 Bienor imaged by theHubble Space Telescope on 3 September 2005 | |
| Discovery[1] | |
|---|---|
| Discovered by | Deep Ecliptic Survey |
| Discovery site | Cerro Tololo Obs. |
| Discovery date | 27 August 2000 |
| Designations | |
| (54598) Bienor | |
| Pronunciation | /baɪˈiːnɔːr/[2] |
Named after | Bienor |
| 2000 QC243 | |
| centaur[3] · distant[1] | |
| Orbital characteristics[3] | |
| Epoch 5 May 2025 (JD 2460800.5) | |
| Uncertainty parameter 0 | |
| Observation arc | 72.28yr (26,401 d) |
| Aphelion | 19.995AU |
| Perihelion | 13.192 AU |
| 16.594 AU | |
| Eccentricity | 0.2050 |
| 67.60yr (24,690 days) | |
| 345.994° | |
| 0° 0m 52.491s / day | |
| Inclination | 20.727° |
| 337.791° | |
| 21 December 2027 | |
| 152.290° | |
| Knownsatellites | 0 |
| Saturn MOID | 4.066 AU[1] |
| Uranus MOID | 0.638 AU[1] |
| Physical characteristics | |
| Dimensions | (254±10) × (110±8) × (90±8) km[4][a] |
| |
Meandensity | 0.55–1.15 g/cm3[5]: 4157 |
| 9.1736±0.0002 h[4] | |
| 30°±3° wrtecliptic[d] | |
| 35°±8°[4] | |
| 50°±3°[4] | |
| 0.065±0.005[4] | |
| ~19[1][9] | |
| 7.47±0.04 (2016 average)[5][e][10]: 7 | |
54598 Bienor (provisional designation2000 QC243) is acentaur orbiting the Sun betweenSaturn andUranus in the outerSolar System. Named after the mythological centaurBienor, it was discovered on 27 August 2000 by theDeep Ecliptic Survey atCerro Tololo Inter-American Observatory in Chile.[1] Bienor has a highly elongated shape that spans up to 254 km (158 mi) across its longest dimension to 90 km (56 mi) across its shortest. It is one of the largest centaurs with a known size and is the fourth centaur whosestellar occultation was detected by multiple people simultaneously, after10199 Chariklo,2060 Chiron, and(95626) 2002 GZ32.[4]
Like other centaurs, Bienor is believed have originated from the region beyondNeptune in the outer Solar System, where thetrans-Neptunian objects reside. The present-day orbit of Bienor is strongly influenced by the gravity of the giant planets, which makes it unstable and subject to ejection within a few million years.[5][11] Observations have shown that the surface of Bienor is dark, colored gray, and contains some waterice, with potentially large amounts oforganic compounds.[4][12] Bienor's brightness periodically fluctuates as it rotates every 9.17 hours, although it exhibits several unusual behaviors such as gradual brightening inabsolute magnitude (intrinsic brightness), which are less easily explained.[5] The unusual behavior of Bienor's brightness alongside other anomalies such asradiometric overestimates of Bienor's diameter have led some astronomers to hypothesize that Bienor might either have an icyring system, anatural satellite, oralbedo variations across its surface.[4][10]
Bienor was discovered on 27 August 2000 by theDeep Ecliptic Survey (DES),[1] anastronomical survey led by astronomersMarc W. Buie,Susan D. Kern,Robert ("Bob") L. Millis,Lawrence H. Wasserman,Eugene I. Chiang,Jessica Lovering,James L. Elliot,Karen J. Meech,David E. Trilling,R. Mark Wagner, andKelly B. Clancy.[13] The DES operated during 1998–2005 with the goal of finding fainttrans-Neptunian objects in theecliptic region of the sky.[14][15] One of the telescopes used for the DES was the 4.0-meterVíctor M. Blanco Telescope atCerro Tololo Inter-American Observatory in Chile,[14] which made the discovery observations of Bienor.[16] Bienor was one of the few objects discovered by the DES whose orbits lay between Jupiter and Neptune.[14]: 118–119 The discovery of Bienor was announced by theMinor Planet Center (MPC) on 14 October 2000, after the object was reobserved fromLowell Observatory earlier in that month.[16] Since then, severalprecovery observations of Bienor have been identified, with the earliest set of observations coming fromPalomar Observatory'sdigitally archived photographs from 12 January 1953.[1]
