How Does the International Space Station Survive a Constant Barrage of Space Debris?

Under Constant Assault

The International Space Station (ISS) travels at over 17,000 miles per hour, circling the Earth in a constant, controlled fall. At this velocity, its orbit in the “emptiness” of space is anything but empty. It moves through a hostile, invisible shooting gallery. This environment, known as low Earth orbit (LEO), is a region of space saturated with millions of tiny, high-velocity projectiles.

The station, a sprawling, football-field-sized outpost, is the largest target ever placed in this environment. It has no choice but to endure a relentless barrage from these particles. This raises a persistent and practical question: Is the exterior of the ISS regularly inspected for damage from this constant assault?

The answer is a definitive yes. The inspection of the ISS is not a casual or occasional task; it’s a fundamental and continuous process necessary for the survival of the station and its crew. But this “inspection” isn’t a simple, scheduled check-up. It is a complex, multi-layered strategy that combines ground-controlled robotics, high-resolution astronaut photography, risky spacewalks, and meticulous laboratory analysis of hardware returned from orbit.

This constant surveillance is not just for maintenance. The station is not only a home and a laboratory inspace; its very structure serves as a massive, passive experiment for space. Every chip, pit, and puncture etched onto its metallic skin is a precious data point. These scars are collected, cataloged, and analyzed to provide “ground truth” for a threat that is largely invisible from Earth. This data, gathered from the station’s long-running gauntlet, is the single most important resource scientists have for understanding the dangers of low Earth orbit and for designing the spacecraft that will one day travel beyond it.

A Hostile Sky: The Micrometeoroid and Orbital Debris Environment

Before understanding the inspections, one must first understand the threat. The danger to the ISS is collectively known as MMOD, which stands for Micrometeoroids and Orbital Debris. While often grouped together, these two terms describe two very different sources of high-velocity projectiles, one natural and one man-made.

The Two-Front Threat: Nature vs. Pollution

Micrometeoroids (MM) are the natural component of the threat. These are tiny, dust-sized particles that have been shed by comets or asteroids, and they have been zipping through the solar system for billions of years. They are tiny grains of cosmic dust, often composed of magnesium-silicates or other minerals, sometimes with icy components. They are part of the background environment of space. The defining characteristic of micrometeoroids is their staggering velocity. Because they are in orbit around the sun, not the Earth, the ISS can slam into them at extreme speeds. Meteoroid impact speeds can range from 11 to 72 kilometers per second (km/s), with an average in Earth’s orbit around 22-23 km/s.

Orbital Debris (OD) is the man-made component, commonly known as “space junk.” This is the pollution of our orbital environment, the accumulated remnants of over 60 years of human space activity. It consists of defunct, abandoned satellites, spent rocket stages, fragments from on-orbit explosions or collisions, solidified liquids expelled from spacecraft, stray bolts, and even flecks of paint that have flaked off rocket bodies.

In the low Earth orbit where the ISS resides, orbital debris is the predominate threat. This man-made debris travels at a “slower” velocity than meteoroids, with an average impact speed for the ISS’s orbit around 9-10 km/s. While this is less than the meteoroid average, it is still a “hypervelocity” speed, equivalent to over 22,000 miles per hour. The debris population is also not a stable, background flux like meteoroids. It is dynamic, growing, and changing, driven by new launches and, more worrisomely, by existing debris colliding with other debris.

The Physics of Danger: Energy, Not Mass

For a non-technical audience, the danger from a “paint fleck” is difficult to grasp. But in orbit, mass is secondary to kinetic energy. At hypervelocity speeds, the laws of physics become extreme and counter-intuitive.

The relevant parameters describing the MMOD environment aren’t just the number of particles, but their velocity, mass, and density. The debris population, for example, is a mix of high-density materials (like steel), medium-density (like aluminum), and low-density (like plastic).

At 10 km/s, the energy released on impact is enormous. NASA’s analysis provides two stark comparisons to make this threat tangible:

1. A 1-centimeter fleck of paint, traveling at these orbital speeds, can inflict the same damage as a 550-pound object traveling at 60 miles per hour on Earth.
2. A 10-centimeter (4-inch) projectile, about the size of a softball, would have the destructive energy equivalent of 7 kilograms of TNT.

