Why space debris is so dangerous for the ISS

The International Space Station had to maneuver to avoid debris caused by the test of a Russian anti-satellite. The event once again reminds us that space debris represents a significant risk for the ISS.

The International Space Station had to change its course, NASA announced on November 15, 2021. Because of space debris, the ISS astronauts took refuge in their ships, so that they could evacuate in the event of a big problem. In a communiqué from NASA, the United States questioned ” debris generated by the destructive test of a Russian anti-satellite (ASAT) ».

This event underlines, once again, that space debris represents an increasingly frequent risk for the crews of the ISS. Last year, the International Space Station had to perform several debris avoidance maneuvers – shifting the station’s orbit relative to the debris path. Since 1999, the ISS has carried out 29 debris avoidance maneuvers, including three in 2020.

What would happen if space debris collided with the ISS?

The term “space debris” includes both natural and man-made debris.

Collisions with debris micrométéoroïdes (a particle smaller than a grain of sand, orbiting the Sun) or orbital debris (made by humans, orbiting the Earth) are one of the main vulnerabilities of the ISS: they could ” injuring a crew member during a spacewalk or causing injury as a result of damage or entry into any of the ISS modules, windows, external replacement orbital units or power lines and fluids », Can we read in a report of NASA from 2007, detailing the weaknesses of the ISS that could compromise the crew and the cockpit.

At the time, the authors of this report believed that “ the risk of micrometeoroid or orbital debris entering the ISS in its fully assembled configuration is 55% with a 9% risk of a catastrophic outcome over a 10 year period. But that was almost 15 years ago: since then, the number of debris has greatly increased.

The ISS has carried out 29 debris avoidance maneuvers since 1999. // Source: Flickr / CC / Roscosmos (cropped photo)

How much debris is there at “low” altitude?

The station operates in low earth orbit, at an altitude of 400 kilometers and a speed of 28,800 kilometers per hour (so that it only takes 92 minutes to complete a complete circle of our planet). In this orbit, also circulates a lot of space debris of all kinds (pieces of rockets, out-of-service satellites, remains of explosive devices).

As of November 9, 2021, the European Space Agency estimated that approximately 29,600 object debris were closely monitored by the Space Surveillance Network. ESA also estimates the number of ” ruptures, explosions, collisions or abnormal events leading to fragmentation To over 630.

According to statistical models, the number of space debris in orbit is estimated at:

  • 36,500 objects over 10 centimeters,
  • 1,000,000 objects larger than 1 centimeter, up to 10 centimeters,
  • And 330 million objects larger than 1 millimeter, up to 1 centimeter.

Of course, the Earth’s atmosphere gradually reduces the energy of the satellites placed in orbit, which then brings them back to our planet and allows them to be destroyed. But it takes time: you have to count under 25 for satellites flying at low altitude (below 500 kilometers). For satellites moving farther away, tens of thousands of kilometers, this duration is of the order of thousands of years.

The “pizza box” and avoidance maneuvers

Guidelines have obviously been put in place to assess the risk represented by the passage of debris near the ISS. An imaginary box (nicknamed ” pizza box “Because of its flattened shape), which is about 4 kilometers deep, 50 kilometers wide and long, with the ISS at its center, serves as a delimitation: if an object passes near this space, it is necessary anticipate what to do.

The “pizza box” drawn around the ISS. // Source: Nasa screenshot, p. 145

When the encounter can be well anticipated, an avoidance maneuver is most often carried out (which ensures that the debris will pass outside the famous box). Avoidance maneuvers take approximately 5 hours and are carried out using the Russian thrusters of the ISS, or the propulsion systems of the vessels which are moored at the station. When the tracking of the debris is not precise enough, it may be decided to relocate the crew in the Russian and American vessels, so that they can be evacuated if the collision damages the ISS.

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Source: Numerama by www.numerama.com.

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