Posted on by Fraser Cain

Mars and Venus

Mars and Venus are the two terrestrial planets most similar to Earth. One orbits closer to the Sun, and one orbits more distant to the Sun. But both are visible with the unaided eye, and two of the brightest objects in the night sky.

Venus orbits at an average distance of only 108 million km from the Sun, while Mars is an average of 228 million km. Venus gets as close to Earth as 38 million km, and Mars gets as close as 55.7 million km.

In terms of size, Venus is almost a twin planet of Earth. Its diameter is 12,104 km, which is 95% the diameter of Earth. Mars is much smaller, with a diameter of only 6,792 km. And again, in terms of mass, Venus is almost Earth’s twin. It has 81% the mass of Earth, while Mars only has 10% the mass of Earth.

The climates of Mars and Venus are very different, and very different from Earth as well. Temperatures on the surface of Venus average 461 °C across the entire planet. That’s hot enough to melt lead. While the average temperature on Mars is a chilly -46 °C. This temperature difference comes from the fact that Venus is closer to the Sun, but also because it has a thick atmosphere of heat trapping carbon dioxide. The atmosphere on Venus is nearly 100 times thicker than Earth’s atmosphere at sea level, while the atmosphere on Mars is 1% the thickness of Earth.

Mars is the most studied planet in the Solar System (after the Earth). There have been dozens of missions sent to Mars, including orbiters and rovers. Although many missions have been lost, there have been several that have successfully orbited the planet and several that have landed on the surface. Missions have also been sent to Venus, and you might be surprised to know that the Soviets sent a series of landers called Venera that actually reached the surface of Venus and survived long enough to send back a few photographs.

Mars has two moons, Phobos and Deimos, while Venus has no moons. And neither planet has rings.

We’ve written many articles about Mars and Venus for Universe Today. Here’s an article about how the atmospheres of Mars and Venus leak into space, and a look at Venus wet past.

If you’d like more information on those two planets, here’s a link to NASA’s Solar System Exploration Guide on Venus, and Hubblesite’s News Releases about Mars.

We’ve also recorded several episodes of Astronomy Cast about the planets. Listen here, Episode 50: Venus and Episode 52: Mars.

Reference:
NASA

Posted on by Fraser Cain

What Color is Venus?

Venus. Image Credit: NASA/courtesy of nasaimages.org



Here’s a question: what color is Venus? With the unaided eye, Venus just looks like a very bright star in the sky. But spacecraft have sent back images of the cloud tops of Venus, and some have even returned images from the surface of Venus.

If you could actually fly out to Venus and look at it with your own eyes, you wouldn’t see much more than a bright white-yellowish ball with no features. You wouldn’t actually be able to see any of the cloud features that you can see in photographs of Venus. That’s because those photos are taken using different wavelengths of light, where differences in the cloud layers are visible. For example, the photo that accompanies this story was captured in the ultraviolet spectrum.

Although the atmosphere of Venus is almost entirely made up of carbon dioxide, the clouds that obscure our view to the surface are made of sulfur dioxide. These are opaque to visible light, and so we can’t see through them to the surface of Venus. These clouds actually rain droplets of sulfuric acid.

Surface of Venus by Venera.
Surface of Venus by Venera.

If you could get down beneath the cloud tops of Venus, you wouldn’t be able to see much either. That’s because the clouds are so thick that most of the light from the Sun is blocked before it reaches the surface. You would see a dim landscape, like you might see at twilight. The surface of the planet is littered with brownish-red volcanic rocks. The bright red color you see in the Soviet Venera images of Venus have been brightened to show more surface detail.

So, what color is Venus? Yellowish-white.

We’ve written several articles about the color of the planets for Universe Today. Here’s an article about the color of Mercury, and here’s an article about the color of Pluto.

If you’d like more info on Venus, check out Hubblesite’s News Releases about Venus, and here’s a link to NASA’s Solar System Exploration Guide on Venus.

We’ve also recorded an entire episode of Astronomy Cast all about Venus. Listen here, Episode 50: Venus.

Posted on by Fraser Cain

When Was Venus Discovered?

Venus captured by Magellan.

Were you wondering when was Venus discovered? Actually, there’s no way to know. Venus is one of the 5 planets visible with the unaided eye. In fact, Venus is the brightest object in the night sky after the Sun and the Moon. When Venus is at its brightest, it even casts shadows. So even ancient people would have been aware of Venus, and so there’s no way to know who that first person was, and when it happened.

