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By Alan Boyle, Science Editor

The planets are aligned to make you smile — thanks to the propitious positioning of space probes orbiting Saturn and Mercury.

Friday's interplanetary photo op should produce a stunning image of Saturn and its rings, plus Earth's pale blue dot, as seen by NASA's Cassini orbiter. There'll be some additional perspectives from NASA's Messenger orbiter, which is expected to snap pictures of Earth and its moon from its vantage point in Mercurial orbit.

But the best place to be during the photo op is sure to be planet Earth — where Carolyn Porco, head of the Cassini mission's imaging team, is orchestrating what she calls "a global moment of cosmic self-awareness."

"My sincere wish is that people the world over stop what they're doing at the time the Earth picture is taken, to revel in the sheer wonder of simply being alive on a pale blue dot of a planet, and to appreciate the ever-widening perspective of ourselves and our world that we have gained from our interplanetary explorations," Porco said in a news release about the event. "We are dreamers, thinkers, and explorers, inhabiting one achingly beautiful planet, yearning for the sublime, and capable of the magnificent. Let's celebrate that, and make this one day a day the whole Earth smiles in unison."

She and her colleagues have created a website for "The Day the Earth Smiled." They've set up a Twitter hashtag, #DayEarthSmiled, to help folks share their sentiments. And they're collaborating with Astronomers Without Borders to cover a world map with celebrations.

NASA also has its official social-media campaign, which it calls "Wave at Saturn." The space agency has set up a Flickr gallery for sharing photos, hashtags for Twitter and Instagram, a Facebook page and a Ustream video hangout that's scheduled for 5 p.m. ET.

Where and when
The Saturn photo op runs from 5:27 to 5:42 p.m. ET on Friday. That's the time period when the light reflected by a toothy smile and a waving hand can make the 898.5 million-mile (1.446 billion-kilometer) journey from Earth to Saturnian orbit, in time to be captured by Cassini's camera 80.4 minutes later. (This blog post by Scott Edgington, the Cassini mission's deputy project scientist, explains the complexity behind the timing.)

The resulting images will be assembled into a mosaic showing a backlit Saturn and its sunlit rings. Earth will show up as just a little more than a single bluish pixel, off to the side.

"A picture like this shows us our place in the solar system and the universe, in a way that other experiences can't," Mike Simmons, president and CEO of Astronomers Without Borders, told NBC News. The view will be similar to the only other picture that Cassini has taken showing Saturn and Earth togethera 2006 image that's one of Simmons' all-time favorites.

The big difference is that this time around, Earth will be smiling for the camera. "It's a metaphorical smile," Simmons said. "The Earth is 'having its picture taken.' It's not a candid shot. It's a portrait."

Messenger gets in on the action
The Mercury images are an example of serendipity at work: When word circulated about Cassini's photo op, the Messenger mission's scientists realized that they too would be pointing their spacecraft's camera in our direction. The picture-taking sessions had already been planned as part of a campaign to look for small satellites of Mercury.

On Friday, Earth will be in the far background in pictures taken at 7:49 a.m., 8:38 a.m. and 9:41 a.m. ET. Earth's moon will be in the frame as well for three more pictures taken at the same times on Saturday.

"It’s important to note that the Earth and Moon are going to be less than a pixel in size, and so no details will be seen,” Hari Nair, a planetary scientist at Johns Hopkins University's Applied Physics Laboratory who planned out Messenger's photo op, said in a news release. "In practice, all we're going to see are two bright dots."

Messenger's scientists expect to release their pictures of Earth and the moon next week. It will take more time for Cassini's team to process the Saturn imagery — probably a few weeks.

At least three other projects are capitalizing on "The Day the Earth Smiled":

Slooh looks back at Saturn: Slooh Space Camera will be sharing imagery of Saturn as seen by the company's remotely operated telescope system in the Canary Islands, starting at 5:30 p.m. ET Friday. "At the exact time the Cassini spacecraft is snapping pics of Earth, Slooh will be snapping images of Saturn — live and in true color, with a live broadcast team," Slooh's president, Patrick Paolucci, said in an email. You can watch the show over the Web, or via Slooh's iPad app, or by clicking on the video window below.

Message to the Milky Way: Carolyn Porco and her colleagues at Diamond Sky Productions are organizing a contest for photos and musical compositions that reflect the spirit of "The Day the Earth Smiled." The photo contest rules specify that the picture has to be taken on Friday. The rules for the music competition are still being worked on. Eventually, the prize-winning images and music will be broadcast to the stars via the Arecibo Radio Observatory in Puerto Rico. Keep an eye on the "Message to the Milky Way" website for complete contest information, including the procedure for submissions.

Saturn Mosaic Project: Astronomers Without Borders is soliciting Saturn-themed pictures from the public through July 29. Those pictures will be woven into a giant mosaic that looks like the Cassini image when you zoom out to the widest scale. When you zoom in, you'll see all the pictures from Earth that made up the mosaic. Check out the Saturn Mosaic Project's website for complete instructions.

Update for 1:30 p.m. ET July 19: Will Messenger's pictures of Earth include a view of Mercury as well? Here's what APL's Hari Nair says in an emailed response to my inquiry: "No part of Mercury will be visible in the images. ... If we had any part of the lit planet in the field of view, it would swamp out the light from any satellites that might be in the image. Some of the satellite images we take are close to the planet, so there is scattered light in the camera, but it's hard to avoid looking close to Mercury if you're looking for satellites close to Mercury!"

Alan Boyle is's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.