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SYFY WIRE Bad Astronomy

A Distant View of Home

By Phil Plait

When I saw the image above, the hair on the back of my neck stood up. Recognize them? Those are the Earth and Moon, as seen from Mars.

That image was taken by the phenomenal HiRISE camera on board the Mars Reconnaissance Orbiter, which was more than 200 million kilometers from Earth at the time. It’s actually a composite of a few separate images, processed to show the relative size and position of our planet and its moon.

HiRISE normally points down to take amazing images of the surface of Mars; it can resolve objects less than a meter across! But it sometimes is pointed at Earth to take calibration images; Earth has known characteristics like color and reflectance that make it a nice test subject for the camera. The image above is composed of ones taken in near-infrared, red, and blue/green, and that’s displayed as red, green, and blue here. Vegetation is highly reflective in near IR, so continents look red; Australia is in the center and Southeast Asia to the upper left. Antarctica is the bright white patch to the lower left.

Also, the contrast has been changed; the Earth is on average more than three times as reflective as the Moon. At the contrast scale Earth is displayed at, the Moon would be almost black, so the brightness of the Moon image has been increased. In this image, you can clearly see features on the Moon, the darker basaltic scars from ancient giant impacts.

This image was taken on Nov. 20, 2016. At the time, the Earth, Sun, and Mars made an isosceles triangle, with Mars at the narrow angle. From Mars, Earth appeared half-lit, at “first quarter,” if you will. From Earth, the Moon was at third quarter, half lit by the Sun and approaching its new phase. That puts the Moon on the far side of Earth as seen from Mars, about 300,000 kilometers farther away. But that’s a drop in the 200 million kilometer bucket, so for all intents and purposes they’re at the same distance. This means the scale of this is right; the Moon is ¼ the size of Earth.

It’s amazing to think that’s home, that we can see our planet and its attendant satellite from so far away. We’ve seen it from farther, of course, including the famous Voyager 1 Pale Blue Dot image (the Moon is invisible in that one), and others taken by the Cassini spacecraft orbiting Saturn and MESSENGER at Mercury (though that last one, I think, was from closer). But in this case, it’s the tantalizing detail that makes it so eerie; there’s just enough there to remind us of home, but not quite enough to make it easy.

Would you have known that was Australia without it having been pointed out? I’m not sure I would have.

It makes me think that there will come a day, perhaps not too long in the future, when we will have an image like this, but it won’t be of the Earth and Moon, or any planet in our solar system. It’ll be an exoplanet, an alien world orbiting an alien star. When that happens, will we see fuzzy continents, pixelated oceans, or a blobby moon?

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