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NASA NTRS2005

A Perfect View of Vesta: Creating Pointing Observations for the Dawn Spacecraft on Asteroid 4 Vesta

The Dawn spacecraft has a timely and clever assignment in store. It will take a close look at two intact survivors from the dawn of the solar system (asteroids 4 Vesta and 1 Ceres) to understand more about solar system origin and evolution. To optimize science return, Dawn must make carefully designed observations on approach and in survey orbit, high altitude mapping orbit, and low altitude mapping orbit at each body. In this report, observations outlined in the science plan are modeled using the science opportunity analyzer program for the Vesta encounter. Specifically, I encoded Dawn's flight rules into the program, modeled pointing profiles of the optical instruments (framing camera, visible infrared spectrometer) and mapped their fields of view onto Vesta's surface. Visualization of coverage will provide the science team with information necessary to assess feasibility of alternative observation plans. Dawn launches in summer 2006 and ends its journey in 2016. Instrument observations on Vesta in 2011 will supply detailed information about Vesta's surface and internal structure. These data will be used to analyze the formation and history of the protoplanet and, therefore, complete an important step in understanding the development of our solar system.

Hay, Katrina M.↗

NASA NTRS2011

Vesta Mineralogy: VIR maps Vesta's surface

The Dawn mission will have completed Survey orbit around 4 Vesta by the end of August 2011. We present a preliminary analysis of data acquired by the Visual and InfraRed Spectrometer (VIR) to map Vesta mineralogy. Thermal properties and mineralogical data are combined to provide constraints on Vesta's formation and thermal evolution. delivery of exogenic materials, space weathering processes, and origin of the howardite. eucrite, and diogenite (HED) meteorites.

Coradina, A.↗

NASA NTRS2014

Probing Asteroid (4) Vesta, Part 2: Meteorites From Vesta

No abstract available

Mittlefehldt, David W.↗

NASA NTRS1996

Workshop on evolution of igneous asteroids: Focus on Vesta and the HED meteorites

Recently, the geology of the surface of Vesta has been coming to light. In 1983 Gaffey first began showing maps of the surface geology of Vesta constructed from numerous spectra obtained at different times as the asteroid rotated. By noting the details of spectral variation with rotation, he was able to develop two possible gross-scale geologic maps of Vesta showing the distributions of mafic and ultramafic materials. These maps were published in 1997. Finally, the capabilities of the Hubble Space Telescope were brought to bear on Vesta and images with a resolution of about 50 km were obtained using four different filters by Binzel and co-workers. Maps produced by this team published in 1997 began to reveal the geology of Vesta in sufficient detail that crude interpretations of the geologic history of the asteroid could be attempted. Additionally, in 1993 Binzel and Xu published a study of small asteroids in the region near Vesta in orbital-element space. In this study, they showed that there are a number of asteroids a few kilometers in size with reflectance spectra like that of Vesta that form a trail in orbital-element space from near Vesta to near resonances that can more easily supply material to near-Earth space. Binzel and Xu thus concluded that these small asteroids were spells of Vesta ejected by impact and that some of their brethren had been perturbed to Earth-approaching orbits. They suggested that these latter were the immediate parents of HED meteorites. This seemed to remove a long-standing dynamical objection to Vesta as the HED parent body, as discussed by Wasson and Wetherill in 1979. Within the last few years, NASA has initiated the Discovery program of low-cost, rapid-timescale development, exploration missions. Vesta has been proposed as an object worthy of study by a Discovery mission, although a Vesta mission has not yet been selected. With all the recent activity aimed at studying Vesta and the HED meteorites, and the possibility of a space mission to Vesta, we felt that time was ripe to convene a workshop bringing together astronomers, meteoriticists, and planetary geologists to focus on what could be learned about the geologic evolution of Vesta through integrating astronomical and HED meteorite studies. This, of course, assumes that the HED meteorites are from Vesta, and this issue was specifically addressed (but not resolved) in the workshop. Indeed, it seems likely that this issue can only be resolved by returning samples from Vesta for detailed study on Earth. The workshop was held at the LPI on October 16-18,1996, and was attended by some 70 scientists. Sessions included a set of talks on Earth- and space-based astronomical observations of Vesta plus the evidence pro and con for Vesta being the HED parent body, talks on the petrology and geochemistry of HED meteorites, talks on the formation and dynamics of ejecta from Vesta, talks on the thermal history of asteroids and HED meteorites, volcanic processes and differentiation history, and a short session devoted to possible missions to Vestal By all accounts, the workshop was considered a great success, although this is the opinion of a biased set of observers.

