.png) Lightcurve-based 3D-model of Aethra | |
| Discovery[1] | |
|---|---|
| Discovered by | James C. Watson |
| Discovery site | Ann Arbor, Michigan, United States |
| Discovery date | 13 June 1873 |
| Designations | |
| (132) Aethra | |
| Pronunciation | /ˈiːθrə/[2] |
Named after | Aethra |
| A873 LA · A922 XB · 1949 MD · 1953 LF[3][a] | |
| Mars crosser | |
| Orbital characteristics[3] | |
| Epoch 21 November 2025 (JD 2461000.5) | |
| Uncertainty parameter 0 | |
| Observation arc | 152.30 yr (55628 d) |
| Aphelion | 3.6230 AU (541.99 Gm) |
| Perihelion | 1.6012 AU (239.54 Gm) |
| 2.6121 AU (390.76 Gm) | |
| Eccentricity | 0.3870 |
| 4.2217yr (1541.9d) | |
Averageorbital speed | 17.72 km/s |
| 113.0611° | |
| 0° 14m 0.24s / day | |
| Inclination | 24.9835° |
| 258.0509° | |
| 255.6188° | |
| Earth MOID | 0.7822 AU |
| Jupiter MOID | 2.2051 AU |
| TJupiter | 3.176 |
| Physical characteristics | |
| 42.87±1.6 km[3] 44.47±0.74 km[4] | |
| Mass | (1.59 ± 0.89/0.42)×1017 kg[4] |
Meandensity | 3.447 ± 1.935/0.923 g/cm3[4] |
| 5.1684 h (0.21535 d)[3] | |
| 0.1990±0.015[3] | |
| M-type (Tholen) Xe-type (Bus)[5]: 142 | |
| 8.96[3] | |
132 Aethra is a metallicasteroid andMars-crosser on an eccentric orbit from theasteroid belt. It was discovered byJames Craig Watson on 13 June 1873 inAnn Arbor, Michigan, and is the first Mars-crossing asteroid to be identified. It subsequently became alost asteroid, and was only rediscovered in December 1922. It is named afterAethra, the mother ofTheseus inGreek mythology.
Aethra measures approximately 43–44 kilometers in diameter and has arotation period slightly longer than five hours. It is the largest and brightest of the Mars-crossers and is classified as an M-type asteroid under theTholen classification scheme. Its surface is relatively reflective and is likely composed of various silicate, hydroxide, and iron-bearing minerals.
Aethra was discovered by astronomerJames Craig Watson on 13 June 1873 atAnn Arbor, Michigan. Its discovery, alongside that of133 Cyrene, was announced in the journalAstronomische Nachrichten on 30 August.[6] Watson only obtained an observation arc of 22 days, insufficient to constrain its orbit very well. The usual method for calculating the orbits of asteroids discovered by Watson were inadequate due to Aethra's largeorbital eccentricity, and the asteroid subsequently becamelost.[7] By 1874,[8] the asteroid was namedAethra after theGreek mythological figureAethra, mother ofTheseus.[9]: 25
On 12 December 1922, Aethra was rediscovered as1922 NA[7] by astronomerBenjamin Jekhowsky atAlgiers Observatory. It was independently observed by G. Beljavsky on 19 December atSimeiz Observatory. Calculations of its orbit strongly indicated that it was the lost asteroid, and Aethra's recovery was announced in the journalNature on 3 February 1923.[10][11] In 1925, the system ofprovisional designation was changed to its current system. TheMinor Planet Center (MPC) retroactively applied the new-style designations to those made before 1925, thus replacing 1922 NA with A922 XB.[12][3]
_Aethra_orbit.png)
Aethra orbits the Sun at an average distance—itssemi-major axis—of 2.61astronomical units (AU), taking 4.22 years to complete one orbit. Due to its eccentricity of 0.39, its distance from the Sun varies from 1.60 AU atperihelion to 3.62 AU ataphelion, crossing the orbit ofMars.[3] It is the first Mars-crossing asteroid discovered.[13]: 4 Itsorbital inclination is high,[7] with a value of 24.98° with respect to theecliptic plane.[3]
Despite crossing Mars's orbit, Aethra appears to be dynamically stable. A 2023 study by Julio Fernández and Michel Helal found that in simulations, its orbit remains stable for 2 billion years (Gyr). Aethra is protected from destabilizing close encounters with Mars by aKozai resonance. This resonance couples variations in its perihelion distance with the precession of itsargument of perihelion such that whenever its perihelion is at a minimum, it is located away from the ecliptic plane, distancing Aethra from Mars.[13]: 5 Occasionally, its perihelion is raised enough that its orbit no longer crosses Mars's.[14]
Aethra has an estimated diameter of 42.87 ± 1.6 kilometres (26.64 ± 0.99 mi)[3] or 44.47 ± 0.74 kilometres (27.63 ± 0.46 mi).[4] Along with itsabsolute magnitude of 8.96,[3] it is the largest and brightest Mars-crossing asteroid.[13]: 4 Observations of Aethra'slightcurve, or variations in its apparent brightness as it rotates, suggests arotation period of about 5.17 hours.[3]
Under theTholen classification scheme, Aethra is classified as anM-type asteroid. TheBus classification scheme meanwhile classifies it as anXe-type asteroid.[5]: 142 Aethra'sgeometric albedo is about 0.2,[13]: 4 and itsspectrum is significantly red. Absorption features in its spectrum indicates the presence ofphyllosilicate andhydroxide minerals and water ice, and the iron-richchamosite may be the primary surface mineral on some regions on the asteroid.[15]: 1923, 1935 These features are broadly consistent with the compositions ofCI andCM chondrites, but albedo and spectral slope dissimilarities rule out Aethra as a parent body of thesemeteorites.[15]: 1931 Variations in Aethra's spectrum seen between May and August 2008 may be caused by variable abundances of phyllosilicates or opaque surface minerals with rotation or an undiscoveredmoon with a distinct composition.[15]: 1923