This is why the MMOD threat is taken so seriously. A particle that is too small to even see from Earth has more than enough energy to puncture a pressurized module, which could be catastrophic for the station and its crew.

Cataloging the Swarm: A Problem of Numbers

The scale of the debris problem is staggering, and it is best understood by breaking it down into size.

• Large Objects (Trackable): Objects larger than 10 centimeters (about 4 inches) are considered “large.” There are more than 21,000 (and by some estimates, over 34,000) of these objects in orbit. These are the items – dead satellites, large rocket bodies, and major collision fragments – that are actively and routinely tracked by the U.S. Space Surveillance Network.
• Medium Objects (Untrackable, Lethal): This is the most dangerous population. The estimated number of particles between 1 centimeter and 10 centimeters in diameter is approximately 500,000 to 900,000. These objects are too small to be reliably tracked by ground-based radar, but as the “TNT” analogy shows, they are more than large enough to cause a catastrophic failure if they hit a critical part of the ISS.
• Small Objects (The Unseeable Swarm): The number of particles smaller than 1 centimeter (0.4 inches) is estimated to exceed 100 million. By some estimates, it is over 128 million. This “swarm” includes the paint flecks, solid rocket motor slag, and tiny fragments that constantly “sandblast” the station.

This catalog of the swarm reveals the central challenge of protecting the ISS, a problem that can be called the “danger gap.” The station’s defenses are split into two categories. For the 34,000+ large objects, the station has an active defense: it dodges them. For the 100+ million tiny objects, the station has a passive defense: armor.

The problem – the “danger gap” – lies with the 500,000+ medium objects. They are too small to dodge and too large for the station’s armor to stop. A direct hit from one of these is the nightmare scenario that keeps mission controllers on edge.

The Kessler Syndrome: A Runaway Problem

The MMOD environment is not static. The man-made orbital debris problem is actively getting worse, and it has a feedback loop.

This concept is known as the “Kessler Syndrome,” proposed in the 1970s by NASA scientist Donald Kessler. He demonstrated that once the amount of debris in a particular orbit reaches a “critical mass,” a runaway chain reaction of collisions can begin, even if no more objects are ever launched.

This “collisional cascading” works like this:

1. A piece of debris hits another piece of debris (or a defunct satellite).
2. That one collision shatters both objects, creating thousands of new pieces of debris.
3. This cloud of new shrapnel dramatically increases the probability of more collisions.
4. Those collisions create even more debris, and the process accelerates.

This cascade was first brought to NASA’s attention in the 1970s when derelict Delta rockets left in orbit began to explode, creating clouds of shrapnel. Today, some experts believe we are already at or near this critical mass in the most crowded orbits, around 900 to 1,000 kilometers in altitude.

The altitude of the debris is also a major factor. In the ISS’s orbit, below 600 kilometers, the faint-but-present atmospheric drag is still strong enough to pull small debris items down to burn up in the atmosphere within several years. But at 800 kilometers, debris can remain in orbit for decades. Above 1,000 kilometers, it can persist for a century or more. The ISS is flying in a relatively “cleaner” part of LEO, but it is still subject to the consequences of breakups in the more persistent, higher-altitude orbits.

The First Line of Defense: Tracking and Dodging

The first line of defense for the ISS is active avoidance. This entire, sophisticated system is designed only to protect the station from the ~34,000 large, trackable objects – the ones bigger than 10 centimeters. It is a defense that is completely blind to the 100+ million smaller particles that must be handled by other means.

Watching the Heavens: The U.S. Space Surveillance Network

The ISS doesn’t watch for debris by itself. Its safety depends on a constant stream of data from Earth. The primary organization responsible for tracking cataloged objects is the U.S. Space Surveillance Network (SSN), which is operated by the U.S. Space Force’s 18th Space Defense Squadron.

