What do the terms milli-g and microgravity mean? How are they used?

Question

I've seen the terms "milli-g" acceleration and microgravity in many questions and answers here, within the context of spaceflight.

Does milli-g refer to accelerations between 0.001 $g_0$ and 1 $g_0$, and microgravity to those between 1E-06 $g_0$ and 0.001 $g_0$, or does the usage of the terms really refer to the underlying concept or application?

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uses of milli-g:

• Is BFR refilling in space purely milli-g “gravity” feed rather than using pumps and/or pressure?
• How might an internal combustion engine generate substantial external thrust?

uses of microgravity:

• Questions tagged with microgravity

Answer 1

tl;dr: milli-G and micro-G refer to literal, specific-ish levels of acceleration. "Microgravity" is a general term for freefall which is not used synonymously with "micro-G." There is no evidence in the literature below that intention has anything to do with referring to "milli-G" or "micro-G".

The only non-opinion-based way I can think to answer is with a literature review. Doing a proper one would probably result in an actual academic paper. I am going to do a half-assed one instead.

I'm starting with a Google Scholar search, with the prompt "milli-g space".

• Multi-parameter rover wheel and grouser optimization for deployment in Phobos' milli-g environment is using "milli-g" for accelerations of some tenths of milli-Gs: per the paper, Phobos' gravity corresponds to between roughly 300 and 700 micro-G (see quote below). The term "microgravity" does not appear in the paper.

  Effective gravity on Phobos is very low and fluctuates roughly between 3 × 10-3 m/s² and 6.8 × 10-3 m/s² [17]. However, due to the probable landing site choice, gravitational acceleration of over 5 × 10-3 m/s² has to be expected [18].

• Microgravity acceleration measurement system for the International Space Station uses milli-G as some specific range as distinguished from nano-G, so likely very literally some thousandths of a G acceleration. I can't read the whole paper but here's an excerpt from the abstract:

  The MAMS instrument will provide highly accurate acceleration measurement data over the nano-g to milli-g range characterizing the Lab Module environment in the frequency spectrum from 10/sup -4/ Hz to 100 Hz.

  And an excerpt from an explainer page on Glenn Research Center's servers:

  The MAMS is a complimentary acceleration measurement system to SAMS. While the SAMS system measures acceleration disturbances from 0.01 to 400 Hz, the MAMS measures accelerations from DC to a maximum of 0.01 Hz. MAMS provides this complimentary function by measuring accelerations caused by the aerodynamic drag of the ISS as it orbits the earth. In addition, MAMS measures accelerations caused by small ISS attitude adjustments and ISS gravity gradient effects.

• Active vibration isolation of ultra-stable optical reference cavity of space optical clock also uses both micro-G and milli-G to refer specifically to a certain magnitude of acceleration:

  Inevitably, the micro-vibration level in the order of micro-g (μg) is required for ultra-stable optical reference cavity of space optical clocks. The realization of such a strict micro-vibration requirement over broadband frequencies is still technically challenging under complex space disturbances in the order of milli-g (mg). This paper thus proposes a multi-degree of freedom (DOF) active vibration isolation approach for the ultra-stable optical reference cavity to satisfy the stringent micro-vibration requirement.

  Elsewhere in the paper "microgravity" is used more generally and informally:

  The Suppression of Transient Accelerations By Levitation (STABLE) system was the first successful flight test of an active isolation device for microgravity science payloads using non-contact electromagnetic Lorentz force actuators [25]

• Inertially stabilized two-axis gimbal for space laser communication systems: design description and test results I also can't get to, but here's the quote Google gave me:

  of 1 milli-G, RMS, (simulating a worst case disturbance environment anticipated for space

  This also seems to refer specifically to literally one-thousandth of a G of acceleration.

Based on this small, somewhat random sampling (the space-related results Google Scholar gave me on the first page of the search above), "milli-g" and "micro-g" (with inconsistent capitalization of G; I prefer capital, but have not edited the quotes above) are distinct terms that refer to actual levels of acceleration, though the Phobos paper demonstrates that the "milli-G" range is probably a couple of orders of magnitude broad. Microgravity is a distinct term referring more generally to freefall / space environments; I admittedly did not search to see if there is some other distinct term for freefall environments with more-severe acceleration perturbations.

If someone else would like to extend this possibly-already-too-long answer (or finds literature that supports a different conclusion than I have come to), I am more than open to turning it into a Community Wiki. Also, sorry/you're welcome to uhoh--I don't know you super well, but I know that giving you papers can send you off on academic journeys, so I hope these are good ones.

Answer 2

Milli-g technically¹ means gravity in the 0.010-0.001 range. However, the term is almost always used for objects on orbit or undergoing very small accelerations in space and is used interchangeably with zero/micro gravity all of which are nonsensical as there is no place in the universe where the gradient of the gravitational field (which determines acceleration due to gravity) is zero. To define the strength of gravity in any meaningful way you need to know what object or collection of objects are generating the gravitational field. I.e. "the gravitational field strength of the sun is X"

TLDR technically it is defined but the definition is nonsensical and ignored so often as to make it meaningless.

For the purposes of the articles you link milli-g is used to mean very small acceleration, in both cases used to cause fuel to settle in the bottom of the tanks as opposed to floating about as it would when in free fall. This is because the liquid rockets rely on either gravity/acceleration or gas pressure to force propellant though the plumbing. You may want to look into ullage motors which is the umbrella term for any rocket engine used for the purposes of settling fuel.

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¹cf. Wikipedia's Milli- and Metric prefix

Answer 3

Milli-g: The definition of Milli-G is one 1 thousandth of a G (the force of gravity on Earth's surface). For practical pruposes, it usually invovles very small accelerations with an acceleration force in the Milli-G's. Microgravity: The literal definition is one millionth of a G. In reality this applied to objects orbiting or not experiencing any (theoretical) acceleration. In practice, there is no such thing as zero gravity as everything everywhere exerts at least a small gravitional pull on everything else. Also, acceleration is accidently caused by, say, on the space station, push your hand off the wall. Conclusion: These terms can be used interchangebly. However, as a rule milligravity is more predictable and intentional, whereas microgravity is not.