ESA's new FLEX satellite measures photosynthesis from 814 kilometres up
It went up on a Vega-C from Kourou on 15 September, the first satellite built specifically to measure photosynthesis. It flies at 814km, has three and a half years of life and revisits the same spot every 27 days.
A struggling plant does not change colour on the day it starts struggling. It slows down internally first, and the leaf gives the game away weeks later. Catching that earlier stage is the whole reason the European Space Agency put a satellite up this week.
It is called FLEX, and it launched on 15 September aboard a Vega-C from Kourou in French Guiana, on flight VV30. It had company. Copernicus Sentinel-3C rode the same rocket, sitting on top of the launch adapter, and was released into orbit roughly an hour earlier.
What FLEX actually measures
Light that went nowhere. When a leaf absorbs sunlight, in good conditions it puts around 80% of it to work in photosynthesis, and the rest leaves as heat or as a very faint glow known as chlorophyll fluorescence. That glow is invisible to the eye and comes in two peaks, one near 685 nanometres and another close to 740.
The useful part is what happens to it under pressure. Frost and drought act directly on the machinery that produces the signal, so the fluorescence shifts with it, usually downwards, because a stressed plant cuts its light absorption to protect itself. Reading the glow means reading photosynthesis as it happens rather than reading appearances.
Doing it takes an imaging spectrometer built for nothing else. This is the first European satellite to fly with that as its main job instead of a by-product of some other survey.
The numbers
FLEX orbits at 814 kilometres, is designed for three and a half years of operations, and passes over the same ground every 27 days. It does not work alone: it flies in tandem with Sentinel-3A, which supplies the atmospheric and surface readings needed to clean up the faint signal, and Sentinel-3C will later take over that slot in the formation.
Why this matters if you live in Portugal
Because Portugal has both of the problems FLEX was designed to spot from above. Earlier this month 26 municipalities hit top fire risk on a single Sunday, and western Europe has just closed out its hottest summer on record, 2.54C above the average. A forest photosynthesising badly in July is fuel in August, and right now that only becomes clear afterwards.
Temper the expectations a little. FLEX data is not arriving tomorrow: the satellite has entered commissioning, with teams at ESA’s operations centre in Germany checking the instrument piece by piece. What the mission offers is a long, global, comparable record of photosynthesis seen from space, and no such record exists.
By Oliver Grant
Image: © ESA–CNES–Avio / Optique Vidéo de CSG – J. Georget (2026)
