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Metro

Among the planets, colors prove tricky

Climbing the eastern sky these evenings is a bright point outshining any star. It’s the planet Jupiter, right there for you to see. If someone asks what color it is, you cah look at it and say white. Because it is white.

Twinkling to its lower right is a lesser spark, Aldebaran, one of the nearest examples of an orange giant star. It’s called an orange giant because it is ­orange. You can see so tonight, weather permitting.

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Look above Jupiter and ­Aldebaran, by about 1½ fist-widths at arm’s length, and (if your sky isn’t too bright) you’ll spot the little Pleiades star cluster, the size of your outstretched fingertip. The stars of Pleiades aren’t bright enough to show color to the unaided eye, but binoculars reveal them as very pale blue-white. They’re so hot that they shine with the color of arc welding.

Back to Jupiter. In a telescope, at first glance it’s a bright white disk, flattened slightly at the poles. A longer look shows that the overall white is a combination of pale tan cloud belts and even brighter white zones between them, dotted here and there with a spot or two of light pastel orange and a few wisps of pale blue-gray. But creamy white predominates.

And that’s what you would see if you were right there looking out the porthole of a spaceship.

But that’s not what a lot of people imagine Jupiter to be. The reason dates back more than 30 years, to the Voyager 1 and 2 flybys of Jupiter in 1979. The whole world was watching. And when NASA released the first pictures, these portrayed Jupiter as a vivid candy planet striped red and blue.

The Voyagers, almost everyone assumed, must be showing what Jupiter really looked like. Wasn’t that the purpose of the mission? But NASA’s imagery people, inexplicably, hadn’t bothered to correct the images for the fact that the Voyagers’ cameras didn’t work like ­human eyes. In came the data, out came the raw photographic prints. They were handed direct­ly to news media, schools, book publishers, and everyone else, and a whole generation came to believe that Jupiter was the candy planet.

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It took backyard amateur ­astronomers, who could see otherwise for themselves, to blow the whistle. Embarrassed by the Jupiter fiasco, NASA has since taken care to show planets in their true colors — or when colors are used to encode something else (such as infrared wavelengths or chemical composition) to explain what’s going on.

But as anyone who communicates science to the public knows, once a picture is set in people’s minds, it’s almost impos­sible to remove. The candy-­planet conception of ­Jupiter probably won’t die until everyone who grew up with those images is in the grave.

Mars, the not-red planet

A similar tale comes from the other side of this month’s evening sky. As twilight fades around 5 p.m., look low in the southwest for a tiny orange-
yellow point barely glimmering through the afterglow of sunset. That’s the planet Mars, and NASA initially goofed its color just as badly.

The first craft to set down successfully on Mars was ­Viking 1 on July 4, 1976. ­Astronomers who looked at Mars with telescopes had ­advised the Viking team about the exact desert-ochre tint of the sunlit surface of Mars, knowing the color vagaries of space-probe cameras.

They were ignored. Before the first images arrived, at least one member of the Viking imag­ing team assumed that the thin-aired sky of Mars would be deep blue. In fact, its atmosphere turned out to be laden with rusty yellow-brown dust. When the first Viking images showed a dust-colored sky, someone twisted the dials to make the sky blue, which turned the ground a vivid, blazing red. Perhaps he or she thought “the red planet” was supposed to look like it was paved with stop signs. That’s how the first pictures went out to the world, and though NASA quickly issued fixed versions, it was years before the bad ones stopped appearing in textbooks.

Nowadays they get it scrupulously right. The teams in charge of the Spirit, Opportunity, and Curiosity rovers have been careful to issue genuine true-color images (as well as less drab versions showing what the landscapes would look like under clear sunlight, rather than Mars’s dust-filtered sun, with clear explanations). Finally, Mars and Jupiter from NASA look like the Mars and Jupiter you can see tonight.

Whole-sky maps

Easy-to-use maps of stars and constellations across the entire evening sky are available at SkyandTelescope.com/
gettingstarted.

Alan M. MacRobert is a senior editor of Sky & Telescope magazine (SkyandTelescope.com). Star Watch appears the first Saturday of every month.

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