Mittlefehldt, D. W.↗

NASA NTRS2024

How Many Vesta-Like Bodies Existed in the Asteroid Belt?

Asteroid 4 Vesta is typically thought to be the parent body of the HED (howardite, eucrite, and diogenite) meteorites due to spectral similarities. The discovery of asteroids far from Vesta with HED-like spectra like (1459) Magnya and HED-like meteorites (e.g., NWA 011) with anomalous oxygen isotopic values compared to typical HEDs is evidence that other Vesta-like bodies formed. We broadly define a Vesta-like body as a differentiated object with a crust composed primarily of low-Ca pyroxene and plagioclase feldspar. We estimate the number of Vesta-like bodies that did form by looking at the astronomical evidence; the oxygen isotopic, chemical, and petrologic evidence; and the iron meteorite evidence. Assuming that fragments of Vesta were scattered from Vesta by giant planet migration, we conservatively estimate that at least two Vesta-like bodies (Vesta and the Magnya parent bodies) existed. From the oxygen isotopic, chemical, and petrologic evidence, we also conservatively estimate that seven Vesta-like bodies formed. Analyses of iron meteorites indicate that there may be as many as 23 Vesta-like bodies (Vesta, 10 magmatic iron groups, South Byron trio, Emsland/Mbosi duo, 10 ungrouped irons). This estimate from iron meteorites is most certainly an overestimation due to the existence of a number of non-HED crustal/mantle fragments that potentially originated from bodies with magmatic iron cores. Using our three estimates as a guide, we predict that there were ~10 Vesta-like bodies (including Vesta) that formed in the early solar system. Only Vesta remains intact with the others being disrupted early in solar system history.

T. H. Burbine↗

NASA NTRS2012

Chemical Mapping of Vesta

Vesta s surface mineralogy and composition have been studied for decades via telescopic spectroscopy and laboratory analyses of the howardite, eucrite, and diogenite (HED) meteorites, which are thought to originate from Vesta. Visible and infrared reflectance measurements by Dawn have broadly confirmed the paradigm established by Earth-based work, strengthening the Vesta-HED connection. The Dawn mission has achieved a milestone by completing the first chemical measurements of a main-belt asteroid using nuclear spectroscopy. Dawn s Gamma Ray and Neutron Detector (GRaND) has globally mapped the composition of Vesta, including the portions of the northern hemisphere not illuminated by solar radiation. GRaND is sensitive to the composition of the bulk regolith to depths of several decimeters. Abundances and/or detection limits for specific elements and elemental ratios, such as H, Fe, Si, Fe/O, Fe/Si, and K, have been measured. Variations in the average atomic mass and neutron macroscopic absorption cross section have been characterized. The measurements constrain the relative proportions of HED whole-rock end-members, providing measurements of the pyroxene and plagioclase content of the regolith, thereby constraining the processes underlying Vesta s differentiation and crustal evolution. The spatial resolution of GRaND is sufficient to determine basin-average compositions of Veneneia and Rheasilvia, which may contain outcrops of Vesta s olivine-rich mantle. While the elemental composition of Vesta s regolith is similar to the meteorites, there are notable departures from HED whole-rock compositions. While these differences are not sufficient to topple the Vesta-HED paradigm, they provide insight into global-scale processes that have shaped Vesta s surface. Questions addressed by the analysis of GRaND data include: (i) Is Vesta the source of the Fe-rich mesosiderites? (ii) Are evolved, igneous lithologies present on Vesta s surface? (iii) What are the origins of exogenic materials found in Vesta s regolith? (iv) Is the vestan mantle exposed within the southern basins?

