The body's observation arc begins 9 days after its official discovery observation at Bergedorf Observatory in Hamburg.[3] On 8 June 1949, Sirene passed 0.049 AU (7,300,000 km; 4,600,000 mi) from Mars.
Aethra group and mass of Mars
Sirene had been of great interest in 1930s.[5] After its discovery, it was observed at Algiers, Bergedorf, Heidelberg, Milan, Vienna and Williams Bay in the United States, where it became too faint to be accurately observed. It was regarded as an excellent example of a member of the then called "Aethra group" – 132 Aethra was the first discovered Mars-crossing asteroid – highly appropriate to measure the exact mass of Mars on its next opposition near perihelion in 1937.[5] However, after its last observation at Williams Bay in 1924, Sirene had only been observed once at Turku Observatory in 1940, and remained unobserved until the 1980s.[3]
Physical characteristics
Rotation period
As of 2017, no rotational lightcurve of Sirene has been obtained from photometric observations. The asteroid's rotation period, shape and poles remain unknown.[1][6]
Diameter and albedo
Sirene has not been observed by any large-scale, space-based surveys such as IRAS, NEOWISE and Akari.[1][6] Based on a generic magnitude-to-diameter conversion, Sirene measures 5 kilometers in diameter for an absolute magnitude of 13.9 and an assumed stony albedo of 0.20.[4] For an assumed albedo of 0.10 and 0.05, its calculated diameter would increase to 7.1 and 10 kilometers, respectively.[4]
