Will space junk hinder humanity's journey to the stars and the sea?

Key Points

★ If space junk grows like an avalanche, it will hinder humanity’s path to the stars and the sea.

★ Low-cost and efficient cleaning of space debris is a technical challenge we face.

"Paul and I used the booster to get close to it. When we were tying the connecting rope, a hole suddenly appeared on Paul's mask... Air rushed out of the mask, and he flew far away. I had to watch him spin... There was a hole the size of a one-dollar coin on the right side of his mouth. It was this hole that made him look like a weird big mouth. But he did have a smile on his face. He must have lost his life in an instant, and that momentary smile solidified into eternity. It seemed to show: how fragile life is."

Fortunately, the tragic space accident above is just a fictional plot in Yuan Yingpei's novel "Space Kamikaze". But if we don't start dealing with the threat of space junk sooner, this scenario may become a reality in the future.

From giphy

01

Who threw the space junk?

Space junk, also known as space garbage or space debris, refers to all abandoned man-made objects in orbit. They range from large debris caused by satellite disintegration to abandoned spacecraft such as rocket stages and abandoned satellites; small debris ranges from coatings dropped from aircraft, particles formed by solidified liquid discharged from aircraft, and unburned solid particles from solid rocket engines.

The sources of space debris are varied, including human dispersion, mission remnants, abandoned spacecraft, anti-satellite tests, etc.

For example, in the 1960s, the United States spread 430 million copper needles less than 2 cm long and thinner than a hair in orbit to form a cloud-like ring to reflect radio signals so that American troops overseas could better communicate with their home country. These copper needles eventually became space junk, and a considerable number of them still remain in orbit today.

Some space debris is the result of anti-satellite weapon tests. For example, in 1985, the United States tried to destroy a satellite with a missile, resulting in thousands of space debris larger than 1 cm. Fortunately, since the experiment was conducted in a relatively low orbit, most of the debris was pulled by gravity and burned up in the atmosphere.

Russia also used a missile to destroy an abandoned military reconnaissance satellite on November 15, 2021. This created thousands of traceable fragments and could generate hundreds of thousands of smaller fragments. These fragments will remain in orbit for years or even decades and could threaten the lives of astronauts on the International Space Station.

In addition to the "space" stuff mentioned above, there are some even more bizarre things among the existing space junk...

For example, a glove lost by astronaut Edward White during an extravehicular activity; a camera lost by Michael Collins during the Gemini 10 mission; and garbage bags, a wrench and a toothbrush discarded during the 15 years of operation of the Mir space station.

Another astronaut lost a pair of pliers while repairing the International Space Station's solar panels, and an astronaut lost a briefcase-sized tool kit during an extravehicular activity on STS-126.

The toolkit lost during STS-126 (center of the picture)

02

Why is space junk so deadly?

Some friends may wonder, these garbage in space don't seem to be much higher than the garbage in daily life? What harm can it do? It's nothing more than avoiding large pieces of garbage and ignoring small pieces, right?

The reason is actually very simple, that is speed. If someone throws it to you with their hand, you can easily catch it. But if it is shot at you from a gun barrel at a speed of hundreds of meters per second, it will result in death. The movement speed of space debris is far greater than that of bullets.

The reason why space debris poses a fatal threat to spacecraft in orbit is precisely because of the high-speed collision of space debris. Relative to the earth, the speed of space debris itself often exceeds 7.5 km/s, while the speed of a rifle bullet is only about 1 km/s. Space debris may collide with spacecraft, and the maximum speed can reach 15 km/s. Although a piece of space debris on the centimeter scale is insignificant, it is enough to destroy an entire satellite. Even debris on the millimeter scale can paralyze a satellite.

Impact crater on the US space shuttle

Solar panels damaged by space junk on the Russian Mir space station

03How to prevent accidents caused by space debris?

Four methods have been proposed to prevent accidental damage caused by space debris: monitoring and early warning, collision avoidance and protection, deorbit and disposal strategies, and active removal.

Monitoring and early warning

Monitoring, cataloguing and warning of collisions of space debris are the first steps to reduce the risk of collisions. Each space power has built a space monitoring network, such as the US Space Surveillance Network, which includes both space-based satellite platforms and ground-based radars and optical telescopes.

The United States’ two generations of space target monitoring “space fence”.