Prettyman, Thomas H.↗

NASA NTRS2012

In-Flight Operation of the Dawn Ion Propulsion System Through the Preparations for Escape From Vesta

The Dawn mission, part of NASA's Discovery Program, has as its goal the scientific exploration of the two most massive main-belt asteroids, 4 Vesta, and the dwarf planet 1 Ceres. The Dawn spacecraft was launched from the Cape Canaveral Air Force Station on September 27, 2007 on a Delta-II 7925H-9.5 rocket that placed the 1218-kg spacecraft into an Earth-escape trajectory. On-board the spacecraft is an ion propulsion system (IPS) developed at the Jet Propulsion Laboratory which will provide a total delta-V of approximately 11 km/s for the heliocentric transfer to Vesta, orbit capture at Vesta, transfer between Vesta science orbits, departure and escape from Vesta, heliocentric transfer to Ceres, orbit capture at Ceres, transfer between Ceres science orbits, and orbit maintenance maneuvers for all Vesta and Ceres science orbits. Full-power thrusting from December 2007 through October 2008 was used to successfully target a Mars gravity assist flyby in February 2009 that provided an additional delta-V of 2.6 km/s. Deterministic thrusting for the heliocentric transfer to Vesta resumed in June 2009 and concluded with orbit capture at Vesta on July 16, 2011. An additional 231 hours of IPS thrusting was used to enter the first Vesta science orbit, called Survey orbit, on August 3, 2011 at an altitude of about 2,735 km. The IPS was then used over the next year to transfer the spacecraft to the other science orbits: a high altitude mapping orbit (HAMO-1) in September 2011 at an altitude of approximately 673 km, a low altitude mapping orbit (LAMO) at approximately 210 km altitude, and a second high altitude mapping orbit (HAMO-2) at approximately 673 km altitude. To date the IPS has been operated for approximately 24,327 hours, consumed approximately 260 kg of xenon, and provided a delta-V of approximately 7 km/s. IPS performance characteristics are very close to the expected performance based on analysis and testing performed pre-launch. Thrusting for escape from Vesta and cruise to Ceres is planned to start in late July 2012 with a planned arrival date at Ceres in February 2015. This paper provides an overview of Dawn's mission objectives and the results of Dawn IPS mission operations through preparations for departure from Vesta.

Garner, Charles E.↗

NASA NTRS2011

Macro vs. Micro: Relating the Spectral Properties of Vesta and the HED Meteorite

We present the main results obtained comparing the visible-near infrared (VIS-NIR) spectra Vesta s surface with howardites, eucrites, diogenites (HED). HEDs are commonly associated with Vesta s composition based on spectral similarities. Because of such association, much effort is being made to merge the information from HEDs as well as Vestoids - with that from Vesta to characterize the lithologic diversity of the surface of this asteroid and to infer clues regarding its thermal history. However, while the HEDs are a class of meteorites well studied in the laboratory, the only spectral data available for Vesta until now were telescopic observations which are limited in terms of observation conditions, spatial resolution and Signal to Noise Ratio. The Dawn spacecraft, orbiting around Vesta since July 2011, is performing detailed observations of this body and thus improving our knowledge of its properties. Dawn s scientific payload includes an imaging spectrometer, VIR-MS, sensitive to the VIS-NIR spectral range. VIR-MS began acquiring spectra during the approach phase started in May 2011 and will continue its observations through July 2012 when the spacecraft will depart Vesta to travel to Ceres. The VIR-MS spatial resolution depends upon the mission phase (approach, survey, high altitude, low altitude). However, spectra acquired by VIR-MS have already exceeded the spatial resolution of ground-based telescopic observations, with resolution in the approach phase ranging from 2.5 up to 0.8 km/pixel. Moreover, the observations are uniformly distributed in latitude and longitude allowing us to have a global view of Vesta s crust spectral properties. Using the information provided by VIR spectra, we studied the distribution of the spectral heterogeneities on the surface and we used our findings to perform a comparison with HEDs spectra in the VIS-NIR spectral range searching for analogies and/or incompatibilities. In our analysis, we focused on a method to compare the results obtained at microscopic scale on HEDs samples and the one obtained at macroscopic scale on the surface of Vesta. Finally, we discuss our results in the context of vestan thermal history. The intent of this study is to improve our understanding of the connection between Vesta and the HED samples, which is one of the primary Dawn scientific objectives. This work is supported by an Italian Space Agency (ASI) grant and by NASA through the Dawn project and a Dawn at Vesta Participating Scientist grant.

