Another ISS Air Leak Strains Aging Hardware
After more than 27 years in orbit, the International Space Station (ISS) is once again exhibiting a troubling air leak. NASA has postponed the upcoming Axiom-4 commercial crew mission at least until June 18, 2025, while engineers on Earth and in orbit work to pinpoint and mitigate the leak source. With multiple docking ports, modules, and over 400 metric tons of hardware, maintaining pressure equilibrium aboard the ISS has become ever more challenging as key structural elements approach three decades of service.
Background: Chronic Air Loss in Zvezda’s PrK Transfer Tunnel
Since 2019, cosmonauts have documented a gradual, yet persistent, pressure drop in the Russian Zvezda service module. This section—one of the station’s first launched in 1998—contains the PrK transfer tunnel, the conduit linking Zvezda to an aft docking port used by Soyuz and Progress vehicles. Early in-flight data logs showed a leak rate of ~0.8–1.0 kPa per month (≈2–3 pounds of air per day).
“We’ve patched small fissures with epoxy-based sealants several times,” said a Russian Flight Engineer, “but each repair only slowed the leak.”
Operational workarounds, such as isolating the PrK hatch when not docking, have kept the leak from escalating. However, the leak’s persistence has raised concerns about fatigue damage in the station’s aluminum alloy structures.
Latest Incident and Mission Delays
On June 12, ground teams observed a renewed decline in cabin pressure despite recent repairs. Roscosmos and NASA declared the PrK tunnel “completely sealed” after last week’s EVA, yet overall station pressure continued to fall at ~0.05 kPa/day. That prompted NASA to delay Axiom-4—carrying four private astronauts—to June 18 or later, pending confirmation that the station’s Environmental Control and Life Support System (ECLSS) can compensate for the anomaly.
In a brief statement, NASA ISS Program Manager Joel Montalbano noted: “Safety remains our top priority. We’re analyzing telemetry from pressure sensors across all modules and evaluating additional troubleshooting steps.”
Material Fatigue and Structural Integrity Analysis
High-cycle fatigue occurs when repeated pressurization cycles induce microcracks in metal. The ISS endures about 14–16 pressure cycles per year—docking, undocking, and routine ECLSS regulation—amassing over 400 cycles since launch. ISS primary structure is primarily aluminum alloys 2219 and 6061, which, under sustained cyclic loads, can develop subcritical crack growth that eventually leads to sudden failure.
- Alloy 2219: Used in pressure vessels; tensile strength ~350 MPa, fatigue limit ~95 MPa.
- Alloy 6061-T6: Common in secondary structures; tensile strength ~290 MPa, fatigue limit ~96 MPa.
Finite element modeling (FEM) of the PrK tunnel—incorporating microfracture mechanics—suggests stress concentrations around joint welds and penetrations for cabling. NASA-classified this risk at the highest severity on its 5×5 risk matrix, meaning a sudden module breach could have catastrophic consequences.
Leak Detection and Monitoring Systems
ISS houses a network of differential pressure transducers and acoustic leak detectors. Sensors in Zvezda’s ISPR racks communicate real-time data via the Russian segment’s onboard computer. Recently, engineers have uplinked software patches to increase sampling rates from 1 Hz to 10 Hz, enhancing the ability to localize micro-leaks.
Additionally, cosmonauts are preparing to deploy an ultrasonic inspection device—similar to those used in the EVA 78 (June 2024)—to scan welds for crack initiation. Combined with infrared thermography from external cameras, the team hopes to triangulate the leak source without further EVAs.
Future Mitigations and Upgrades
- EVA-based Reinforcement: Installation of composite overwrap repairs (carbon-fiber patches) over high-stress areas.
- Module Replacement: Accelerating the arrival of the NASA-funded ISS-Ready Module (IRM) in 2026, featuring advanced aluminum-lithium alloys with superior fatigue resistance.
- Enhanced Health Monitoring: Integrating fiber-optic strain gauges into station hull panels to detect sub-micron deformations in real time.
Expert Opinions
- Dr. Elena Ivanova, Materials Engineer at the Moscow Aviation Institute: “Monitoring crack propagation with acoustic emission is critical. If left unchecked, microcracks coalesce rapidly under cyclic loading.”
- Dr. Mark Langley, Chief Scientist at the Fatigue Institute (UK): “The ISS is a unique lab for long-duration fatigue studies; lessons learned here inform future deep-space habitats.”
For now, Mission Control continues to validate leak repairs and consults with Roscosmos on further actions. Meanwhile, Axiom-4’s launch date remains tentative, reflecting the high stakes of maintaining one of humanity’s most complex engineering feats in orbit.