If I'm not mistaken, the smallest asteroid 2015 TC25 was discovered in 2015 and is about 2 meters across. After that discovery, the division size between asteroids and meteoroids was revised from 10 m to 1 m. But I don't understand why 2015 TC25 wasn't just considered as a meteoroid in the first place? Why the need to include it as an asteroid and move the division line.
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2$\begingroup$ I don't think that there was any deliberate reclassification. Can you give any example of a source from before 2015 that says "10 m" (in a reasonably authoritative way) I think the definition is and has always been small = meteroid, big=asteroid. Perhaps "if you can see it in space from the ground" then it's an asteroid. $\endgroup$– James KCommented Feb 29 at 18:09
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1$\begingroup$ For example physics.stackexchange.com/questions/25645/… Meteoroid: A small particle from an asteroid or comet orbiting the Sun [...] There is no clear line between an asteroid and a meteoroid. Both are rocky/dusty; it's a matter of size as to how you refer to it. However, there is no clear-cut dividing line. (written 2014) $\endgroup$– James KCommented Feb 29 at 18:10
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$\begingroup$ "This is the first time we have optical, infrared and radar data on such a small asteroid, which is essentially a meteoroid. You can think of it as a meteorite floating in space that hasn't hit the atmosphere and made it to the ground — yet." Quoting the discoverer of 2015 TC25: news.arizona.edu/story/its-bird-its-plane-its-tiniest-asteroid. Since the body is in the vicinity of Earth's orbit, and near-Earth object are a thing, they refer to it as asteroid. $\endgroup$– Nilay GhoshCommented Mar 1 at 1:57
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Thus, the boundary between meteoroids and asteroids is soft and will only shrink with improved observational capabilities.
The defining characteristic of an asteroid is that it is observed in space from the ground, or from Earth orbit. Any body that is observed as a "point of light" (except moons and known artificial bodies) can be classified as an asteroid. Meteoroids are, therefore, those bodies that are too small to be observed until they enter the atmosphere and form a meteor (or are discovered as stones: meteorites)
So as observational technology improves, and especially as space telescopes are tasked with systematically finding asteroids, the boundary between meteoroids and asteroids moves.
In 1995, Beech and Steel suggested a value of 10m as the smallest size that could reasonably be observed from the ground. But 2010 Rubin and Grossman, noting that sub 10\ m objects were being discovered, proposed moving the lower boundary to 1\ m.
Offical sources don't place a specific number, they only say that meteoroids are small, natural objects that orbit the sun.
So there was no reclassification in response to 2015 TC25, but a general shifting of the boundary in response to improved observational technology.
Perhaps, if observation technology improves to the point that we can routinely observe 10cm objects as moving points of light in space, we will move the goalposts again. Or perhaps 10cm will be considered just "too small". Ultimately, it doesn't really matter much. Everyone agrees that there is no natural difference between a meteoroid and an asteroid.
Beech M. and Steel D. 1995. On the definition of the term ‘meteoroid’. Quarterly Journal of the Royal Astronomical Society 36: 281–284.
Rubin, A.E. and Grossman, J.N. (2010), Meteorite and meteoroid: New comprehensive definitions. Meteoritics & Planetary Science, 45: 114-122.
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$\begingroup$ "The defining characteristic of an asteroid is that it is observed in space from the ground, or from Earth orbit. " Ah this makes so much sense! Now I see why it's tied to observational technique. Thanks! $\endgroup$– CosmoCommented Mar 3 at 6:39
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$\begingroup$ And thank you for the references! Very helpful! $\endgroup$– CosmoCommented Mar 3 at 6:45