Ammannito, E.↗

NASA NTRS2012

Geochemistry at 4 Vesta: Observations Using Fast Neutrons

Dawn is currently in orbit around the asteroid 4 Vesta, and one of the major objectives of the mission is to probe the relationship of Vesta to the Howardite, Eucrite, and Diogenite (HED) meteorites. As Vesta is an example of a differentiated planetary embryo, Dawn will also provide fundamental information about planetary evolution in the early solar system [1]. To help accomplish this overall goal, the Dawn spacecraft carries the Gamma-Ray and Neutron Detector (GRaND). GRaND uses planetary gamma-ray and neutron spectroscopy to measure the surface elemental composition of Vesta and will provide information that is unique and complementary to that provided by the other Dawn instruments and investigations. Gamma-ray and neutron spectroscopy is a standard technique for measuring planetary compositions [2], having successfully made measurements at near-Earth asteroids, the Moon, Mars, Mercury and now Vesta. GRaND has made the first measurements of the neutron spectrum from any asteroid (previous asteroid measurements were only made with gamma-rays). Dawn has been collecting data at Vesta since July 2011. The prime data collection period for GRaND is the Low-Altitude Mapping Orbit (LAMO), which started on 12 December 2011 and will last through spring 2012. During LAMO, the Dawn spacecraft orbits at an average altitude of ~210 km above the surface of Vesta, which allows good neutron and gamma-ray signals to be detected from Vesta. A description of the overall goals of GRaND and a summary of the initial findings are given elsewhere [3,4]. The subject of this study is to present the information that will be returned from GRaND using fast neutron measurements. Here, we discuss what fast neutrons can reveal about Vesta s surface composition, how such data can address Dawn science goals, and describe fast neutron measurements made in the early portion of the Vesta LAMO phase.

Lawrence, David J.↗

NASA NTRS2014

Bulk Composition of Vesta as Constrained by the Dawn Mission and the HED Meteorites

Of the objects in the main asteroid belt, Vesta is of particular interest as it is large enough to have experienced internal differentiation (520 km diameter), and it is known to have a basaltic surface dominated by FeO-bearing pyroxenes. Furthermore, visible-IR spectra of Vesta and associated Vestoids are remarkably similar to laboratory spectra of Howardite-Eucrite-Diogenite (HED) meteorites, leading to the paradigm that the HEDs ultimately came from Vesta. Geochemical and petrological studies of the HEDs confirm the differentiated nature of the near-surface region of their parent body, and imply that crust extraction occurred well within the first 10 Ma of solar system history Vesta is therefore a prime target for studies that aim to constrain the earliest stages of planet building, and it is within this context that the NASA Dawn spacecraft orbited Vesta from July 2011 to September 2012. The results of the Dawn mission so far have significantly reinforced the HED-Vesta connection, confirming a significant degree of internal differentiation, a surface mineralogy compatible with that of the HEDs, and near-surface ratios of Fe/O and Fe/Si consistent with HED lithologies. The combination of data from the HED meteorites and the Dawn mission thus presents an unprecedented opportunity to use Vesta as a natural laboratory of early differentiation processes in the early solar system. However, the bulk composition of Vesta remains a significant unknown parameter, but one that plays a key role on the physical and chemical properties of the internal and surface reservoirs (core, mantle, crust). Several attempts have been made to constrain the bulk composition of the eucrite parent body, early endeavours relying on petrological or cosmochemical constraints. More recently, individual chondrite class compositions, or mixtures thereof, have been considered, constrained by considerations such as O-isotopes, trace-element ratios and siderophile element concentrations of the eucrites. The work presented here builds upon these latter studies, with the primary aims of: i) illustrating the potential diversity of the geochemical and geophysical properties of a fully differentiated Vesta-sized parent body, and ii) assessing which, if any, of the known chondritic bulk compositions are plausible analogues for proto-Vesta.

