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

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?

uses of milli-g:

uses of microgravity:

• <curmudgeon mode> I dislike "microgravity", an object in orbit is affected by normal gravity, it just doesn't resist it. To me the old term "free fall" is more accurate so I try to use that. </curmudgeon mode> Mar 16, 2018 at 12:18
• I know it's because someone decided zero-gravity was wrong, but to me micro-gravity is just as wrong. Mar 16, 2018 at 12:32
• Not robust enough for an answer, but: in my personal experience, "microgravity" is the term people use for "gravity is negligible" rather than referring to any specific magnitude of force.
– Bear
Mar 16, 2018 at 12:37
• @Bear I think it's used when acceleration is very small but can not or at least should not be neglected.
– uhoh
Mar 16, 2018 at 15:12
• @Starshipisgoforlaunch that isn't a close reason in Stack Exchange. Good SE answers generally draw from authoritative sources (i.e. "look stuff up") but add context and personal knowledge as well. After sitting here for five years with all these folks knowledgable in the field without a definitive answer, that means that there is not likely a simple answer to this question that everyone will agree upon. Have you tried to "look it up" and found the same or similar answers in several places? If so, I invite you to post it as an answer. I'll even add a bounty to the question. Thanks!
– uhoh
Feb 23, 2023 at 23:56

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.

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.

1cf. Wikipedia's Milli- and Metric prefix

• Interesting! So perhaps in practice, the milli/micro distinction is more like intentional/desired vs unintentional/undesired? And loosely speaking those often fall into the magnitude ranges suggested by those prefixes?
– uhoh
Feb 24, 2023 at 1:25
• In this context definitely, In general I have found that unless someone is reporting scientific results, or the distinction makes a significant difference (e.g. milli vs micro kelvin in ultra-cold matter physics) the two terms are used interchangeably. Feb 24, 2023 at 2:50
• I really don't think this addresses the core of uhoh's question, which is whether or not "milli-g" and "microgravity" have defined meanings (i.e. are they jargon) in the context of spaceflight. Your only sources are about inferring the meaning from the prefixes, which isn't the point. Feb 24, 2023 at 2:52
• @ErinAnne as I thought I'd made pretty clear they don't. As cited prefixes in the metric system have defined meanings and nobody in the scientific community uses metric prefixes for anything other than metric because it would be way too confusing. Technically milli-g has a defined meaning but that definition is ignored so often that functionally it does not. In the same way a tonne has a defined meaning and if someone says "8.8t" you know what they mean and it is well defined but if they say "tonnes" then you just know they mean a lot. Feb 24, 2023 at 3:12

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.

• why do you conclude these terms can be used interchangeably? Who is using them as synonyms? How do you arrive at the "rule" that "milligravity is more predictable and intentional, whereas microgravity is not?" Feb 24, 2023 at 3:02
• Milligravity is caused by spacecraft or other known causes (if the sation passes near a large asteroid). Microgravity is something like hitting a wall or the gravity from another star on us. So yes, no one really usually bothers to predict that, and it definetly isnt intentional. I say they can be used incherchangebely because they are used that way (although microgravity is more common) Feb 24, 2023 at 10:01
• typo sorry has been fixed @OrganicMarble Feb 24, 2023 at 15:05
• how about some finding some citations to support your view instead of just repeating your opinion? Frankly, this is not a Space-SE quality answer. Feb 24, 2023 at 19:28
• Not only do citations have to support the arguments you're making, you have to actually add them to the answer (and hopefully call out which parts you're referring to. Maybe even use a quotation). There is no Wikipedia article for milli-G, and the article for microgravity does not use the term "milli-G." MAYBE THEY ARE NOT INTERCHANGEABLE AFTER ALL. Feb 24, 2023 at 23:42