However, a better question might be to ask, when did we know that Venus was a planet? This happened about the same time that astronomers first realized the Earth was a planet too. In ancient times, astronomers used to think that the Earth was the center of the Universe, and everything orbited around it: the Sun, the Moon, the planets and the stars. One problem with this model was the strange behavior of the planets. Sometimes they would speed up, and then slow down, stop, and even go backwards in the sky.

But then in the 1500s, Nicolaus Copernicus developed his model of a Sun-centered Solar System. The Earth was just a planet, and all of the planets orbited around the Sun instead. This model explained how the planets could have such strange movements. Since the Earth is moving too, we’re really just seeing them from different perspective in they sky.

The first person to see Venus in a telescope was Galileo. Although he wasn’t able to resolve anything but a bright disk (astronomers can’t do any better today), he saw that Venus went through phases like the Moon. This was further evidence that Venus orbits around the Sun – closer than the Earth, and so we see it in various phases of illumination.

Because Venus is shrouded in clouds, astronomers weren’t able to get a better view of Venus until the first spacecraft arrived from Earth. The first spacecraft to visit Venus was NASA’s Mariner 2, which arrived at Venus in 1962. But even then the planet was still blocked by clouds. The Russian Venera landers were able to pierce through the clouds and landed on the surface to send back a few quick images of the planet’s surface. They showed a hellish world, with thick atmosphere, clogging clouds, and blasting heat, hot enough to melt lead. NASA’s Magellan spacecraft (launched in 1989) was equipped with a radar instrument that allowed it to pierce through the clouds on Venus and show the planet’s landscape, craters and volcanoes.

We’ve written many articles about the discovery of planets for Universe Today. Here’s an article about the discovery of Uranus, and here’s an article about the discovery of Neptune.

If you’d like more information on Venus, check out Hubblesite’s News Releases about Venus, and here’s a link to NASA’s Solar System Exploration Guide on Venus.

We’ve also recorded an entire episode of Astronomy Cast about Venus. Listen here, Episode 50: Venus.

Posted on by Jerry Coffey

How Far is Venus from Earth?

The are varying numbers for the Venus distance from Earth. Each number depends on the relative position of each planet in its elliptical orbit. The point when the planets are at their closest approach to each other is called opposition. The distance between the planets can even vary at different oppositions. The closest possible opposition distance between Earth and Venus is 38 million kilometers. This is the closest that any planet comes to Earth.

The farthest that Venus ever gets from Earth is 261 million km. The means that the Venus distance from Earth can vary by an incredible 223 million km. While that seems like an amazing distance, it is nothing compared to the numbers attributed to other planets. Try to imagine how far it is between Earth and Neptune. Here is a link that tells you how to figure those distances out.

The relative proximity of Venus helps to explain why it is the second brightest object in the night sky. The planet has an apparent magnitude of about -4.9 at its brightest. It can also completely disappear from the night sky when it is at its most distant, because the Sun is between it and the Earth. The planet’s apparent magnitude is also helped by the reflectivity of the sulfuric acid clouds that dominate its atmosphere. The clouds reflect a great deal of visible light, increasing the planet’s albedo and making it more readily seen.

Venus will periodically pass across the face of the sun. This is called a transit. These transits of Venus occur in pairs with more than a century separating each pair. Since the advent of the telescope, transits have been observed in 1631, 1639; 1761, 1769; and 1874, 1882. the most recent occurred on June 8, 2004. The second in this pair will occur on June 6, 2012, so mark your calenders and prepare your telescopes. The planet can also be seen to go through phases much like the Moon when you observe it through powerful binoculars or small telescopes.

Venus is always brighter than any star. It is at its brightest when the Venus distance from Earth is the smallest. The planet can be easy to see when the Sun is low on the horizon, it always lies about 47° from the Sun. The planet orbits faster than the Earth, so overtakes it every 584 days. When this happens Venus is more easily seen in the morning, just after sunrise. Hopefully, you have found quite a bit of useful information here.

We have written many articles about Venus for Universe Today. Here are some interesting facts about Venus, and here’s an article about Venus compared to Earth.

If you’d like more information on Venus, check out Hubblesite’s News Releases about Venus, and here’s a link to NASA’s Solar System Exploration Guide on Venus.

We’ve also recorded an entire episode of Astronomy Cast all about Venus. Listen here, Episode 50: Venus.

References:
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Venus&Display=OverviewLong
http://curious.astro.cornell.edu/question.php?number=564