6 Hebe

6 Hebe (Template:IPAc-en) is a large main-belt asteroid, containing around 0.5% of the mass of the belt. However, due to its apparently high bulk density (greater than that of the Moon), Hebe does not rank among the top twenty asteroids by volume. This high bulk density suggests an extremely solid body that has not been impacted by collisions, which is not typical of asteroids of its size – they tend to be loosely-bound rubble piles.

In brightness, Hebe is the fifth-brightest object in the asteroid belt after Vesta, Ceres, Iris, and Pallas. It has a mean opposition magnitude of +8.3, about equal to the mean brightness of Saturn's moon Titan,<ref>The Brightest Asteroids Template:Webarchive</ref> and can reach +7.5 at an opposition near perihelion.

Hebe may be the parent body of the H chondrite meteorites, which account for about 40% of all meteorites striking Earth.

History

Hebe was discovered on 1 July 1847 by German astronomer Karl Ludwig Hencke in the town of Driesen, Brandenburg, Prussia (now Drezdenko, Poland).<ref name=jpldata/> It is the sixth asteroid discovered. It was the second and final asteroid discovery by Hencke, after 5 Astraea. The name Hebe, after the Greek goddess of youth, was proposed by Carl Friedrich Gauss at Hencke's request.<ref name=wudfagw-1947/> The first asteroids discovered had widely been considered planets by astronomers, but the rapid discoveries of several new asteroids in the late 1840s complicated the classification of asteroids.<ref name="minorplanets"/> In the years following its discovery, Hebe was variously labelled as a planet,<ref name=wudfagw-1947/> small planet, or asteroid.<ref name="Gould1848"/> Eventually, throughout the latter half of the 19th century, the terms "asteroid" and "minor planet" became favored,<ref name="minorplanets"/> although some astronomers continued referring to Hebe as a planet in this period.<ref name="Luther1889"/>

Gauss chose a wineglass as Hebe's astronomical symbol.<ref name=wudfagw-1947/><ref name=steger/> It was encoded in Unicode 17.0 as U+1CEC0 𜻀 (File:Hebe symbol (fixed width).svg).<ref name=astunicode/><ref name="Unicode-1CEC0">Template:Cite web</ref> As asteroids and their symbols grew in number, the practicality of assigning unique astronomical symbols to each asteroid was questioned. In 1851, astronomer Johann Franz Encke proposed a simpler system of a numberTemplate:Mdashdenoting the order of discoveryTemplate:Mdashinscribed in a circle. For Hebe, this would be ⑥.<ref name="minorplanets"/><ref name="Gould1852"/>Template:Rp This system was widely adopted by astronomers, though astronomers eventually switched to using parentheses enclosing the numberTemplate:Mdashthus (6) Hebe<ref name="MPC-object"/> or 6 Hebe<ref name=jpldata/> in modern notation.<ref name="minorplanets"/>

On 5 March 1977, Hebe occulted the star γ Ceti A (Kaffaljidhma).<ref name=Dunham1977/>

Orbit

The orbit of 6 Hebe compared with the orbits of the inner planets and Jupiter

Hebe orbits the Sun with an average distance (or semi-major axis) of 2.426 astronomical units (AU),<ref name=jpldata/> placing it in the inner section of the main asteroid belt.<ref name="Gaffey1998"/>Template:Rp Its distance to the Sun varies from 1.935 AU at perihelion to 2.917 AU at aphelion due to its moderately elliptical orbit, indicated by its orbital eccentricity of 0.202. It has an orbital period of 3.778 Earth years, following an orbit inclined by 14.74° with respect to the ecliptic plane.<ref name=jpldata/>

