Every few days now, a giant rocket blasts a fiery trail through the atmosphere, lofting satellites into orbit, carrying out space missions or performing other tasks. Since 2017, both the number of launches and the size of their cargo loads have grown dramatically, with ever bigger rockets carrying ever larger numbers of satellites and other objects per launch. In 2016, a total of 221 objects were launched into space; in 2023, the number was 2,644. There are now about 10,000 satellites in low-Earth orbit, and thousands more that have stopped working. About 6,000 belong to the SpaceX company alone, and its owner, Elon Musk, has vowed to increase those to 40,000. On top of that, there are more than 130 million chunks of “space junk” of varying sizes floating around, the spawn of larger craft that have broken up.

Launches have local impacts, including huge though temporary clouds of pollution from fuel combustion, and brief hails of sheet metal, insulation and other debris from disintegrating rocket stages. And, of course what goes up must come down—everything sent into orbit will eventually fall back to Earth, and as launches increase, so will re-entries of failed or decommissioned spacecraft. Most burn up as they hit the atmosphere, but not always completely. Just last year, sizable metal components landed, albeit harmlessly, in the woods of rural North Carolina, on the roof of a Florida home, in a farm field in Saskatchewan, and near a small village in Kenya.

Kostas Tsigaridis is an atmospheric scientist at the Columbia Climate School’s Center for Climate Systems Research and its affiliated NASA Goddard Institute for Space Studies. He does not worry much about short-lived local pollution in the lower atmosphere, nor about the still-rare falling of intact debris. He is, however, concerned about byproducts from combustion of fuels as rockets travel through the upper atmosphere, and the byproducts from burning of re-entering debris there. Both could alter upper atmospheric chemistry, temperature and circulation, with possible effects on planetary climate.

He and his colleagues are trying to understand the potential for such effects. Preliminary models they have used to make estimates assume that by 2050 there might be 1,000 or even 10,000 yearly launches. We spoke with Tsigaridis about this relatively new field of study.

Why are you investigating the possible effects of rocketry?

Rocket launches and satellite reentry are increasing at an unprecedented rate. Given the lack of any sort of regulation on their number, we expect they can start becoming a noticeable pollutant in the near future. Even more, they comprise a unique anthropogenic source of short-lived chemicals in the upper atmosphere. Given the rapidly expanding activity, we shouldn’t be waiting around for something to happen and then study it. Better to try and figure this out as best we can starting now.

What do most rockets use for fuel, and what are the typical byproducts?

Kerosene, which is the most popular one, and solid fuels, are carbon-rich and produce black carbon as a byproduct, much as automobiles do. Hydrogen, used by Blue Origin, does not contain carbon and has mostly water as a byproduct, but it has less lifting capacity per fuel mass than carbon-based fuels. Liquefied natural gas (LNG), which is mostly methane, is expected to dominate space travel in the future. LNG is still carbon based, but it burns much more efficiently than kerosene, and forms much less black carbon.

Black carbon is important because of its long lifetime in the upper atmosphere. Near the surface, it quickly gets rained out by precipitation, but in the absence of clouds or surfaces higher up, including in the stratosphere, only gravity and atmospheric circulation can eventually remove it. Both of these are very slow processes. Hence it accumulates, multiplying its impact on chemistry and climate. The cleaner LNG fuel does reduce the amount of black carbon per launch, but given the projected number of launches in the future, we are still talking about quite significant amounts injected in the upper atmosphere.

Are there any perceptible effects yet? What could happen in the future?

Let me explain first why we care about the stratosphere, which starts at aircraft cruising altitudes. This is where the ozone layer is, which protects life on this planet from the harmful solar radiation. It is sandwiched between the troposphere, the part of the atmosphere that we live in, and the mesosphere, above it. The troposphere is very moist, hence we have clouds, rain, humidity, etc. The stratosphere is extremely dry, due to its extremely cold temperatures. The coldest part between the troposphere and the stratosphere is called the tropopause, which blocks water from entering the stratosphere. There are many more features that make the stratosphere unique, but water and ozone are the ones of importance here. And, as I already said, the new component introduced by rocketry is black carbon.

As its name says, black carbon is black, so it absorbs solar radiation. That heats up the environment around it. This is a very well understood effect. What we found recently, which is new, is that the heating of the stratosphere from black carbon heats up the tropopause, allowing water to “leak” into the stratosphere. This alters the chemistry and destroys stratospheric ozone. The erosion of ozone appears to not be a large effect on the global scale, but ongoing analysis implies that enhanced polar ozone destruction will happen. Furthermore, normal atmospheric circulation will bring most of the black carbon down near the polar regions, where it can land on snow and ice, and accelerate melting by making those surfaces less reflective. This is an effect that has not yet been studied, but we have plans on delving into it soon.

What about debris re-entering the atmosphere―what does that produce, and what might be the results?

Space debris is a very different story. There is no black carbon involved, and the burning (called ablation, to be more exact) happens higher up, in the mesosphere. The temperatures of ablation are high enough that they break down molecular nitrogen, the most abundant chemical of our atmosphere. This then forms nitrogen oxides, which can affect mesospheric chemistry. Also, the satellites we send into orbit contain large amounts of aluminum in their structures. This oxidizes to alumina, which then forms very small particles that reflect sunlight and affect chemistry. In principle, these are similar but not exactly identical to gases from major volcanic eruptions, which we know can reflect sunlight and cool the planet. These particles, as well as multiple other satellite-originating metals, have been detected in the stratosphere, on their way from the mesosphere down to the surface. Their role is yet to be exactly quantified.

What are your next steps?

While finalizing our work on the role of black carbon on stratospheric water, we are getting ready to study debris reentry, both in terms of nitrogen oxides and alumina. We are also looking into regional effects, rather than global, since our preliminary analysis shows that the polar atmosphere will be disproportionately affected.