6 Hebe (/ˈhiːbiː/) is a largemain-beltasteroid, containing around 0.5% of the mass of the belt. However, due to its apparently high bulk density (greater than that of theMoon), 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-boundrubble piles.
Inbrightness, Hebe is the fifth-brightest object in the asteroid belt afterVesta,Ceres,Iris, andPallas. It has a mean opposition magnitude of +8.3, about equal to the mean brightness of Saturn's moonTitan,[12] and can reach +7.5 at an opposition near perihelion.
Hebe may be the parent body of theH chondrite meteorites, which account for about 40% of allmeteorites striking Earth.
Simulations (top) and direct images (bottom) of 6 Hebe[17]
Hebe was once thought to be the probable parent body of theH chondritemeteorites and theIIE iron meteorites. This would imply that it is the source of about 40% of all meteorites striking Earth. Evidence for this connection includes the following:
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 adifferentiated asteroid.
The IIE irons and H chondrites likely come from the same parent body, due to similar trace mineral and oxygenisotope ratios.
Asteroids with spectra similar to theordinary 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 3:1Kirkwood gap at 2.50AU and the nearbysecular resonance which determines the high-inclination edge of theasteroid belt at about 16°inclinations hereabouts.
Of the asteroids in this "well-placed" orbit, Hebe is the largest.
An analysis of likely contributors to Earth's meteorite flux places 6 Hebe at the top of the list,[18] due to its position and relatively large size.[19]
However, observations by theVLT in 2017 indicate that the depressions caused by impacts on 6 Hebe are only 20% the volume of the nearby H-chondrite asteroid families, suggesting the Hebe is not the most likely or primary source of H-chondrite meteorites.[17]
This article needs to beupdated. Please help update this article to reflect recent events or newly available information.(October 2019)
Lightcurve-based 3D-model ofHebeSize comparison: the first 10 asteroids profiled against theMoon. Hebe is sixth from the left.
Lightcurve analysis suggests that Hebe has a rather angular shape, which may be due to several large impact craters.[7] Hebe rotates in aprograde direction, with the north pole pointing towardsecliptic coordinates (β, λ) = (45°, 339°) with a 10° uncertainty.[7] This gives anaxial tilt of 42°.
It has a bright surface and, if its identification as the parent body of the H chondrites is correct, asurface composition ofsilicatechondritic rocks mixed with pieces ofiron–nickel. 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.
As a result of the aforementioned 1977 occultation, a smallmoon around Hebe was reported byPaul D. Maley.[20] It was nicknamed "Jebe". This was the first modern-day suggestion that asteroids have satellites. It was 17 years later when the first asteroid moon was formally discovered (Dactyl, the satellite of243 Ida). The discovery of Hebe's moon was never confirmed.
^abcdeP. Vernazza et al. (2021) VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis.Astronomy & Astrophysics 54, A56
^abJames Baer, Steven Chesley & Robert Matson (2011) "Astrometric masses of 26 asteroids and observations on asteroid porosity."The Astronomical Journal, Volume 141, Number 5
^Steger, Franz (1847).Ergänzungs-conversationslexikon [Supplementary Conversational Lexicon] (in German). Vol. 3. p. 442.Hofrath Gauß gab auf Hencke's Ansuchen diesem neuen Planetoiden den Namen Hebe mit dem Zeichen (ein Weinglas).
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