The controlled re-entry of spacecraft into Earth's oceans, known as splashdown, has long been the Gold standard for mission safety, but new research is challenging the assumption that these high-energy impacts are harmless to marine ecosystems.
While the immediate physical damage from a spacecraft splashdown is limited to a small spatial footprint, the cumulative effects of decades of orbital reentries are under renewed scrutiny. As NASA's Artemis II mission nears its final recovery phase in the Pacific, marine biologists and environmental lawyers are raising concerns over acoustic shockwaves, residual propellants, and the long-term accumulation of debris on the seafloor.
According to NASA's environmental assessment, water landings are favoured because the ocean effectively absorbs the kinetic energy of a descending capsule, protecting both the crew and the vehicle's structure. However, the propagation of underwater shock waves and acoustic energy through the water column remains a variable that could influence the behaviour of sensitive marine species.
What Happens When Spacecraft Enter The Ocean?
A splashdown occurs when a spacecraft slows through the atmosphere and deploys parachutes before landing in the ocean. The method has been used for missions including Mercury, Gemini, and Apollo, and continues today with modern crew capsules such as Orion and SpaceX's Dragon systems.
According to NASA documentation on spacecraft recovery systems, water landings are favoured partly because the ocean absorbs impact energy, reducing structural damage to the vehicle and crew risk during descent.
However, research into ocean impacts highlights that entry into water generates shock waves and acoustic energy that can propagate through the marine environment. A NASA environmental assessment notes that these effects are generally short-lived, with most biological responses expected to be limited to startle reactions in fish and marine mammals rather than physical harm.
Immediate Effects On Marine Life
Scientific modelling of spacecraft and sonic boom interactions with water suggests that pressure waves diminish rapidly with depth, reducing exposure levels for most marine species.
A long-term study conducted by Hubbs-SeaWorld Research Institute (HSWRI) beginning in 1978 examined the impact of sonic booms from Space Shuttle launches on wildlife across California's Channel Islands.
Researchers monitored seal and sea lion populations, focusing on San Miguel Island, where sonic booms were strongest, and comparing it with San Nicolas Island as a control site. The study provided rare, long-term insight into population trends among six pinniped species.
Findings showed that between 40% and 100% of male seals reacted to sonic booms by lifting their heads in an alert posture, but there was no evidence of movement, increased aggression, or threat displays. Female seals responded similarly, though fewer reacted, particularly after the early breeding season.
Nursing pups showed limited disturbance, with feeding interrupted on only three occasions and lasting no more than seven minutes. Weaned pups exhibited almost no response.
Similarly, a US Federal Aviation Administration environmental assessment concluded that splashdowns involving capsule-sized spacecraft are unlikely to adversely affect marine species due to the limited spatial footprint of the impact zone.
These findings have historically been used to justify the continued use of ocean recovery zones, particularly in remote regions of the Pacific.
The Hidden Concern: Cumulative Impacts
While individual splashdowns appear to have minimal environmental consequences, some researchers argue that cumulative effects remain poorly understood.
Environmental law analyses of ocean disposal of spacecraft components highlight concerns about long-term contamination risks, including the potential for residual propellants and rare materials to enter deep-sea ecosystems.
The South Pacific Ocean Uninhabited Area, commonly known as the spacecraft cemetery or Point Nemo, used for controlled re-entries of defunct spacecraft, has already accumulated more than 263 pieces of space debris on the seafloor since 1971, according to a 2019 study.
Experts warn that, while each event is isolated, the cumulative effects of multiple decades of missions could introduce chemical and physical disturbances to deep-ocean environments that are difficult to monitor or reverse.
Noise, Shockwaves, and Scientific Uncertainty
One of the most studied impacts is acoustic disturbance. Data from earlier spacecraft programmes indicate that underwater shock waves generated during descent are brief and typically below thresholds associated with injury to marine fauna.
However, the same NASA-derived research acknowledges uncertainties about duration and repeated exposure effects, noting that while short impulses have not demonstrated significant harm, longer-term behavioural changes in sensitive species cannot be entirely ruled out.
Marine biologists caution that deep-sea ecosystems remain among the least understood on Earth, meaning baseline data is limited when assessing disturbance.
Balancing Space Operations And Ocean Protection
The growing use of controlled splashdowns reflects broader trends in reusable spacecraft design and mission safety strategy. Modern capsules are engineered to survive ocean landings, and recovery operations are optimised for minimal surface disruption.
Yet legal and environmental frameworks are evolving. Under international marine protection principles, states are required to minimise pollution of the marine environment and to assess potential risks where reasonable grounds for concern exist, including in remote ocean regions.
As more spacecraft return from orbit in the coming decades, environmental monitoring is likely to become more central to mission planning.
Low Risk, But Not Zero Uncertainty
Current evidence suggests that splashdowns are unlikely to cause significant harm to marine life in the immediate aftermath of a landing. Most documented effects appear to be temporary and behavioural rather than physical.
However, scientists and legal experts increasingly point to gaps in long-term ecological data, particularly around cumulative impacts and deep-sea contamination risks.
In short, while space agencies consider splashdowns a safe and controlled method of spacecraft recovery, the full environmental cost, especially beneath the ocean surface, remains only partially understood.