This global network of ground-based radars and optical telescopes feeds its data into a massive catalog. NASA’s own Orbital Debris Program Office (ODPO), located at the Johnson Space Center, takes the lead in modeling the debris environment and assessing the risk to NASA assets. The ODPO develops the engineering models, but it’s the SSN that provides the real-time “eyes on the sky” for tracking specific objects.

The “Pizza Box” and Conjunction Assessment

The process of determining if the ISS and a tracked object will have an uncomfortably close encounter is called “Conjunction Assessment.” This analysis is performed by NASA’s Conjunction Assessment Risk Analysis (CARA) team.

This assessment is run three times a day, every day. The team uses a volumetric region, or a virtual safety perimeter, that is established around the ISS. This volume is often referred to as the “pizza box” – a flat, wide box that is larger in the horizontal plane (to account for tracking uncertainties) and thinner vertically.

If the SSN’s tracking data predicts that a cataloged object will penetrate this “pizza box,” it is flagged as a conjunction threat.

But a simple penetration isn’t enough to trigger an alarm. The early “shoebox” method used for the Space Shuttle was found to be statistically inefficient for the ISS. It produced too many false positives and would have required an unsustainable number of maneuvers for a long-duration station with limited maneuverability.

Instead, the ISS team uses a strict, probability-based method. Flight rules dictate a specific risk threshold. A Debris Avoidance Maneuver (DAM) is typically considered if the probability of a collision is calculated to be higher than one-in-10,000. This threshold is chosen to efficiently balance risk, maximize protection, and minimize the number of maneuvers, which are costly in terms of fuel and operational disruption.

The Debris Avoidance Maneuver (DAM)

If the 1-in-10,000 risk threshold is met, NASA and its international partners will coordinate to execute a Debris Avoidance Maneuver, or DAM.

It’s important to understand that the ISS itself is not an agile spacecraft. It’s a massive, sprawling structure of trusses and modules, and it does not “fly” in a conventional sense. It doesn’t have a primary propulsion system for large maneuvers.

Instead, the DAMs are executed by other vehicles. The maneuver is typically performed using the thrusters on the Russian Zvezda module, which is a permanent part of the station. More commonly, the maneuver is performed by an uncrewed Russian Progress cargo spacecraft that is docked to the station. These cargo ships are used for routine “reboosts” about once a month to raise the station’s orbit, which constantly decays due to atmospheric drag. For a DAM, these same thrusters can be fired in a pre-planned burn to nudge the entire station, raising or lowering its orbit slightly to get out of the way.

These maneuvers are not handled by the astronauts. They are normally executed by computer, under the control of flight directors on the ground in Houston (NASA) and Moscow (Roscosmos).

This process is not theoretical; it’s a regular part of station operations. As of early 2024, the ISS had performed over 30 DAMs in its history. A recent example occurred in April 2025. A fragment from a Chinese Long March rocket launched in 2005 was projected to come within 0.4 miles of the station – a dangerously close pass. After coordinating the maneuver, Roscosmos flight controllers commanded the docked Progress 91 spacecraft to fire its thrusters for 3 minutes and 33 seconds, raising the station’s orbit and moving it clear of the debris path.

This entire system – the global tracking network, the probabilistic risk analysis, the ground-controlled maneuvers – is a “leaky” shield. It’s a sophisticated and necessary defense, but it works only for the ~0.02% of debris objects that are large enough to be tracked. For the 99.98% of the threat – the 100+ million objects in the “unseeable swarm” – the station has no choice but to absorb the hit. This is where the passive defense comes in: armor.

Armor in Orbit: The ISS Shielding System

The ISS is the most heavily-armored spacecraft ever flown. This armor is the station’s only defense against the millions of MMOD particles that are too small to track. It is this armor that is the subject of the inspections, as it is designed to be damaged.

A “Good Enough” Defense: Accepting Risk

The first thing to understand about the ISS’s armor is that it isn’t perfect, nor is it meant to be. NASA and its partner agencies have formally “accepted some risk from MMOD.”

An Office of Inspector General report on the subject noted that NASA does not intend to add further protective exteriors to the ISS, even as the debris environment worsens. The reason is a simple, hard-constraint trade-off: mass, cost, and technical challenges.

