Mercury Imaging Earth

Mysteries Hidden in a Few Pixels

Last week many (if not all) folks on planet Earth had the chance to glance up at the skies and wave at a distant planet. Around each of the two worlds–Saturn and Mercury–are circling spacecraft with cameras powerful enough to look across the solar system and take images of our planet and its moon. We saw the images from Saturn last week (and if you didn’t, check them out at the Cassini page).  A couple of days ago the team working with the MESSENGER mission at Mercury released their spacecraft’s view of our planet. Not surprisingly, our planet looks like a tiny outpost set against a backdrop of stars.

MESSENGER took this image while it was searching out any natural satellites that might be orbiting Mercury. As far as we know, Mercury doesn’t have any moons or we would have spotted them by now.  However, it might have some very small satellites that haven’t yet been spotted; they would be only a few kilometers across at most. It just so happened that Earth and the Moon would be photobombing during one visual sweep, so astronomers announced the time and date for everybody to smile and wave at Mercury.

As you can see from this distance, our home world is pretty small. The moon is a blip next to it. Yet, MESSENGER’s camera caught the pair largely because they were looking for dim objects and the Earth/Moon combo appear very bright by comparison to small rocky moonlets.

The Earth and Moon appear very large in this picture because they are overexposed. When looking for potentially dim satellites, long exposures are used to capture as much light as possible. Consequently, bright objects become saturated and appear artificially large. In this image, Earth and the Moon are each less than a pixel in size. At that tiny scale, it’s impossible to make out any details on the surface of either world.

(The “tails” pointing downward from the Earth and Moon are artifacts caused by the image saturation. These can be seen clearly in the zoomed image in the center lower panel. Anyone who has ever aimed a digital camera at a bright object has likely seen these “blooms” that happen when cameras are overburdened with light.)

MESSENGER took more than one image of the Earth-Moon system. On July 20th it caught our two worlds in the spotlight again, this time on the 44th anniversary of the Apollo 11 landing on the Moon. If you could see the landing sites at 1-pixel resolution, they would appear as they do in the image on the lower right.

There’s SO much hidden in a few pixels of a distant world. As we look out at other stars with planets, it’s important to know that they, too, hide some incredibly cool things to discover. So far, Earth is the only planet we KNOW harbors life, and it’s not even evident in this image. Makes me wonder just what we’ll find when we figure out ways to explore worlds around other stars!