Setting off on a Russian Soyuz rocket will be Sentinel-1B with its Earth surveillance radar, and Microscope, a French-built orbiter seeking to poke a hole in Einstein's theory of general relativity.
They will be hoisted from Europe's launch pad in Kourou, French Guiana, at 2102 GMT Friday.
Sentinel-1B is the twin of Sentinel-1A, launched two years ago.
The pair are equipped with sophisticated, cloud-penetrating radar with which to monitor Earth's surface by day and night, regardless of the weather conditions.
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Sentinel-1A and 1B are part of the 3.8-billion-euro (USD 4.3-billion) Copernicus project, which will ultimately sport six orbiters in all. It is a joint undertaking of the European Space Agency and the European Commission.
Between them, the twin satellites will be able to take a picture of anywhere on Earth every six days from an altitude of nearly 700 kilometres (435 miles).
The images can be used to spot icebergs and oil spills, illegal logging or landslides.
They will help scientists monitor changes in forest cover, water and crop health.