Toplis, M. J.↗

NASA NTRS2011

The Dawn of Vesta Science

The Dawn mission is part of NASA's Discovery Program and has as its goal the scientific exploration of the two most massive main-belt asteroids, Vesta and Ceres. The Dawn spacecraft was launched from the Cape Canaveral Air Force Station on September 27, 2007 on a Delta-II 7925H-9.5 (Delta-II Heavy) rocket that placed the 1218-kg spacecraft onto an Earth-escape trajectory. On-board the spacecraft is an ion propulsion system (IPS) developed at the Jet Propulsion Laboratory which will provide a total ?V of 11.3 km/s for the heliocentric transfer to Vesta, orbit capture at Vesta, transfer between Vesta science orbits, departure and escape from Vesta, heliocentric transfer to Ceres, orbit capture at Ceres, and transfer between Ceres science orbits. Fullpower thrusting from December 2007 through October 2008 was used to successfully target a Mars gravity assist flyby in February 2009 that provided an additional ?V of 2.6 km/s. Deterministic thrusting for the heliocentric transfer to Vesta resumed in June 2009 and concluded with orbit capture at Vesta on July 16, 2011. An additional 210 hours of IPS thrusting was used to enter the first Vesta science orbit, called Survey orbit, on August 3, 2011 at an altitude of approximately 2,735 km. To date the IPS has been operated for 23,621 hours, consumed approximately 252 kg of xenon, and provided a delta-V of approximately 6.7 km/s. The IPS performance characteristics are very close to the expected performance based on analysis and testing performed pre-launch. The only significant issue in over the almost four years of IPS operations in flight was the temporary failure of a valve driver board in the Digital Control and Interface Unit-1 (DCIU-1), resulting in a loss of thrust of approximately 29 hours. Thrusting operations resumed after switching to DCIU-2, and power cycling conducted after orbit capture restored DCIU-1 to full functionality. After about three weeks of Survey orbit operations the IPS will be used to transfer the spacecraft to the other planned science orbit altitudes. After approximately one year of science operations the IPS will be used for escape from Vesta and then for thrusting to Ceres with a planned arrival date at Ceres in February 2015. This paper provides an overview of Dawn's mission objectives and the results of Dawn IPS mission operations through the start of science operations at Vesta.

electric propulsion↗

NASA NTRS2011

In-Flight Operation of the Dawn Ion Propulsion System Through Orbit Capture at Vesta

Garner, Charles E.↗

NASA NTRS2012

Vesta and the HED Meteorites: Comparison of Spectral Properties

We present the main results obtained comparing the visible-near infrared (VIS-NIR) spectra Vesta s surface with howardites, eucrites, diogenites (HEDs). HEDs are commonly associated with Vesta based on spectral similarities. Because of such association, much effort is being made to merge the information from HEDs as well as Vestoids with that from Vesta to characterize the lithologic diversity of the surface of this asteroid and to infer clues regarding its thermal history. The Dawn spacecraft, orbiting around Vesta since July 2011, is performing detailed observations of this body and thus improving our knowledge of its properties. Dawn s scientific payload includes an imaging spectrometer, VIR-MS, sensitive to the VIS-NIR spectral range. VIR-MS began acquiring spectra during the approach phase that started in May 2011 and will continue its observations through July 2012 when the spacecraft will depart Vesta to travel to Ceres. The observations are uniformly distributed in latitude and longitude, allowing a global view of Vesta s crustal spectral properties. Using the information provided by VIR spectra, we studied the distribution of the spectral heterogeneities on the surface and used our findings to perform a comparison with HED spectra in the VIS-NIR spectral range searching for analogies and/or incompatibilities. In our analysis, we utilized a method to compare the results obtained at microscopic scale on HED samples and the one obtained at macroscopic scale on the surface of Vesta. The intent of this study is to improve our understanding of the connection between Vesta and the HEDs, which is one of the primary Dawn scientific objectives. Dawn VIR spectra are characterized by pyroxene absorptions and most of the surface materials exhibit howardite-like spectra. However, some large areas can be interpreted to be material richer in diogenite (based on pyroxenes band depths and band centers) and some others like eucrite-rich howardite terrains. In particular, VIR data strongly indicate in the south polar region (Rheasilvia) the presence of Mg-pyroxene-rich terrains. The hypothesis that Vesta is the HED parent body is consistent with, and strengthened by, the geologic and spectral context for pyroxene distribution provided by VIR on Dawn.

Ammannito, E.↗

NASA NTRS2010

In-Flight Operation of the Dawn Ion Propulsion System: Status at One Year from the Vesta Rendezvous

