Hello,
Hmm, the meteoroid question still appears to be quite open, so I did some further research and here’s what I found:
Reference sources:
Ref A: http://neo.jpl.nasa.gov/faq/#diff
Ref B: http://en.wikipedia.org/wiki/Meteoroid
Ref C: On the Definition of the Term Meteoroid (from Ref B)
Definitions:
Ref A: NASA
A small particle from a comet or asteroid orbiting the Sun.
Ref B: IAU
A solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom or molecule.
Ref C:
“…we wish to point out here that the term `meteoroid', as presently defined by the IAU, is out-dated and requires some refinement”.
"It is argued that the term `meteoroid' is best defined as being applicable to any natural solid object moving in space, and having a size between 100 µm and 10 m (10-4 ~ 10+1 m) This definition reflects modern-day instrumentation limitations and recognizes the physical constraints that dictate the production of a meteor in the Earth's atmosphere. Our argument also has implications for the definitions of the terms `asteroid' (synonym `minor planet'), `micrometeorite', and `interplanetary dust.'"
Historical Note from Ref C:
"The term `meteoroid' was first coined by H.A.Newton in 1864 (Newton 1865). Newton explained that:
It will be assumed that the phenomenon called a shooting or falling star is caused by a small body (probably solid) which originally was moving in its own orbit in the solar system, or in space; that this body coming into the atmosphere of the earth elicits light by the loss of velocity, and is usually dissipated before reaching the earth's surface. The term meteoroid will be used to designate such a body before it enters the earth's atmosphere".
"Not quite a century after Newton published the above — remarkably insightful — description, the IAU adopted the following definition for the term `meteoroid' (Millman 1961, Sadler 1962):
Meteoroid — a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom or molecule".
Ref C seems to discuss meteoroids in terms of asteroids and (arguably) comets, here is an example:
“The upper size limit of a meteoroid may reasonably be set according to the minimum detectable size of an asteroid. The lower asteroid size limit is in turn determined by the present-day observational limitations”.
“In light of the above discussion it does not seem unreasonable to adopt 10 m as the dividing line for an object to be considered to be either an asteroid or a meteoroid. We therefore propose that any natural object in space, which is solid and neither cometary in nature or planetary in size, but larger than ten metres, be defined as being an `asteroid', `minor planet' being a synonym; any object smaller than 10 m (which might indeed be cometary in nature, although volatiles are rapidly lost by such small objects), but larger than the proposed 100 µm limit discussed below, would be defined as being a `meteoroid'”.
This previous paragraph appears to discuss meteoroids in the context of asteroids and objects of a cometary nature. Perhaps this is where the folks at NASA derived their definition?
As Alice said, “Curiouser and curiouser”
Cheers
Dennis