconic ‘Pale Blue Dot’ Photo Is 30 Years Old. So NASA Made A New One

Earth, described by scientist Carl Sagan as a "Pale Blue Dot," as seen by Voyager 1 from a distance . [+] of more than 4 billion miles (6.4 billion kilometers).

“Look again at 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.”

“Every saint and sinner in the history of our species lived there—on a mote of dust suspended in a sunbeam.”

The words of American astronomer Carl Sagan, published in his 1994 book “Pale Blue Dot: A Vision of the Human Future in Space” are some of the most stirring and enduring ever penned about Earth’s place in space.

The image of our planet “suspended in a sunbeam” was taken on Valentine's Day, February 14, 1990, as a distant satellite heading out of the solar system turned its cameras back and took some one final image.

Or, at least, that’s the story. In fact, there were a lot more than one image taken that day by Voyager 1 as it took not just one image of Earth, but dozens—and of every other planet in the solar system, too. And now “Pale Blue Dot” has been updated for a new generation to gaze in awe at.

What does the ‘Pale Blue Dot’ image show?

Earth appears in the original “Pale Blue Dot” as a tiny blueish-white speck of a crescent a mere 0.12 pixel in size in an image that contains 640,000 pixels. It appears afloat in a great expanse of deep space. “Pale Blue Dot” is a magnified image, and the placing of Earth seemingly “suspended in a sunbeam” that’s brownish in color (in the original image, above) is a mere coincidence.

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Position of Voyager 1 on February 14, 1990. The vertical bars are spaced one year apart and indicate . [+] the probe's distance above the ecliptic.

Joe Haythornthwaite and Tom Ruen, Wikimedia (CC BY-SA 4.0)

Where was Voyager 1 and Earth when it took ‘Pale Blue Dot?’

Voyager 1 was 4 billion miles/6.4 billion kilometers from Earth and 32º above the ecliptic—the plane of the solar system where all of the planets orbit, more or less—when it snapped the images that became known as “Pale Blue Dot.” Earth is pictured within a light ray from the sun because, as seen from Voyager 1’s perspective, Earth was close to the sun. So Voyager 1’s wide-angle camera had to take a photo that also included the sun. Voyager 1 is now 13 billion miles distant, firmly in interstellar space.

Who was the first person to see ‘Pale Blue Dot?’

That would be Dr. Candy Hansen-Koharcheck, who was part of the Voyager Imaging Team. “It gave me shivers down my spine the first time I saw the Earth, and it still gives me shivers to think about it,” she told me in an email earlier this week.

The new 30th anniversary version of "Pale Blue Dot".

What does the new ‘Pale Blue Dot’ look like?

For the 30th anniversary of one of the most iconic images taken by NASA's Voyager mission, a new version of the image (above) uses modern image-processing software and techniques to revisit the well-known Voyager view while attempting to respect the original data and intent of those who planned the images.

How significant was (and is) ‘Pale Blue Dot?’

“At the time the US and the Soviet Union were still in the Cold War, and that image spoke to us . we are all people on a tiny planet, and in a nuclear war everyone would lose,” said Hansen-Koharcheck. “The perspective that the image gives us was so important at the time, but it is still just as important now. Today the existential threat to humanity is climate change. We must fix that—there is no other planet to move to. We are all in it together.”

“So the image is really a timeless reminder to love and take care of our home world.”

What was Voyager 1?

Voyager 1 is the most distant man-made object from Earth and, at 13.8 billion miles, it’s now beyond the influence of the sun. It was part of NASA’s “Grand Tour” of the outer solar system, launching in September 1977 a few weeks after Voyager 2. It visited Jupiter in 1979 and Saturn (and Titan, its largest moon) in 1980 before heading on a trajectory out of the solar system while Voyager 2 visited Uranus and Neptune. So its mission was pretty much complete almost a decade before it took “Pale Blue Dot.”

Almost 43 years later, Voyager 1 is still sending back data to NASA’s Deep Space Network, though it will run out of electric power by 2025.

Voyager 1's position in the night sky in February 14, 1990 (left), and today (right) in the . [+] constellation of Ophuicus.

How was the ‘Pale Blue Dot’ image assembled?

