132 Aethra

132 Aethra is a metallic asteroid and Mars-crosser on an eccentric orbit from the asteroid belt. It was discovered by James Craig Watson on 13 June 1873 in Ann Arbor, Michigan, and is the first Mars-crossing asteroid to be identified. It subsequently became a lost asteroid, and was only rediscovered in December 1922. It is named after Aethra, the mother of Theseus in Greek mythology.

Aethra measures approximately 43–44 kilometers in diameter and has a rotation 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 the Tholen classification scheme. Its surface is relatively reflective and is likely composed of various silicate, hydroxide, and iron-bearing minerals.

History and naming

Aethra was discovered by astronomer James Craig Watson on 13 June 1873 at Ann Arbor, Michigan. Its discovery, alongside that of 133 Cyrene, was announced in the journal Astronomische Nachrichten on 30 August.<ref name="Watson1873"/> 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 large orbital eccentricity, and the asteroid subsequently became lost.<ref name=Herget1938/> By 1874,<ref name="Watson1874"/> the asteroid was named Aethra after the Greek mythological figure Aethra, mother of Theseus.<ref name=Schmadel2003/>Template:Rp

On 12 December 1922, Aethra was rediscovered as 1922 NA<ref name=Herget1938/> by astronomer Benjamin Jekhowsky at Algiers Observatory. It was independently observed by G. Beljavsky on 19 December at Simeiz Observatory. Calculations of its orbit strongly indicated that it was the lost asteroid, and Aethra's recovery was announced in the journal Nature on 3 February 1923.<ref name="Nature19230203"/><ref name="NYT19230203"/> In 1925, the system of provisional designation was changed to its current system. The Minor Planet Center (MPC) retroactively applied the new-style designations to those made before 1925, thus replacing 1922 NA with A922 XB.<ref name="MPC-provisional"/><ref name=jpldata/>

Orbit

File:(132) Aethra orbit.png

An orbital diagram of Aethra, with the orbits of the inner planets and the ecliptic grid shown.

Aethra orbits the Sun at an average distance—its semi-major axis—of 2.61 astronomical 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 at perihelion to 3.62 AU at aphelion, crossing the orbit of Mars.<ref name=jpldata/> It is the first Mars-crossing asteroid discovered.<ref name="Fernandez2023"/>Template:Rp Its orbital inclination is high,<ref name=Herget1938/> with a value of 24.98° with respect to the ecliptic plane.<ref name=jpldata/>

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 a Kozai resonance. This resonance couples variations in its perihelion distance with the precession of its argument of perihelion such that whenever its perihelion is at a minimum, it is located away from the ecliptic plane, distancing Aethra from Mars.<ref name="Fernandez2023"/>Template:Rp Occasionally, its perihelion is raised enough that its orbit no longer crosses Mars's.<ref name="Jeongahn2015"/>

Physical characteristics

Aethra has an estimated diameter of Template:Convert<ref name=jpldata/> or Template:Convert.<ref name="FiengaEtAl2020"/> Along with its absolute magnitude of 8.96,<ref name=jpldata/> it is the largest and brightest Mars-crossing asteroid.<ref name="Fernandez2023"/>Template:Rp Observations of Aethra's lightcurve, or variations in its apparent brightness as it rotates, suggests a rotation period of about 5.17 hours.<ref name=jpldata/>

Under the Tholen classification scheme, Aethra is classified as an M-type asteroid. The Bus classification scheme meanwhile classifies it as an Xe-type asteroid.<ref name="Hardersen2005"/>Template:Rp Aethra's geometric albedo is about 0.2,<ref name="Fernandez2023"/>Template:Rp and its spectrum is significantly red. Absorption features in its spectrum indicates the presence of phyllosilicate and hydroxide minerals and water ice, and the iron-rich chamosite may be the primary surface mineral on some regions on the asteroid.<ref name="Hardersen2011"/>Template:Rp These features are broadly consistent with the compositions of CI and CM chondrites, but albedo and spectral slope dissimilarities rule out Aethra as a parent body of these meteorites.<ref name="Hardersen2011"/>Template:Rp 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 undiscovered moon with a distinct composition.<ref name="Hardersen2011"/>Template:Rp

External links

• Template:AstDys
• Template:JPL small body