Can space exploration be environmentally friendly?
Lift-off is usually the most environmentally harmful stage of any space mission, with vast quantities of fuel burnt up in a matter of minutes. For instance, SpaceXs Falcon 9 gets through 112 tonnes of refined kerosene, emitting about 336 tonnes of CO2 (the equivalent produced by your average car driving almost 70 times around the world). Advertisement  As well as greenhouse gases, rocket engines emit chlorine and particles of soot and aluminium oxide that destroy ozone. These issues are growing more pressing with the advent of commercial spaceflight. There were 114 space launches in 2020, but there may be up to 1,000 per year in future. Sustainable fuels are the top priority to enable greener space travel. Current spacecraft use a variety of fuels, but most are based on fossil fuels. One potentially greener option is liquid hydrogen and oxygen, used by the New Shepard Propulsion Module from private spaceflight company Blue Origin. Hydrogen can be obtained sustainably by using solar power to break water down into oxygen and hydrogen molecules. In 2019, NASAs Green Propellant Infusion Mission (GPIM) road-tested AF-M315E, a green alternative to hydrazine (a toxic component of many types of rocket fuel) and hopes to use this to power future missions. Reusable rockets can cut down on some of the waste associated with spaceflight. Traditionally, boosters, fuel tanks and other components are treated as expendable. But guiding them back to Earth in a controlled manner opens new possibilities  most components from the Falcon 9 can be reused up to 100 times. Truly environmentally friendly space travel is still some way off. But we already have many of the technologies needed to start limiting its impact on our planet. Read more:  Advertisement  Every week on BBC World Service, CrowdScience answers listeners questions on life, Earth and the Universe. Tune in every Friday evening on BBC World Service, or catch up online at bbcworldservice.com/crowdscience