Photosynthesis itself cannot be measured directly by satellites. However, when plants absorb sunlight for photosynthesis, a small fraction of this energy is re-emitted as a very faint fluorescence signal known as solar-induced chlorophyll fluorescence.

This fluorescence, which is invisible to the naked eye, occurs when chlorophyll molecules release absorbed light at longer red and far-red wavelengths. Because its intensity varies with environmental conditions and the physiological state of vegetation, fluorescence provides a unique window into plant activity and health — and, remarkably, can be detected from orbit. Using cutting-edge technology, ESA’s FLEX mission is designed to measure this faint signal of fluorescence to assess photosynthetic activity and plant health.

The animation illustrates how this works. In this case, healthy plants emit a strong fluorescence signal as they photosynthesise. When water becomes scarce, photosynthetic activity is affected and the fluorescence signal begins to decrease – before visible signs of stress appear in the leaves. FLEX can detect these subtle changes from orbit, providing an early indication of water stress.

As the plant withers, its fluorescence signal changes further, demonstrating how fluorescence can reveal the progression of plant stress and provide valuable information about vegetation health.