Category: Astronomy

Although Galileo wasn’t able to see the phases of Mercury (like the phases of Venus and the Moon) with his first crude telescope, the astronomers that carried on his discoveries did. This was powerful evidence that both Mercury and Earth are orbiting the Sun.

You can also check out these amazing books for more information about the planet Mercury.

Since Mercury orbits the Sun, and it follows a very elliptical path on its journey around the Sun, our two planets can vary their distance significantly.

When Mercury is at its closest point to Earth, astronomers call this opposition (from the point of view of Mercury). This would happen when Mercury was at its furthest from the Sun, and Earth is at its closest. When this happens, Mercury and Earth would be separated by only 77 million km (48 million miles).

Their maximum distance occur when Earth is at its furthest point from the Sun, and Mercury is at its maximum on the other side of the Sun. The three objects then line up perfectly. At this point, Mercury and Earth can be 222 million km (138 million miles) apart.

Distancia desde la Tierra a Mercurio

Earth and Venus share many physical characteristics. Atmosphere is not one of them. The atmosphere of Venus is so harsh that it is the main reason that no one has ever been able to make optical observations of anything other than the planet’s upper atmosphere.

Visualization of the planet is made impossible be the high amounts of sulfuric acid in the atmosphere. Clouds in the upper atmosphere are full of sulfuric acid droplets. Sulfur is highly reflective of visible light, thus preventing observation much deeper than the upper limits of the clouds themselves.

The next challenge that the atmosphere of Venus presents is its composition. It is made up of 96% carbon dioxide. Oxygen can not be detected even as a trace element. At the surface the atmospheric pressure is 92 times that of Earth. If you were able to find a way to survive the intense pressure and had enough oxygen, you would be standing on a surface that is hot enough to melt lead. The temperature across Venus, from pole to pole, is a steady 462°C as a result of the atmosphere’s greenhouse qualities. In the hottest parts of the hottest deserts here on Earth, the temperatures never top 50°C.

The sweltering surface of the planet is covered by volcanoes, impact craters, and old lava flows. There are 1,000 volcanic craters and crater remnants that are over 20 km in diameter. The surface has never been impacted by a small meteorite. The atmosphere burns up anything small. It could take an asteroid in excess of 50 km to survive long enough to hit the surface and then it would have lost close to half of its mass in the atmosphere. The entire planet was resurfaced by volcanic flows between 300 and 500 million years ago.

Several spacecraft have entered the atmosphere of Venus. The first successful probe was the Soviet mission Venera 3, but its instruments failed before entry. Venera 4successfully deployed a number of science experiments. They showed a surface temperature of almost 500°C and an atmosphere that was 90 to 95% carbon dioxide. The atmosphere was much denser than anticipated and its slower descent caused its batteries to run down before reaching the surface. NASA’s Mariner 5 reached the planet one day after Venera 4, but only flew by. The data from the two missions were compared and were an early step in international space exploration cooperation. The Soviet space agency also launched Venera’s 5 and 6. They returned a great deal of information, but were crushed by atmospheric pressure within 20 km of the surface.

There have been over a dozen missions to study the surface and atmosphere of Venus since the Venera program. Currently, the Venus Express mission is in orbit. Bepicolumbo and the Venus in-Situ Explorer are planned. Each mission returns new data, it is hard to imagine what mysteries may be unraveled with a surface explorer.

We have written many articles about Venus for Universe Today. Here are some interesting facts about Venus, and here are some pictures of planet Venus.

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://www.nasa.gov/audience/forstudents/5-8/features/F_The_Planet_Venus_5-8.html
http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Venus&Display=OverviewLong

Venus is one of the 5 planets visible to the unaided eye, and so it has been seen in the sky since prehistoric times. After the Sun and the Moon, Venus is the brightest object in the sky – brighter than any stars.

Venus is the Roman name for the Greek goddess of love; Aphrodite. But there are references to Venus as far back as Babylonian cuneiformic texts, like the Venus table of Ammisaduqa which could be as old as 1600 BC. Ancient Egyptians thought that Venus was actually two separate objects… a morning star, and then a completely different evening star. The tradition carried on with the ancient Greeks, as well.

The early mathematician Pythagoras was one of the first to recognize that the morning and evening stars were actually the same object: Venus. So perhaps he was the first to “discover Venus”.

The Maya civilization held Venus in high regard, and the planet figured prominently in their religious calendar.

