How We Came to Know the Cosmos: Space & Time

Discover How We Came to Know the Cosmos

Chapter 16. The planet Venus

16.1 Characteristics of Venus

Venus is the second-closest planet to the Sun. It takes about 225 days to orbit the Sun and almost 2 days pass on Venus during this time.[1] Apart from the Sun and Moon, Venus is the brightest natural object in space that is visible from Earth. Venus is never far from the Sun and so like Mercury, it is best observed in the early morning or at twilight. The name Venus is also Roman in origin, referring to the goddess of love and beauty.[2]

Venus is sometimes called Earth’s ‘sister planet’ because it’s a similar size, mass, and density to the Earth.[3] It may have once contained large oceans, but it has undergone an extreme greenhouse effect and the water has evaporated, leaving a dry, desert landscape.[4]

Venus is now covered in a thick atmosphere of carbon dioxide with sulfuric acid clouds. The atmospheric pressure on Venus is almost 100 times that of Earth’s, and the atmosphere reflects most sunlight back into space before it can reach the planet’s surface. Sunlight that does enter cannot escape and this has made Venus the hottest planet in the Solar System. It is almost three times the temperature of Mercury despite being almost twice the distance from the Sun.

The four inner planets, Mercury, Venus, Earth, and Mars, and the Moon. Sizes are approximately to scale, and the composition is shown.

Figure 16.1
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The four inner planets to scale.

A photograph of Venus above the clouds.

Figure 16.2
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An ultraviolet image of Venus taken using the Pioneer Venus Orbiter.

A photograph of Venus taken below the clouds.

Figure 16.3
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An image of Venus below the clouds taken using data from Magellan.

Venus is covered in volcanoes but there are relatively few craters, which means that something - like lava - must be filling them in. It’s estimated that its surface must be fairly new, at about half a billion years old.[2]

Venus has the most circular orbit of all the planets in the Solar System, however, it’s one of only two planets to rotate on its axis in a clockwise direction. The only other planet to do this is Uranus (discussed in Chapter 23). Venus can be considered to be tilted by almost 180°, and so compared to most of the other planets, it appears to be upside down. It’s still not known why this is, or whether Venus really did tip upside down.[5]

The fact that Venus orbits in the opposite direction to the other planets and the Sun means that the time between sunrises is shorter than the rotation period. Venus takes the longest of all the planets in the Solar System to rotate on its axis, taking about 243 Earth-days to rotate. Over two day and night cycles will occur during this time.

Venus Fact Sheet[1]

Designation = Terrestrial (rocky) planet
Mass = 4.9×1024 kg (81.6% mass of Earth)
Radius = 6052 km (94.9% radius of Earth)
Density = 5243 kg/m3 (95.1% density of Earth)
Length of Day = 2802.0 hours (116.8 Earth-days)
Length of year = 224.7 Earth-days
Days per year = 1.92 days on Venus per year on Venus
Distance from the Sun = 1.1×108 km (0.72 AU)
Orbital Velocity = 35.0 km/s
Orbital Eccentricity = 0.007
Obliquity (tilt) = 177.4°
Mean Temperature = 464 °C
Moons = None
Ring System = None

16.2 Missions to Venus

Venus is the closest planet to the Earth and over 40 attempts have been made to send probes to Venus. This began in 1961 when the Soviet Union launched Sputnik 7. This would have been the first space probe to be sent to another planet, but it failed before it left low Earth orbit. Later that year, the Soviets’ Venera 1 became the first probe to be sent to another planet, although they lost contact with it before it arrived at Venus.

NASA launched their first probe to Venus, Mariner 1, in 1962. This was lost too, but Mariner 2 completed its mission later that year, passing about 35,000 km from the surface of Venus.

The Soviet Union also attempted to send multiple probes to Venus in 1962 - Sputnik 19, Sputnik 20, and Sputnik 21 - but they all failed to escape the Earth’s orbit. They attempted 10 more failed missions between 1963 and 1967, including Venera 3, which was launched in 1965 and crash-landed on Venus in 1966, becoming the first human-made object to strike the surface of another planet.

The Soviets’ first successful mission to Venus occurred in 1967 with Venera 4, which became the first human-made device to successfully land on another planet, and the first to successfully transmit data back to Earth. NASA’s Mariner 5 flew past Venus the day after Venera 4 landed, and data from the Soviet and American missions were shared.

The Soviet Union completed 12 more successful Venera missions, Venera 5 - Venera 16, between 1969 and 1983. They also launched two unsuccessful Cosmos missions to Venus, with Cosmos 359 in 1970, and Cosmos 482 in 1972. Finally, they had two more successes in 1985, with Vega 1 and Vega 2, which went on to fly past Halley’s Comet.

NASA’s Mariner 10 successfully flew past Venus in 1974, on its way to Mercury. NASA’s Pioneer 1, the Pioneer Venus Orbiter, and Pioneer 2, the Pioneer Venus Multiprobe, were launched in 1978 and reached Venus later that year. The Pioneer Venus Multiprobe released four probes into the atmosphere, and the Pioneer Venus Orbiter went into orbit and then used radar to map the surface. It transmitted data back to Earth until 1992. The remainder of Venus was mapped by NASA’s Magellan spacecraft, which was launched in 1989 and reached Venus in 1990, where it remained in orbit until 1994.

NASA’s Galileo spacecraft flew past Venus in 1990, before travelling to Jupiter, and the Cassini spacecraft flew past Venus in 1998 and 1999, before travelling to Saturn. The Cassini mission was a joint project between NASA, the ESA, and the Italian Space Agency (ASI).

The ESA launched their own Venus orbiter in 2005, the Venus Express, and this remained in orbit around Venus from 2006 until 2015. JAXA launched three probes to Venus in 2010, Akatsuki (PLANET-C), IKAROS, and Shin’en (UNITEC-1). Akatsuki and Shin’en failed, but IKAROS successfully flew past Venus using solar sail technology. JAXA remained in contact with Akatsuki, and it made a second attempt to fly past Venus in March 2016. This time it was successful.

16.3 References

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