5 Astraea
5 Astraea is a large main-belt asteroid classified as S-type, notable for being the fifth asteroid discovered and one of the brightest objects in the asteroid belt due to its high reflectivity. Discovered on 8 December 1845 by German amateur astronomer and postmaster Karl Hencke after a systematic search lasting over 15 years, it was named after the Greek goddess of justice, Astraea.[1] With a mean diameter of 115 ± 6 km determined from a stellar occultation event, it exhibits a geometric albedo of 0.227, consistent with its silicaceous composition rich in silicates and metals. Astraea orbits the Sun at a semi-major axis of 2.58 AU, with a moderate eccentricity of 0.19 that carries it from 2.09 AU at perihelion to 3.06 AU at aphelion, and an orbital inclination of 5.4° relative to the ecliptic plane, yielding a sidereal orbital period of 4.13 years.[2] The asteroid rotates rapidly with a sidereal period of 16.8 hours, producing lightcurve variations of about 0.2 magnitudes, indicative of an elongated shape. Its S-type spectral classification, based on reflectance spectra showing absorption features from olivine and pyroxene, places it among the most common asteroid types, suggesting origins in the inner main belt. As the namesake parent body of the Astraea asteroid family, comprising approximately 6,000 members identified through dynamical clustering, 5 Astraea is believed to have fragmented from a major collisional event approximately 400 million years ago, contributing to the population of S-type fragments in the inner main belt.[3] Observations, including radar imaging and Hubble Space Telescope imaging in 2001, have revealed a potato-like irregular shape without prominent craters, and its high albedo has made it a target for photometric and polarimetric studies to understand surface regolith properties.[4] No dedicated spacecraft missions have visited Astraea, but its brightness (apparent magnitude reaching 8.7 at opposition) allows frequent ground-based observations for refining orbital parameters and family dynamics.Discovery and Naming
Discovery
5 Astraea was discovered on December 8, 1845, by the German amateur astronomer Karl Ludwig Hencke from his private observatory in Driesen, Brandenburg (now Drezdenko, Poland).[1] Hencke, a former postmaster, had begun a systematic visual search for additional asteroids in 1830 using a small refractor telescope, persisting despite the prevailing belief that no more such bodies existed after the four known at the time. His methodical comparisons of the night sky against star charts finally yielded this result after 15 years of effort.[5] This discovery ended a 38-year hiatus since the finding of 4 Vesta by Heinrich Olbers on March 29, 1807, during which professional astronomers had largely abandoned the hunt due to inadequate star charts and the assumption that the known objects represented a complete group.[1] Initially classified as planets like its predecessors, the rapid increase in asteroid discoveries following Hencke's find—reaching 15 by 1851 and 100 by 1868—prompted their reclassification as minor planets in astronomical catalogs starting in 1868.[6] Hencke's observation was quickly confirmed by independent astronomers at observatories including Berlin, leading to the computation of its orbit and official designation as the fifth asteroid, (5) Astraea.[7] Post-discovery analysis placed it firmly in the main asteroid belt between Mars and Jupiter.[1]Naming and Symbolism
5 Astraea derives its name from Astraea (Ancient Greek: Ἀστραία, romanized: Astraía, meaning "star-maiden"), the virgin goddess of justice, innocence, purity, and precision in Greek mythology, who was the last immortal to live among humans during the Golden Age before fleeing to the heavens amid increasing wickedness, with prophecies foretelling her return to usher in a new era of virtue.[8] The name was proposed by its discoverer, the German amateur astronomer Karl Ludwig Hencke, shortly after the object's identification on December 8, 1845, and was promptly accepted by the international astronomical community.[9] It adheres to the established convention of the era for naming asteroids after figures from classical mythology, as seen with predecessors like 1 Ceres (the Roman goddess of agriculture) and 2 Pallas (the Greek goddess of wisdom).[10] Hencke also devised the original astronomical symbol for 5 Astraea: an inverted anchor (, Unicode U+1F778), intended to evoke the scales of justice held by the goddess, though some later representations stylized it as a pair of balances (⚖, Unicode U+2696).[11] This glyph appeared in early 19th-century discovery announcements, almanacs like the Berliner Astronomisches Jahrbuch, and nautical ephemerides such as the Nautical Almanac, persisting in some publications into the mid-20th century before numbered designations supplanted symbolic notation for minor planets.[11] In contemporary astrological usage, 5 Astraea is denoted by a stylized percent sign (⯙, Unicode U+2BD9), a simple adaptation reflecting its position as the fifth discovered asteroid and evoking the shift-5 key on keyboards.[11]Orbital Properties
Orbital Elements
The orbit of 5 Astraea is characterized by the following Keplerian orbital elements, as determined for the epoch October 16, 2024 (JD 2460600.5): a semi-major axis of 2.577 AU, an eccentricity of 0.1874, an inclination of 5.36° relative to the ecliptic, a longitude of the ascending node of 141.47°, an argument of perihelion of 340°, and a mean anomaly of 180° (approximate).[12] These parameters define an elliptical path that classifies 5 Astraea as a main-belt asteroid.[12] From these elements, the orbital period is calculated to be 4.14 years, or 1,512 days, with a perihelion distance of 2.094 AU and an aphelion distance of 3.060 AU.[12] The mean motion is 0.238° per day.[12] An extensive observation arc spanning 179 years enables the precise determination of these elements, minimizing uncertainties in the orbital solution.[12]| Parameter | Value | Unit |
|---|---|---|
| Semi-major axis (a) | 2.577 | AU |
| Eccentricity (e) | 0.1874 | - |
| Inclination (i) | 5.36 | ° |
| Longitude of ascending node (Ω) | 141.47 | ° |
| Argument of perihelion (ω) | 340 | ° |
| Mean anomaly (M) | 180 | ° |
| Orbital period (P) | 4.14 (1,512) | years (days) |
| Perihelion distance (q) | 2.094 | AU |
| Aphelion distance (Q) | 3.060 | AU |
| Observation arc | 179 | years |
| Mean motion (n) | 0.238 | °/day |