The narrow-angle color image is was taken by Voyager’s 1’s 1,500 mm high-resolution narrow-angle camera using violet, blue and green colour spectral filters, and stored on the spacecraft’s tape recorder for a few weeks because NASA’s Deep Space Network was busy downloading images from its Magellan spacecraft at Venus and its Galileo spacecraft at Jupiter. When it was sent to Earth, the 60 images—which took five and a half hours to cross the solar system as radio signals—were then recombined as a single image. The image looks lo-res and noisy because it’s been zoomed-in on and magnified, which causes image artifacts.

You can freely download “Pale Blue Dot” as a 1304 × 1152 pixel JPEG or TIFF file.

Was ‘Pale Blue Dot’ the only image taken?

No—“Pale Blue Dot” is just one of a number of images that comprised the first ever “Family Portrait” of the solar system. All images were taken by Voyager 1, and NASA published a mosaic comprising its images of each planet. Voyager 1 actually took 60 photos of the solar system to create it.

Voyager 1's "Family Portrait". The cameras of Voyager 1 on Feb. 14, 1990, pointed back toward the . [+] sun and took a series of pictures of the sun and the planets, making the first ever "portrait" of our solar system as seen from the outside.

A diagram of the Voyager 1 "Family Portrait."

What did Carl Sagan say when it was published?

Although the image was taken on Valentine’s Day in 1990, Carl Sagan actually unveiled the Pale Blue Dot image at a press conference about the Voyager missions in the summer of 1990. This is what he said at the time of the image’s unveiling to journalists:

“This is where we live—on a blue dot. Astronomy has always been said to be a humbling experience and I think these pictures underscore that point.”

What else did Carl Sagan say about ‘Pale Blue Dot?’

Somewhat unusually for a scientist, Sagan had a real gift for language, so here are some of his other passages about the significance of the “Pale Blue Dot” photo, taken from “Pale Blue Dot: A Vision of the Human Future in Space.”

“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. 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.”

Taken at 4:48 GMT on Feb. 14, 1990, “Pale Blue Dot” and other images that made-up the “Family Portrait” collection were the last thing Voyager 1’s cameras ever did. Just 34 minutes later, NASA mission managers sent commands to Voyager 1 to power its cameras down to save power for its long, long journey into interstellar space.


Pale Blue Dot (read comment)

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, 40.5 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.

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 one last photograph of Earth across a great expanse of space, at the request of astronomer and author Carl Sagan.

During a public lecture at Cornell University in 1994, Carl Sagan presented the image to the audience and shared his reflections on the deeper meaning behind the idea of the Pale Blue Dot.

"We succeeded in taking that picture [from deep space], and, if you look at it, you see a dot. That's here. That's home. That's us. On it, everyone you ever heard of, every human being who ever lived, lived out their lives. The aggregate of all our joys and sufferings, thousands of confident religions, ideologies and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilizations, every king and peasant, every young couple in love, every hopeful child, every mother and father, every 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 in 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 the dot on scarcely distinguishable inhabitants of some other corner of the dot. 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. It is up to us. It's been said that astronomy is a humbling, and I might add, a character-building experience. To my mind, 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 and compassionately with one another and to preserve and cherish that pale blue dot, the only home we've ever known."


Contents

In September 1977, NASA launched Voyager 1, a 722-kilogram (1,592 lb) robotic spacecraft on a mission to study the outer Solar System and eventually interstellar space. [3] [4] After the encounter with 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 travelling at 64,000 km/h (40,000 mph), is the most distant human-made object from Earth and the first one to leave the Solar System. [5] 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. Operating for 43 years, 9 months and 18 days as of 23 June 2021, it receives routine commands and transmits data back to the Deep Space Network. [3] [6] [7]

Voyager 1 was 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. [8] 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 into practice, 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. Finally, NASA Administrator Richard Truly interceded to ensure that the photograph was taken. [5] [9] [10] A proposal to continue to photograph Earth as it orbited the Sun was rejected. [11]

Voyager 1 ' s Imaging Science Subsystem (ISS) consists of two cameras: a 200 mm focal length, low-resolution wide-angle camera (WA), used for spatially extended imaging, and a 1500 mm high-resolution narrow-angle camera (NA) – the one that took Pale Blue Dot – intended for detailed imaging of specific targets. Both cameras are of the slow-scan vidicon tube type and were fitted with eight colored filters, mounted on a filter wheel placed in front of the tube. [12] [13]

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. [14]

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. [15]

