A Russian rocket is about to carry perhaps the most important European space mission into space.
The first of two ExoMars missions will set off to see if they can find evidence that there is or was life on the red planet.
The first mission will see an orbiter carried off by a heavy-lift proton rocket towards Mars. When it gets there it will look for methane and try and work out whether it is a sign of biological processes.
It will be followed in two years by another British-built rover, which will land on the planet itself. That will be full of new technology, including a special drill that can bury deeper into the planet and look for signs of living things.
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.
Additional reporting by agencies
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