The Dawn mission, part of NASA's Discovery Program, has as its goal the scientific exploration of the two most massive main-belt asteroids, Vesta and Ceres. The Dawn spacecraft was launched from Cape Canaveral Air Force Station on September 27, 2007 on a Delta-II 7925H-9.5 (Delta-II Heavy) rocket that placed the 1218 kg spacecraft into an Earth-escape trajectory. On-board the spacecraft is an ion propulsion system (IPS) developed at the Jet Propulsion Laboratory which will provide most of the delta V needed for heliocentric transfer to Vesta, orbit capture at Vesta, transfer among Vesta science orbits, departure and escape from Vesta, heliocentric transfer to Ceres, orbit capture at Ceres, and transfer among Ceres science orbits. The Dawn ion thruster [I thought we only called it a thruster. Both terms are used in the paper, but I think a replacement of every occurrence of "engine" with "thruster" would be clearer.] design is based on the design validated on NASA's Deep Space 1 (DS1) mission. However, because of the very substantial (11 km/s) delta V requirements for this mission Dawn requires two engines to complete its mission objectives. The power processor units (PPU), digital control and interface units (DCIU) slice boards and the xenon control assembly (XCA) are derivatives of the components used on DS1. The DCIUs and thrust gimbal assemblies (TGA) were developed at the Jet Propulsion Laboratory. The spacecraft was provided by Orbital Sciences Corporation, Sterling, Virginia, and the mission is managed by and operated from the Jet Propulsion Laboratory. Dawn partnered with Germany, Italy and Los Alamos National Laboratory for the science instruments. The mission is led by the principal investigator, Dr. Christopher Russell, from the University of California, Los Angeles. The first 80 days after launch were dedicated to the initial checkout of the spacecraft followed by cruise to Mars. Cruise thrusting leading to a Mars gravity assist began on December 17, 2007 and was successfully concluded as planned on October 31, 2008. During this time period the Dawn IPS was operated mostly at full power for approximately 6500 hours, consumed 71.7 kg of xenon and delivered approximately 1.8 km/s of delta V to the spacecraft. The thrusting to Mars was followed by a coasting period of approximately 3.5 months that included a Mars flyby in February of 2009. The Mars flyby provided a gravity assist (MGA) for a plane change and approximately 1 km/s of heliocentric energy increase and is the only part of the mission following launch in which a needed velocity change is not accomplished by the IPS. During the coast period IPS was operated for a trajectory correction maneuver and for engineering tests but was not operated for primary propulsion. Closest approach to Mars occurred as planned on February 17, 2009 and was followed by another coasting period of just under 4 months in duration. During this last coasting phase IPS was operated only for routine maintenance activities and for system engineering tests. Deterministic thrusting for heliocentric transfer to Vesta resumed on June 8, 2009. Since resumption of cruise to Vesta IPS has been operated at throttled power levels, most of the time at full power, and with a duty cycle of approximately 93%, leading to an arrival at Vesta in July of 2011 and arrival at Ceres in February 2015. This paper provides an overview of Dawn's mission objectives and the results of Dawn IPS mission operations through one year from the spacecraft's rendezvous with Vesta.

Garner, Charles E.↗

NASA NTRS2016

An Iron-Rain Model for Core Formation on Asteroid 4 Vesta

Asteroid 4 Vesta is differentiated into a crust, mantle, and core, as demonstrated by studies of the eucrite and diogenite meteorites and by data from NASA's Dawn spacecraft. Most models for the differentiation and thermal evolution of Vesta assume that the metal phase completely melts within 20 degrees of the eutectic temperature, well before the onset of silicate melting. In such a model, core formation initially happens by Darcy flow, but this is an inefficient process for liquid metal and solid silicate. However, the likely chemical composition of Vesta, similar to H chondrites with perhaps some CM or CV chondrite, has 13-16 weight percent S. For such compositions, metal-sulfide melting will not be complete until a temperature of at least 1350 degrees Centigrade. The silicate solidus for Vesta's composition is between 1100 and 1150 degrees Centigrade, and thus metal and silicate melting must have substantially overlapped in time on Vesta. In this chemically and physically more likely view of Vesta's evolution, metal sulfide drops will sink by Stokes flow through the partially molten silicate magma ocean in a process that can be envisioned as "iron rain". Measurements of eucrites show that moderately siderophile elements such as Ni, Mo, and W reached chemical equilibrium between the metal and silicate phases, which is an important test for any Vesta differentiation model. The equilibration time is a function of the initial metal grain size, which we take to be 25-45 microns based on recent measurements of H6 chondrites. For these sizes and reasonable silicate magma viscosities, equilibration occurs after a fall distance of just a few meters through the magma ocean. Although metal drops may grow in size by merger with other drops, which increases their settling velocities and decreases the total core formation time, the short equilibration distance ensures that the moderately siderophile elements will reach chemical equilibrium between metal and silicate before metal drop merger becomes important. In this model, there must be at least 30 percent melting of the silicate phase when metal melting is complete, corresponding to a crust thickness of at least 30 kilometers on Vesta, consistent with Dawn gravity observations. Greater degrees of silicate melting and a correspondingly thicker crust are possible if Vesta accreted sufficiently rapidly.