Once he built his first rudimentary telescope, Galileo turned it towards Venus, was was astonished to find that the planet goes through phases like the Moon, going from crescent to gibbous to full and then back again. This was one of the strongest pieces of evidence that Venus goes around the Sun, and not the Earth as others originally believed.

Because Venus has always been obscured by thick clouds, science fiction writers have been free to imagine whatever they wanted existed on the surface of Venus. The clouds helped them to imagine a warm tropical world with constant rainfall and lush vegetation. The truth, of course, is that Venus is a hellish hot world, devoid of life.

We’re familiar with the close-up images of Earth captured by orbital satellites and astronauts on the International Space Station. But here are a few pictures of the Earth and Moon captured at a distance, while passing around the Moon, or orbiting distant Saturn. In the words of Carl Sagan, “Look again at that dot. That’s here. That’s home. That’s us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives.”

Since Sagan commented on Voyager 1’s image of Earth, several more pics of the Earth and Moon have been captured that really put our tiny spot in the Universe into perspective.

Voyager 1 – the Pale Blue Dot
This image, captured by NASA’s Voyager 1 spacecraft on February 14, 1990, is the one Sagan was talking about. It was taken when Voyager 1 was about 6.4 billion kilometres (4 billion miles) from Earth. In this image, the entirety of the planet takes up less than a single pixel; in fact, NASA calculated that we’re only 0.12 pixel in size. Sagan pushed the agency to capture images of all the planets in the Solar System, to create a family portrait seen from Voyager’s distant position.

Cassini – the Earth from Saturn
This is easily the best picture ever captured by NASA’s Cassini spacecraft, and in the running for the best space pictures of all time. It shows Saturn in all its glory, backlit by the Sun. From this view, the rings glow with an ethereal light.

But this image is extra special. Take a look towards the upper left of the image, and you’ll notice a tiny blue-white dot. Once again, there’s Earth, seen from a distance of 1.4 billion km (1 billion miles).

Mars Express – the Earth and Moon from Mars
ESA’s Mars Express captured this image of Earth and the Moon on July 3, 2005 when it was 8 million km (5 million miles). This was actually the first observational data sent back by the probe. A fitting introduction to Martian system. Of course, Mars Express was designed to image the relatively nearby surface of Mars, and not such distant objects, so the resolution of the image isn’t very good. And yet, they’re instantly recognizable as the Earth and Moon.

Opportunity Rover – the Earth from the Surface of Mars
Here’s an image of Earth captured from the surface of another planet. In this case, we’re seeing Earth from Mars, as photographed by NASA’s Mars rover Opportunity on April 29, 2005. Normally operating only in the Martian day, Opportunity woke up a little after sunset, and captured this image of Earth as the stars were starting to come out. The image of Earth looks a little elongated because it was moving slightly during the 15-second exposures. Imagine what some future Martian might see, stepping out onto the surface of the Red Planet in the dusky twilight to see our bright planet in the sky.

Rosetta – Earth and Moon during a flyby

Now this is what you might have been expecting to see. A picture of Earth, with the continents clearly visible. This photograph of Earth and the other of the Moon were taken by ESA’s Rosetta spacecraft during a flyby on November 15, 2007. Rosetta’s primary mission is to visit Comet 67P/Churyumov-Gerasimenko in May 2014, but this flyby was a great opportunity for Rosetta to test out its OSIRIS camera system. The image of the Moon was taken separately.

Earthrise – seen from the Moon
This is the classic “Earthrise” photograph, captured by astronaut William Anders during the historic Apollo 8 mission – the first human mission to the Moon. This photograph was taken on December 24, 1968. If you’re standing on the surface of the Moon, the Earth doesn’t actually “rise”. Since the Moon is tidally locked to the Earth, our planet always hangs in exactly the same point of the sky. It takes an orbiting spacecraft to see the Earth actually rise from the horizon.

This photograph was recreated by the Japanese Kaguya spacecraft, which captured a similar sequence of the Earth using its high-definition video camera.

Earth from Galileo
And finally, I think I’ve saved one of the best pictures for last. This is a photograph of the Earth and Moon (not to scale) captured by NASA’s Galileo spacecraft as it was speeding away towards Jupiter. Galileo captured these images in 1992, and finally reached the Jovian system in 1995-97. You’re looking at the Pacific Image, with South America visible as well as the Caribbean.