Hebe resides near but does not participate in several orbital resonances. It orbits close to the 3:1 mean-motion resonance (MMR) with Jupiter at approximately 2.50 AU.<ref name="Gaffey1998"/>Template:Rp Asteroids caught in the 3:1 Jovian MMR have orbital periods one-third that of Jupiter's; their orbits are destabilized and they are eventually removed by encounters with the planets, creating the 3:1 Kirkwood gap.<ref name="Yoshikawa1990"/>Template:Rp Hebe is also located near the destabilizing <math>\nu_{\rm 6}</math> (<math>g = g_{\rm 6}</math>) secular resonance with Saturn, which at Hebe's average distance of 2.426 AU is located at 15–16° inclination.<ref name="Gaffey1998"/>Template:Rp

Possible Hebe family

In the 1990s, Hebe was identified as a possible source for H chondrites and IIE iron meteorites.<ref name="Gaffey1998"/>Template:Rp<ref name="Fieber-Beyer2020"/>Template:Rp Its location near the 3:1 Jovian MMR and the <math>\nu_{\rm 6}</math> Saturnian secular resonance means that fragments created by impact events are easily destabilized into Earth-crossing orbits, where they could eventually impact Earth as meteorites.<ref name="Farinella1993a"/>Template:Rp Indeed, the H chondritic surface compositions of two near-Earth objectsTemplate:Mdash(4953) 1990 MU and 2007 LETemplate:Mdashpoint towards Hebe as their parent body.<ref name="Kelley2014"/>Template:Rp<ref name="Fieber-Beyer2015"/>Template:Rp Spectral observations of asteroids near the 3:1 Kirkwood gap in the early 2010s identified 695 Bella, 1166 Sakuntala, and 1607 Mavis as potential relatives of Hebe.<ref name="Fieber-Beyer2011"/>Template:Rp<ref name="Gaffey2013"/> However, they are located on the opposite side of the 3:1 Kirkwood gap, indicating that they jumped across the gap.<ref name="Fieber-Beyer2020"/>Template:Rp In 2020, a study led by Sherry K. Fieber-Beyer identified nine additional candidate members of the tentative Hebe family, with some located on Hebe's side of the 3:1 Kirkwood gap.<ref name="Fieber-Beyer2020"/>Template:Rp

Physical characteristics

Simulations (top) and direct images (bottom) of 6 Hebe<ref name=mother/>

Size comparison: the first 10 asteroids profiled against the Moon. Hebe is sixth from the left.

Hebe is a large asteroid, with a volume-equivalent spheroidal diameter of Template:Convert. Though Hebe's shape approximates an oblate spheroid, it hosts numerous extreme topographical features. Five large depressions have been identified on its surface, possibly representing deep impact craters.<ref name="Marsset2017"/>Template:Rp The depressions range from around Template:Convert to over Template:Convert in size, with depths between Template:Convert.<ref name="Marsset2017"/>Template:Rp Hebe additionally has a large, flattened face, giving it the appearance of a "lopped-off tooth". This large facet may represent a section of the asteroid that was blasted away into space by an ancient impact event.<ref name="Fieber-Beyer2020"/>Template:Rp

Based on Hebe's lightcurve, or variations in its observed brightness, it rotates in a prograde (counterclockwise) direction with a rotation period of 7.27 hours. Its north pole pointing towards ecliptic coordinates (β, λ) = (45°, 339°) with a 10° uncertainty.<ref name=Torppa2003 />Template:Rp

Spectrum and composition

Hebe is classified as an S-type asteroid<ref name="Migliorini1997"/>Template:Rp under the Tholen classification scheme.<ref name="DeMeo2015"/> Planetary scientist Michael James Gaffey further subdivided S-type asteroids into seven mineralogical subclasses,<ref name="Gaffey1993b"/> categorizing Hebe as an S(IV)-type asteroid. Hebe's S(IV) classification indicates its surface is silicate (or stony) in composition and undifferentiated or partially differentiated.<ref name="Gaffey1998"/>Template:Rp

See also

• List of former planets

External links

• shape model deduced from lightcurve
• MNRAS 7 (1847) 283 (discovery announcement)
• MNRAS 8 (1848) 103
• JPL Ephemeris
• Template:AstDys
• Template:JPL small body