The object is named after thecentaurBienor fromGreek mythology.[1] This follows the officialastronomical naming convention where centaurs in the Solar System are named after centaurs from mythology.[17]: 8 In Greek myth, Bienor was one of the centaurs who attempted to kidnapPirithous's brideHippodamia at her wedding, but was killed by the heroTheseus.[1][18]: 17 The name was suggested by Elaine K. Elliot, the wife of DES team member James ("Jim") Elliot.[18]: 17 The naming of this object was announced by theInternational Astronomical Union'sMinor Planet Center on 7 January 2004.[19]: 37
Before Bienor was officially named, it was known by itsprovisional designation2000 QC243,[1][5]: 4147 which indicates the year and half-month of the object's discovery date.[20] Bienor'sminor planet catalog number of 54598 was given by the Minor Planet Center on 16 February 2003.[21]
Bienor orbits the Sun between Saturn and Uranus[11] with an average orbital distance of 16.6 astronomical units (AU) and anorbital period of 67.6 years.[3] It follows anelliptical orbit that brings the object within13.2 AU from the Sun atperihelion to as far as20.0 AU ataphelion—slightly crossing outside the orbit of Uranus.[3] The orbit of Bienor isinclined 20.7° with respect to the Earth's orbital plane (also known as theecliptic).[3] Bienor seems to be in a 5:4mean-motion orbital resonance with Uranus.[11] If confirmed, this resonance would not last longer than a million years because the resonance is destabilized by thegravitational perturbations of other planets.[11]
Bienor is classified as acentaur, a type ofsmall Solar System body generally[f] defined as orbiting between Jupiter and Neptune and sharing features of bothasteroids andcomets.[4]: 1 [10]: 1 Centaurs are believed to have originated from the region beyond Neptune (which includes theKuiper belt andscattered disk), but weregravitationally scattered closer to the Sun due to close encounters with thegiant planets, mostly with Neptune.[5]: 4147 [10]: 1 The centaurs are still heavily influenced by the giant planets' gravitational perturbations, which makes their orbits unstable and susceptible to ejection within a few million years.[5]: 4147 [10]: 1 This suggests that the current population of centaurs, including Bienor, entered the Solar System within Neptune's orbit recently in the past few million years.[10]: 1
As of 2025[update], Bienor is located in thenorthern celestial hemisphere of the sky[g] with a currentapparent magnitude of around 19.[1][9] Its apparent magnitude depends on its distance from Earth, approaching 18 at perihelion to nearly 21 at aphelion.[23] As of 2025[update], Bienorhas been observed for over 72 years—longer than its orbital period.[1][3]
Bienor's intrinsic brightness, orabsolute magnitude, has been observed to vary on short timescales of hours due to the rotation of Bienor's elongated shape, and on long timescales of years for less clear reasons.[5] From 2001 to 2016, Bienor's rotationally averaged absolute magnitude brightened from 8.1 to 7.4—the exact cause of this unknown, though possible explanations include either a bright spot on Bienor's surface or aring system orbiting it.[5]: 4154, 4157 Both scenarios invoke a gradual appearance due to the changing viewing angle of Bienor from Earth during this time interval.[5]: 4157
While moving across the sky, Bienor may serendipitouslyoccult a background star—passing in front of the star and briefly blocking out its light from Earth.[18]: 18 Observing stellar occultations by Bienor can provide accurate measurements for its position, shape, size, and can potentially uncover rings ornatural satellites,[4]: 2 [5]: 4157 although predicting occultations requires extensiveastrometry from observational campaigns beforehand.[4]: 2 [18]: 18 The first positive detection of an occultation by Bienor was reported by a single person on 29 December 2017.[18]: 18 The first Bienor occultation that was simultaneously detected by multiple people occurred on 11 January 2019, which involved eight different telescope locations inPortugal andSpain (with four of them detecting the occultation).[10]: 2 Bienor was the fourth centaur whose stellar occultation has been positively detected by multiple people simultaneously, after10199 Chariklo,2060 Chiron, and(95626) 2002 GZ32.[10]: 1 More stellar occultations by Bienor were observed by multiple people in 2022, 2023, and 2024.[4][24]
