Cybele was discovered on 8 March 1861, by German astronomerWilhelm Tempel from theMarseilles Observatory in southeastern France.[2] A minor controversy arose from its naming process. Tempel had awarded the honour of naming the asteroid toCarl August von Steinheil in recognition of his achievements intelescope production. Von Steinheil elected to name it "Maximiliana" after the reigning monarchMaximilian II of Bavaria. At the time, asteroids were conventionally given classical names, and a number of astronomers protested this contemporary appellation. The nameCybele was chosen instead, referring to thePhrygian goddess of the earth.[4] (The previously discovered45 Eugenia,54 Alexandra, and64 Angelina had nevertheless also been given non-classical names; 64 Angelina had also been discovered by Tempel, but its name stood despite similar protests.)
The first Cybelian stellaroccultation was observed on 17 October 1979, in theSoviet Union. The asteroid appeared to have an irregular shape, with the longestchord being measured as 245 km, closely matching results determined by the IRAS satellite in 1983(see below). During the same 1979 occultation, a hint of a possible 11 km wideminor-planet moon at 917 km distance was detected,[26] but has since never been corroborated. As of 2017, neither theAsteroid Lightcurve Data Base norJohnston's archive considerCybele to be abinary asteroid.[23][27]
Mean diameter estimates forCybele range between 218.56 and 300.54 kilometers. According to observations by the Infrared Astronomical SatelliteIRAS in 1983, the asteroid has a diameter of 237.26 km.[22] TheNEOWISE mission of NASA'sWide-field Infrared Survey Explorer gave a diameter of 218.56 and 276.58 km.[20][21] The largest estimates of 300.54 km is from the JapaneseAkari satellite.[18] In 2004, Müller estimatedCybele using thermophysical modelling (TPM) to have dimensions of 302 × 290 × 232 km, which corresponds to a mean-diameter of 273.0 km.[19]
Observations in 2021 show that Cybele's present shape very closely matches what would be expected if it were inhydrostatic equilibrium, in contrast to the other large Cybele asteroids87 Sylvia and107 Camilla. This suggests a transition point between irregular small asteroids and equilibrium larger bodies at around 1.5 × 1019 kg, and makes it possible that outer Solar System bodies at least 260 km in diameter might have formed in equilibrium (note Saturn IXPhoebe, which at 212 km diameter probably formed in equilibrium). However, it is not certain if Cybele's current shape is its primordial one, or if it is the result of a large impact as on the very round10 Hygiea and31 Euphrosyne. Today no Cybele family is observable, but since Cybele orbits in a rather unstable region, any such family would be dispersed within 2 billion years. Cybele's larger density compared to Sylvia and Camilla does somewhat favour the first hypothesis, though. (Sylvia and Camilla experienced impacts, as evidenced by their satellites and by theSylvia family.)[28]
Examination of the asteroid's infrared spectrum shows an absorption feature that is similar to the one present in the spectrum of24 Themis. This can be explained by the presence of water ice. The asteroid may be covered in a layer of fine silicate dust mixed with small amounts of water-ice and organic solids.[29]
On August 24, 2008, Cybeleocculted 2UCAC 24389317, a 12.7-magnitude star in the constellationOphiuchus which showed a long axis of at least 294 km.[30] On 11 October 2009, Cybele occulted a 13.4-magnitude star in the constellationAquarius.[31]
^Pilcher (2014m). Rotation period for (65) Cybele:6.081±0.001 hours with a brightness amplitude of0.03±0.01 mag. Quality Code of 3−. Summary figures at theLCDB; not available as2014MPBu...41..250F at ADS
