Ground teams track the junk and decide when the station must move
Space junk, also called orbital debris, means human-made objects in orbit that no longer do anything useful. Because they move at orbital speed, even small ones can do real damage. Space.com's report points out that even paint flecks can cause trouble at those velocities [1].
Ground radar and telescopes follow the larger pieces. In the ISS's orbit, the pieces that can be tracked are roughly 5 centimeters (2 inches) across [1]. Warnings, called conjunction messages, reach the station's controllers from the US Space Force [2].
Controllers do not react to every close pass. NASA draws an imaginary safety zone around the station that space engineers call a "pizza box". It is a flat box about 50 kilometers long, 50 kilometers wide, and 4 kilometers tall (roughly 30 by 30 by 2.5 miles), with the station in the middle [1] [2]. When a tracked object is predicted to pass through the box, that starts a closer check, and teams keep working until they know whether the risk is real [2].
The bar for acting is low because people live on board. A space-junk modeler quoted by Live Science said the team typically looks at events where the chance of a collision is greater than about 1 in 10,000 [2].
When a piece of junk comes close, the station fires thrusters to change its orbit
If the check says the risk is real, and the tracking data is good enough to trust, mission control centers in Houston and Moscow work together to choose what to do [1]. The usual answer is a debris avoidance maneuver. The station, or a docked spacecraft pushing on it, fires thrusters to change the orbit slightly. Live Science describes the case on November 19, 2024, when the station raised its orbit to avoid a fragment from a satellite that broke up in 2015. NASA officials said the fragment could otherwise have passed within about 4 kilometers (2.5 miles) [2].
How often has this happened? NASA records, as reported by Live Science, count that 2024 maneuver as the 39th since the first piece of the station was launched in November 1998. There were five in 2023, and at least two a year from 2020 to 2022 [2]. An earlier NASA report, from December 2022, gave 32 since 1999 [1]. The two counts use different starting years, so they are not a direct year-by-year comparison.
A maneuver takes planning, so the warning has to come early enough. That raises the next question: what if it does not?
If there is no time to move, the crew waits in a docked spacecraft
When notice comes too late for a maneuver, there is a standard backup. The crew goes into a docked spacecraft, which works like a lifeboat that is already attached. On March 24, 2012, the European Space Agency (ESA) reported that a piece of junk was predicted to pass the station at about 23 kilometers (14 miles). The warning had come too late to boost the station's orbit. The six crew members woke an hour early, entered their two Soyuz spacecraft, and waited until flight controllers in Houston gave the all-clear [3]. ESA called this a standard precaution.
The idea behind it is simple. If a fragment did hit the station, the damage could affect a module. A docked spacecraft gives the crew a separate place to shelter while controllers wait for the danger to pass.
In November 2021, a missile test created a cloud of fragments and the crew sheltered again
On November 15, 2021, Russia fired a missile from the ground to destroy one of its own old satellites, Cosmos 1408. This kind of test is called an anti-satellite (ASAT) test: a test that uses a weapon to destroy a satellite. The satellite had been in orbit since 1982, and the blast left at least 1,500 pieces of trackable debris [6].
NASA's statement that day said controllers had been told of a satellite breakup that could produce enough debris to threaten the station. The crew was woken and told to close the hatches to the modules on the station's radial ports, including Europe's Columbus and Japan's Kibo. They also took the extra step of sheltering in their spacecraft for two passes through or near the debris cloud. They went in shortly before 2 a.m. US Eastern time and came out about 4 a.m. [4].
The station passes through or near the cloud again and again as it circles Earth. NASA said the need to shelter for only the second and third passes came from a risk assessment by its debris office and specialists at Johnson Space Center in Houston [4]. A follow-up post said the largest risk was in the first 24 hours, that data from the station showed no problems, and that the crew and station were safe and back to normal operations [5].
The NASA administrator at the time, Bill Nelson, called the test "reckless and dangerous" [4]. Notice what the episode shows: controllers were warned, the risk was assessed pass by pass, and the crew sheltered in their spacecraft.
Pieces too small to track are stopped by a wall made of layers
Tracking has a limit. The ground cannot follow every speck, and a small piece cannot be dodged if nobody knows it is coming. For those, the station relies on its walls.
One basic design is the Whipple shield, which NASA's Johnson Space Center lists among its shield concepts [7]. A simple way to picture it is a thin outer sheet standing a gap away from the main wall. A speck hits the thin sheet first and breaks up into a spreading cloud of smaller bits. By the time the cloud reaches the main wall, its force is spread over a wider area, so the wall can take it. This is a simplified explanation of how the shield works.
Layering is the trick. One thick plate has to absorb everything at one spot, while a thin plate plus a gap spreads the blow out before it matters.
After the 2021 test, countries began promising not to do it again
The test drew strong criticism around the world. In April 2022, the United States became the first nation to commit to a self-imposed moratorium on destructive anti-satellite missile tests. Australia, Canada, Germany, Japan, New Zealand, South Korea, Switzerland, and the United Kingdom have since followed [6].
The idea also moved to the United Nations. On November 1, 2022, the UN General Assembly's First Committee, which handles disarmament and international security, approved a resolution calling on countries to avoid such tests. The vote was 154 to 8, with 10 abstentions. The resolution is not legally binding, and several countries, including Russia and China, voted against it [6]. I did not read the later vote of the full General Assembly, so I am not quoting it.
How to look up a station maneuver yourself
Search the NASA space station blog or a space news site for "debris avoidance maneuver". You will find when the station last changed course, and often what the fragment was. Try the 2024 example above and ask yourself: why would a fragment from a 2015 breakup still be a danger nearly ten years later?
Some questions are still open. How should the growing number of satellites be handled? Who should warn the station if the US warning service changes? Those are good ones to follow up on your own.