In this answer to What do ISS astronauts do while the ISS gets reboosted? I wrote

They strap everything down first, then make videos about what it's like to be in a significant amount of microgravity when the ISS gets reboosted.

My assumption

The term "microgravity" prima facie admits that there is non-zero gravity or acceleration, and micro just indicates that it's small.

Microgravity environments admittedly have some acceleration. It's just that it's quite small compared to 1 g gravity on Earth.

When we are talking about how astronauts experience the acceleration during the altitude boost maneuver, we can say (if we want to) that they experience "a significant amount of microgravity".


Can we say that? Can we suggest that there can be an amount of microgravity, that it is smaller in some cases and larger in others? If so, does higher/lower microgravity mean more/less acceleration, or less/more? Is the state of minimum acceleration "high microgravity" or "low microgravity"?

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    $\begingroup$ I despise the term utterly. There's nothing micro about the gravity at ISS altitude; it's 89% of Earth surface gravity. It just confuses humans, who aren't used to their surroundings being in freefall. $\endgroup$ – Russell Borogove Apr 18 at 2:42
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    $\begingroup$ @RussellBorogove The term makes sense, eminent sense. There is no local experiment that can sense gravitation. This is patently obvious from the perspective of general relativity, and if one thinks about it, even from the perspective of Newtonian mechanics. All that can be sensed locally are accelerations due to non-gravitational forces. "Microgravity" addresses the magnitude of those non-gravitational accelerations. That said, the ISS is not a micro-g environment. It is a best a sub milli-g environment. $\endgroup$ – David Hammen Apr 18 at 3:42
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    $\begingroup$ "Gravity" seems like a strange term to apply to "non-gravitational accelerations". I'd have no beef with "micro-acceleration environment." $\endgroup$ – Russell Borogove Apr 18 at 5:43
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    $\begingroup$ @DavidHammen thermodynamics (for example) makes sense, eminent sense, and yet I still despise it :-) $\endgroup$ – uhoh Apr 18 at 13:33
  • $\begingroup$ Again voicing my support for the superior "free fall". $\endgroup$ – Organic Marble Apr 18 at 17:25

One can talk about "levels of microgravity". In fact, there is (or was - I see nothing recent - their last annual report was in 2001 and I can find no web site for them) a scholarly group called the International Microgravity Measurements Group. Their charter was to discuss the levels and "quality" of microgravity in the various facilities and vehicles available at the time.

Perusing their 17th annual report, it's all about the levels. Here's part of the table of contents:

enter image description here

Here's a sample table from the document

enter image description here

They don't really get into characterizing it as "high or low". It's more quantitative.

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    $\begingroup$ excellent, I didn't make a space words faux pas this time $\endgroup$ – uhoh Apr 18 at 13:35

1 micro-g is defined as a millionth g, or 0.000001 g. The atmospheric drag alone produces 3 micro-g onto the ISS. I'm defining the upper border of microgravity at 0.0005 g, so everything just below that value can be referred to as "high microgravity", e.g. Deimos' surface gravity of 0.0003 g. Phobos on the other hand has almost twice Deimos' gravity, and I define 0.0005 g to 0.007 g as "minigravity", 0.007 g is from where on humans start to perceive linear gravity properly, as per this document (page 9).

There is also a "nanogravity" experimented with on the Space Shuttle which is a billionth g. So anything between one millionth and 0.5 billionth g you can call a rather "low microgravity" and everything beneath "nanogravity", and perfect weightlessness at exactly 0 g (performed approximately by Gravity Probe B).

  • $\begingroup$ Minor nitpick: I doubt that Gravity Probe B does exact 0 g experiments. If anything its 0 g plusminus some measurement precision, which you should quote. Futhermore, the point of the probe is to measure the definitively nonzero geodesic tensor elements of spacetime around earth, each of them has the dimension of a nonzero "g". $\endgroup$ – AtmosphericPrisonEscape Apr 18 at 23:29
  • $\begingroup$ @AtmosphericPrisonEscape I now added "approximately" but you seem to have more knowledge about it so feel free to add any info into my answer. $\endgroup$ – Giovanni Apr 19 at 5:25
  • $\begingroup$ @AtmosphericPrisonEscape When viewed as a satellite with a satellite Gravity Probe B was extremely close to drag free, to less than $10^{-8}\,g$. The inner satellite was suspended in a tank of liquid helium. As that liquid helium boiled off, the boiled-off gas was used propulsively to keep the inner satellite as close to drag-free as possible. $\endgroup$ – David Hammen Apr 19 at 10:59

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