It’s a 12-meter radar reflector.
Watch it unfold in space on a real video from a camera mounted to the satellite, and another, artistic, video, both quite impressive:
www.esa.int/Applications...
esa.int
Forest satellite’s big antenna opens up
Just a week after its launch, ESA’s Biomass mission has reached another critical milestone on its path to delivering unprecedented insights into the world’s forests and their vital role in Earth’s car...