Throughout history, humanity has looked up at the night sky, captivated by the Moon. This celestial body has inspired awe and wonder, shaped myths and beliefs, guided sailors across oceans and marked the passage of time.
From nearly 400,000 kilometers away, the Moon plays a crucial role in supporting life on Earth. Its gravitational pull generates tides that oxygenate coastal waters, transport nutrients and sustain some of the planet's most diverse ecosystems, like the Great Barrier Reef.
With such a significant influence on human life, the Moon has been the subject of scientific study for thousands of years, and understanding it helps provide context for the entire solar system. Our enduring fascination with the Moon is also reflected in International Moon Day, observed by the United Nations on 20 July.
Recently, the world has seen a resurgence in lunar exploration. Earlier this year, NASA's Artemis II mission sent four astronauts on a 10-day journey around the Moon as part of a programme aiming to establish a long-term human presence on its surface by the 2030s. China, India, Japan, and several private companies are also pursuing their own lunar mission programmes.
"Decades of rapidly expanding space activity have shown that each launch leaves an environmental footprint, and it can take a long time to understand the impact fully. This is why it is so crucial to consider the sustainability of these programmes early on," says Jason Jabbour, Senior Officer at the United Nations Environment Programme (UNEP).
Space sustainability means that activities in outer space and on the Moon must be carried out in a way that will preserve them for future generations. International agreements such as the 1967 Outer Space Treaty established essential principles for the responsible exploration and use of outer space, obliging states to avoid harmful contamination.
Building on these treaties, UN Member States have adopted guidelines on space debris mitigation and the long-term sustainability, including practical measures like minimizing debris, preventing explosions, avoiding intentional destruction of spacecraft, and safely removing defunct satellites and spent rocket stages from orbits.
Still, an estimated 140 million debris larger than one milimeter, but most too small to track are circling our planet today. Here's a closer look at what they are and at their potential environmental impact.
Defunct satellites
Every satellite eventually reaches the end of its operational life. Most re-enter Earth's atmosphere, where they burn up, releasing aluminum, lithium, copper and other metals into the upper atmosphere. Scientists have already detected spacecraft-derived metals in stratospheric particles and are actively investigating their potential effects on atmospheric chemistry, cloud formation and ozone.
Abandoned rocket stages
Rocket bodies are among the largest objects left in orbit after launches. Some remain in space for years before re-entering the atmosphere, while others fragment into thousands of pieces. Hundreds of metric tonnes of rocket bodies and spacecraft are estimated to re-enter Earth's atmosphere annually, and their number is expected to rise as large satellite constellations are replenished, creating the risk of metallic emissions and of surviving fragments reaching Earth.
The UN Space Debris Mitigation Guidelines urge operators to vent leftover propellant, discharge onboard batteries, and remove retired satellites and rocket stages from orbit after their missions end to reduce the risk of explosions and the buildup of space debris.
Collision fragments and debris from explosions and anti-satellite tests
When satellites or rocket stages collide, they can shatter into thousands of high-speed fragments, traveling at roughly 28,000 kilometres per hour. At this speed, even a small piece can destroy another spacecraft, with each collision generating even more debris. Explosions caused by leftover fuel or batteries, as well as deliberate destruction during anti-satellite weapon tests, create dense clouds of fragments that can remain in orbit for decades.
The UN debris mitigation guidelines advise against intentional destruction of orbiting spacecraft, and since 2022 a growing number of states have committed not to conduct destructive anti-satellite missile tests.
Mission-related operational debris and dust
Space missions can leave behind smaller objects, such as protective covers, bolts, lens caps and other hardware, released during launch or satellite deployment. Although individually small, these objects travel at orbital speeds and contribute to the growing congestion in low Earth orbit. Limiting the release of such objects during normal operations is the first of the UN debris mitigation guidelines.
Repeated collisions, explosions and gradual erosion produce microscopic particles that form a diffuse cloud around Earth. Scientists estimate that sunlight reflected and scattered by the accumulated mass of orbiting objects and fragments is already contributing to increased night-sky brightness.
Surviving re-entry debris
Most spacecraft are designed to burn up during re-entry; however, larger or more heat-resistant components can survive and fall back to Earth. These objects may threaten people and infrastructure on land, while debris landing in the ocean could damage marine habitats or introduce hazardous materials into marine ecosystems.
Under the 1972 UN Liability Convention, every launching Member State is responsible for damage caused by its space objects on Earth. The term includes the country that commissions the launch and the country from whose territory or facilities it takes place. In practice, this means that a nation purchasing a launch abroad for its satellite shares responsibility alongside the country providing the launch services.
UNEP and the UN Office for Outer Space Affairs are working together to improve our understanding of environmental risks, enhance the scientific evidence base, and ensure that environmental factors are included in space governance. What happens in space affects life on Earth. Through strong global cooperation and action, countries can prevent environmental crises in this new frontier.
