Vega-C lifts FLEX and Sentinel-3C from Kourou, Europe’s first dual flight on the rocket

A Vega-C rocket flew two European Earth-observation satellites into orbit from French Guiana on 15 September, the first time the launcher has carried a dual stack. Flight VV30 lifted off at 03:21 CEST (22:21 local time on 14 September). A Vespa adapter held Copernicus Sentinel-3C on top and ESA’s Fluorescence Explorer, FLEX, below. Sentinel-3C separated first. FLEX left the adapter about an hour later. ESA’s operations centre in Darmstadt received first signals from both spacecraft and confirmed they were alive.

Simonetta Cheli, ESA’s director of Earth observation programmes, called the flight the end of years of design work inside the FutureEO programme. FLEX is the research mission in the pair. It carries a Fluorescence Imaging Spectrometer meant to pick up the faint red glow plants emit when they photosynthesise. That signal is weak. The instrument will try to read it from about 814 kilometres up and turn it into global maps at 300 metres by 300 metres. The maps are supposed to show plant stress before leaves change colour, and to feed studies of carbon and water moving through vegetation.

FLEX is built to fly in tandem with a Sentinel-3 satellite, though not at first with Sentinel-3C itself. The older Sentinel-3 craft already measure sea-surface temperature, ocean colour, sea ice and land. FLEX needs that context so scientists can place a fluorescence reading against the temperature and water story of the same patch of ground.

Sentinel-3C is the third satellite in the Copernicus Sentinel-3 family, operated with EUMETSAT for the European Commission. It keeps the operational series whole as earlier satellites age. Its data run straight into Copernicus services that track oceans, land, ice and atmosphere. The flight therefore mixed a research instrument with a workhorse.

Vega-C stands 35 metres tall and weighs about 210 tonnes on the pad. It can put roughly 2,300 kilograms into low Earth orbit. Three solid stages lift the stack. A liquid upper stage finishes the job. Avio is prime contractor. This flight used Vespa so two passengers could share one fairing, a configuration the rocket had not flown before.

The science case for FLEX is specific. Chlorophyll fluorescence is a known laboratory signal. Seeing it from orbit, over farms, forests and grasslands, at a few hundred metres of resolution, would give agronomists and climate modellers a number they now infer from greener proxies. If the spectrometer cannot pull the signal out of scattered light and instrument noise, the maps will be pretty and unused. The first months of commissioning in Darmstadt will decide which way that goes.

Sentinel-3C faces a simpler test. The series already works. The new satellite has to match its siblings and take over when one of them fades. Ocean temperature and sea-ice extent are not experimental products. Weather services and shipping desks already consume them.

Europe now has both satellites on the same plane of the sky after a single firing from Kourou. That is the operational fact of 15 September. The scientific fact will arrive later, when FLEX either publishes a fluorescence map that farmers can recognise or it does not.