6 Hebe

Not to be confused with Thebe (moon).

6 Hebe

Hebe imaged by the European Southern Observatory's Very Large Telescope
Discovery
Discovered by	Karl Ludwig Hencke
Discovery site	Driesen, Prussia
Discovery date	1 July 1847
Designations
MPC designation	(6) Hebe
Pronunciation	/ˈhiːbiː/
Named after	Hēbē
Alternative designations	A847 NA; 1847 JB
Minor planet category	Main belt
Adjectives	Hebean /hiːˈbiːən/ (trad.)
Symbol	(historical)
Orbital characteristics
Epoch 13 September 2023 (JD 2453300.5)
Uncertainty parameter 0
Aphelion	2.92 AU (437 million km)
Perihelion	1.93 AU (289 million km)
Semi-major axis	2.43 AU (364 million km)
Eccentricity	0.2027
Orbital period (sidereal)	3.78 yr (1379.85 d)
Average orbital speed	18.93 km/s
Mean anomaly	144.0°
Inclination	14.736°
Longitude of ascending node	138.63°
Time of perihelion	10 March 2022
Argument of perihelion	239.59°
Earth MOID	0.97 AU (145 million km)
Proper orbital elements
Proper semi-major axis	2.4252710 AU
Proper eccentricity	0.1584864
Proper inclination	14.3511092°
Proper mean motion	95.303184 deg / yr
Proper orbital period	3.77742 yr (1379.702 d)
Precession of perihelion	31.568209 arcsec / yr
Precession of the ascending node	−41.829042 arcsec / yr
Physical characteristics
Dimensions	205 km × 185 km × 170 km
Mean diameter	195±3 km
Flattening	0.25
Mass	(1.24±0.24)×1019 kg (1.27±0.13)×1019 kg
Mean density	3.18±0.64 g/cm3
Equatorial surface gravity	~0.079–0.099 m/s2
Equatorial escape velocity	~0.127–0.135 km/s (457–486 km/h)
Synodic rotation period	7.274 h
Pole ecliptic longitude	339°
Pole ecliptic latitude	45°
Geometric albedo	0.268
Temperature	~170 K max: ~269 K (−4°C)
Spectral type	S
Apparent magnitude	7.5 to 11.50
Absolute magnitude (H)	5.61
Angular diameter	0.26" to 0.065"

6 Hebe (/ˈhiːbiː/) 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, 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). 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. 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. In the years following its discovery, Hebe was variously labelled as a planet, small planet, or asteroid. Eventually, throughout the latter half of the 19th century, the terms "asteroid" and "minor planet" became favored, although some astronomers continued referring to Hebe as a planet in this period.

Gauss chose a wineglass as Hebe's astronomical symbol. It was encoded in Unicode 17.0 as U+1CEC0 𜻀 (). 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 number—denoting the order of discovery—inscribed in a circle. For Hebe, this would be ⑥. This system was widely adopted by astronomers, though astronomers eventually switched to using parentheses enclosing the number—thus (6) Hebe or 6 Hebe in modern notation.

On 5 March 1977, Hebe occulted the star γ Ceti A (Kaffaljidhma).

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), placing it in the inner section of the main asteroid belt. 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.

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. 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. Hebe is also located near the destabilizing ${\nu }_{\mathrm{6}}$ ($g=g_{\mathrm{6}}$) secular resonance with Saturn, which at Hebe's average distance of 2.426 AU is located at 15–16° inclination.

Possible Hebe family

In the 1990s, Hebe was identified as a possible source for H chondrites and IIE iron meteorites. Its location near the 3:1 Jovian MMR and the ${\nu }_{\mathrm{6}}$ 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. Indeed, the H chondritic surface compositions of two near-Earth objects—(4953) 1990 MU and 2007 LE—point towards Hebe as their parent body. 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. However, they are located on the opposite side of the 3:1 Kirkwood gap, indicating that they jumped across the gap. 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.

Physical characteristics

Simulations (top) and direct images (bottom) of 6 Hebe

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 193 ± 6 kilometres (119.9 ± 3.7 mi). 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. The depressions range from around 50 km (31 mi) to over 100 km (62 mi) in size, with depths between 7 and 18 kilometres (4.3 and 11.2 mi). 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.

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.

Spectrum and composition

Hebe is classified as an S-type asteroid under the Tholen classification scheme. Planetary scientist Michael James Gaffey further subdivided S-type asteroids into seven mineralogical subclasses, 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.

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
• 6 Hebe at AstDyS-2, Asteroids—Dynamic Site
  • Ephemeris · Observation prediction · Orbital info · Proper elements · Observational info
• 6 Hebe at the JPL Small-Body Database