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See other posts from July 2013

Headshot of Emily Lakdawalla

Pretty picture: Looking backward

Posted By Emily Lakdawalla

2013/07/23 05:03 CDT

Topics: pics of Earth by planetary missions, Cassini, pretty pictures, Earth, the Moon, Saturn, Saturn's rings

Here it is: the view from Saturn of our Earthly home, one and a half billion kilometers away. We see Earth and the Moon through a thin veil of faintly blue ice crystals, the outskirts of Saturn's E ring. Earth is just a bright dot -- a bit brighter than the other stars in the image, but no brighter than any planet (like Saturn!) in our own sky.

Of course, Earth isn't the only thing in the photo; we have a nearly 180-degree-phase Saturn and its rings, too. Sunlight is being refracted through Saturn's uppermost atmosphere -- that's what makes the sliver of brightness at lower left -- but the sliver of brightness breaks up as you follow it counterclockwise around the edge of Saturn's disk. That's where Saturn's own rings are casting shadows on the planet, so there's no sunlight falling on the sun-facing side to be refracted through the sky.

NASA / JPL / SSI

"The Day the Earth Smiled"
In this rare image taken on July 19, 2013, the wide-angle camera on NASA's Cassini spacecraft has captured Saturn's rings and our planet Earth and its Moon in the same frame.

This is a wide-angle camera photo. There was a narrow-angle camera sequence, too. Unfortunately, the images appear to have been hopelessly overexposed (creating the rays visible in, for example, this photo), so they used the color from the wide-angle photos to colorize one of the narrow-angle camera images, something that's standard operating procedure in spacecraft image processing:

Earth and the Moon from Cassini

NASA / JPL / SSI

Earth and the Moon from Cassini
Taken during the "Day the Earth Smiled" imaging sequence on July 19, 2013, this narrow-angle camera photo of Earth and the Moon has been colorized with the wide-angle camera data.

One paragraph in the caption released with a zoomed-in version of the Earth-Moon narrow-angle camera image caught my eye. It said: "Image scale on Earth is 5,382 miles (8,662 kilometers) per pixel. The illuminated areas of neither Earth nor the moon are resolved here. Consequently, the size of each 'dot' is the same size that a point of light of comparable brightness would have in the narrow angle camera." Of course, Earth is larger than 8,662 kilometers in diameter: its actual diameter is nearly 13,000 kilometers across. So shouldn't it be more than one pixel, if only slightly?

It's not, because Earth is closer to the Sun than Saturn is, and consequently didn't appear full. In fact, it only a bit more than half-phase at the time. Its light may span more than one pixel, but there's less than a pixel's worth of sunlit planet visible. Here is a lovely comparison photo assembled by the Planetary Habitability Laboratory using image data from Earth's weather satellites acquired at about the same time, from about the same angle, as the Cassini photo -- only from much, much closer. The GOES image was grayscale; it has been colorized with data from NASA's Visible Earth project using the Planetary Habitability Laboratory's Scientific Exoplanets Renderer software.

Earth at the moment of the

PHL @ UPR Arecibo, NASA/JPL-Caltech/Space Science Institute, NERC Satellite Station, Dundee University, Scotland

Earth at the moment of the "Day the Earth Smiled" photo

They released a similar photo of Earth from the perspective of the MESSENGER photo I blogged about yesterday; I've updated the blog entry with that picture.

 

Read more blog entries about: pics of Earth by planetary missions, Cassini, pretty pictures, Earth, the Moon, Saturn, Saturn's rings

Comments:

Raymond Scholer: 07/28/2013 12:14 CDT

awesome!

Dennis Davis: 07/28/2013 12:19 CDT

That is such a beautiful photo.

Hartson Doak: 07/28/2013 01:01 CDT

What is the large bright object upper center right by the rings?

Massimo Bonanno: 07/28/2013 03:21 CDT

Simply fantastic! I'm wordless!!!

Bernard El Hadad: 07/28/2013 03:52 CDT

Good opportunity to remember we have to live together on this small pale blue dot lost in the black sky

Terri Middlemiss: 07/28/2013 04:09 CDT

What a view of us! Spectacular!

Craig Potter: 07/28/2013 06:51 CDT

I don't see any other Earths out there, so let make sure we take care of this one OUR HOME, before it's too late. Spectacular picture.

Robin Sturgeon: 07/28/2013 07:03 CDT

I just wish my dad, Theodore Sturgeon, had lived to see this image. He wold have been so thrilled! What an accomplishment this is...Cassini rocks!

paul-suncoast: 07/28/2013 07:55 CDT

I remember the image taken from Voyager looking back from the edge of our planetary system. That was an astonishing view but this seems more emotive somehow. Not simply a pixel but a living planet and the moon we see each day - "us".

Fidel Soto: 07/28/2013 08:58 CDT

Does the Earth reflects back more light to the outer space because its atmosphere composition? If so, a picture of it should more shinning that one of Mars. Am I right? Thank you Cassini; wonderful job. How far is New Horizon from Saturn, now? My comments are more questions than something else.

Roy Mckenzie: 07/28/2013 11:51 CDT

So the Earth and Moon are in the lower third of the hazy band, almost centered from left to right of the frame, correct? Does anyone answer these questions? Someone should make that photo available for purchase. I'd buy it.

Ruby: 07/29/2013 08:41 CDT

It makes me feel how homesick I would be if I were on Saturn looking at my home planet.

T. MacDonald: 07/30/2013 09:03 CDT

it just totally blows me away

Emily Lakdawalla: 08/05/2013 05:28 CDT

@Hartson: it's probably a star. @Fidel: Earth is fairly dark, with an albedo of about 37%, but that's brighter than either Mars or Mercury (or the Moon, for that matter). But Earth's clouds are very bright, and of course it's larger than any of the other inner planets. So it's a fairly bright spot in the heavens. As for how far New Horizons is from Saturn, you can use the website space.jpl.nasa.gov to answer that question for yourself. Roy: No, Earth and the Moon are the brightest spot in the picture, above the hazy band of the E ring but below the narrower, slightly sharper but still hazy band of the G ring.

David Voros: 08/14/2013 03:13 CDT

It is fantatick to see the earth as a small dot from Saturn and too think that wee are so fare.

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