It takes an enormous amount of energy and money to launch every pound into orbit. A shield capable of stopping all MMOD threats would be impractically thick and heavy. The “brute force” approach of armoring the station with a single, thick “monolithic” slab of aluminum was never an option.

The shielding that is on the station is already a significant part of its mass. It’s estimated that 25 metric tons (over 55,000 pounds) of MMOD shielding are installed on the ISS. This represents approximately 6% of the station’s total mass. This armor is a compromise, a calculated balance of risk versus the engineering reality of building a habitat in orbit.

From “Brute Force” to “Smart” Armor: The Whipple Shield

The solution to stopping hypervelocity particles with minimal mass is not a new one. It was first proposed in the 1940s by astronomer Fred Whipple, long before the first satellite was launched.

This “Whipple Shield” is a counter-intuitive and “smart” design. Instead of one thick, heavy plate, the Whipple shield consists of a thin, sacrificial “bumper” (usually a sheet of aluminum) that is spaced at a distance from the main spacecraft wall (the “rear wall”).

The magic of the Whipple shield is in how it handles the impact’s energy:

1. The MMOD particle, traveling at 10 km/s, first strikes the thin outer bumper.
2. The bumper’s job is not to stop the particle. Its job is to shatter it. The hypervelocity impact completely vaporizes the particle and a small part of the bumper, creating an expanding “debris cloud” of smaller, pulverized, and often molten fragments.
3. This debris cloud crosses the empty standoff gap between the bumper and the rear wall.
4. As it travels, the cloud spreads out, “diluting” the impact’s force.
5. By the time this cloud of fragments reaches the rear wall, the force is distributed over a much larger area.

The difference is analogous to being hit by a single rifle bullet versus being hit by a shotgun blast filled with sand. The sand might sting over a wide area, but the bullet will punch right through. The Whipple shield turns the “bullet” into “sand.”

The “Stuffed Whipple” Shield: The ISS Standard

The ISS takes this concept a step further, using an advanced design known as the “Stuffed Whipple” shield. This design inserts additional layers of high-tech “stuffing” into the standoff gap, between the outer bumper and the rear wall.

This “stuffing” isn’t insulation; it’s a dedicated ballistic blanket. Its job is to “further shock and pulverize” the debris cloud that was created by the first bumper impact, slowing it down and breaking it up even more before it hits the final pressure wall.

The materials used are high-performance ballistic fabrics:

• Nextel: This is a fabric woven from alumina-boria-silica ceramic fibers. It’s exceptionally strong and heat-resistant.
• Kevlar: This is a well-known aramid fabric, the same material used in bulletproof vests on Earth.

These “stuffed” shields are the most capable MMOD protection ever flown. They are also an example of international collaboration, as the exact “recipe” for the shields differs slightly between the modules. The shields on the U.S., Japanese (JAXA), and European (ESA) modules all use the “Stuffed Whipple” concept, but with different configurations:

• NASA Configuration: 2-millimeter aluminum bumper, followed by 6 layers of Nextel, 6 layers of Kevlar, and finally the 4.8-millimeter aluminum rear wall.
• JAXA Configuration: A 1.3-millimeter aluminum bumper, 3 layers of Nextel, 4 layers of Kevlar, and the 4.8-millimeter rear wall.
• ESA Configuration: A 2.5-millimeter aluminum bumper, 4 layers of Nextel, a Kevlar-Epoxy layer, and the 4.8-millimeter rear wall.

The effectiveness of this design is proven. This “Stuffed Whipple” configuration is capable of defeating a 1.3-centimeter (about 0.5-inch) diameter aluminum sphere traveling at 7 km/s. In ground tests, a projectile of this size will punch cleanly through a solid aluminum block nearly 2 inches thick. But the far lighter “Stuffed Whipple” shield, which has the same mass as a much thinner 3/8-inch aluminum plate, can stop it.

This is the armor that is getting hit, day after day. And because it’s designed to be the “sacrificial” layer, it’s the armor that must be inspected.

Finding the Damage: A Multi-Pronged Inspection Strategy

This brings us to the core of the query: how is this armor inspected? The inspection of the ISS exterior is not a single, scheduled event. There is no “annual MMOD inspection” on the calendar.