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. [9] The command sequence was then compiled and sent to Voyager 1, with the images taken at 04:48 GMT on February 14, 1990. [16]

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 travelling at the speed of light for nearly five and a half hours to cover the distance. [5]

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. [17] [18]

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. [5] [19] 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. [ citation needed ] [18]

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. [18]

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. [20] [21] (The ocean also contributes to Earth's blueness, but to a lesser degree than scattering. [20] ) Earth is a pale blue dot, rather than dark blue, because white light reflected by clouds combines with the scattered blue light. [21]

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. [21] 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. [22] Earth's plentiful atmospheric oxygen, which is produced by photosynthetic life forms, causes the atmosphere to be transparent to visible light, which allows for substantial Rayleigh scattering and hence stronger reflectance of blue light.

According to NASA's Jet Propulsion Laboratory's HORIZONS tool, the distances between Voyager 1 and the Earth on February 14 and May 15, 1990, were as follows: [23]

Distance of Voyager 1 from Earth
Unit of measurement February 14, 1990 May 15, 1990
Astronomical units 40.472229 40.417506
Kilometers 6,054,587,000 6,046,400,000
Miles 3,762,146,000 3,757,059,000

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

Look again at 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.

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." [26]

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. [16] [27]

To celebrate the same occasion, the Carl Sagan Institute released a video with several noted astronomers reciting Sagan's "Pale Blue Dot" speech. [28]


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Pale Blue Dot?

Water is high on the ingredients list of astrobiologists hunting for signs of prebiotic conditions elsewhere in the universe. Evidence for an ocean-covered world, or pale blue dot, would provide some of the most compelling evidence that at least simple microbial life might find safe harbor outside of the Earth. Indeed, in looking back at our own planet from space–the "Pale Blue Dot"-Carl Sagan used the Viking photograph of Earth from over 3 billion miles to highlight both the vastness of space and the relative importance of water for life on Earth. Water detection on extrasolar planets has long been considered a daunting challenge for astronomers. The 1996 NASA-Ames Workshop, called the Pale Blue Dot, summarized the prospects: "The presence of H2O bands in a planet’s atmosphere, combined with knowledge of the amount of stellar radiation it absorbs and its effective radiating temperature, should provide a good indication of whether liquid water exists at the planet’s surface. Liquid water is considered essential for life as we know it. This direct information about the presence of water would augment purely theoretical estimates of whether the planet orbited within the star’s liquid water habitable zone."

The view back toward Earth from 3 billion+ miles, Viking photo of Pale Blue Dot underscores the difficult task of imaging planets at all, much less water presence using anything but inferred evidence like spectral data
Credit: Viking NASA

New tantalising results for a star called Upsilon Andromedae may suggest that such water detection is possible in exoplanetary searches. The findings derived from observations using the 32-metre Medicina radio telescope near Bologna, Italy. The reception on the large telescope was narrowed to a microwave frequency (22 gigahertz, GHz) known to signal a sharp spike specific to vibrations in water molecules. The Italian team of scientists, headed by Cristiano Cosmovici, from the Institute for Cosmic and Planetary Sciences in Rome, recently reported their preliminary research this week at the Second European Workshop on Exo/Astrobiology in Graz, Austria. "This would be a historic discovery – the first detection of a prebiotic molecule in an extrasolar planet," reported Cosmovici.

Water searches are done by a global enterprise of radio telescopes. For instance, largely because of the presence of frozen water in many comets (often characterized as "dirty snowballs"), comet clouds do present opportunities for water detection in the neighborhood of distant stars. In July of last year, a team of astronomers from Harvard, NASA/Ames and Johns Hopkins reported in Nature on circumstellar water around an aging carbon star (IRC+10216) and inferred the presence of icy bodies and cometary debris. So while the Italian team is seeking confirmation of their most recent conference publication from other researchers around the world, whether the water detection is a viable signal or not will hinge greatly on what is already known about their primary target, a triple-planet system orbiting Ups Andromedae.