Left: The first generation deployed in the United States; right: The second generation deployed overseas

Collision Avoidance and Prevention

Collision avoidance and protection is to deal with the possible threat of collision with space debris by "hiding" and "preventing". When there is a high risk of collision between an in-orbit spacecraft and larger space debris (larger than 10 cm) or other objects, the simplest and most direct way to avoid it is to change orbit to leave the collision orbit. But this comes at the cost of consuming fuel and losing its working life. From its deployment in 1999 to September 2020, the International Space Station changed orbit 28 times to avoid space debris, including 3 collision avoidance orbit changes from January to September 2020.

For smaller debris, the spacecraft's own protection level can be improved to directly resist the impact of space debris.

Contrary to common sense, simply increasing the thickness of the spacecraft shell is not an efficient approach. In many cases, the multi-layer protection measures proposed by Fred Whipple in 1947 for space micrometeoroids are adopted. Whipple proposed that micrometeoroids will vaporize and ionize due to high temperature after penetrating a piece of material with a thickness comparable to its own size. Therefore, it is only necessary to wrap a layer of material about 6-7 mm thick at a distance of 1 inch outside the spacecraft to protect against the impact of space micrometeoroids. This protection measure is called Whipple protection. At present, the International Space Station and my country's Tiangong Space Station are both equipped with Whipple protection panels.

The left side shows the simulation of only thickening the warehouse wall, while the right side shows the effect of adding Whipple protective plates. Image from NASA

Deorbit and disposal

Due to atmospheric drag, low-orbit satellites will naturally lower their orbits. If a low-orbit satellite can re-enter the atmosphere and fall within 25 years after its service life ends, it does not need to be disposed of. If the orbital altitude is not too low, appropriate deorbiting measures need to be taken to lower the orbit, in accordance with the recommendation of falling within 25 years.

For geostationary orbit satellites, their orbits can be raised to "graveyard orbits" after their working life ends. The graveyard orbit can ensure that the abandoned satellites do not intersect with the normal geostationary orbit, thus ensuring the safety of the working satellites in geostationary orbit. In addition, in order to further mitigate space debris and reduce unnecessary disintegration incidents, the spacecraft or rocket body needs to exhaust its internal energy when it is abandoned, including: (1) emptying or burning the propellant; (2) draining the battery; and (3) releasing the cabin pressure.

Active removal

For the increasing number of space debris in near-Earth space, the more appropriate way to deal with it is to actively remove it. Currently, active removal is still in the theoretical research and experimental stage. Common solutions include pushing it off the orbit after attachment, pulling it off the orbit with a flexible net, and pushing it off the orbit by laser ablation.

04

For the sake of the stars and the sea, we really can't increase the amount of garbage

At present, there are more than 20,000 pieces of space debris catalogued, and there are more than 100 million pieces of debris smaller than centimeters that cannot be tracked due to observation limitations. In fact, as early as the 1970s, scientists proposed that the disintegration of satellites and other spacecraft due to collisions could trigger a chain reaction.

The number of larger debris of each type counted by the Orbital Debris Project Office at NASA's Johnson Space Center

This is known as the "Kessler syndrome." In simple terms, unless the number of large-scale waste objects is significantly reduced, the debris generated by random collisions between objects will become an important source of new debris, causing a chain reaction similar to an atomic bomb explosion, with more and more collisions. Space junk will gradually surround the Earth, making new space launches impossible.

If the amount of space debris is too much and exceeds the critical density, the "Kessler effect" will occur, leading to an avalanche-like growth, which will hinder mankind's path to the stars and the sea.

To this end, the United States, Europe, Russia, and my country are all developing active space debris removal technology. Low-cost and efficient cleaning of space debris is a technical challenge we face. I believe that with the increase in human space activities in the future, there will be a space debris recycling industry like the one in the novel "Space Kamikaze" and the anime "Starry Sky Cleaner".

Author | Zhou Binghong

Ambassador of China's Space Science Popularization, Deputy Director of the Space Science Communication Expert Studio, Researcher of the National Space Science Center of the Chinese Academy of Sciences

Audit | Liu Yong

Ambassador of China's Space Science Popularization, researcher and doctoral supervisor at the National Space Science Center of the Chinese Academy of Sciences, Doctor of Science from the University of New Hampshire, director of the China Science Writers Association, special expert of the Space Science and Technology Education Alliance

Editor | Ding Zong

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