FLEX and Sentinel-3C ready on Vega-C: the European launcher's eighth flight carries a photosynthesis satellite and Copernicus's next relay

In the early hours of 15 September 2026, at 01:21 UTC (03:21 CEST; 22:21 on the 14th in Kourou), a Vega-C is due to lift off from Europe's Spaceport in French Guiana with two Earth-observation satellites that could hardly be more different and yet are built to fly together. On top, Sentinel-3C, the third satellite of the Copernicus Sentinel-3 series, which has been watching the oceans and the land surface since 2016. Below it, FLEX, a 400 kg ESA Earth Explorer whose single instrument is designed to measure the faint glow that plants emit as they photosynthesise. This is flight VV30, the eighth for Vega-C and the launcher's second of 2026.

ESA declares both satellites ready after more than six weeks of preparations at the spaceport. They travel stacked under the same fairing on the Vespa adapter, which lets Vega-C release two main payloads on a single flight: Sentinel-3C sits in the upper position and will be the first to separate; then, after the launcher manoeuvres, FLEX will be deployed. Both are bound for the same Sun-synchronous orbit at about 814 km, and that is no coincidence: FLEX is designed to fly in tandem with a Sentinel-3, between 40 and 100 km ahead of it, so that the Copernicus satellite's atmospheric and surface measurements complement its own.

Vega-C is a four-stage launcher, 34.80 m tall and 3.40 m in diameter, weighing 210 tonnes on the pad and able to lift up to 2,300 kg, according to the vehicle's entry in this encyclopaedia. ESA leads the programme and Avio, in Italy, builds the rocket; by the same entry's account, May's SMILE mission was the first Vega-C launch operated by Avio without Arianespace.

FLEX, short for Fluorescence Explorer, is what ESA calls its "photosynthesis mission". When a plant absorbs light to fix carbon, a small fraction of that energy is re-emitted as fluorescence, a glow invisible to the eye but measurable from orbit if the instrument is fine enough. That is the job of FLORIS, the Fluorescence Imaging Spectrometer, built by Leonardo in Italy, which observes between 500 and 780 nanometres and concentrates its spectral resolution on the two oxygen absorption bands — O2-A, from 759 to 769 nm, and O2-B, from 686 to 697 nm — where the vegetation signal stands out against reflected sunlight. To these it adds the chlorophyll absorption band, from 600 to 677 nm, and the photochemical reflectance index band, from 500 to 600 nm.

The output will be fluorescence maps at 300 m by 300 m resolution with a 27-day repeat cycle. ESA wants them to gauge the health of the planet's vegetation and, above all, to better understand how carbon moves between plants and the atmosphere and how photosynthesis feeds into the carbon and water cycles. The satellite, with Thales Alenia Space in France as prime contractor, has a planned lifetime of three and a half years plus three months of commissioning.

Sentinel-3C is a different matter: not a science mission but the continuation of an operational service. The Sentinel-3 series measures sea-surface topography, sea- and land-surface temperature and ocean and land colour, with data that are free and open to anyone. Sentinel-3A launched on 16 February 2016 and Sentinel-3B on 25 April 2018, and since then the mission has operated as a constellation of two satellites 140 degrees apart in the same orbit. ESA develops the satellites, Eumetsat operates the satellite and its marine payload data ground segment, and the two agencies manage the mission jointly: ESA processes the land products and Eumetsat the marine ones for the Copernicus services, funded by ESA's Member States and the European Union.

It carries four instruments. OLCI, the Ocean and Land Colour Instrument, with 21 spectral bands between 400 and 1,020 nm and a 1,270 km swath; SLSTR, the surface-temperature radiometer, with nine bands between 550 and 12,000 nm and swaths of 1,400 km at nadir and 740 km in the oblique view; SRAL, a Ku- and C-band radar altimeter with 300 m resolution; and MWR, a dual-frequency microwave radiometer that accompanies the altimeter. Satellites of the series weigh about 1,150 kg, 130 kg of it propellant, in a body measuring 2.2 by 2.2 by 3.7 m; they are designed for seven years of operations with consumables for twelve. The prime contractor is Thales Alenia Space France, which also builds OLCI and SRAL. ESA has not spelled out what technical difference the C unit introduces over its two sisters; its role is to make sure the constellation is not left short when the 2016 and 2018 satellites begin to run down.

For the launcher, VV30 is one more step in a recovery of credibility that is not yet complete. The Vega-C entry records seven launches up to May 2026, six successful and one failed: VV22, on 21 December 2022, when a loss of pressure in the second-stage Zefiro-40 motor, caused by nozzle erosion, cost the two Pléiades Neo satellites on board. The return to flight, with a redesigned nozzle, did not come until 5 December 2024 with Sentinel-1C; this year the rocket has flown once already, on 19 May, with the Sino-European SMILE mission.

That ESA entrusts Vega-C with an Earth Explorer and a Copernicus satellite on a single flight says a good deal about where confidence in the vehicle stands two years after its return to service. It also says something about pace: the eighth flight of a launcher that debuted on 13 July 2022 comes more than four years into its life, a cadence Europe will need to raise if Vega-C is to carry the continent's light and medium institutional payloads on its own.

Three things. First, the Zefiro-40 itself, still the stage under closest scrutiny on every flight since VV22. Second, the double separation sequence on the Vespa adapter, Sentinel-3C first and then FLEX, with the fourth stage manoeuvring in between. Third, signal acquisition from both satellites by ESA's ground stations in the minutes that follow: until that comes, the launch is not over. This piece will be updated with the outcome of the flight.

Main release: "FLEX and Sentinel-3C ready for liftoff on Vega-C", ESA, September 2026. https://www.esa.int/Applications/Observing_the_Earth/FLEX_and_Sentinel-3C_ready_for_liftoff_on_Vega-C

Encapsulation: "FLEX and Sentinel-3C sealed within Vega-C fairing", ESA, September 2026. https://www.esa.int/Applications/Observing_the_Earth/FLEX_and_Sentinel-3C_sealed_within_Vega-C_fairing

FLEX mission: "FLEX" and "Facts and figures" pages, ESA FutureEO. https://www.esa.int/Applications/Observing_the_Earth/FutureEO/FLEX

Sentinel-3 mission: "Sentinel-3" and "Facts and figures" pages, ESA Copernicus. https://www.esa.int/Applications/Observing_the_Earth/Copernicus/Sentinel-3/Facts_and_figures

Vehicle context: the Vega-C entry on Above Ten Thousand (flight record, VV22 failure and return to flight).