Deorbiting Sails: The “Scavenger” of Space Junk

In the future, fireworks may not only rise from the ground, but also fall from the sky.

Dutch artist Daan Roosegaarde is working with the European Space Agency to build a space junk lab. It is envisioned that space junk will be turned into artificial meteors that light up the sky like fireworks. The plan seeks to guide the debris into the Earth's atmosphere and cause it to burn up at a predetermined time.

The prerequisite for completing this "fireworks show" is how to guide the debris into the atmosphere? This is actually a topic that aerospace scientists and engineers at home and abroad have already begun to study. Among them, deorbit sails, as a relatively convenient and economical way to dispose of space debris, are increasingly entering the public eye.

Not long ago, the deorbit sail, which is currently the largest in China and the first to be used in a rocket compartment, independently developed by the Eighth Academy of China Aerospace Science and Technology Corporation, was successfully deployed in orbit for the first time, bringing the last stage of the rocket that completed its mission back to the Earth's atmosphere for burning. So, what are the main advantages of the deorbit sail?

Actual photo of the deorbit sail independently developed by the Eighth Academy being deployed in space

The new killer of space junk

In 2021, more than 100 orbital rockets were launched around the world. After the mission is completed, the rocket debris will either fall to the ground, fall into the earth's atmosphere and burn up, or become a permanent individual wandering in space as "space junk".

According to statistics, since humans began space activities, tens of billions of space debris of various sizes have been left in space. They are not stationary, but are flying at extremely fast speeds. For example, the flying speed of space debris in low-Earth orbit can reach 7 to 8 kilometers per second. If they collide with a spacecraft, it will cause a spacecraft system failure at the very least, and may even cause complete disintegration or explosion.

It is generally believed that it is a very convenient and economical way for tiny satellites with orbital altitudes below 600 kilometers to use "de-orbit sails" to increase drag to achieve autonomous cleaning of space debris after their missions.

Deorbit sails use a large area of ​​thin film sails to increase aerodynamic drag and accelerate the deorbit process of spacecraft. They are a simple, passive deorbit method suitable for low-orbit spacecraft and have the advantages of ease of use and low cost.

Ground test of the deorbit sail of the Eighth Academy

Taking the payload cabin of a launch vehicle as an example, although it has been in orbit for a long time, it has no propulsion system and no electrical and chemical energy supply. At this time, only a weak electric current needs to be given, and the sail of the off-orbit sail can be unfolded, carrying the payload cabin out of orbit and entering the atmosphere to burn up.

During the launch phase of the carrier rocket, the deorbit system's deorbit sail device is always in a folded and compressed state. After the satellite enters orbit, the deorbit sail is automatically unlocked and deployed under the remote control command from the ground. The spacecraft increases the force-bearing area by deploying the "sail surface", slowly decelerates with the help of the thin atmospheric resistance, and is eventually dragged into the atmosphere by the earth's gravity and burns up. Simply put, this is like giving the spacecraft an "umbrella". After completing the mission, the umbrella surface opens, the resistance increases, and the spacecraft slows down to achieve deorbit.

The reason why deorbit sails are convenient and economical is that compared with active deorbit devices such as thrusters, the development cost of deorbit sails is less than 5% of the spacecraft. Moreover, for some satellites with lower orbits and smaller masses, the device can be further miniaturized to reduce costs.

Space junk's path out of orbit

There are generally two ways to deorbit a spacecraft: active and passive.

On July 19, 2019, the Tiangong-2, which had been traveling in space for nearly three years, was deorbited and re-entered the atmosphere in a controlled manner. A small amount of debris fell into the designated safe waters in the South Pacific. This is a successful example of active deorbit, that is, at the end of its life, the spacecraft uses its own power device to perform orbital maneuvers, reduce its flight speed, leave its orbit, and gradually fall into the atmosphere.

Passive deorbiting is to allow an unpowered spacecraft to lower its orbital altitude with the help of a membrane sail, an electric tether, an inflatable ball, etc. When the orbital speed is lower than the normal operating speed, the spacecraft will automatically fall into the atmosphere.

How to quickly deorbit the orbital compartment of a rocket that has completed a satellite launch is one of the key research directions in the field of space debris mitigation.

Not long ago, the Eighth Academy conducted a deorbit system test on the "Long March 2D", and for the first time applied the deorbit sail to the orbital compartment of a launch vehicle. This provided an effective means for the subsequent launch vehicle's final stage, payload compartment, payload adapter, etc. to quickly deorbit after completing their missions, and at the same time laid a technical foundation for the development of autonomous debris removal technology for large spacecraft at the end of their life cycle.

