When the 165-foot-tall (50 meters) SpaceX Starship upper stage crashed into the Indian Ocean last week during its third test flight, environmentally conscious observers wondered whether the stainless-steel vehicle, perhaps containing hundreds of kilograms of residual fuel, could endanger marine life. The good news is — not really. Starship uses one of the most environmentally friendly fuel combinations available. Still, sustainability experts warn that the rocket is not without its problems.
"Debris and fuel [from the latest Starship launch] are a drop in the ocean," Tommaso Sgobba, executive director of the International Association for the Advancement of Space Safety, told Space.com.
Starship's Raptor engines burn liquid oxygen and liquid methane, neither of which, fortunately, is toxic to the environment.
Related: Relive SpaceX Starship's 3rd flight test in breathtaking photos
Still, dumping garbage into the ocean is not the most respectable behavior, although the world's space agencies and launch operators have been doing that for decades.
"The stuff actually dumped is similar to other industrial materials contributed by shipping and fishing," space sustainability researcher Vitali Braun told Space.com. "Theoretically, the United Nations Convention on the Law of the Sea binds all states 'to protect and preserve the marine environment in all zones of the sea.' So, effectively, states dumping their trash into the ocean is a violation of this treaty."
What concerns Braun most are Elon Musk's plans to ramp up the frequency of Starship launches to perhaps hundreds per year. Each of those flights could loft more than 100 tons' worth of satellites into space. These satellites will not only be going up, but at some point, they will be also falling back down.
"Those numbers are insane," Braun said. "We have already seen an exponential increase in reentering satellites and rocket stages in the past years. With that perspective, I am quite concerned about the consequences."
Satellites and old rockets burn up when they spiral back to Earth, leaving behind metallic ash, the composition of which raises concerns. Some researchers think that these remnants of satellite incineration could damage Earth's protective ozone layer or even affect the planet's magnetic field. Most of the reentering material vaporizes some 75 to 50 miles (120 to 80 kilometers) above Earth's surface. At these altitudes, the ash particles may remain essentially forever, which means their concentrations will only continue to rise.
Green or not green?
But there are other concerns about frequent Starship launches. Methane, although not toxic, is a gas commonly found in nature and is used to power everything from city heating through electricity generation to transport. When burned, it converts into carbon dioxide and water vapor. Although the overall greenhouse gas emissions of methane combustion are lower than those of burning oil or coal, the gas still contributes to the warming greenhouse effect that humankind is currently working hard to thwart.
According to Andrew Wilson, assistant professor in environmental management at Glasgow Caledonian University in Scotland, one Starship launch produces 76,000 metric tons of carbon dioxide equivalent (a measure combining different types of greenhouse gases in one unit). That's 2.72 times more emissions than those produced by a single SpaceX Falcon 9 launch but only 0.96 of the emissions produced by a Falcon Heavy liftoff. Both the Falcon 9 and the Falcon Heavy combust the much dirtier oil-based rocket fuel RP-1, so their carbon footprint per ton launched is much higher. The Falcon 9, for example, has less than one-sixth the payload capacity of Starship.
Carbon emissions are not Starship's only contribution to global warming. Water vapor too is a potent greenhouse gas. The higher layers of Earth's atmosphere naturally contain very little of it, and scientists don't know what water vapor produced by rocket exhaust at high altitudes can do to the planet. On top of that, Starship emissions contain soot, which spreads throughout the upper atmosphere and absorbs incoming heat. This effect, known as radiative forcing, can contribute to additional warming. And let's not forget that methane itself is a greenhouse gas up to 90 times more potent in trapping heat in the atmosphere than carbon dioxide. Methane leaks from processing and storage facilities and gas pipelines are known to be a major contribution to global greenhouse gas emissions.
A full accounting of the atmospheric impact would also include the greenhouse gases generated by the manufacture of Starship vehicles, which occurs at SpaceX's Starbase facility in South Texas. But that's an even tougher number to pin down.
Related: Climate change: Causes and effects
Carbon footprint
Wilson cautions that the sheer size of Starship and the frequency at which SpaceX plans to launch it mean that the giant rocket's environmental footprint in the long term is unlikely to remain just a drop in the ocean.
"Historically, the space sector has been granted a lot of exemptions from different legislations, and, as a result, they have essentially been able to get away with doing what they want," Wilson said. "And Starship, because it's the biggest rocket ever built, is also one of the dirtiest."
Right now, the overall amount of greenhouse gas emissions from spaceflight is negligible, equivalent to 1% or 2% of the carbon footprint of aviation, which by itself makes up about 2.5% of overall global greenhouse gas emissions. But the number of rocket launches has been rising steeply in recent years. 2023 saw a record-breaking 223 attempted spaceflights worldwide, according to astronomer and space age historian Jonathan McDowell. That's more than double the 85 attempts made in 2016. SpaceX alone launched a record 96 orbital rockets last year and aims for nearly 150 in 2024. SpaceX's bold ambitions are what worry scientists like Wilson.
"If it scales up and SpaceX launches Starship more and more and more, as they say they will, then there will be an accumulation of those effects on our environment," said Wilson.
Apart from enabling humankind to colonize Mars, as Elon Musk envisions, Starship has also been proposed as a next-generation means of intercontinental travel that could shorten the duration of the longest journeys here on Earth to a mere hour.
"The amount of pollution that would cause in comparison to aircraft is orders of magnitude of a difference," said Wilson.
While engineers in most industries are scratching their heads, pondering how to decarbonize by the middle of this century as required by international climate protection pledges, Musk and his company are developing a carbon-intensive business with uncertain impacts on the environment, Wilson suggests.
The Paris Agreement, signed at the 2015 United Nations Climate Change Conference in the French capital, binds nations to limit global warming to 2.7 degrees Fahrenheit (1.5 degrees Celsius) compared to preindustrial times. So far, that goal appears to be slipping through our fingers. In 2023, in fact, global temperatures breached the 2.7 degrees F threshold on 50% of days, according to the European Copernicus Climate Change Service.
"We are in a climate crisis — we already saw warming that is close to the targets of the Paris Agreement — and here we go, creating a massive launcher which is going to just add more carbon dioxide into the atmosphere," Wilson concluded.