A Triple-Planet System Orbiting Ups Andromedae

Cristiano Cosmovici Institute for Cosmic and Planetary Sciences in Rome

Of the planets catalogued so far, a number of surprising patterns have been revealed. About half have small orbits and reside very close to their parent star (about 1/5th the distance from the Earth to the Sun, or 0.2 AU). Such proximities would suggest extreme temperatures. The majority also have highly elliptical or eccentric orbits, not circular ones. Prior to the flurry of post-1996 planet discoveries, most theories suggested that as a parent star’s disk began to evaporate during planet formation, circular orbits would dominate mainly as in our own solar system. One explanation for the large number of elliptical orbits posits that planet’s interact gravitationally with each other strongly to generate stable, non-circular orbits. In the case of Ups Andromedae, a 3-planet system has been proposed by Marcy, et al. with small and eccentric orbits. Thus the discovery of this multiple planet system suggested a new paradigm for planet formation where many small seed planets known as planetesimals might develop in the disk of matter surrounding a star. "The Upsilon Andromedae system suggests that gravitational interactions between Jupiter-mass planets can play a powerful role in sculpting solar systems," said Butler, staff astronomer at the Anglo-Australian Observatory.

The trio of planets and an overlay orbital scale of our own solar system
Credit: Harvard-Smithsonian

Located 44 light-years from Earth, the parent star itself (HD 9826) is probably a single star, although the Bright Star catalog lists it as a possible binary, but the star’s motion is more consistent without including a companion. The star is approximately 1.2 times as massive as our Sun and around 2.6 billion years old (2/3 the age of our Sun). With its trio of planets, Upsilon Andromedae harbors the first planetary system that is reminiscent of our own Solar System.

"The usual picture is that gas giant planets form at least four AU away from a star, where temperatures are low enough for ice to condense and start the process of planet formation," said Timothy Brown of the High Altitude Observatory (HAO). "But all three giant planets around Upsilon Andromedae now reside inside this theoretical ice boundary." The planets may have formed close to the host star, or, like balls on a billiard table, the planets may have scattered off of each other, migrating into their current orbits from a more distant place of origin.

The inner planet, or "hot Jupiter" candidate, was first announced by Butler et al. in Jan 1997. This innermost of the three planets contains at least three-quarters of the mass of Jupiter and orbits only 0.06 AU from the star. It traverses a circular orbit every 4.6 days. The middle planet contains at least twice the mass of Jupiter and it takes 242 days to orbit the star once. It resides approximately 0.83 AU from the star, similar to the orbital distance of Venus. The outermost planet has a mass of at least four Jupiters and completes one orbit every 3.5 to 4 years, placing it 2.5 AU from the star. The two outer planets have elliptical (oval) orbits which is characteristic of other extrasolar planets in distant orbits around their stars.

All 3 planets are predicted roughly to have very high temperatures, based on standard reflection estimates (albedo like Jupiter, or 0.35) and their theorized orbits. From inner to outer planets found, the trio register temperatures of 1097 C (1947 F), 92 C (198 F), and -63 C (-81 F). Previous publications (Butler, et al. Astrophys. J) have noted that "Internal heating probably raises the temperature of the middle companion above the boiling point of water, while the outer companion is probably pushed up to near the water freezing point at a pressure of 1 atmosphere".

The trio of planets as they might appear around Ups Andromedae: artist rendering
Credit: Harvard-Smithsonian

No current theory predicted that so many giant worlds would form around a star. "I am mystified at how such a system of Jupiter-like planets might have been created," said Geoffrey Marcy, San Francisco State University’s (SFSU’s) Distinguished Professor of Science. "This will shake up the theory of planet formation."

Robert Noyes, a professor of astronomy at Harvard-Smithsonian CfA and a member of the CfA-HAO team, said, "A nagging question was whether the massive bodies orbiting in apparent isolation around stars really are planets, but now that we see three around the same star, it is hard to imagine anything else."

If these Jupiter-mass planets are like our own Jupiter, they would not be expected to have solid Earth-like surfaces. But, Peter Nisenson of the Harvard-Smithsonian Center for Astrophysics (CfA) noted, "our observations can’t rule out Earth-sized planets as well in this planetary system, because their gravity would be too weak for them to be detectable with present instruments."

Water Maser Emissions: Shock Physics

The 32 meter radio telescope in Bologna, Italy
Credit: Italian Astronomy

For radio astronomers, the 22 GHz water maser line is the brightest spectral line in the radio universe. Water production is a very common natural result from shockwaves when stars are formed, and thus are of keen interest for studying stellar evolution in radio bands. The birth of a massive star generates strong stellar winds (with velocities up to a few thousand km/s), which shock the ambient cloud material. Shocks with velocities exceeding some 20 km/s running into high-density magnetized material successfully explain water maser emission around some stars.

