Europe's historic search for life on Mars is set to take off on Monday as a heavy-lift Russian rocket blasts into space from Kazakhstan.
The Proton rocket will launch the first of two ExoMars missions, together costing 1.2 billion euro (£924 million), designed to uncover signs of past or present life on the Red Planet.
An orbiter carried on the ExoMars 2016 mission will hunt for methane in the Martian atmosphere and show if it is likely to have been generated by geology or biological processes.
Then in two years' time, ExoMars 2018 will send a British-built rover bristling with cutting- edge technology to Mars.
Equipped with a drill that can burrow deep below the radiation-baked Martian surface, it will look for the chemical fingerprints of life.
If the scientists find evidence of life - even primitive life that existed billions of years ago - it will be one of the biggest discoveries of all time.
Humanity will have to re-assess its place in the universe, just as it did when Copernicus showed that the Earth and its sister planets orbited the Sun.
While American rovers have paved the way by investigating whether the Martian environment is or ever was suitable for living microbes, none of them has been equipped to search for life itself.
Planetary scientist Dr Peter Grindrod, from Birkbeck, University of London, who is funded by the UK Space Agency, said: "It's incredibly exciting.
"This is a series of missions that's trying to address one of the fundamental questions in science: is there life anywhere else besides the Earth?
"Finding that life exists elsewhere in the solar system would be a huge discovery, so the evidence has to be strong.
"As they say, extraordinary claims require extraordinary evidence."
The ExoMars missions are being undertaken jointly by the European Space Agency (Esa) and Russian Federal Space Agency, Roscosmos.
Monday's launch from the Baikonur Cosmodrome will send two unmanned probes on a journey across space lasting seven months.
One Trace Gas Orbiter (TGO) will sniff the Martian atmosphere in search of methane, water vapour and other trace gases.
The other, Schiaparelli, is a lander whose primary job is to undertake a trial run of the parachute and retro-rocket technology needed for the later rover mission.
Methane is quickly broken down by sunlight and has to be continually regenerated from some source to persist in a planet's atmosphere.
On Earth the primary source is life. Billions of microbes, including many that thrive in the guts of animals such as cattle and termites, belch out the gas.
But methane can also be released by volcanic activity and geological chemistry.
TGO is equipped to look for methane hotspots on the Martian surface, and using highly sensitive instruments, test whether it is likely to be the product of biology or geology.
Sue Horne, head of space exploration at the UK Space Agency, based in Swindon, said: "We hope TGO will answer once and for all the question of whether the methane has a biological or geological origin.
"If it is shown to be biological, created by life, that would be amazing."
The orbiter also carries a high resolution colour 3D camera called CaSSIS (Colour and Stereo Surface Imaging System) that can snap photos of surface objects as small as 15 to 20 metres across.
CaSSIS will look ahead and behind the spacecraft to build up stunning 3D maps of craters, mountains, dunes and other surface features.
Dr Grindrod said: "We can definitely expect some spectacular pictures."
Scientists will need to be patient. Although the TGO is due to reach Mars in October, it will have to undergo a lengthy series of orbital manoeuvres before the five year scientific mission can begin in December 2017.
Schiaparelli meanwhile is programmed to detach from the orbiter on October 16 and land on a flat region of Mars known as Meridiani Planum three days later.
The disc-shaped descent and landing demonstrator module measures 7.8ft (2.4m) across with its heatshield and weighs 1,300lb (600kg).
It will deliver a small science package to the surface designed to probe the weather around it - measuring wind speed, humidity, pressure, and the amount of dust in the air.
But its chief purpose is to test technology to be incorporated into the ExoMars 2018 landing system.
Landing on Mars is so fraught with difficulty, the scientists are leaving nothing to chance.
Like the rover-carrying spacecraft, Schiaparelli will use a parachute to brake its descent through the atmosphere and then fire clusters of retro rockets shortly before touch-down.
The six-wheeled rover, weighing 680lb (310kg), is being built by the British division of Airbus Defence and Space in Stevenage.
It will have a drill capable of extracting soil samples from a depth of 6.5ft (2m).
If bugs do live on Mars, they could only be found beneath the radiation-baked surface.
Instruments on the rover will analyse the samples and look for signatures of life, including mineral residues left by long dead microbes and organic molecules with the correct left or right "handed" configuration.
The ExoMars mission has had a bumpy ride so far. Originally it was supposed to have been a partnership between ESA and Nasa, but the American space agency dropped out in 2012 because of budget cuts.
A year later, ESA signed a new deal with Roscosmos, which is now responsible for the rover landing system.
Dr Manish Patel, from the Open University, who heads TGO's small team of British scientists and is in charge of the probe's ozone-mapping ultraviolet (UV) spectrometer instrument, said: "This is a fantastic mission; massive.
"I spent the last 13 years of my life working on it so I am somewhat excited and nervous.
"You're strapping an instrument you've devoted your life to on top of a great big bomb.
"It's scary but it's why I'm in this business.
"There won't be many nails left on launch day."