Who is the best "catcher" of space junk?

As the "artery industry" of transporting supplies to space continues to grow and strengthen, the development space for the accompanying "vein industry" of clearing up debris and fragments is also expanding.

01

Dangerous Space Debris

On July 11th, the ignition failure of the second stage of the "Falcon 9" rocket led to an explosion, marking the first failure for the "Falcon 9" in seven years. This explosion turned more than 20 Starlink satellites into the latest space debris. On June 26th, a 6-ton Russian satellite disintegrated, producing at least 250 fragments that have already struck multiple GPS and Starlink satellites, posing a potential threat to the space station.

In 2021, a fragment not under surveillance punctured a hole in the International Space Station's robotic arm. In 2022, a 0.8-millimeter hole appeared on the hull of the Soyuz spacecraft docked at the space station, resulting in the leakage of 44 kilograms of coolant.

For the International Space Station, if a space debris larger than 10 centimeters is detected, entering an area within 1.25 kilometers above and below the space station's orbit and 25 kilometers in front and behind, or if the collision probability is greater than one in ten thousand, the space station must maneuver to avoid it. During the maneuver, astronauts must hide in the manned spacecraft, ready to evacuate at any time. Since its launch in 1998, the International Space Station has performed more than 30 maneuvers to avoid debris, most of which occurred in the last decade. In 2021, the Chinese space station also conducted two emergency maneuvers to avoid Starlink satellites.

Advertisement

Since the launch of the first artificial satellite, human space activities have generated a large amount of space debris. By 2021, humans had launched more than 6,000 rockets, placing 10,000 satellites into Earth's orbit, with 3,600 still operational and over 2,600 becoming orbital space debris. They have produced a large number of fragments, with 36,500 larger than 10 centimeters in diameter and approximately 1 million between 1 centimeter and 10 centimeters (according to data from the European Space Agency). The total weight of these near-Earth orbit (below 2,000 kilometers) materials exceeds 9,200 tons. With the dense deployment of Starlink satellites, the debris in near-Earth orbit will also grow rapidly.Currently, the space debris monitoring network regularly tracks and catalogs over 28,000 pieces of debris, 74% of which are defunct satellites or rocket bodies and other space debris. However, these only account for 4% of the total amount of space debris.

These pieces of space debris are traveling at high speeds of 6 to 8 kilometers per second in the low Earth orbit (LEO), which is the most frequently used orbit by humans, posing a threat to the safety of other satellites and the International Space Station, and hindering human exploration of outer space.

02

Preventing Kessler Syndrome

Kessler Syndrome is a hypothesis about space debris proposed by Donald J. Kessler. He pointed out that when the quantity of space junk in low Earth orbit (LEO) exceeds a certain threshold, a collision event could trigger a chain reaction, producing more debris and forming a vicious cycle that leads to the destruction of a large number of spacecraft. For example, a defunct Russian satellite, "Cosmos 1934," collided with another satellite, "Cosmos 926." The impact caused Cosmos 1934 to split in two, while Cosmos 926 was shattered into multiple fragments, too many to track.

To prevent the occurrence of Kessler Syndrome, it is necessary to take space debris mitigation measures, such as establishing tracking and early warning systems, limiting the generation of new debris, actively removing debris from orbits, and strengthening international legislation.

For instance, the United States plans to reduce the post-mission deorbiting period from 25 years to 5 years for near-Earth orbits. This requires spacecraft design to consider ease of capture and deorbiting, and to establish a priority list for clearing orbital debris, as well as investing in the development of commercial technologies for the removal of hazardous space junk.China's Space Station in Protective Armor

The numerous space debris pieces, each less than 1 centimeter in size, are difficult to monitor. To withstand the impact of micro space debris, China's space station has been "clad in a layer of protective armor" on its exterior. This armor is made from special materials that can absorb energy and mitigate damage to the space station. On July 3, 2024, astronauts from the Shenzhou-18 mission conducted an extravehicular activity to install space debris protection devices on the space station's external pipelines, cables, and key equipment.

Tracking Space Debris

LeoLabs, a near-Earth orbit laboratory company, was established in 2016. It operates six S-band active phased array radars in the United States, Costa Rica, Australia, and other locations to track space debris larger than 10 centimeters and below 2000 kilometers in altitude. The company utilizes artificial intelligence technology, through its global phased array radar network and data analysis platform, to monitor activities in orbit, providing clients with near-Earth orbit mapping and space situational awareness services, and predicting the probability of collisions between spacecraft and space debris.

Equipping Satellites with Giant Sails

If inactive satellites are not actively managed, a 15-kilogram satellite at an orbital height of 700 kilometers may take up to 120 years to re-enter the atmosphere. The de-orbit sail technology from the 805th Institute of China's Eighth Academy involves equipping satellites or rockets with a thin-film sail that can be unfolded to a maximum area of 25 square meters.

When a spacecraft becomes inoperative, the de-orbit sail can be deployed with a small amount of electrical energy. By increasing friction with the atmosphere and solar wind, the satellite slows down and leaves its orbit, freeing up valuable orbital resources. According to calculations, a de-orbit sail of just 2 square meters can reduce the re-entry time from 120 years to 10 years.Space Catchers and Shooters

In January 2022, China used the "Practice 21" spacecraft to capture a retired Beidou 2-G2 satellite from the Geostationary Earth Orbit (GEO) and dragged it to a higher "graveyard orbit" (300 kilometers above the GEO, at an orbital altitude of 36,000 kilometers), marking the first time that space debris was removed from the GEO. Similarly, Japan's Astroscale company is also developing artificial satellites for capturing space debris.

Using lasers to shoot down small or rapidly spinning space debris that is unsuitable for capture is also a direction being researched by multiple countries. This includes different research paths such as using ground-based laser launchers and space-based laser satellites. For example, Perfect Sky - a Japanese satellite consortium is developing technology to launch lasers from satellites to bring debris down into the atmosphere.

07

Cleanup Costs and Benefits

Reducing space debris requires a significant cost. In May 2024, NASA released a report comparing the costs and benefits of various cleanup methods. According to the assessment, the overall cost-to-benefit ratio ranges from 20 to 1000 times. The graph shows the benefit-to-cost ratio after 30 years of investment in different methods, with the left side having high costs and low benefits, and the right side having low costs and high benefits. Taking the last item, radar/optical tracking of centimeter-sized debris, as an example, the benefit after a 30-year investment of 1 dollar ranges from a few cents to 5 dollars.