Taxonomy — Rocks in Space

Axis one

Dynamical classes

Every object gets exactly one dynamical class, assigned from its orbit. Our schema wraps the source's 34-node code set in a curated hierarchy, so that a question like "how many near-Earth asteroids?" is answered by one query over the whole branch instead of a hand-maintained list of leaf codes that goes stale.

Counts below are hierarchy-aware: a group row includes everything beneath it.

Class	Source code	Objects (including sub-classes)
Small body		1,557,369
› Asteroid		1,553,300
› Main-belt asteroid		1,457,561
› Middle main-belt	`MBA`	1,374,787
› Outer main-belt	`OMB`	49,943
› Inner main-belt	`IMB`	32,831
› Near-Earth asteroid		42,062
› Apollo	`APO`	23,933
› Amor	`AMO`	14,666
› Aten	`ATE`	3,425
› Atira (Interior-Earth Object)	`IEO`	38
› Mars-crosser		29,739
› Mars-crossing asteroid	`MCA`	29,739
› Trojan		16,356
› Jupiter Trojan	`TJN`	16,356
› Trans-Neptunian object		6,387
› Trans-Neptunian object	`TNO`	6,387
› Centaur		1,038
› Centaur	`CEN`	1,038
› Unclassified asteroid	`AST`	152
› Unbound asteroid		5
› Hyperbolic asteroid	`HYA`	5
› Comet		4,069
› Long-period comet		3,011
› Parabolic comet	`PAR`	1,763
› Long-period comet	`COM`	730
› Hyperbolic comet	`HYP`	518
› Short-period comet		1,058
› Jupiter-family comet		846
› Jupiter-family comet (Tisserand-defined)	`JFc`	829
› Jupiter-family comet (period-defined)	`JFC`	17
› Halley-type comet	`HTC`	111
› Encke-type comet	`ETc`	79
› Chiron-type comet	`CTc`	22

A boundary worth knowing about

"Near-Earth object" is not one definition. The IAU's geometric version is perihelion inside 1.3 AU, which 42,062 asteroids satisfy. JPL's operational flag is narrower for comets — it takes in short-period ones only — so a long-period comet that comes well inside 1.3 AU is not flagged. The schema here carries both and documents the difference rather than picking one silently.

Axis two

Spectral classes — and two incompatible grammars

Reflectance spectroscopy sorts surfaces into classes. Two schemes are in common use, and the trap is that multiple letters mean opposite things in each.

In the Tholen system, extra letters mean ambiguity: CX says "C, or else X, with C fitting better", ordered by goodness of fit. A trailing U means the spectrum is unusual, and colons flag noisy data — so FBCU:: reads as "F, else B, else C; unusual; very noisy".

In the Bus/SMASS II system, extra letters name a subclass: Ch is a single class in its own right — C-like with a 0.7 µm absorption band from hydrated silicates, direct evidence that liquid water once altered the rock. It is not "C or h".

Parse one with the other's rule and you invent classifications that nobody measured. Our ingest runs a separate parser for each, preserves the raw string, and keeps every ambiguous alternative as its own ranked row rather than collapsing it to a single answer the spectroscopy does not support.

Objects with a Bus classification, grouped by complex. Note the vertical scale: this is 2,173 objects out of 1,557,369. Surface composition is the single most under-measured thing about the solar system's small bodies.

The X-complex admits what it cannot measure

Tholen's E, M and P classes are spectrally indistinguishable — all featureless and reddish. What separates them is albedo: E-types are very bright, M-types moderate and metallic, P-types very dark. When the albedo is unknown, the object is called X. It is a rare case of a classification scheme carrying its own uncertainty in the label.

Albedos of the 138,346 asteroids with a measured value between 0.005 and 0.8, which is all but 134 of them. The distribution is **bimodal**, not continuous: a dark peak near 0.05 and a bright one near 0.20. That is composition showing through a single number — carbon-rich surfaces reflect a twentieth of the light that falls on them, stony ones a fifth. Objects whose spectra are degenerate can often be separated by albedo alone.

Bus / SMASS II

26 classes in three complexes plus the end members. Group rows in gold.

