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Digital Trends' Watch a NASA astronaut jettison part of the ISS into space which was linked in says:

Writing in Air & Space last year about the process of jettisoning objects, veteran NASA engineer Mike Engle explained how launching decommissioned parts from the space station can be a risky process, a fact that prompted him to help create an official ISS Jettison Policy to ensure that such activities are carried out safely.

“Jettisoning trash from a spacecraft is no mere stroll to a dumpster,” Engle wrote. “First and foremost, you have to make sure that whatever you throw away doesn’t come back and hit you — a frightening possibility in the weird realm of orbital mechanics.”

The engineer added, “Simple trigonometry led to the conclusion that pushing an object away at two inches per second within a 30-degree cone centered on a line directly opposite the direction that the ISS was traveling as it orbited the Earth would be enough” to send the part safely on its way.

The same speed is mentioned in the Air and Space link it cites. From Tossing Out Trash From the Space Station Takes More Planning Than You’d Think:

Our idea was to have EVA astronauts manually push jettisoned items away in the direction opposite the station’s orbit. Analysis showed that a surprisingly small retrograde change in velocity was required: only about 1 to 1.5 inches per second would ensure no recontact. The drag of the jettisoned object would be greater than that of the ISS, further ensuring that the jettisoned object would keep moving behind and below the ISS until it eventually burned up in the atmosphere. In the case of the EAS, however, we scheduled a thruster burn to raise the ISS orbit after jettison just to make sure. Safety engineers insisted that we define a jettison “cone” to account for any directional errors, so that even if an object were at the edge of the cone, it would still fly away safely. Simple trigonometry led to the conclusion that pushing an object away at two inches per second (a rate easily achievable by an EVA astronaut) within a 30-degree cone centered on a line directly opposite the direction that the ISS was traveling as it orbited the Earth would be enough.

This seems to be a lot faster than that

In the video in this International Space Station tweet it looks more like two feet per second than two inches, an order of magnitude difference. I estimate that the antenna cover moves more than it's own length in one second.

.@AstroVicGlover jettisons a science antenna cover into space since it is no longer needed. It will eventually enter Earth's atmosphere and burn up safely.

Question: What's the root cause of the disparity between what the article says and what's shown in the video?

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    $\begingroup$ I believe the 2 inches per second are the minimum required, but more than that is better, and easier to execute manually. $\endgroup$ – Speedphoenix Mar 13 at 8:48
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    $\begingroup$ Essentially any speed above 2 inches per second will do... apart from 15km/s which would be a really bad idea. $\endgroup$ – asdfex Mar 13 at 17:58
  • $\begingroup$ you don't want to push it away fast enough to unintentionally and non-trivially increase the velocity of your space station, either - although my guess is that this is not ever a concern since the mass of the rubbish compared to the space station itself is quite small. $\endgroup$ – user45266 Mar 14 at 6:10
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    $\begingroup$ @asdfex " Essentially any speed above 2 inches per second will do... apart from 15km/s which would be a really bad idea. " Why is that? Would that put the debris in an orbit opposing the ISS? $\endgroup$ – MacGuffin Mar 14 at 13:43
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    $\begingroup$ @MacGuffin Exactly. 15.6km/s to be more precise, that gives the same orbit as ISS but in the opposite direction. $\endgroup$ – asdfex Mar 14 at 15:12
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What's the root cause of the disparity between what the article says and what's shown in the video?

There is no disparity. The article says you need a minimum velocity of two inches per second [bold emphasis mine]:

The engineer added, “Simple trigonometry led to the conclusion that pushing an object away at two inches per second within a 30-degree cone centered on a line directly opposite the direction that the ISS was traveling as it orbited the Earth would be enough” to send the part safely on its way.

Analysis showed that a surprisingly small retrograde change in velocity was required: only about 1 to 1.5 inches per second would ensure no recontact.

[The word "only" can be used different ways. As I interpret the sentence, this does not mean that 1 to 1.5 inches per second is the "only possible" velocity. Rather, it is used in the sense of "a dose of only 5mg can kill you", which does not mean that a dose of 5g will not kill you.]

Simple trigonometry led to the conclusion that pushing an object away at two inches per second (a rate easily achievable by an EVA astronaut) within a 30-degree cone centered on a line directly opposite the direction that the ISS was traveling as it orbited the Earth would be enough.

The actual velocity is two feet per second, which is more than two inches per second, and thus meets the minimum velocity requirement.

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    $\begingroup$ Great answer, thank you! One thought; since we are trying to do something right rather than wrong, I think that a dose of medicine is a better analogy than a dose of radiation. And a dose of medicine can certainly have a range of doses that both below and above which undesired things can happen. $\endgroup$ – uhoh Mar 13 at 20:13
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    $\begingroup$ you can mentally replace 'only about' with 'as little as' $\endgroup$ – eps Mar 13 at 22:10
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    $\begingroup$ @eps We should not as a rule do that. As a rule, technical writing should be clear, and so if something is bounded on the lower side only we can then say 'as little as' if we want, and if it is bounded on both sides, and fairly tightly, we should say 'about'. 'only about' is the same as about; the 'only' simply remarks that the amount is small. At least that's my take on this. $\endgroup$ – uhoh Mar 14 at 10:53
  • $\begingroup$ @uhoh Agreed, technical writing should do better. But this is not technical writing, is it? At least the sections you quoted appear to be targeted at the general population, rather than a technical audience. $\endgroup$ – Brian Drake Mar 14 at 12:01
  • $\begingroup$ @BrianDrake the general public understands the difference between a one-sided limit and a two-sided limit; the difference between "must be greater than x" and "must be about x". An author writing about technical information should not think that it's okay to say something ambiguously when slightly different and equally simple wording can avoid said ambiguity. It's not okay to do something wrong when doing it right isn't any harder, no. $\endgroup$ – uhoh Mar 14 at 12:23

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