A sustainable use of space

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Science  16 Jul 2021:
Vol. 373, Issue 6552, pp. 259
DOI: 10.1126/science.abk3135

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Last month, at the G7 Leaders' Summit in Cornwall, United Kingdom, the leading industrial nations addressed the sustainable and safe use of space, making space debris a priority and calling on other nations to follow suit. This is good news because space is becoming increasingly congested, and strong political will is needed for the international space community to start using space sustainably and preserve the orbital environment for the space activities of future generations.

There are more than 28,000 routinely tracked objects orbiting Earth. The vast majority (85%) are space debris that no longer serve a purpose. These debris objects are dominated by fragments from the approximately 560 known breakups, explosions, and collisions of satellites or rocket bodies. These have left behind an estimated 900,000 objects larger than 1 cm and a staggering 130 million objects larger than 1 mm in commercially and scientifically valuable Earth orbits.

Today's already active satellite infrastructure provides a multitude of critical services to modern society, including communication, weather, navigation, and Earth-monitoring missions. Its loss would severely damage modern society. Furthermore, a new era in space has just started, driven by commercial, low-latency broadband services that rely on large constellations of satellites in low Earth orbit. These will revolutionize connectivity on the ground and in the air. However, they will also increase space traffic. The satellites to be launched over the next 5 years will surpass the number launched globally over the entire history of spaceflight. Congestion in space is only going to get worse.

It is apparent that debris mitigation strategies—defined two decades ago by experts in the world's leading space agencies—are ever more important. They aim to prevent explosive breakups by venting residual energy from space systems at the end of their missions, and to “dispose” of a space object through a final maneuver that causes it to reenter Earth's atmosphere. Although these strategies are widely recognized, dozens of large space objects are still stranded every year in critical orbital regions where they will remain for several hundred years. And an average of eight fragmentation events in orbit occur annually, adding more pollution and increasing the likelihood of more collisions. Operations in space are themselves facing the burden of increasing evasive maneuvers to prevent losing a mission. In the most densely populated orbital altitudes, space objects are receiving dozens of collision warnings per day, of which only the most critical can be avoided. The number of such alerts will grow as large constellations of satellites come online.

Another important facet of the debris problem is the risk on Earth from reentering objects. Between 100 and 200 metric tons of human-made hardware reenters Earth's atmosphere every year in an uncontrolled fashion. Heat-resistant material, like titanium or stainless steel, can survive the harsh reentry conditions.

Progress can be made by advancing technology to ensure spaceflight safety. For example, the European Space Agency's Space Safety Programme is developing solutions that make disposal and energy passivation actions more fail-safe. “Deorbiting kits” will provide redundant propulsion and communication to ensure disposal of a spacecraft even after it ceases to function. A new field of “design-to-demise” will aim to replace critical components with less heat-resistant material to limit their chance of reaching ground upon reentry. In addition, a more systematic deployment of ground-based laser tracking could increase the accuracy of space surveillance data and consequently limit the number of collision avoidance alerts. Laser power could even transfer a small amount of momentum to objects to prevent their collisions. On top of that, missions, such as Clearspace-1, will aim to remove targeted debris through robotic capture.

An internationally binding regime for the management of debris and space traffic is pending. Thus far, space missions have been supervised on the national level only, and states have been encouraged to translate the nonbinding space debris guidelines into national regulations. Space, however, is a commonly used resource with a limited capacity. International harmonization of space traffic would be required for an efficient and interference-free use of space. The coordinated use of the available radio frequencies could serve as a template. Furthermore, the implementation of space debris mitigation requirements should be tracked, following internationally binding principles. New and affordable technical solutions might stimulate more ambitious steps in international regulation to preserve space for the spacefarers of tomorrow.

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