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Transit of Venus: Spotting Earth’s ‘evil twin’

About the author

Richard is a science journalist and presenter of the Space Boffins podcast. He edits Space:UK magazine for the UK Space Agency, commentates on launches for the European Space Agency and is a science presenter for BBC radio. You can also follow him on Twitter or Facebook.

  • Exceptional view
    Transits of Venus are a rare event. The last one occurred on 8 June 2004, but before that the last one occurred in 1884. (Copyright: Nasa)
  • Sun block
    A transit occurs when Venus passes directly between the Sun and Earth. It appears as a small black disk moving across the face of the Sun. (Copyright: SPL)
  • Up close
    Transits allowed early astronomers to begin to characterise Venus, including its size. Now, probes such as Europe’s Venus Express offer a close up view. (Copyright: Esa)
  • Acid bath
    A shot from Nasa’s Pioneer-Venus Orbiter shows the planet’s yellow-white cloud cover of sulphuric acid, which permanently obscures the surface. (Copyright: SPL)
  • What lies beneath
    The Magellan mission in the early 1990s imaged more than 98% of the planet’s surface, producing vivid maps that showed what lay beneath the clouds. (Copyright: Nasa)
  • Volcanic rock
    Radar images showed that the planet was volcanically active. The brighter lines on this image show the upwelling of magma from the interior of the planet. (Copyright: Nasa)
Looking forward to the transit of Venus, Richard Hollingham explains why the second planet from the Sun deserves your attention.

On 5-6 June many of us will have the chance to witness a cosmic event that won’t happen again for another 105 years. On those two days our planetary neighbour, Venus, will pass in front of the Sun. If you’re lucky enough to live in the right part of the globe, and the skies are clear, then you too can be part of space history.

The transit is the sort of thing us astronomical twitchers love to try to see. It’s a similar idea to a solar eclipse — Venus will move between us and the Sun, blocking out some of its light. But there are none of the long shadows and spooky ethereal darkness you get with solar eclipses. Instead, observers (please don’t look directly at the Sun) will see just a tiny black dot move across the face of our nearest star.

But this is no ordinary black dot. Venus is one of the most extraordinary places in the Solar System. The surface temperature on the planet, at 470C (880F), is enough to melt lead and the pressure, of some 90 atmospheres, is roughly equivalent to diving a kilometre beneath the ocean on Earth. Venus is surrounded by a toxic soup of gases with sulphuric acid clouds and winds of up to 350km/h (220 mph). This dense atmosphere prevents heat escaping into space – a process often described as a ‘runaway’ greenhouse effect.

What’s weird is that Venus is only slightly smaller than the Earth and has a very similar chemical composition and density. The fact that it’s turned out so differently has led those who study the planet to refer to it as ‘Earth’s evil twin’. The one the family don’t like to talk about. But that hasn’t stopped scientists trying to figure it out. So far, there have been more than twenty missions to Venus, including two Russian landers, which survived for a couple of hours before getting frazzled by the extreme conditions. The most recent mission, Europe’s Venus Express, is in orbit right now analysing the planet’s atmosphere. But despite all this international effort, the second planet from the Sun still holds many mysteries.

‘Cloud city’

“When we look at Venus we think this is a planet that once, in the very early solar system, might have been very Earth-like,” says Geoffrey Landis from Nasa’s Glenn Research Centre in Cleveland, Ohio. “But obviously it changed, it overheated, it turned into the very hot thick atmosphere planet. Nevertheless, when we study Venus we can get a view of the very different ways that a rocky planet like the Earth can evolve.”

And, he says, there are basic things that we don’t understand. “It seems to be a planet that doesn’t have plate tectonics – but the Earth does – so we’d like to know how come the Earth is so different. We don’t know if it has a very thin crust… or maybe it has a very thick crust; we see a lot of evidence of volcanoes but we don’t even know if any of them are still active.” In short, Venus turns much of what we understand about Earth-sized rocky planets on its head.

There might even be life. “It’s not out of the question,” says Landis. “Although the surface is very hot – and not very hospitable to life – as you go up into the middle atmosphere it becomes very Earth-like... researchers have proposed that maybe the middle atmosphere does have life.”

But, as you’ll have figured, visiting Venus isn’t easy. Fortunately, as well as a Nasa scientist, Landis is also a respected science fiction writer – a fearsome combination that allows him to think big thoughts about future mission concepts. He’s even worked on plans to send humans.

“We’ve looked at an interesting new way of exploring Venus,” he tells me. “Our concept is to put the humans into a habitat that orbits the planet and they operate rovers on the surface via tele-robotics.” By immersing these human operators in a high-definition virtual reality on board an orbiting spaceship, Landis reckons it “would be just like being there.”

This idea has major advantages over the current rovers used to explore Mars. With a time delay of up to 20 minutes each way, controlling any rover usually involves transmitting commands from Earth and letting the rover get on with it. “If you could be there and adapt to what happens when it happens, then you could be much more interesting science,” Landis explains. If, for example, you spotted an interesting feature, you could head straight there to examine it – just as the Apollo astronauts were able to on the Moon.

If you think that’s ambitious, then how about... a human colony on Venus? It’s not as crazy as it first sounds. About 50km (31 miles) above the planet’s surface the atmospheric composition and pressure become very Earth-like. Landis envisages balloons filled with a breathable mixture of nitrogen and oxygen. Because of the atmospheric density on Venus, these gases would act as a lifting gas allowing the balloon to float through the clouds. “You could fill them up and walk around inside the envelope of these balloons.”

With power from solar arrays, Landis calculates that these cloud bases could be largely self-sufficient. “In my vision you could make enormous balloons – perhaps a kilometre in diameter. It’d be like being outdoors inside this balloon. You could put whole cities in the clouds of Venus in this very Earth-like environment!”

Okay, so none of this is going to happen any time soon, but it’s worth contemplating the possibilities. Imagine what life would be like living in a city in the clouds adrift in the atmosphere of an alien world. Rather than distant exoplanets, could Venus be the real Earth 2.0?

So on 5 and 6 June, as that little black dot makes its way across the face of the Sun (and I want to emphasise again, please don’t look directly at the Sun) consider what a remarkable place our twin is. Not evil, just misunderstood.

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