The growth of megaconstellations is rapidly increasing the number of rocket launches. While Low Earth Orbit (LEO) broadband satellites help to connect unconnected communities and achieve the Sustainable Development Goals (SDGs), there are also significant environmental emissions impacts from burning rocket fuels. We present sustainability analytics for phase 1 of the three main LEO constellations including Amazon Kuiper (3,236 satellites), Eutelsat Group`s OneWeb (648 satellites), and SpaceX Starlink (4,425 satellites). We find that LEO megaconstellations provide substantially improved broadband speeds for rural and remote communities, but are roughly 6-8 times more emissions intensive (250 kg CO2eq/subscriber/year) than comparative terrestrial mobile broadband. In the worst-case emissions scenario, this rises to 12-14 times more (469 kg CO2eq/subscriber/year). Policy makers must carefully consider the trade-off between connecting unconnected communities to further the SDGs and mitigating the growing space sector environmental footprint, particularly regarding phase 2 plans to launch an order-of-magnitude more satellites.