Bienor is a highly elongated object whose shape is approximated as atriaxial ellipsoid with dimensions 254 km × 110 km × 90 km (158 mi × 68 mi × 56 mi).[4] The shortest dimension of 90 km (56 mi) is parallel to Bienor'srotation axis, whereas the two other axes lie along Bienor's equatorial plane.[4]: 8 Along Bienor's equator, the longest dimension of 254 km (158 mi) is more than twice as long as the shortest equatorial dimension of 110 km (68 mi). Astronomers were able to determine Bienor's shape by observing its silhouette from stellar occultations and by measuring periodic changes in its brightness over time.[4] Observations of a stellar occultation on 6 November 2024 suggest that Bienor may be more elongated than the aforementioned ellipsoid approximation, with a maximum length of 275 km (171 mi).[24] However, a formal analysis of the 2024 occultation has not yet been published.[24]
The size of Bienor may be represented with anarea-equivalent diameter (which is based on the amount of surface area visible from Earth), although it can vary due to rotation of its three-dimensional shape.[4]: 6 Radiometric measurements of Bienor'sthermal emission by theHerschel Space Observatory andAtacama Large Millimeter Array in 2011 and 2016 have determined an area-equivalent diameter of up to 179 to 184 km (111 to 114 mi), which would make Bienor one of the largest centaurs with measured diameters, after Chariklo, Chiron, and2002 GZ32.[18]: 18 [25] However, calculations of Bienor's area-equivalent diameter from occultations have consistently found smaller values between 130 and 170 km (81 and 106 mi),[10]: 8 even when Bienor's rotation is taken into account.[4]: 6 The discrepancy between the radiometric and occultation estimates of Bienor's area-equivalent diameter could be caused by either shape irregularities, surfacealbedo variation, or an unknown satellite of Bienor.[4]: 10
Although the mass and density of Bienor have not been measured, it could be estimated from its shape and rotation rate by assuming it is a purely self-gravitating fluid body inhydrostatic equilibrium.[4]: 2 [5] However, it is uncertain whether Bienor is actually in hydrostatic equilibrium.[10]: 8 Nevertheless, estimates for Bienor's possible density range from0.55 to 1.15 g/cm3, depending on whether Bienor's brightness is affected by some external factor like surface albedo variations or an orbiting ring system.[5]: 4157 Since the densities of small Solar System bodies are expected to scale with size, Bienor's likely density is predicted to be somewhere around0.7 g/cm3—denser than a smallcomet nucleus, but less dense than a typical trans-Neptunian object around 500 km (310 mi) in diameter.[5]: 4156–4157
Due to Bienor's elongated shape, its apparent brightness oscillates as it rotates every 9.17 hours.[4]: 2, 5 Bienor'srotation period was determined by measuring its brightness over time and plotting it as alight curve; this technique was first applied to Bienor in 2001,[26] although it was not until 2003 that astronomers recognized that Bienor's correct rotation period should be around nine hours.[27]: 1151 Over the duration of Bienor's rotation, its light curve exhibits two distinct pairs of peaks and troughs whoseminima differ by roughly 0.1 magnitudes;[10]: 11 this could be caused by either an irregular shape, surface albedo variations at Bienor's poles,[5]: 4157 or an orbiting satellite or ring.[4]: 9