Instead, the process is a constant, multi-faceted, and opportunistic strategy. It’s a combination of four distinct methods, each with its own advantages and limitations, that together build a comprehensive picture of the station’s health. The process is primarily opportunistic and responsive rather than proactive and scheduled. Damage is found, documented, and analyzed while crews (on the ground or in orbit) are performing other routine tasks.

Method 1: The Robotic Eye (Remote & Ground-Controlled)

The most common and safest method for external surveys is the use of the station’s sophisticated robotic systems. The workhorse of the ISS exterior is the Mobile Servicing System (MSS), the Canadian Space Agency’s primary contribution to the station.

This system has two key components:

• Canadarm2: This is the 57.7-foot-long robotic arm that is the public face of the MSS. It can “walk” end-over-end along the station’s main truss by latching onto grapple fixtures, giving it enormous reach.
• Dextre (Special Purpose Dexterous Manipulator): This is the station’s “hand.” Dextre is a 11.4-foot-tall, two-armed robot that attaches to the end of Canadarm2. It is equipped with high-resolution lights, video equipment, and four tool holders, allowing it to perform delicate, human-scale maintenance tasks.

A fleet of other external television cameras – mounted on the station’s truss, the Japanese Experiment Module (JEM), and the robotics themselves – provides a near-complete view of the station’s exterior.

The most significant aspect of this robotic system is that it’s almost exclusively operated by flight controllers on the ground. Teams at NASA’s Johnson Space Center in Houston and the Canadian Space Agency’s headquarters in Saint-Hubert, Quebec, command the arms. This is a massive force-multiplier. It allows for detailed external surveys to be conducted “routinely” without using a single minute of an astronaut’s valuable time. This ground-controlled capability allows astronauts inside to focus on scientific research.

This method is so effective that it’s how the most famous recent MMOD strike was discovered. In May 2021, ground operators conducting a routine inspection with Canadarm2 spotted a hole in the arm itself.

Method 2: The Human Element (Spacewalks)

The second method is direct, human inspection. Astronauts conduct visual inspections of the exterior during Extravehicular Activities (EVAs), or spacewalks.

While robotics are powerful, an astronaut’s eyes and brain remain the best tools for pattern recognition and close-up analysis. During an EVA, an astronaut can get their helmet camera or their own eyes inches from a surface, allowing them to spot and document much smaller features than a robotic camera positioned many feet away.

The limitation of this method is the immense risk and complexity of EVAs. Spacewalks are among the most dangerous tasks an astronaut performs. They are scheduled only for critical maintenance, assembly, or science tasks. An EVA is almost never scheduled just for inspection. Instead, inspection is a “task of opportunity.” While an astronaut is traversing from one worksite to another, they will be “eyes-on” for any unusual damage, reporting it to the ground.

This method is also essential for the mitigation of damage. If a leak were detected, spacewalking astronauts are trained to use patch kits and perform repairs on the station’s pressure shell.

Method 3: Looking from Within (Crew Photography)

A significant amount of MMOD damage is first identified and cataloged from inside the station. The ISS is not a windowless container; it features many high-quality optical windows, most notably the seven-window Cupola observation module.

Astronauts use a suite of high-powered, handheld digital SLR cameras (such as Nikon D-series cameras with a selection of powerful zoom lenses) to take detailed photographs of the station’s exterior from these windows.

This is the primary way that damage to the windows themselves is found, documented, and monitored over time. An astronaut will spot a new chip, photograph it, and downlink the imagery to ground teams. These engineers will then analyze the chip, compare it to previous photos, and evaluate whether it poses a threat to the window’s integrity. This method is also used to document damage on nearby solar arrays and radiators that are visible from the windows.

Method 4: The Gold Standard (Returned Hardware)

This is the most scientifically valuable inspection method. It provides the “ground truth” that no camera in orbit can. This method involves the detailed, microscopic, and chemical analysis of components that were exposed to the space environment for years and then returned to Earth.

NASA’s Hypervelocity Impact Technology (HVIT) group, based at Johnson Space Center, is responsible for these meticulous post-flight inspections.

