Most missions probing deep space keep their cameras pointed towards their planetary targets and communications arrays squarely pointed back to Earth. Occasionally, just a bit of precious mission time is devoted to turn those cameras back on Earth. The results can be stunning and give us a unique perspective on our place in our solar system.
In 1990, the Voyager 1 spacecraft imaged Earth from 4 billion miles away as it passed the orbit of Neptune. The resulting grainy image, dubbed "Pale Blue Dot," showed Earth in a way not seen before.
In 2006, Cassini positioned itself for three hours to capture a mosaic of an eclipse of the sun by Saturn creating an iconic image of the back-lit planet. Earth appeared to be nestled with Saturn's rings but was actually 761 million miles behind.
This Friday, NASA will again turn the cameras aboard the Cassini mission toward Earth. For 15 minutes, the spacecraft will capture images of Saturn and all its rings as it eclipses the sun.
For the first time, back-lit Saturn will be imaged in natural color. It's also the first time we Earthlings have known in advance that a spacecraft so far away is snapping photos of us. NASA is encouraging us to "Wave at Saturn" that evening.
Look to the southeast between 5:27 and 5:42 p.m. EDT and give a little wave at the camera nearly 900 million miles away.
You probably won't be able to make yourself out in the resulting image. Earth will only be a pixel or two wide. Still, it is a good excuse to go outside and take a look up at the sky and reflect on how big our solar system is.
Don't be disappointed if you can't make out Saturn either. The planet is easy to spot a few hours after sunset but sunset is still hours away when North America will be waving.
NASA encourages everyone to photograph friends and family waving at Saturn and share those photos on the Facebook event page or via twitter using the hashtag #waveatsaturn. A collage of the submitted images is planned.
The mosaic of images being created has scientific goals as well. The solar eclipse allows researchers to look for changes in the more diffuse rings that wrap Saturn. That 2006 mosaic revealed that unexpected variations in the the outer dusty ring are fed by water-ice plumes jetting from the moon Enceladus (which my spell checker insists should have been named Enchiladas.)
Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on twitter @rtphokie.