Abstract The growth of mega-constellations is rapidly increasing the number of rocket launches required to place new satellites in space. While Low Earth Orbit (LEO) broadband satellites help to connect unconnected communities and achieve the Sustainable Development Goals, there are also a range of negative environmental externalities, from the burning of rocket fuels and resulting environmental emissions. We present sustainability analytics for phase 1 of the three main LEO constellations including Amazon Kuiper (3,236 satellites), OneWeb (648 satellites), and SpaceX Starlink (4,425 satellites). In baseline scenarios over five years, we find a per subscriber carbon dioxide equivalent (CO2eq) of 0.70±0.34 tonnes for Kuiper, 1.41±0.71 tonnes for OneWeb and 0.47±0.15 tonnes CO2eq/subscriber Starlink. However, in the worst-case emissions scenario these values increase to 3.02±1.48 tonnes for Kuiper, 1.7±0.71 tonnes for OneWeb and 1.04±0.33 tonnes CO2eq/subscriber for Starlink, more than 31-91 times higher than equivalent terrestrial mobile broadband. Importantly, phase 2 constellations propose to increase the number of satellites by an order-of-magnitude higher, highlighting the pressing need to mitigate negative environmental impacts. Strategic choices in rocket design and fuel options can help to substantially mitigate negative sustainability impacts.