Kiefer, Walter S.↗

DOE OSTI2025

A small core in Vesta inferred from Dawn’s observations

Vesta’s large-scale interior structure had previously been constrained primarily using the gravity and shape data from the Dawn mission. However, these data alone still allow a wide range of possibilities for the differentiation state of the body. The moment of inertia is arguably the most diagnostic parameter related to the radial density distribution of a planetary body, making it crucial for assessing the body’s state of internal differentiation. Determining the moment of inertia requires additional measurements of the amplitudes of small rotational motions, such as precession and nutation. Here we report an updated estimate of the moment of inertia of Vesta inferred from Dawn’s Doppler tracking via the Deep Space Network and onboard imaging data. The recovered value for Vesta’s normalized polar moment of inertia is $\overline{C}$/MR 2 = 0.4208 ± 0.0047 (where M is the mass of Vesta and R is the reference radius), which is only 6.6% lower than the homogeneous value of 0.4505. This value, combined with the gravity field and global shape, suggests that Vesta’s interior has limited density stratification beneath its howardite–eucrite–diogenite-dominated crust. We propose two possible origin scenarios that are consistent with the observed constraints. In the first scenario, Vesta’s interior did not undergo full differentiation due to late accretion. In the second scenario, Vesta originated as an impact remnant of a larger differentiated body re-accreted with non-chondritic bulk composition produced from a catastrophic impact. Vesta did not experience complete differentiation in either scenario, suggesting that its current state reflects a complex interplay between its accretion timing, thermal evolution, redistribution of 26 Al bearing melt and/or impact processes.

CNEOS 2014-01-08 bolide↗

NASA NTRS1993

The asteroid-meteorite connection: Forging a new link to Vesta as the parent body of basaltic achondrite (HED) meteorites

Asteroid 4 Vesta has been at the center of the debate over the identity of the howardite eucrite diogenite (HED) parent body since the early 1970s. Despite its unique (among the 500 largest asteroids) compositional match to HED meteorites, substantial dynamical difficulties in delivering fragments from Vesta to the Earth have precluded any conclusive HED parent body link. These dynamical difficulties arise because Vesta's orbital location is far from known resonances. Consequently, it has been argued as dynamically improbable that meteoroid-sized (1 km) fragments could be excavated from Vesta with sufficient velocities to reach the resonances. Through new astronomical observations, numerous small (4-7 km) asteroids between Vesta and the 3:1 resonance have been discovered to have eucrite and diogenite compositions. Based on similar orbital elements to Vesta, all of these new asteroids are likely large impact fragments excavated from Vesta. Their current orbits imply ejection velocities in excess of 700 m/sec. Smaller (1 km) fragments can therefore be expected to have been ejected with velocities greater than 1 km/sec, sufficient to reach the 3:1 and v6 resonances. Thus it now appears to be dynamically viable for Vesta to be linked as the HED parent body.

Binzel, R. P.↗

NASA NTRS1994

Grain size of the surface regolith of asteroid 4 Vesta estimated from its reflectance spectrum in comparison with HED meteorites

The grain-size distribution of the regolith of asteroid 4 Vesta has been estimated by comparing its reflectance spectra (0.3-2.6 microns) with those of HED meteorites. The finest grain-size separate (less than 25 micrometers) of a particular howardite has a reflectance spectrum most similar to Vesta's. In order to better simulate Vesta's surface mineralogy, reflectance spectra of those finest HED meteorite powders were linearly combined, and Vesta's spectrum was scaled for the best fit between them. Both the albedo and the shape of reflectance spectrum of Vesta were well reproduced by regional mixtures of the finest (less than 25 micrometers) powders of HED meteorites. The result suggests the heterogeneity of Vesta's surface and provides an estimate of the visible reflectance of Vesta that is close to its Infrared Astronomical Satellite (IRAS) albedo. Thus, this suggests that fine grains can be generated and retained by relatively small bodies (Vesta is approximately 500 km in diameter).

Hiroi, Takahiro↗