Earth, the marvellous blue orb that it is

Humans have not exactly left Earth. It has been 40 years or so, in fact, since representatives of this erstwhile space faring species have been more than a few hundred miles from our planet’s surface.

Today, human spaceflight, such as it is, is centered on the International Space Station. It lumbers around Earth about 250 miles out, going in and out of darkness every 90 minutes on average. It is staffed by six astronauts who spend their time - when they are not singing David Bowie songs – doing maintenance and conducting scientific and medical experiments.

A new set of investigations will examine the cosmic Kelly twins, Scott and Mark, as a pair of genetically synchronised watches – one on the station, one on Earth – for a year to see what happens. The International Space Station has cost roughly $100 billion. Ever since this giant Tinkertoy began to come together, pundits, scientists, politicians and fans of space exploration have argued over what it is for, and whether it is worth it.

One of the little-known benefits of the space station is that it is at just the right altitude to photograph things on Earth. Every week, the astronauts aboard the space station record images of things that would spew headlines if we saw them on any one of the thousands of planets that we now know circle other stars.

Earth Day is coming up, and so it’s worth taking a look. From above, we can see how geology has become destiny. This is the one place we know in the universe where primordial forces – the ones that gave rise to our wrinkled mountain ranges, gouges of canyons, winding ribbons of water and mud, circus spirals of clouds, volcanos, continents slipping under one another, a crust that splits and belches fire – have conspired to bring about life.

Earth has been habitable – that is, temperate enough for liquid water on the surface – for four billion years, geologists say, regulated by the ebb and flow of carbon dioxide in the atmosphere and the crust. Washed out of the atmosphere by rain, that greenhouse gas weathers rocks, carrying silica, calcium and carbon compounds into the ocean and eventually under the ocean floor. Millions of years later, carbon dioxide is disgorged in volcanos, replenishing the atmosphere.

The thermostat is self-regulating. The more carbon dioxide in the atmosphere, the more violent the weather, and the faster the gas is rained out. When it cools off, there is less rain and carbon dioxide builds back up.

Given a chance in the form of a stable environment, life thrives in unexpected ways. From above, the entire world seems like one organism, one market exchange for genes – and, as demonstrated by lights seen from above, an increasingly complicated and gaudy nervous system more and more apparent to the rest of the cosmos.

The glory of the Earth is its blue oceans, the cradle of life as we know it. Someday, we might find another one Out There – Earth 2.0, it is sometimes called.

Our planet’s marine blue is so striking that it showed up in Voyager 1’s camera from four billion miles away on February 14, 1990, and again posing in the far distance beyond Saturn’s rings for the Cassini spacecraft’s camera on July 19, 2013.

The third rock from the sun is a pale blue dot, the only one we know. As time passes and the roll call of exoplanets found by NASA’s Kepler telescope and other efforts grows, I can’t help thinking that it is looking rarer and rarer.

As T S Eliot wrote in “Four Quartets”: We shall not cease from exploration, and the end of all our exploring will be to arrive where we started and know the place for the first time.