MESSENGER Looks at Earth

Earth and moon as it looks from the planet Mercury a la MESSENGER. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Yesterday we had a look at Earth from the Cassini spacecraft which is in the Saturn system. Today we get a look from the MESSENGER spacecraft which is orbiting around Mercury.

The MESSENGER spacecraft was actually doing a survey looking for moons. You would at first think if Mercury had a moon it should be easy to spot. Not so! Mercury is so elusive owing to it being so close to the sun. The only time we get to see Mercury is when it is reaches eastern or western elogation, the point at which it is either east or west of the Sun from our perspective. When at elongation it is either leading or lagging the Sun so it becomes visible before sun rise or after sun set for a short time.

If you have a good look at the eastern horizon Mercury will in fact be at western elongation on July 30, so you should be able to see it just before sunrise.  Be VERY careful not to include the Sun in your sights accidently!

No doubt some of you are going to notice the “Earth” has a bit of a tear-drop shape, this is from over exposure and the way a chip handles the light. You’ve heard me say planetary photography is very difficult, for me, this is why.

so here is part of the MESSENGER caption and if you would like to see a larger version of the image pay the MESSENGER site a visit — go ahead, it’s worth the click.

The pair of bright star-like features in the upper panel are not stars at all, but the Earth and Moon! MESSENGER was at a distance of 98 million kilometers (61 million miles) from Earth when this picture was taken. The computer-generated image in the lower left shows how the Earth appeared from Mercury at the time. Much of the Americas, all of Europe and Africa, the Middle East, and much of Asia were visible.

MESSENGER took this image as part of a campaign to search for natural satellites of Mercury. Mercury has no moons that we know of. If any exist, they must be small (less than a few kilometers), or we would have seen them by now. The strategy for the satellite search involves taking multiple images of locations at predetermined distances from Mercury, from 2.5 to 25 times the planet radius. Pictures of these points in space are captured at intervals ranging from seconds to nearly an hour, depending on their distances from Mercury. A moving satellite will appear at different positions in images of the same region of space taken at different times.

The Earth and Moon appear very large in this picture because they are overexposed. When looking for potentially dim satellites, long exposures are required to capture as much light as possible. Consequently, bright objects in the field of view become saturated and appear artificially large. In fact, the Earth and Moon are each less than a pixel in size, and no details on either can be seen. 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.

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