But for the more aged stars like Ups Andromedae, shockwaves are long subsided and another water source must be proposed. Detection of water registering as microwaves (or maser) signals at 22 gigahertz (Gz), Cosmovici wrote, yields "a powerful diagnostic tool for planetary search outside the Solar system".

In deep space, as the tiny wishbone-shaped water molecule (H2O) rotates around its central oxygen atom, a very high frequency resonance reaches microwave detectability. To record (or generate) these molecular rotations, a "maser" is the microwave equivalent of a laser, operating on (higher frequency) rotational energy states instead of vibrational states or electron shell jumps as in an ordinary light laser. (The name MASER stands for "Microwave Amplification by Stimulated Emission of Radiation", while laser is the "Light Amplification…" equivalent. )

Comparison of Mars, Venus and Earth in water bands, showing the clear presence of water on Earth uniquely
Credit: NASA Workshop, Pale Blue Dot

Other than water detection, a number of other signatures are of interest for probing the characteristics of extrasolar planets. In the near future, medium-resolution spectroscopy will be obtained of about 75 of the most interesting planetary systems detected, enabling the measurement of broad water-vapour and carbon dioxide spectral features – the signatures of an Earth-like oxidising atmosphere – if present. Finally, even higher resolution spectra will be obtainable for up to 30 of the most promising planets, and will be capable of detecting the infrared 10-micron ozone band, a life-signature on Earth.

Other possible targets for water detection around exoplanets included in Cosmovici’s survey yield less certain probabilities of true planetary systems. Epsilon Eridani, a Sun-like star 10 light years away, and Lalande 21185, a red dwarf about 8 light years away, are both closer than the 44 light-year distance to Ups Andromedae but offer a less complete picture of what might be going on orbitally around those parent stars.

The star, Ups Andromedae should make an ideal target for NASA’s upcoming Space Interferometry Mission (SIM). Expected to launch in 2005, SIM will spend five years probing nearby stars for Earth-sized planets and will test technology slated for future planet-searching telescopes. The ongoing ground-based planet search will enable SIM to home in on those stars most likely to harbor small planets.


Contents

The first part of the book examines the claims made throughout history that Earth and the human species are unique. Sagan proposes two reasons for the persistence of the idea of a geocentric, or Earth-centered universe: human pride in our existence, and the threat of torturing those who dissented from it, particularly during the time of the Roman Inquisition. However, he also admits that the scientific tools to prove the Earth orbited the Sun were (until the last few hundred years) not accurate enough to measure effects such as parallax, making it difficult for astronomers to prove that the geocentric theory was false.

After saying that we have gained humility from understanding that we are not literally the center of the universe, Sagan embarks on an exploration of the entire Solar System. He begins with an account of the Voyager program, in which Sagan was a participating scientist. He describes the difficulty of working with the low light levels at distant planets, and the mechanical and computer problems which beset the twin spacecraft as they aged, and which could not always be diagnosed and fixed remotely. Sagan then examines each one of the major planets, as well as some of the moons—including Titan, Triton, and Miranda—focusing on whether life is possible at the frontiers of the Solar System.

Sagan argues that studying other planets provides context for understanding the Earth—and protecting humanity's only home planet from environmental catastrophe. He believes that NASA's decision to cut back exploration of the Moon after the Apollo program was a short-sighted decision, despite its expense and declining popularity among the American public. Sagan says future exploration of space should focus on ways to protect Earth and to extend human habitation beyond it. The book was published the same year comet Shoemaker-Levy 9 crashed into Jupiter, an event Sagan uses to highlight the danger Earth faces from the occasional asteroid or comet large enough to cause substantial damage if it were to hit Earth. He says we need the political will to track large extraterrestrial objects, or we risk losing everything. Sagan argues that in order to save the human race, space colonization and terraforming should be utilized.

Later in the book, Sagan's wife, Ann Druyan, challenges readers to pick one of the other planetary dots photographed and featured in the book, and imagine that there are inhabitants on that world who believe that the universe was created solely for themselves. She shared Sagan's belief that humans are not as important as they think they are.

The first edition of the book includes an extensive list of illustrations and photographs, much of it taken from public archives of information released by NASA.