6 Hebe

6 Hebe  

Discovery
Discovered by	Karl Ludwig Hencke
Discovery date	July 1, 1847
Designations
Named after	Hebe
Alternate name(s)	1947 JB
Minor planet category	Main belt
Adjective	Hebean
Orbital characteristics
Epoch November 26, 2005 (JD 2453700.5)
Aphelion	2.914 AU (435.996 Gm)
Perihelion	1.937 AU (289.705 Gm)
Semi-major axis	2.426 AU (362.851 Gm)
Eccentricity	0.202
Orbital period	3.78 a (1379.756 d)
Average orbital speed	18.93 km/s
Mean anomaly	247.947°
Inclination	14.751°
Longitude of ascending node	138.752°
Argument of perihelion	239.492°
Proper orbital elements[4]
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	205×185×170 km[1][2][3] 186 km (mean)
Mass	1.28×1019 kg[1]
Mean density	3.81±0.26 g/cm³[1]
Equatorial surface gravity	~0.087 m/s2
Escape velocity	~0.13 km/s
Rotation period	0.3031 d[5]
Albedo	0.268 (geometric)[2]
Temperature	~170 K max: ~269 K (-4° C)
Spectral type	S-type asteroid
Apparent magnitude	7.5[6] to 11.50
Absolute magnitude (H)	5.71
Angular diameter	0.26" to 0.065"

6 Hebe ( /ˈhiːbiː/ HEE-bee; Greek: ‘Ήβη) is a large main-belt asteroid, containing around half a percent of the mass of the belt. Its apparently high bulk density (greater than that of the Earth's Moon or even Mars), however, means that by volume it does not rank among the top twenty asteroids. 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 Titan^[7] and can reach +7.5 at an opposition near perihelion.

Hebe is probably the parent body of the H chondrite meteorites, which account for a remarkable 40% of all meteorites striking the Earth.

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, goddess of youth, was proposed by Carl Friedrich Gauss.

Major meteorite source

6 Hebe is the probable parent body of the H chondrite meteorites and the IIE iron meteorites.

The orbit of 6 Hebe compared with the orbits of Earth, Mars and Jupiter

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 Gaffey and Sarah L. Gilbert.^[8]):

• 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 asteroid 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,^[9] due to its position and relatively large size. If Hebe is not the H-chondrite parent body, then where are the meteorites from Hebe?^[8]

Physical characteristics

Lightcurve analysis suggests that Hebe has a rather angular shape, which may be due to several large impact craters.^[3] Hebe rotates in a prograde direction, with the north pole pointing towards ecliptic coordinates (β, λ) = (45°, 339°) with a 10° uncertainty.^[3] 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.

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.^[10] It was nicknamed "Jebe" (see Heebie Jeebies). However, the discovery has not been confirmed.

References

• Yeomans, Donald K.. "Horizons system". NASA JPL. Archived from the original on 28 March 2007. http://ssd.jpl.nasa.gov/?horizons. Retrieved 2007-03-20.  — Horizons can be used to obtain a current ephemeris

1. ^ ^{a b c} Jim Baer (2008). "Recent Asteroid Mass Determinations". Personal Website. http://home.earthlink.net/~jimbaer1/astmass.txt. Retrieved 2008-11-28. 
2. ^ ^{a b} Supplemental IRAS Minor Planet Survey
3. ^ ^{a b c} J. Torppa et al. Shapes and rotational properties of thirty asteroids from photometric data, Icarus, Vol. 164, p. 346 (2003).
4. ^ "AstDyS-2 Hebe Synthetic Proper Orbital Elements". Department of Mathematics, University of Pisa, Italy. http://hamilton.dm.unipi.it/astdys/index.php?pc=1.1.6&n=6. Retrieved 2011-10-01. 
5. ^ Planetary Data System Small Bodies Node, lightcurve parameters
6. ^ Donald H. Menzel and Jay M. Pasachoff (1983). A Field Guide to the Stars and Planets (2nd edition ed.). Boston, MA: Houghton Mifflin. pp. 391. ISBN 0-395-34835-8. 
7. ^ The Brightest Asteroids
8. ^ ^{a b} M. J. Gaffey & S. L. Gilbert Asteroid 6 Hebe: The probable parent body of the H-Type ordinary chondrites and the IIE iron meteorites, Meteoritics & Planetary Science, Vol. 33, p. 1281 (1998).
9. ^ A. Morbidelli et al. Delivery of meteorites through the ν₆ secular resonance, Astronomy & Astrophysics, Vol. 282, p. 955 (1994).
10. ^ W. R. Johnston Other reports of Asteroid/TNO Companions

External links

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