Actual photo of the deorbit sail independently developed by the Eighth Academy being deployed in space

Zheng Qi, technical leader of the launch vehicle payload module deorbit system team of the 805th Institute of the Eighth Academy, introduced that the deorbit sail is a lightweight accelerated deorbiting measure with low rocket carrying margin overhead and little impact on the launch mission, making it very suitable to be a standard deorbiting device for launch vehicles. In addition, from the perspective of new technology verification, compared with satellites with longer on-orbit mission cycles, using the last stage and payload module that remain in orbit after the launch mission is completed to implement such new space technology experiments will make task organization and management more efficient, enable faster on-orbit technology verification, achieve continuous iterative improvements in new technologies, and quickly improve product maturity. It is a new way of thinking for conducting new space technology experiments.

In September 2019, the team conducted a technical demonstration of the deorbit sail on the Taurus micro-nano satellite. On-orbit data showed that the deorbit rate of Taurus increased 10 times under the action of the deorbit sail, which was consistent with the simulation results.

In comparison, the deorbit sail used by the Chang Zheng 2D rocket has been significantly upgraded. Not only has the sail surface area been extended from 2.25 square meters to 25 square meters, but the volume of the spacecraft it can fit has also increased from 10 kilograms to 50-500 kilograms. In addition, the sail surface deployment method has been upgraded from elastic strain deployment to active drive deployment to ensure the flatness and integrity of the large-area film sail surface, and the deployment time has also been extended from less than 0.5 seconds to 20 minutes.

Actual photo of the deorbit sail independently developed by the Eighth Academy being deployed in space

In order to fold up the 25-square-meter sail and fit it into the rocket compartment where every inch of space is valuable, the research team put a lot of effort into thin film materials, not only ensuring that the deorbit sail can meet the requirements of toughness and durability, but also reducing the thinness to less than 1/10 of a hair. Finally, the sail is stored in a box nearly the size of a football through high-density compression technology.

Yun Weidong, the person in charge of the deorbit sail device of the 805 Institute of the Eighth Academy, introduced that the Eighth Academy started to research and develop various types of space debris cleaning technologies at the beginning of this century. In addition to the 2.25 square meters and 25 square meters of deorbit sails that have completed in-orbit verification, they are currently developing larger-scale deorbit sail devices, which are suitable for low-orbit spacecraft weighing 500 to 3,000 kilograms. In the near future, they will also complete the on-board verification and form a spectrum of deorbit sail devices for low-orbit spacecraft.

The “Space Defense War” Across National Borders

In recent years, although commercial space travel has developed rapidly and the cost of entering space has become lower and lower, orbital resources in near-Earth space have been further squeezed.

In China's aerospace field, in addition to scientific research institutes and other units, universities have also made many attempts at deorbiting sail technology. In 2018, the "Huaian" Enlai satellite developed by Nanjing University of Science and Technology planned to use deorbiting sails for active deorbiting tests.

At present, the United States, the United Kingdom, Germany and other countries have also launched a series of projects to study drag sail devices, which are mainly used to achieve controlled deorbiting of satellites. Key technologies such as support arm design, sail membrane design, and sail membrane folding have been verified in the projects.

Concept image of the deployment of the UK's DeOrbitSail off-orbit sail on orbit

Thanks to the efforts of scientists from various countries, the relevant technologies of deorbiting sails have been continuously verified and matured. In June 2017, the technology verification satellite "InflateSail" developed by the Surrey Space Center at the University of Surrey in the UK successfully deployed the deorbiting sail about 1 hour after entering orbit, achieving rapid orbit reduction within 72 days, and the sail surface unfolded with an area of ​​about 10 square meters. In July 2019, the LightSail-2 of the Planetary Society of the United States successfully deployed in orbit, with a sail surface unfolded with an area of ​​about 32 square meters.

Today, deorbit sail technology is becoming one of the most popular technologies for space debris mitigation due to its advantages of low cost, high technical maturity, and applicability to low-orbit spacecraft of different specifications. A series of successful flight tests have shown that deorbit sails are feasible for deorbiting low-orbit spacecraft.

In order to make the protection of space "rules-based", the world's major space powers have also reached a consensus on mitigating space debris and formulated a series of rules and treaties.

Today, the growth of space debris remains an increasingly serious problem for governments and the commercial space sector, and a comprehensive approach, including deorbiting sails, is needed to mitigate space debris and clean up space junk.