Nestled below the foothills of the San Gabriel mountains, the Jet Propulsion Laboratory outside Pasadena has a surprisingly low-tech feel.
For more than 40 years, space missions to the planets have been controlled from its operations rooms, yet the place is still striking for its bucolic charm. Mule deer crisscross its paths, pausing only to nibble plants, while its buildings, erected during the heyday of the US space programme, now have a settled, slightly worn aspect.
For a Californian campus, it is all very laid-back - on most occasions. On August 5 last year JPL was far from being peaceful. Thousands of anxious scientists and engineers had gathered to track the fate of the Curiosity Mars rover, the most sophisticated interplanetary probe in history, which was about to plunge toward the surface of the Red Planet after a journey of 570 million km.
A decade earlier, Nasa had decided to ramp up its efforts to study Mars and construct the granddaddy of all mobile laboratories. The one-tonne craft would be nuclear-powered and be fitted with a plethora of instruments constructed to answer one simple question: had Mars ever possessed conditions that could support life? More than 7000 scientists and engineers - an unprecedented number - were called in to help overcome the challenges in designing, building and operating Curiosity.
Just selecting a site for the giant rover's landing had proved a major headache, for example. More than 60 places were considered by mission staff, a list from which Gale Crater - near the Martian equator - was eventually selected. It is deep, boulder-strewn and has strong drafts of air sweeping up from its floor. "That made it a difficult place to land in," admits Bill Dietrich, a member of the Curiosity science team. "On the other hand it is geologically fascinating."
At the centre of Gale, Mt Sharp rises from the crater floor to a height of 4875m. Satellite photographs show it is made of layers that seem to delineate Mars' entire geological history. Set down at its base, Curiosity could trundle up its flanks to study how the Red Planet had changed over the past 4.5 billion years. The trick would be getting there.
In the past, US space engineers had adopted the bouncy castle approach to putting their rovers on Mars. A robot vehicle was secured inside a bag which was inflated during descent so that it simply bounced over the surface until it came to a standstill. For Curiosity, this was not an option. Five times bigger than its predecessors, it would have required an airbag so big, the stresses of descent would have ripped its fabric apart. Similarly, landers that use legs were considered too unstable to settle on Mars.
It took a marathon brainstorming session of Curiosity scientists and engineers at the Jet Propulsion Laboratory in September 2003 to come up with a solution: a rover on a rope. According to the plan proposed by Adam Steltzner, leader of the probe's Entry Descent and Landing (EDL) team, a platform of rocket thrusters - subsequently dubbed the Sky Crane - would hover above the Martian surface before lowering Curiosity by cable. The system was accurate, manoeuvrable and could drop the rover precisely where scientists wanted it to go. The concept was novel, untested and contained dozens of potential flaws, any one of which could doom the mission.
Not surprisingly many mission scientists were doubtful. "I remember when I was first shown the plan. I thought: they must be joking!" says Dietrich. Many of Nasa's bigwigs were alarmed. The mission had a US$2.5 billion ($3 billion) price-tag, after all. Steltzner - a former rock musician-turned-astrophysicist - was summoned to Washington by Mike Griffin, then the head of the agency, to justify his plan. Griffin announced he still thought the concept was crazy but "maybe just the right sort of crazy". The Sky Crane was approved and on November 26, 2011, the device - attached to Curiosity - was blasted into space. On August 5, 2012, it reached its target.
It takes between four and 21 minutes for radio signals from Earth to reach Mars, depending upon the planets' relative positions. Last August, the distance between the two was such that radio signals took around 14 minutes to reach Mars. Guiding a complex landing sequence from California was never a prospect for this reason. The whole process would, instead, be run by on-board computers whose performance would determine the fate of the device created by the thousands of nervy scientists and engineers who had gathered in rooms and halls round JPL.
The future of their brainchild depended on a sequence of parachutes and rocket thrusters being deployed automatically and accurately to slow Curiosity from its atmosphere-entry speed of 20920 km/h to a landing velocity of 2.7km/h.
Each stage of the descent passed perfectly and the landing was greeted with cheers and screams. Like an athlete limbering up before a race, the rover's components were gently put through their paces before the six-wheeled Curiosity trundled off on its search for clues to the past habitability of Mars. It did not have to travel far.
"We could see from satellite photographs, that a fan of sedimentary material - like an alluvial delta produced by a river of water on Earth - appeared to have spread through the Gale Crater from one point on its northern rim and had passed very close to Curiosity's landing place," says Dietrich. "It was a perfect target."
On Earth, our relatively strong gravitational field plus our relatively intense magnetic field protect our atmosphere from battering by the solar wind, a constant stream of particles that pours from the sun. Without such fields, Mars could not hold on to its atmosphere or its water, which were swept into space.
Earth remained blue and watery while Mars turned to dust. Over the next few years, Curiosity's Sam detector will sniff out delicate isotope variations in the painfully thin remnants of Mars' atmosphere for clues to the timing and nature of this disaster. The key question is: did life get a chance to make its appearance before catastrophe struck?
For Michael Meyer, lead scientist for Nasa's Mars Exploration Programme, this issue is of critical importance. "We know life appeared on Earth. But we do not know how easy that process was. It could have been straightforward or it could have been a highly fraught business filled with all sorts of unlikely contingencies."
Mars provides the perfect place to solve that mystery. If its ancient watery, organic soils eventually led to the appearance of primitive living beings before catastrophe struck, we can conclude that life could be relatively commonplace in the cosmos, he argues. "Life appearing separately on two neighbouring worlds would suggest it is a straightforward phenomenon."
But if the sands of Mars turn out never to have supported life, despite their initially attractive properties, life will look a far less likely outcome in the universe. "From that perspective, life on Earth - including humans - may turn out to be a cosmic improbability," adds Meyer.
Finding out which version is the right one underscores the importance of Curiosity and future rover missions.
There is more, adds Meyer. Our knowledge about life's appearance on our own planet is abysmal. Billions of years of biological, chemical, meteorological and geological activity on Earth have obliterated all evidence of its origins. "We do not know where it started, how it started, when it started, or what biochemical precursors led to its appearance."
By contrast, on Mars, life may have flourished for only a very brief period before being extinguished so that signs of its existence may well have been preserved in the planet's dead dust. Their discovery would be like finding biological snapshots frozen in time. "If we can find evidence of organisms' first appearance on Mars, we will be provided with a cookbook for the ingredients of life itself," says Meyer.