6 Hebe

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6 Hebe

Discovery A
Discoverer	Karl Ludwig Hencke
Discovery date	July 1, 1847
Alternate designations B	1947 JB
Category	Main belt
Orbital elements C
Epoch November 26, 2005 (JD 2453700.5)
Eccentricity (e)	0.202
Semi-major axis (a)	362.851 Gm (2.426 AU)
Perihelion (q)	289.705 Gm (1.937 AU)
Aphelion (Q)	435.996 Gm (2.914 AU)
Orbital period (P)	1379.756 d (3.78 a)
Mean orbital speed	18.93 km/s
Inclination (i)	14.751°
Longitude of the ascending node (Ω)	138.752°
Argument of perihelion (ω)	239.492°
Mean anomaly (M)	247.947°
Physical characteristics D
Dimensions	205×185×170 km[1][4]
Mass	~1.4×1019 kg [5]
Density	~4.1 g/cm3
Surface gravity	~0.087 m/s2
Escape velocity	~0.13 km/s
Rotation period	0.3031 d[2]
Spectral class	S-type asteroid
Absolute magnitude	5.71
Albedo (geometric)	0.268 [1]
Mean surface temperature	~170 K max: ~269 K (-4° C)
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6 Hebe (hee'-bee (key), Greek ‘Ήβη) is one of the largest Main belt asteroids, and is probably the parent body of the H chondrite meteorites, which account for a remarkable 40% of all meteorites striking the Earth.

Hebe is the eleventh largest asteroid by mass, containing around 0.5 percent of the mass of the entire asteroid belt. Its high bulk density (greater than that of the Earth's Moon or even that of Mars), however, means that by volume it does not rank among the top twenty asteroids.

[edit] Discovery

Size comparison: the first 10 asteroids profiled against Earth's Moon. Hebe is fifth from the right.

Hebe was the sixth asteroid to be discovered, on July 1, 1847 by Karl Ludwig Hencke. It was the second and final asteroid discovery by Hencke, who had previously found 5 Astraea. The name "Hebe" was proposed by Carl Friedrich Gauss.

[edit] Major meteorite source

6 Hebe is the probable parent body of the H chondrite meteorites and the IIE iron meteorites. Remarkably, this would imply that it is the source of about 40% of all meteorites striking the Earth. Evidence for this connection includes the following (after Michael J. Gaffey and Sarah L. Gilbert ^[6]):

• The spectrum of Hebe matches a mix of 60% H chondrite and 40% IIE iron meteorite material.
• The IIE type are unusual among the iron meteorites, and probably formed from impact melt, rather than being fragments of the core of a differentiated asteroid.
• The IIE irons and H chondrites likely come from the same parent body, due to similar trace mineral and oxygen isotope ratios.
• Asteroids with spectra similar to the ordinary chondrite meteorites (accounting for 85% of all falls, including the H chondrites) are extremely rare.
• 6 Hebe is extremely well placed to send impact debris to Earth-crossing orbits. Ejecta with even relatively small velocities (~280 m/s) can enter the chaotic regions of the 3:1 Kirkwood gap at 2.50 AU and the nearby secular resonance which determines the high-inclination edge of the main belt at about 16° inclinations hereabouts.
• Of the asteroids in this "well-placed" orbit, Hebe is the largest.
• An analysis of likely contributors to the Earth's meteorite flux places 6 Hebe at the top of the list ^[7], due to its position and relatively large size. If Hebe is not the H-chondrite parent body, then where are the meteorites from Hebe? ^[6]

[edit] Physical characteristics

Lightcurve analysis suggests that Hebe has a rather angular shape, which may be due to several large impact craters ^[4]. Hebe rotates in a prograde direction, with the north pole pointing towards ecliptic coordinates (β, λ) = (45°, 339°) with a 10° uncertainty ^[4]. This gives an axial tilt of 42°.

It has a bright surface and, if its identification as the parent body of the H chondrites is correct, a surface composition of silicate chondritic rocks mixed with pieces of nickel-iron metal. A likely scenario for the formation of the surface metal is as follows:

1. Large impacts caused local melting of the iron rich H chondrite surface. The metals, being heavier, would have settled to the bottom of the magma lake, forming a metallic layer buried by a relatively shallow layer of silicates.
2. Later sizeable impacts broke up and mixed these layers.
3. Small frequent impacts tend to preferentially pulverize the weaker rocky debris, leading to an increased concentration of the larger metal fragments at the surface, such that they eventually comprise ~40% of the immediate surface at the present time.

[edit] Moon

On March 5, 1977 Hebe occulted Kaffaljidhma (γ Ceti), a moderately bright 3rd magnitude star. No other observed occultations by Hebe have been reported.

As a result of that occultation, a small Hebean moon was reported by Paul D. Maley ^[3]. It was nicknamed "Jebe" (see Heebie Jeebies). However, the discovery has not been confirmed.

[edit] Aspects

[edit] References

1. Supplemental IRAS Minor Planet Survey
2. Planetary Data System Small Bodies Node, lightcurve parameters
3. R. Johnston Other reports of Asteroid/TNO Companions
4. J. Torppa et al Shapes and rotational properties of thirty asteroids from photometric data, Icarus, Vol. 164, p. 346 (2003).
5. G. Michalak Determination of asteroid masses Astronomy & Astrophysics, Vol. 374, p. 703 (2001).
6. M. J. Gaffey & S. L. Gilbert Asteroid 6 Hebe: The probable parent body of the H-Type ordinary chondrites and the IIE iron metorites, Meteoritics & Planetary Science, Vol. 33, p. 1281 (1998).
7. A. Morbidelli et al Delivery of meteorites through the ν₆ secular resonance, Astronomy & Astrophysics, Vol. 282, p. 955 (1994).

[edit] External links

• shape model deduced from lightcurve
• MNRAS 7 (1847) 283 (discovery announcement)
• MNRAS 8 (1848) 103