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On 2023 June 22, during an ISS spacewalk, Sergey Prokopyev and Dmitry Petelin "tossed overboard" three no longer needed devices, "off the back of the space station in a direction that ensured that the gear could never make recontact with the outpost" before burning up in the atmosphere.

At that time, the ISS was about 400 km above ground. Below it are quite a few other satellites, and quite a lot of tracked debris. What is the risk of these three new pieces of litter hitting something before burning up, thereby possibly damaging a satellite or further scattering orbital debris? Why didn't they instead retrieve the devices, to be loaded onto the next "garbage truck" whose departing trajectory could and would be controlled?

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tl;dr It's not dangerous or they don't do it.

As you might expect, this gets studied to death before it happens.

Anything planned to be jettisoned or deployed from the ISS or any ISS Visiting Vehicle must show compliance with the various safety requirements documented in ISS Partner Program Directive (PPD) 1011, the ISS Jettison Policy. The ISS Program’s analysis and approval process for jettison candidates typically takes roughly 3-6 months, depending on the complexity of the jettison candidate.

Some excerpts -

The Jettison Policy requires all candidates be trackable by the Space Surveillance Network

In addition to trackability, the policy also requires candidates demonstrate they don’t pose a significant risk of on-orbit fragmentation.

There are a number of detailed requirements in the Jettison Policy dealing with re-contact risk and how it is assessed.

Source: ISSPO Best Practices for Satellite Payload Developers

The Directive is available in its entirety here. The rest of the answer comes from there.

Why don't they retrieve the devices? The following items are candidates for jettison.

  1. Items that pose a safety issue for the ISS or for return onboard an ISS Visiting Vehicle (contamination, materials degradation, etc.).
  2. Items that negatively impact ISS utilization, return manifest or on-orbit stowage manifests.
  3. Items that represent an Extravehicular Activity (EVA) timeline savings large enough to reduce the sum of the risks of EVA exposure time and the orbital environment’s hazardous debris population, compared to the sum of such risks without a jettison.
  4. Items that are designed for jettison
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