VESTA – A HISTORICAL PERSPECTIVE MICHAEL J. GAFFEY Space Studies Department John D. Odegard School of Aerospace Sciences University of North Dakota Grand Forks, ND 58202-9008 gaffey@space.edu

Roman sestertius (~165 AD) Discovery and Rank Asteroid (4) Vesta was discovered in 1807 by Heinrich Olbers. The asteroid was named after Vesta, the Roman goddess of home and hearth. Vesta is second in mass after (1) Ceres and before (2) Pallas. Vesta is 3rd in size after (2) Pallas Roman sestertius (~165 AD) Heinrich Oblers

Vesta is Unique Vesta has a unique spectral reflectance curve among the larger asteroids. The basaltic surface lithology of Vesta indicates strong post-accretionary heating and at least partial melting. The preservation of a basaltic surface indicates a largely intact body. Since 1970 Vesta has been identified as the possible / probable parent body of the basaltic HED meteorites.

Vesta is Important Asteroid (4) Vesta has been called the smallest terrestrial planet (Keil 2002). But its unique place in the study of the solar system goes far beyond that designation. At opposition Vesta is generally the brightest asteroid. As a result, it has been an early target of virtually every new optical observational technique applied to asteroids.

Lightcurves and Rotation Period Photometric observations of Vesta were made at the Harvard College Observatory in 1880–1882 and at the Observatoire de Toulouse in 1909. Uncertainty as to whether the Vesta lightcurve was single or double peaked led to ambiguities in the rotation period: 5.34 hours versus 10.68 hours (e.g. Chapman, Williams and Hartmann, 1978) This ambiguity was not fully resolved until HST imaging the early 1990’s.

Showed a variety of asteroid spectra Bobrovnikoff (1929) used photographic spectrophotometry to investigate the spectra of 12 minor planets. Showed a variety of asteroid spectra Suggested that Vesta’s spectrum varied with rotation 4860 A Moon 6 Hebe 12 Victoria Halley’s Comet 7 Iris 9 Metis 2 Pallas Vesta 8 Flora Vesta

Hall and Green at Amherst October 15-16,1939 Large = Photographic observations in Blue light (0.43 mm) Small = Photoelectric observations in infra-red light (0.81 mm) Figure was published in the Harvard Books on Astronomy Series “Between the Planets” by Fletcher G. Watson (1st edition, 1941)

Searching for color variations Stephenson (1951) Groeneveld and Kuiper (1954) Gehrels (1967) McCord, Adams and Johnson (1970) Chapman et al. (1971) Larson and Fink (1975) Feierberg et al. (1980) Feierberg (1985, personal comm.) Blanco and Catalano (1979) Gradie et al. (1978) Gradie (1982, personal comm.) McFadden et al. (1981) McCheyne et al. (1985) Vdovichenko et al. (1990) Festou et al. (1991) Lagerkvist and Oja (1991) Reynoldson et al. (1993). Between 1951 and 1993, fourteen groups either reported or showed rotational color or spectral variations for Vesta. Two investigations reported no variations.

VNIR Spectrum of Vesta compared to Nuevo Laredo Eucrite Established that the surface of Vesta had a significant pyroxene component comparable to that in an eucrite McCord, Adams and Johnson (1970)

Confirmation of the Expected 2 mm Pyroxene Feature Larson and Fink (1975) Derived location on Adam (1975) Band I vs. Band II plot. Located on “long” edge of Eucrite zone, possibly due to the use of Moon as standard.

Variations in the spin-forbidden Fe2+ feature at 0.506 mm Shifts in the feature was attributed to compositional variations in the pyroxene Cochran and Vilas (1998)

VNIR Spectrum of Vesta Gaffey (1997)

Effect of Phase Geometry ~7% increase in band depth from phase angle ~4° to ~17 °

Map Derived from Rotational Variations Note: North and South are swapped. Gaffey (1997)

The Vesta – HED Meteorite Link? Mineralogy derived from VNIR spectra of Vesta is a clear match for the HED meteorites. Wisdom (1985) showed that the chaotic zones associated with the proper motion (e.g., 3:1, 5:2, etc.) and secular (e.g. nu6) resonances were the primary escape hatches for meteoroids exiting the main belt. The HEDs are the most common non-ordinary chondrite meteorite type (~6.2% of falls). HEDs must come from a favorably situated parent body. Vesta is NOT favorably located as a meteorite parent body!

You can’t get here from there --- 4:1 2:1 nu6 Lifetimes of stony meteoroids is too short to allow significant numbers of Vesta-derived meteoroids to reach the 3:1 zone.

By comparison, 6 Hebe is favorably located Hebe is the probable parent body of the H-chondrites and IIE iron meteorites.

Vestoids to the rescue Binzel and Xu (1993)

With a little help from your friends --- Vestoids from Binzel & Xu (1993)

Basaltic NEOs 3551 Verenia 3908 Nyx 4055 Magellan Cruikshank et al. (1991)

HST Images of South Pole Basin Thomas et al. (1997)

Synopsis Studies of Vesta using ground-based and orbiting telescopes helped the DAWN mission to be better prepared to explore this asteroid. Many of the pre-DAWN characterizations of Vesta were confirmed by the mission itself. Even so, DAWN @ Vesta has revealed a myriad of surprises which could only be discovered by an orbital mission. Future asteroid missions would benefit by focused ground-based investigations of their target bodies. Ground-based studies increase our inducement for missions. Such studies do not reduce the need for missions.