Feb 19, 2001

Blazing Venus




Fiery Venus is a wonderful planet to look at, but you wouldn't want to live there! This is a good time to keep an eye on the second planet from the Sun as it approaches Earth and delivers a dazzling sky show.


Marshall Space Flight Center


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From low-hanging clouds fell eternal misty rain; squat rubbery vegetation stretched away in all directions. Now and then a Hop-scotch Bird fluttered wildly above them. Karl turned to gaze at the tiny dome of Aphrodopolis, the largest city on Venus! 
(from "The Weapon Too Dreadful to Use," a 1939 short story by Isaac Asimov)


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February 20, 2001 -- During the "Golden Age of Science Fiction" in the 1930's and 40's, Venus was a frequent setting for space adventure stories. After all, cloud-covered Venus is nearly the same size as Earth and it's only a little closer to the Sun than our planet is. Readers and writers alike fancied Venus as an enticing safari planet -- a steaming world-wide jungle filled with unknown and exotic creatures.


Nowadays we know better, thanks to a parade of U.S. and Soviet spacecraft that visited Venus dozens of times between 1961 and 1997.

Above: This 3-D radar image shows Venusian lava flows extending for hundreds of kilometers. Some of Venus's volcanoes may still be active. Image credit: NASA's Magellan Mission to Venus.

Venus is indeed warm, but more so than early sci-fi authors suspected. The surface temperature is ~860 F (460 C) -- hot enough to melt lead! The air is thick and steamy, too. The atmospheric pressure is about 90 times that of Earth. And the steam .... it's sulfuric acid, a corrosive mist that floats cloud-like through Venus's 96% carbon dioxide atmosphere.

Carbon dioxide is a greenhouse gas that traps infrared radiation beneath Venus's thick cloud cover. A runaway greenhouse effect is what makes Venus even hotter than Mercury! The clouds also hide a forbidding terrain, strewn with craters and volcanic calderas. There are no rivers, lakes, or oceans on Venus -- like Mars, Venus is bone dry.


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Above: Venera 13, a Soviet spacecraft that survived for 127 minutes on Venus's surface, captured this picture of its landing site in 1982. [more]

If you were deposited on Venus by some unscrupulous space-tour company, you would immediately suffocate, melt and be crushed. Which might happen first is debatable, but it hardly matters. Venus is an awful vacation spot no matter how you arrange the itinerary.

The best way to see Venus -- surely one of the most hellish worlds in the solar system -- is from afar. And now is a great time to do just that.

On Thursday, February 22nd, Venus reaches its maximum brightness (visual magnitude -4.6) this year. Hovering about 30 degrees above the western horizon after sunset, Venus will be at least 7 times brighter than any other star or planet in the night sky. It can even cast weak shadows! You can see them before the Moon rises if you happen to live in a very dark area.




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Venus is frequently mistaken for a bright star, an airplane, or even a UFO. Indeed the planet is probably the most-often reported Unidentified Flying Object! But if you peer at Venus for more than a few seconds it's easy to see that it must be a planet. Venus doesn't twinkle like a star, nor does it move rapidly across the sky as an airplane or a flying saucer might.

Why is Venus so bright? Although it's not the largest planet in the solar system, Venus is usually the one nearest to Earth. Furthermore, Venusian clouds -- the same ones that hide Venus's fiery surface from inquisitive astronomers -- are excellent reflectors of sunlight. As much as 72% of the light that shines down on Venus is bounced back into space.

When the Sun goes down on February 25th, sky watchers can spot Venus and the slender crescent Moon pleasingly close together in the western sky. Be sure to look before 8 p.m. local time, because that is roughly when the Moon will set at mid-northern latitudes. If city lights are a problem in your area, don't worry. The pair are so bright that even city-dwellers can enjoy the show.

Using a telescope you can see that there are actually two crescents in the sky that night -- the Moon and Venus. Even small telescopes are adequate to reveal what Galileo first saw through a spyglass 400 years ago: Venus looks like a croissant!


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Above: The little crescents show Venus's changing phase (at exaggerated scale) as well as its altitude and azimuth in the western sky from late January through March. The planet is plotted for an observer at latitude 40 degrees north and at the time each evening when the Sun is 3 degrees below the horizon. Credit: Sky & Telescope magazine, copyright 2001, all rights reserved. For more information see "Venus's Evening Sky Show" in the March 2001 issue of Sky & Telescope.

Like the Moon, Venus has a full range of phases. It can be "Full" when Venus is on the far side of the Sun, "New" when Venus is between the Sun and Earth, and a crescent at points in between.

Venus will become New on March 30th. Its darkened night side will face Earth as it moves almost directly between our planet and the Sun -- a situation astronomers call inferior conjunction. During the days around inferior conjunction, when Venus is as close as it can be to us, its whisker-thin crescent will appear through the lens of a 34-power telescope as large as the Moon does to the unaided eye!


phases of Venus
But be careful, because Venus in late March will be less than 10 degrees from the Sun. It is possible to see Venus during the day through a telescope, and the temptation to look can be irresistible to amateur astronomers. Observers should take precautions not to accidentally sweep their telescopes across our star -- focused sunlight can cause severe eye damage.

Right: Selected phases of Venus and where they occur along that planet's orbit.

If you do take care to watch Venus safely on March 30th -- twilight is the best time -- you could be rewarded by something fantastic. At inferior conjunction, the horns of Venus's fading crescent can join tip-to-tip to form a complete circle -- the result of sunlight trickling through the upper layers of Venus's cloud cover. The short-lived phenomenon depends critically on the Sun-Earth-Venus geometry and how light is bent through Venus's dynamic atmosphere -- there's no guarantee it will happen at all. But with a little luck you can spot the rare and marvelous illusion of a ring-shaped planet.

It seems those early sci-fi writers were right: Venus truly is an exotic world, even if we prefer to appreciate it from a safe distance!

To learn more about Venus's evolving phases and how to view them, see "Venus's Evening Sky Show" by Adrian Ashford in the March 2001 issue of Sky & Telescope.

Editor's Note: The author mentions that Venus is Full when it is on the opposite side of the Sun. Can we ever see such a thing? Wouldn't the Sun's disk hide that phase of the second planet? In fact, we can see a Full Venus through a telescope, although it can be difficult with the bright Sun so nearby. The Sun's disk rarely blocks Venus, though, because that planet's orbit is tilted 3.3 degrees with respect to our own.


Web Links

Venus -- a nice overview of our "sister planet" from the University of Michigan/NASA Ames "Windows to the Universe" web site.

Why is Venus so bright when it's only a crescent? -- find out here! (external link)

Sky and Telescope -- Find out more about observing the phases of Venus in the March 2001 issue of this magazine. (external link)

Magellan Mission to Venus -- During its four years in orbit around Earth's sister planet, NASA's Magellan spacecraft radar-mapped 98 percent of the surface and collected high-resolution gravity data of Venus.

Venus (again) -- Another overview of Venus from the SEDS Nine Planets web site. (external link)

Venus Image Gallery -- from Windows to the Universe


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