6 Hebe

Infobox Planet | discovery=yes | physical_characteristics = yes | bgcolour=#FFFFC0
name=6 Hebe
symbol=
discoverer=Karl Ludwig Hencke
discovered=July 1, 1847


alt_names=1947 JB
mp_category=Main belt
adjectives=Hebean
epoch=November 26, 2005 (JD 2453700.5)
semimajor=362.851 Gm (2.426 AU)
perihelion=289.705 Gm (1.937 AU)
aphelion=435.996 Gm (2.914 AU)
eccentricity=0.202
period=1379.756 d (3.78 a)
inclination=14.751°
asc_node=138.752°
arg_peri=239.492°
mean_anomaly=247.947°
avg_speed=18.93 km/s
dimensions=205×185×170 km [http://www.psi.edu/pds/archive/astdata04/simps04/diamalb.tab Supplemental IRAS Minor Planet Survey] ] [http://www.rni.helsinki.fi/~mjk/thirty.pdf J. Torppa et al "Shapes and rotational properties of thirty asteroids from photometric data"] , Icarus, Vol. 164, p. 346 (2003).]
mass=~1.4×10¹⁹ kg [http://www.edpsciences.org/articles/aa/pdf/2001/29/aa10228.pdf?access=ok G. Michalak "Determination of asteroid masses"] Astronomy & Astrophysics, Vol. 374, p. 703 (2001).]
density=~4.1 g/cm³
surface_grav=~0.087 m/s²
escape_velocity=~0.13 km/s
rotation=0.3031 d [ [http://www.psi.edu/pds/archive/lc.html Planetary Data System Small Bodies Node, lightcurve parameters] ]
spectral_type=S-type asteroid
magnitude = 7.5cite book | author=Donald H. Menzel and Jay M. Pasachoff | year=1983 | title=A Field Guide to the Stars and Planets | edition=2nd edition | publisher=Houghton Mifflin | pages=p. 391 | location=Boston, MA | isbn=0395348358 ] to 11.50
abs_magnitude=5.71
albedo=0.268 (geometric)
angular_size = 0.26" to 0.065"
single_temperature=~170 K
"max:" ~269 K (-4° C)

6 Hebe (pronEng|ˈhiːbi respell|HEE|bee, _gr. "‘Ήβη)" is a large Main belt asteroid. Hebe is the thirteenth largest asteroid by mass, containing around 0.5 percent of the mass of the entire asteroid 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 behind Vesta, Ceres, Iris and Pallas. It has a mean opposition magnitude of +8.3, about equal to the mean brightness of Titan [ [http://jas.org.jo/ast.html The Brightest Asteroids] ] 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

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.

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 Gaffey and Sarah L. Gilbert. [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1998M%26PS...33.1281G&db_key=AST&data_type=HTML&format=&high=438c93072f28336 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).] ):
* 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 $u\_6,!$ 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, [http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1994A%26A...282..955M&data_type=PDF_HIGH&type=PRINTER&filetype=.pdf A. Morbidelli et al "Delivery of meteorites through the ν₆ secular resonance"] , Astronomy & Astrophysics, Vol. 282, p. 955 (1994).] due to its position and relatively large size. "If Hebe is not the H-chondrite parent body, then where are the meteorites from Hebe?"

Physical characteristics

Lightcurve analysis suggests that Hebe has a rather angular shape, which may be due to several large impact craters. Hebe rotates in a prograde direction, with the north pole pointing towards ecliptic coordinates (β, λ) = (45°, 339°) with a 10° uncertainty. 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, asurface 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:
# 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.
# Later sizeable impacts broke up and mixed these layers.
# 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. [ [http://www.johnstonsarchive.net/astro/asteroidmoonsq.html W. R. Johnston "Other reports of Asteroid/TNO Companions] ] It was nicknamed "Jebe" (see Heebie Jeebies). However, the discovery has not been confirmed.

References

* — Horizons can be used to obtain a current ephemeris

External links

* [http://www.rni.helsinki.fi/~mjk/thirty.pdf shape model deduced from lightcurve]
* [http://adsabs.harvard.edu//full/seri/MNRAS/0007//0000283.000.html MNRAS 7 (1847) 283] (discovery announcement)
* [http://adsabs.harvard.edu//full/seri/MNRAS/0008//0000104.000.html MNRAS 8 (1848) 103]