The “golden age” for this data collection was the Space Shuttle program. The Shuttle was, and remains, the only vehicle ever built that could bring large pieces of hardware back from the ISS. Since the Shuttle’s retirement in 2011, this capability has been severely limited. Today, the return of space-exposed hardware is limited to small items that can fit inside a SpaceX Cargo Dragon capsule. As one NASA document notes, “no analogous high-quality, large area sensors have provided in situ data since the cessation of the Shuttle program.”

This makes the database collected during the Shuttle era incredibly precious. This database, which includes analysis of returned ISS components as well as the Shuttles themselves (which were also massive MMOD targets), contains over 1,400 documented impact records.

Two key examples from the ISS highlight the value of this method:

1. Airlock Shields (2010): After 8.75 years in orbit, two MMOD shield panels were removed from the Airlock module (to make room for other hardware) and returned to Earth on a Space Shuttle. Post-flight inspection found 58 MMOD impacts on these two panels. The largest was 1.8mm.
2. PMA-2 Cover (2015): A temporary thermal cover from the Pressurized Mating Adapter 2 (PMA-2) was returned on a SpaceX CRS-6 mission. After just 1.6 years of exposure, ground inspection found 26 impact features.

This database of thousands of documented, ground-truth impacts is the absolute foundation upon which all MMOD risk modeling is built.

The Future: Real-Time Impact Detection

All four of the current methods are “after-the-fact.” They find damage after it has occurred. For unmanned spacecraft, this is a problem, as there are no human assets to conduct surveys. And even on the ISS, a dangerous impact might not be noticed for months.

To solve this, NASA is developing new technologies to provide real-time impact detection. The most promising is a strain-sensing system that can be affixed to a spacecraft’s shielding. This system uses Fiber Bragg Grating (FBG) technology, where multiple strain sensors are encoded into one or more optical fibers.

When an MMOD particle strikes the shield, it imparts a transient shock wave. These sensors “feel” that shock wave as a “transient strain” moving through the structure. By analyzing the time signature from multiple sensors, a data collection device can instantly determine the “occurrence, time, location and severity” of the strike. This would alert ground crews immediately that a potentially harmful impact has occurred, allowing them to point robotic cameras to the exact location and assess the damage.

A Record of Violence: Cataloged Evidence of MMOD Impacts

The inspection strategy is not just a theoretical exercise. It has produced a long and detailed catalog of the violence the ISS endures. This evidence is written all over the station, from its robotic arms to its windows, radiators, and solar arrays.

Case Study 1: A Hole in the Arm (Canadarm2 Strike)

The most high-profile recent example of MMOD damage was the strike on Canadarm2.

• Discovery: On May 12, 2021, ground operators at NASA and the Canadian Space Agency spotted the damage during a routine robotic inspection.
• Damage: The images were stark, showing a clean, ~5-millimeter-wide puncture. The impactor had punched through the arm’s boom segment and its thermal blanket.
• Impactor: The object was too small to be tracked. Based on analysis of the hole, experts estimated the impactor was likely a 1-millimeter object, traveling at over 25,000 km/h.
• Significance: The event was called a “lucky strike.” The arm is a relatively small target – just 14 inches in diameter – and the object missed all of its critical components, such as joints or wiring. An analysis by both agencies concluded that the arm’s performance “remains unaffected.” The damaged arm is still fully operational, but it serves as a powerful, visible reminder of the constant, random threat.

Case Study 2: The Cupola’s Chip (A Famous Flaw)

The most-seen piece of MMOD damage is a small chip in one of the windows of the Cupola, the station’s 360-degree observation dome.

• Discovery: Around 2012, astronauts spotted and photographed a small, circular chip in Window #2 of the seven-window module.
• Damage: The chip is a small pit in the outermost pane of glass. This is not a major failure; it is the system working perfectly.
• Context: The Cupola windows are not a single pane of glass. They are a sophisticated, multi-layer assembly. The outermost pane is a sacrificial “debris pane.” Its entire job is to be the first line of defense, absorbing the impact and shattering the MMOD particle before it can reach the inner, structural “pressure panes” that actually hold the station’s atmosphere in. The particle that caused this chip – believed to be a tiny paint fleck or a sub-millimeter piece of debris – was stopped exactly as intended. The Cupola windows also have external aluminum shutters that are kept closed when the module isn’t in use, providing an additional layer of MMOD protection.

