The story so far:
Millions of naturally occurring Micrometeoroids and Orbital Debris (MMOD) orbit the earth, posing a constant threat to all spacecraft and space stations. The menace attracted global attention recently when a piece of debris struck the Chinese crewed vehicle Shenzhou-20, causing a minor crack in the window of its return capsule, rendering it unusable for crew travel.
What is MMOD?
Micrometeoroids are typically extremely small, with sizes ranging from a few micrometres (a millionth of a meter) — the approximate size of a grain of dust — to up to about two millimetres. Each weigh less than a dried grape. Most of them (about 80 to 90%) originate from collisions between asteroids in the Asteroid belt (between Mars and Jupiter) with a small portion coming from comets. They also travel at extremely high velocities (about 11 to 72 km/s).
Orbital debris (also called space debris, space junk or space trash) consists of human-made objects in the Earth’s orbit that no longer serve any useful purpose. All orbital debris originated primarily from exploded rocket stages, satellites, accidental collisions and intentional anti-satellite weapon tests. The typical average speed of orbital debris is about 10 km/s. With increase in density of space debris, there could be a theoretical scenario wherein collisions between them may create a cascade of further collisions, eventually making space travel impossible, a phenomenon known as the Kessler Syndrome.
The Inter-Agency Space Debris Coordination Committee (IADC) — an international forum of major space agencies such as NASA, ESA, ISRO, JAXA, etc. — plays a vital technical role by generating the foundational standards for space debris mitigation. These technical standards form the basis for the space debris mitigation guidelines adopted by the United Nations Committee on the Peaceful Uses of Outer Space (UNCOPUOS). However, the guidelines are considered as “soft law” implying that they are voluntary, and have no legally binding enforcement mechanism to compel the countries to adopt them.
How is MMOD distributed in space?
Orbital debris is mostly concentrated in a “shell” around the Earth in Low earth Orbit (LEO) ranging from about 200 km up to 2,000 km altitude. In contrast, micrometeoroids exist everywhere in space, but due to the Earth’s gravity pull their distribution is slightly higher near our planet. There are hundreds of millions of pieces of orbital debris in the LEO — an estimated 34,000 objects larger than 10 cm (and are accurately tracked) and over 128 million pieces greater than 1 mm in size. The micrometeoroids in Earth’s orbital environment are effectively uncountable and they deliver billions of impacts to orbiting spacecrafts annually.
How are space systems designed for MMOD impact?
The risk of being hit by MMOD in the LEO is not uniform; it is highly directional. The face of the spacecraft in the direction of travel experiences the maximum overall hazard because the debris collides directly with the highest relative speed. Because of the very high velocity of the debris, even tiny fragments carry enough kinetic energy to cause catastrophic failure or critical damage to onboard systems.
Space agencies utilise complex engineering models that process tracking and statistical data to determine the MMOD flux, which is the expected number of particles of a given size that will hit a spacecraft over its mission lifetime. This data with other relevant information is fed into specially developed software tools to perform a vulnerability analysis and to calculate the probability of loss or failure of critical components under the impact of MMOD. If the calculated risk exceeds the established safety standard, then the spacecraft is protected by physical shielding against the impact.
How are satellites protected from MMOD?
MMOD threats are countered using a strategy of design and operational methods. Engineers rely on Whipple shields to protect the space vehicle from MMOD where energy dissipation is achieved through fragmentation and turbulence, analogous to sea waves breaking against tetra pods and splitting its energy. Whipple shields have an outer “bumper” and inner “rear wall” with a stand-off or gap between two. The bumper shatters the incoming high-velocity debris into a cloud of fragments. As the cloud expands across the standoff distance, the momentum is distributed over a wide area, allowing the rear wall to absorb the energy without failing.
For avoiding the risk from larger debris, space agencies maintain detailed tracking catalogs of objects larger than 10 cm. When a potential collision with a trackable object is projected, a debris avoidance manoeuvre is executed by firing the spacecraft’s thrusters to slightly alter its orbit and move out of the projected impact zone.
How is Gaganyaan crew protected?
The main difference in Gaganyaan compared to other ongoing human space missions is that it is a standalone mission as there is no space station where the orbital module can dock and seek help in case of any exigency during the orbital phase. Since the mission duration is very short (less than a week), the probability of being hit by a catalogued space debris is extremely low, though the risk from small un-catalogued, high-velocity fragments still necessitates protective measures.
The MMOD protection scheme for Gaganyaan is based on internationally accepted standards, such as employing passive defences like Whipple shields. To ensure that these shields meet stringent human-rating requirements, ISRO uses specialised facilities and software tools for design and validation. The gas gun facility at DRDO’s Terminal Ballistics Research Laboratory (TBRL), Chandigarh, is one of the facilities used for validating the design where a 7 mm spherical projectile can be accelerated to achieve an impact velocity of up to 5 km/s.
The era of expanding human presence beyond the Moon, driven by both nations and commercial entities, can only be secured if the global community collectively addresses the risks of debris and adopts stringent zero-junk practices to de-escalate the MMOD menace for ensuring a safe, sustainable orbital highway for all future endeavours.
Unnikrishnan Nair S. is Former Director, VSSC & IIST; founding director, HSFC; and an expert in launch vehicle systems, orbital re-entry and human spaceflight technologies.
Published – December 23, 2025 08:30 am IST
