So much debris from past missions has accumulated in low Earth orbit that between one-third and half of the zone's capacity to sustain long-term space activities has already been filled, scientists have reported.

Staff researchers at the Italian National Research Council's Institute of Information Science and Technologies in Pisa analyzed how space junk has built up over the past 25 years, and concluded that the 2020s will be a critical juncture for enforcing steps to deal with the problem. The team published the findings April 2 in Acta Astronautica.

"If we are interested in the sustainability of space operations in the coming decades, some strong mitigation measures and rules of conduct must be introduced in this decade; the sooner, the better," said Luciano Anselmo, the last author of the paper.

There are millions of pieces of space debris from dead satellites, bits of machinery and paint flecks from spacecraft and rockets and other detritus from human activities circling the planet.  

"Space is certainly big, and the human-made objects occupy only an infinitesimal fraction of it and are on average very distant from each other," Anselmo acknowledged. 

However, these objects can travel at speeds of 36,000 kilometers per hour through low Earth orbit, and the energy released when they collide can be huge. A collision with a centimeter-sized piece of debris can disable a working satellite, and a decimeter-sized object can completely destroy a spacecraft and create hundreds or thousands of new potential projectiles, Anselmo says.

In recent years, space activities in low Earth orbit have entered a "revolution," he and his co-author Carmen Pardini wrote in the paper. From 2014-20, the total mass of artificial objects in orbit grew by 22% and the number of operational spacecraft more than doubled. 

"Today, with the impetuous flowering of the so-called New Space Economy, characterized by the deployment of countless satellites and mega-constellations, the problem is becoming really urgent, because some wrong decisions or actions taken now may have lasting adverse consequences on space operations, in particular in low Earth orbit," Pardini said.

She and Anselmo have been studying space debris since the 1980s, and wanted to evaluate the status of space debris from the last quarter-century. They examined catalogues of space debris from the U.S. Space Surveillance Network, analyzing how the amount of debris grew over time by altitude and object type.

The team found that the overall evolution of space debris matched forecasts from 20 years ago of a business-as-usual scenario with no mitigation measures. 

International groups such as the Inter-Agency Space Debris Coordination Committee have recommended steps to limit the junk, such as removing objects from crowded regions of space at the end of their missions. Although unevenly applied, these actions did help rein in the accumulation of space debris, Pardini and Anselmo found.

Unfortunately, this progress was undone by several catastrophic collisions in the 2000s. In January 2007, China destroyed the satellite Fengyun 1C during an anti-satellite weapon test. Two years later, the satellites Cosmos 2251 and Iridium 33 crashed accidentally. These two events produced a number of trackable fragments equivalent to the objects that had accumulated in low Earth orbit from 1970 to 2007.

Pardini and Anselmo didn't find any signs of an exponential increase in space debris, which could have indicated that a chain reaction of collisions known as the Kessler syndrome, also called the Kessler effect, was underway. This means that the evolution of space debris in low Earth orbit is still driven by actions such as new launches and collision avoidance maneuvers, rather than by collisions caused by fragments of previous collisions, Anselmo says.

Worryingly, though, above altitudes of 700 kilometers, "The density of objects capable of causing catastrophic collisions is already higher than the 'critical' one capable of causing, sooner or later, the Kessler Syndrome," Pardini said.

Additionally, this cascade of collisions would probably not become apparent immediately.

"Looking at space debris, the movie 'Gravity' — in addition to several serious inaccuracies, which are, however, evident only to those who have basic knowledge of orbital dynamics — is especially wrong in defining the time scale of the Kessler Syndrome, which is not a few hours, but at least several decades," Anselmo noted.

The researchers also used indicators such as the debris density at given altitudes to "evaluate where we are now and the sustainability of space activities in the future," Pardini said. "While there is still 'room' for new space activities, the 'room' left is not so much considered what it is planned to do compared to what has been done in the previous six decades."

Extreme care will be required in planning and executing new missions from now on, she and Anselmo wrote. Enhanced maneuvering capabilities and other measures for collision avoidance and debris cleanup will be particularly vital as more mega-constellations of thousands of satellites come online, and private companies such as SpaceX have a larger impact on exploration.

"Any true progress in the field of space debris will be the result of a deep and wide amount of international cooperation," Anselmo said. "But countries, governments and international organizations alone are not enough anymore. Private companies must be involved in the loop as soon as possible."

The study, "Evaluating the impact of space activities in low earth orbit," published April 2 in Acta Astronautica, was authored by Carmen Pardini and Luciano Anselmo, Institute of Information Science and Technologies (ISTI).