A UK-assembled satellite is set to go into orbit shortly to monitor air quality around the globe.
Its Dutch-designed instrument will make daily maps of polluting gases and particles known to be harmful to health.
S5P will ride to orbit on a converted Russian intercontinental ballistic missile called a Rockot.
Lift-off from the Plesetsk Cosmodrome is scheduled for 12:27 local time (10:27 BST; 09:27 GMT).
Controllers will know they have a functioning satellite in position above the planet about an hour-and-a-half later when they first receive a radio communication from S5P.
The EU, with the help of the European Space Agency (Esa), is developing a constellation of satellites as part of its Copernicus programme.
Five of the platforms are already up; many more will follow in the next few years.
All called Sentinels, they are tasked with taking the pulse of the planet and gathering data that can inform the policies of member states – everything from fisheries management to urban planning.
The Sentinels, in number and capability, dwarf anything planned elsewhere in the world, and Sentinel-5 Precursor, to give it its full title, is one of the big UK contributions to the whole endeavour.
The satellite’s TROPOMI instrument has been developed by a consortium led from the Netherlands’ national meteorological agency (KNMI), and will build daily global maps of key gases that contribute to pollution.
These include nitrogen dioxide, ozone, formaldehyde, sulphur dioxide, methane and carbon monoxide. All affect the air we breathe and therefore our health, and a number of them also play a role in climate change.
The “Precursor” in the spacecraft’s name references the fact that the TROPOMI instrument comes before a near-identical sensor that will eventually fly on Europe’s next-generation weather satellites