Case Study 3: The “Cut Glove” Hazard (EVA Handrails)

This is one of the most interesting and non-obvious consequences of MMOD damage. The threat isn’t to the station’s structure, but directly to the astronauts.

• Damage: Spacewalking astronauts and post-flight analysis of returned hardware (like handrails) have documented hundreds of tiny MMOD impact craters on the station’s external handrails. Craters as large as 1.85 millimeters have been observed.
• The Hazard: A hypervelocity impact doesn’t just make a clean hole. It vaporizes the metal, which then re-solidifies around the pit. This process creates a “raised lip” around the crater that is exceptionally sharp.
• The Consequence: Studies have shown that crater lips as small as 0.25 millimeters are sharp enough to “snag and tear” the high-tech fabric of a spacesuit glove. This is a critical failure risk. A glove tear during an EVA could lead to a loss of pressure, forcing an emergency end to the spacewalk. There have been several reported incidents of glove tears; at least one, during the STS-118 mission, was cut short and later attributed to a sharp edge on the station, likely created by MMOD.

Case Study 4: Solar Array “Bullet Holes”

The station’s massive solar arrays are, by design, thin, lightweight, and completely unshielded. They are the station’s most exposed components.

• Discovery: Astronauts, most famously Canadian Chris Hadfield, have taken dramatic photographs of clean “bullet holes” punched through the solar array wings.
• Damage: The particles, often micrometeoroids, pass right through the thin panels.
• Significance: This is not just cosmetic. This is a functional threat. A strike can break the “bypass diode” on the backside of a solar panel. This can cause an electrical short or an “overheat,” creating a dead spot in the array. Over time, this constant “sandblasting” effect slowly but surely degrades the station’s ability to generate power.

Case Study 5: Punctures in the Radiators (The “Smoking Gun”)

The station’s radiators are also large, exposed, and critical for shedding the immense heat generated by the station’s systems.

• Discovery: Damage has been observed on both U.S. and Russian radiator panels, often during EVAs.
• Damage: A particularly significant impact was found on the P4 photovoltaic radiator in June 2014.
• Significance: This case is the “smoking gun” that connects on-orbit damage to ground-based science. Ground teams took the on-orbit photos of the P4 damage, which clearly showed both the entry and exit holes. They then went to the hypervelocity impact lab and fired projectiles at a test panel. They found a “good comparison” between the on-orbit damage and a test shot from a 4.5-millimeter aluminum sphere traveling at 7.08 km/s. This was a perfect, tangible example of how a specific piece of damage can be directly linked to a specific type of threat.

This ongoing analysis has produced a rich database of known impacts, summarized in the table below.

This brings us to the final, and most important, part of the story. The inspections and the damage they find are not just for maintenance. The ISS is, by its very existence, the most important MMOD experiment ever launched. Every scar is a data point, and this data is invaluable.

Differentiating the Source: Debris vs. Meteoroid

When a piece of hardware – like the PMA-2 cover or the airlock shields – is returned to Earth, the inspection goes far beyond just counting and measuring the impacts.

Scientists take these components into a lab and use powerful tools like scanning electron microscopy and energy dispersive X-ray spectroscopy. These tools allow them to analyze the microscopic “projectile residue” that was melted and fused into the crater lip upon impact.

This chemical analysis is a fingerprint.

• If the residue is rich in magnesium-silicates, they know the impactor was a natural micrometeoroid.
• If the residue contains aluminum alloys, steel, or traces of plastic, they know it was man-made orbital debris.

This analysis is the only way to get a true “breakdown” of the risk, to determine the real ratio of the natural threat versus the man-made one. This data is critical for refining the models that predict the orbital environment.

Validating the Models: How Damage Data Fuels ORDEM

This is the most important takeaway. The ISS is not just a laboratory in space; its own structure is a passive, long-term laboratory for space.