Bienor rotates in theprograde direction with respect to Earth's orbit (theecliptic), with its rotational north pole pointing in the direction of theecliptic coordinates (λ,β) = (35°, +50°).[4]: 10 This translates to anaxial tilt of 30° with respect to the ecliptic.[d] The direction of Bienor's rotation pole was determined via observations of stellar occultations and long-term changes in Bienor's light curve.[4] In particular, the amount of brightness variation oramplitude of Bienor's light curve was observed to gradually decrease between 2001 and 2016, due to the changing viewing angle of Bienor's rotation from Earth.[5]: 4152 Bienor's light curve amplitude was about 0.6 magnitudes in 2001, and reached a minimum of 0.08 magnitudes in 2015—when its pole was nearly pointed towards Earth.[5]: 4152, 4156 Bienor's light curve amplitude will maximize around 2030, when Bienor's equatorial plane aligns with Earth's line of sight—this will be an edge-on configuration.[5]: 4156
Bienor is a dark object with a lowgeometric albedo of 6.5%.[4]: 2, 10 It is spectrally neutral with respect to the Sun invisible light,[4]: 2 [h] which gives it a gray color.[7]: 181 [29] Based on Bienor's color in visible light, astronomers have categorized it as a member of theBR ("blue-red") group of trans-Neptunian objects[7]: 186 [8] and the "dark-neutral" group of centaurs.[4]: 1 Since Bienor is believed to have transferred from the trans-Neptunian region to its current orbit a few million years ago, Bienor's surface is expected to be similar to those of trans-Neptunian objects.[10]: 1 Near-infraredspectroscopy by telescopes on Earth have detected waterice in the surface of Bienor.[4]: 2 Analysis of Bienor's near-infrared spectrum in 2009 suggests that 13% of its surface is composed ofcrystalline water ice with grains39 μm in size,[30]: 278 although the concentration of water ice has been observed to vary across different years, possibly due to changes in the viewing angle of Bienor from Earth.[5]: 4157 The remaining fraction of Bienor's surface composition has been tentatively suggested to be largelyorganic compounds.[12]: 413 TheJames Webb Space Telescope has observed Bienor viamid-infrared spectroscopy in 2024,[31]: 2 but was unable to find any recognizable spectral features that could be attributed to known compounds.[31]: 12
Bienor has received scientific interest from astronomers because it was suspected that Bienor may share certain features with Chariklo and Chiron—particularly their rings.[5]: 4147 [10]: 1 In 2017, a team of researchers led byEstela Fernández-Valenzuela proposed that a narrow ring of icy material encircling Bienor could explain its various peculiarities, such as its overestimated area-equivalent diameter from radiometry, long-term change in absolute magnitude with viewing angle, and variation in water ice concentration.[5]: 4156–4157 [10]: 10 Alternatively, Bienor's peculiarities could instead be explained by the existence of a natural satellite ormoon.[4]: 2, 10 [10]: 11 No signs of rings or satellites were detected in stellar occultations by Bienor in 2019–2024, although this does not rule out their possible existence around Bienor.[4]: 10 [10]: 10 [24] It is possible that a ring of Bienor could have evaded detection because it was either too narrow or too transparent; observations of the 2019 occultation suggest that any possible ring around Bienor would either have to be narrower than 1.7 km (1.1 mi) in radial width at 100% opacity, or narrower than 3.4 km (2.1 mi) at 50% opacity.[10]: 10–11 On the other hand, the properties of a hypothetical satellite around Bienor remain unconstrained by the available observational data on Bienor.[4]: 10
Thespacecraft exploration of Bienor was discussed in a 2008–2011NASAcase study on a hypotheticalradioisotope electric propulsion (REP)-powered orbiter mission to a centaur.[32]: 6 [33]: 71 Bienor was initially considered a potential exploration target for this REP centaur orbiter mission, but it ended up being replaced by32532 Thereus because Bienor was deemed incompatible with the mission's time and mass constraints.[32]: 6 [33]: 17, 71 In another centaur mission concept proposed for NASA's 2023Planetary Science Decadal Survey, dubbed theCentaur ORbiter And Lander (CORAL), Bienor was listed as a "top-interest" target alongside other centaurs like Chiron and Chariklo.[22]: 10 However, all of these were dropped in favor of the more accessible target(433873) 2015 BQ311.[22]
