With rockets blasting off almost weekly, space tourism gaining momentum and plans to return to the moon within the decade, scientists are becoming concerned that hot exhaust gasses from rockets could have a negative impact on the atmosphere — from the ground all the way to the edge of space.
For more than half a century, rockets have been punching through the atmosphere on the way to space. Until now the effect of their exhaust plumes on the atmosphere has not been a major concern. Because rockets travel so fast, they only spend a matter of minutes in the atmosphere, so their impact has been considered minimal.
Now, with private companies such as SpaceX, Blue Origin, Virgin Galactic, Rocket Lab — along with space agencies in China, Japan, Europe and Russia — all sending more rockets into space than ever before, scientists are saying it’s time to look at the long-term impact on the atmosphere of all those launches.
According to estimates, 2021 was a record year for launches with 160 attempted and 146 successes; these are only estimates because some military launches are unannounced because they are classified.
A report in the journal, Physics of Fluids, shows how scientists at the University of Nicosia in Cyprus modelled the effects of chemicals and heat released by rocket exhausts up to an altitude of 67 kilometres and found there are significant effects on the Earth’s climate.
The rocket used for the model was the SpaceX Falcon 9, which has been making regular trips to space delivering satellites to orbit, as well as bringing cargo and crew to the International Space Station. It is not the largest rocket, nor is it the smallest, making it a good average example of rocket flight.
Rockets do not spend much time in the atmosphere, taking only about eight minutes to go from a standing start all the way to orbit. In fact, one reason they rise straight up off the launch pad is to get through the air as quickly as possible to reduce drag. But during that short time they burn tonnes of fuel.
The nine engines on the Falcon 9 burn RP-1 fuel, which is a refined version of kerosene similar to jet fuel. Since this is a fossil fuel, it produces carbon dioxide, a greenhouse gas, in its exhaust.
The researchers found the amount of carbon dioxide emitted as the rocket climbs one kilometre into the mesosphere is equivalent to that contained in 26 cubic kilometres of ambient atmosphere air at that same altitude.
Another aspect of rocket flight is the effect on the surrounding air of heat and pressure from the exhaust plume. This can heat the atmosphere directly, possibly affecting ozone, and produces nitrogen oxides, pollutants that are harmful to human health.
Currently, these nitrogen oxide gasses created under high temperatures at lower altitudes are carried away by upper winds so the impacts are temporary at higher altitudes. But scientists are concerned that the expected increase in rocket travel in the future could have a cumulative effect.
These results suggest more study is needed on atmospheric response at all altitudes, plus consideration should be made to future designs of rocket engines and alternate fuels to minimize their impact.
One change that could be made is to use liquid hydrogen instead of kerosene to eliminate carbon emissions, although that comes with its own set of challenges, since the fuel is super cold and difficult to handle. But rockets in the past, such as the main engines of the space shuttles and the upper stages of the Saturn V moon rockets, used that fuel very successfully.
Rockets have been carrying people and goods off the planet for decades; now it is time to consider what they leave behind.