Ground-based radars, as a rule, cannot reliably detect objects smaller than 10 centimeters. And ground-based sensors have an even harder time in the millimeter-to-1-centimeter range. This creates a massive data gap for the 100+ million particles that are too small to see but large enough to cause damage.

The only way to understand the threat from this “unseeable swarm” is to use in-situ measurements. That means putting a large target in orbit, leaving it there for decades, and then counting the holes.

The ISS and the data from the retired Space Shuttle program are those in-situ measurements.

This “ground truth” data – the 58 impacts on the airlock shield, the 26 on the PMA-2 cover, the thousands from the Shuttle era – is the lifeblood of NASA’s Orbital Debris Engineering Model (ORDEM).

ORDEM is the state-of-the-art, data-driven software model that NASA, the Department of Defense, and other agencies use to predict the orbital debris environment for any satellite, at any altitude, at any point in the future.

This creates a “closed loop” of knowledge:

1. Scientists use ORDEM to predict the number of impacts a component, like the PMA-2 cover, shouldreceive during its 1.6 years in orbit.
2. The component is returned to Earth, and scientists observe the actual damage (26 impacts).
3. They compare the prediction to the reality. If the model’s prediction was wrong, they “adjust the parameters” of the model – tweaking the assumed production rate or particle distribution – until the model’s output does match the real-world, “ground truth” data from the ISS.

This means the damage surveys are not just for station maintenance. They are the primary method NASA uses to validate and improve its fundamental understanding of the space debris environment.

Building Better Spacecraft: Informing Future Shield Design

This “closed loop” of knowledge – where ISS damage data refines the ORDEM model – directly informs the design and safety of all future spacecraft.

The validated ORDEM model is fed into another piece of software, the BUMPER MMOD risk assessment code. This code allows engineers to model a new spacecraft and, using the ORDEM-defined threat, determine the “Probability of No Penetration.”

This isn’t a historical or academic exercise. It’s happening right now. The lessons learned from the ISS’s 25-ton shielding are being applied directly to the Artemis program, which aims to return astronauts to the Moon.

The new spacesuit for the Artemis missions, the Exploration Extravehicular Mobility Unit (xEMU), must be able to protect astronauts from MMOD impacts. To determine its “ballistic performance,” NASA engineers performed over 100 hypervelocity impact tests on the suit’s fabric layers. The threat they tested against – the size, speed, density, and number of particles the suit had to survive – was defined by the MMOD environment models. And those models were built and validated using the real-world, empirical data from the scars on the International Space Station.

Engineers are also using this data to test new, lighter, and more effective shielding concepts, such as carbon nanotube composites and “self-healing” materials, to avoid the massive weight penalty of the ISS’s armor.

Summary

The exterior of the International Space Station is, in fact, under constant and meticulous inspection for space debris damage. This is not a single, scheduled event but a continuous, multi-pronged strategy that is fundamental to the station’s survival. This strategy combines the high-tech (ground-controlled robotic cameras), the human (astronaut photography and spacewalks), and the forensic (meticulous laboratory analysis of hardware returned to Earth).

The evidence of damage from this “MMOD” environment is extensive and undeniable. It is written in the form of millimeter-sized punctures in the station’s robotic arm, chips in its strongest windows, “bullet holes” in its power-generating solar arrays, and sharp-lipped craters on its handrails that pose a direct threat to spacewalking astronauts.

This relentless barrage is caused by a swarm of over 100 million untrackable particles, ranging from natural micrometeoroids to man-made paint flecks and aluminum fragments, all traveling at hypervelocity speeds many times faster than a bullet.

The data gathered from these surveys is far more than a simple maintenance log. These “scars” are a vital scientific resource. By analyzing the physical and chemical traces of each impact, scientists can differentiate the natural and man-made threats. This “in-situ” data is the “ground truth” used to build and validate NASA’s official Orbital Debris Engineering Model (ORDEM). This, in turn, allows engineers to design and test the shielding for the next generation of spacecraft and spacesuits, ensuring that the lessons learned from the ISS’s long and violent gauntlet in orbit will protect future explorers.