Class	Sits under	What it means	Objects
C-complex	—	Dark primitive surfaces. Bus/SMASS II resolves the Tholen C group into six classes, several distinguished by a 0.7 um hydrated-silicate band	0
S-complex	—	Stony silicate surfaces, resolved into a core S class plus five transitional classes bridging toward A, K, L, Q and R	0
X-complex	—	Moderately reddish featureless spectra, resolved into a core X plus three classes with subtle features	0
end-member	—	Distinctive mineralogies outside the three complexes	0
`A`	end-member	Very red shortward of 0.75 um with a deep 1 um olivine band	17
`B`	C-complex	Negative (blue) spectral slope	66
`C`	C-complex	Neutral and flat longward of 0.55 um	155
`Cb`	C	Transitional between C and B	37
`Cg`	C	C-like with a strong UV dropoff	9
`Cgh`	C	Combines the Cg UV dropoff with the 0.7 um hydration band	15
`Ch`	C	C-like with the 0.7 um absorption band of hydrated silicates - direct evidence that liquid water once altered the rock. One of the most common C-complex classes	139
`D`	end-member	Very red and featureless; Trojans and comet-like surfaces	13
`K`	end-member	Between S and X with a weak 1 um feature. Characteristic of the Eos family	38
`L`	end-member	Steeply reddish shortward of 0.75 um, then flat	41
`Ld`	L	L-like but with an even steeper red slope	15
`O`	end-member	Unusual deep 1 um absorption. 3628 Boznemcova is the sole archetype	7
`Q`	end-member	Strong 1 um band from olivine plus pyroxene - an unweathered, freshly exposed surface	20
`R`	end-member	Deep 1 um and 2 um bands	5
`S`	S-complex	Reddish shortward of 0.7 um with a moderate 1 um band	462
`Sa`	S	Transitional from S toward A	38
`Sk`	S	Transitional from S toward K	29
`Sl`	S	Transitional from S toward L	56
`Sq`	S	Transitional from S toward Q, indicating a fresher surface	116
`Sr`	S	Transitional from S toward R	27
`T`	end-member	Moderately red, flat, and dark	19
`V`	end-member	Deep 1 um pyroxene band: basaltic crust, from Vesta and the vestoids	49
`X`	X-complex	Featureless with a moderate reddish slope	144
`Xc`	X	X-like, curving toward C	67
`Xe`	X	X-like with a 0.49 um absorption feature, the Bus counterpart of Tholen E	30
`Xk`	X	X-like, curving toward K	47

Tholen (ECAS)

The older scheme, based on eight-colour photometry. Still the only classification available for many objects.

Class	Sits under	What it means	Objects
C-complex	—	Dark, primitive, carbon- and volatile-rich surfaces. Dominates the outer main belt and is the likely parent of the carbonaceous chondrites	0
S-complex	—	Stony, moderately bright silicate surfaces rich in olivine and pyroxene; the likely parent of the ordinary chondrites. Dominates the inner main belt	0
X-complex	—	Featureless reddish spectra that cannot be told apart spectroscopically. E, M and P are separated ONLY by albedo; X is used when the albedo is unknown - a rare case of a taxonomy admitting what it cannot measure	0
end-member	—	Distinctive mineralogies that sit outside the three big complexes	0
`A`	end-member	Olivine-rich: very red shortward of 0.7 um with a deep 1 um band	7
`B`	C-complex	Like C but with a slightly blue (negative) slope and somewhat higher albedo. 2 Pallas is the archetype	32
`C`	C-complex	Carbonaceous: albedo near 0.05, flat to slightly reddish spectrum with a UV dropoff	305
`D`	end-member	Very red and dark, organic-rich. Dominates the Jupiter Trojans and resembles cometary nuclei	84
`E`	X-complex	Enstatite: very high albedo above 0.3, featureless. 44 Nysa is the archetype	13
`F`	C-complex	Flat and featureless, lacking the UV absorption that C-types show	78
`G`	C-complex	C-like with a strong UV absorption below 0.4 um. 1 Ceres is the archetype	31
`I`	end-member	Not a mineralogical class: the ECAS photometry for this object was internally inconsistent	3
`M`	X-complex	Metallic: moderate albedo, featureless reddish, interpreted as iron-nickel cores of disrupted parent bodies. 16 Psyche is the archetype	43
`P`	X-complex	Pseudo-M / primitive: very low albedo below 0.07, featureless reddish. Common among the Hildas and Trojans	66
`Q`	end-member	Broad 1 um absorption from olivine plus pyroxene, indicating an unweathered surface. Common among near-Earth asteroids	6
`R`	end-member	Deep 1 um and 2 um pyroxene bands. 349 Dembowska is the archetype	4
`S`	S-complex	Silicaceous: albedo 0.15-0.25, reddish slope with a 1 um mafic absorption band	382
`T`	end-member	Moderately red and dark, intermediate between D and X	29
`V`	end-member	Basaltic, with a strong 1 um pyroxene band. 4 Vesta and its family; the parent of the HED meteorites	5
`X`	X-complex	Featureless and moderately reddish, with no albedo measurement available to break the E/M/P degeneracy	151

Nothing is dropped silently

Symbols the parsers produce but our curated hierarchy does not define become flagged dictionary rows, and raw values no grammar accepts are recorded separately. In the current release that is 2 symbols and one unparsed value — small, known, and visible, rather than quietly missing. The predecessor project's hardest-won lesson was that silently skipping unknown types is invisible data loss.

Research & development — not peer-reviewed. The catalog, analysis, and software behind this site were developed in collaboration with AI and have not been validated by the scientific community. No claims are made as to scientific validity. Source measurements are credited to NASA/JPL and the Minor Planet Center; the interpretation is ours.