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Exploring Main Belt Asteroids - Draft White paper - NRC Decadal Study

Status Report From: National Research Council
Posted: Thursday, October 18, 2001

Draft 10/18/01

Mark V. Sykes, Steward Observatory, University of Arizona; Stephen M. Larson, Robert Whiteley, Uwe Fink, Robert Jedicke, Josh Emery, and Ronald Fevig, Lunar and Planetary Laboratory, University of Arizona; Michael Kelley, NASA Johnson Space Center; Allen W. Harris and Steven J. Ostro, Jet Propulsion Laboratory; Kevin L. Reed, Raytheon Corporation; Richard P. Binzel and Andrew Rivkin, Massachusetts Institute of Technology; Christopher Magri, University of Maine ; Wlliam Bottke and Dan Durda, Southwest Research Institute; Russell Walker, Vanguard Research; Don Davis and William K. Hartmann, Planetary Science Institute; Derek Sears, University of Arkansas; Hajime Yano, Institute of Space and Aeronautical Science; James Granahan, BAE Systems; Alex Storrs, Towson University; Schelte J. Bus, Jeffrey F. Bell, and David Tholen, University of Hawaii; Alberto Cellino, Torino Observatory; M. Cristina De Sanctis, Instituto di Astrofizica Spaziale CNR; and; David J. Lien, Oklahoma State University

I. PERSPECTIVE

Planet formation in our solar system followed a variety of paths, leading in most cases to finished planets: rocky bodies in the inner solar system, gas giants in the middle region, and icy planets in the outer solar system. However, the planet-forming process was interrupted in two regions - the Kuiper Belt (with the exception of Pluto) and the asteroid belt - leaving a vast population of small bodies instead of a single planet or planets. Accretion formed numerous protoplanets in the region between Mars and Jupiter, certainly Ceres-sized bodies formed, and perhaps even Mars-sized bodies. These large bodies would have gravitationally perturbed the local population, increasing collision speeds, and terminating accretion among small bodies. Fragmentation and disruption commenced.

Some asteroidal protoplanets were large enough that they were not fragmented, but continued to grow in this new environment. Jupiter was also forming, accreting (in addition to rocky material) great amounts of ices and gases which were unable to condense in the warmer region of the inner solar system. Jupiter's large mass and orbital location created strong resonance locations in the asteroid region that quickly removed any body unlucky enough to move into one. This depletion mechanism removed most of the primordial mass in the asteroid belt and finally cut off the material needed there for further planetary growth.

Today, main belt asteroids consist of a few unshattered protoplanets (e.g., Ceres, Pallas, Vesta) and disrupted protoplanets whose pieces were shattered and dispersed, then further shattered and dispersed leaving the hundreds of thousands of fragments that dominate the main asteroid belt today (e.g., Eros, Gaspra, Ida). While the boundary between the two populations may have some fuzziness (containing perhaps small shattered protoplanets that were not dispersed), these fragments tend to be relatively homogeneous, and protoplanets are more complex (particularly the differentiated Vesta).

The pot has been stirred dynamically and collisionally over the age of the solar system, yet it retains evidence of initial conditions and planet formation and evolution. Some asteroids have shifted into Earth-crossing orbits, causing concerns about the hazard of impact and interest in their potential as resources for human activity in space. The primoridial outer main belt may have been the source of Earth's water (Moridelli et al. 2000), thus critical to the existence of life on Earth. Destructive asteroid collisions have also been a source of interplanetary dust, generating much of our zodiacal cloud. The detection of non-primordial dust disks around other stars suggests that asteroids and the collisional processes we observe in our solar system occurs elsewhere in the universe.

II. KEY SCIENCE QUESTIONS

There are a heierarchy of questions that motivate almost all of asteroid science which are rooted in our knowledge of the present. In earlier decades asteroid science focussed primarily on the gathering of basic data from which derives our modern understanding of the processes operating in the asteroid belt at present and in the past. This is a neverending process. Our continuously expanding capabilities to gather a wider range of more detailed data over a larger number of objects continues both allows for more sophisticated, detailed, and overarching models, as well as more challenges to them. Asteroid science is dynamic.

On the basis of what we know today of the asteroid belt as a site of interrupted terrestrial planet formation and of complex collisional and dynamical activity, there are three fundamental questions that we consider to be key. They are listed below along with the next level of questions that derive from them:

A. What was the compositional gradient of the asteroid belt at the time of initial protoplanetary accretion?

  • What is the compositional structure of the main asteroid belt today?
  • How do dynamics and collisions modify this structure over time?
  • What are the physical properties of asteroids?
  • How do surface modification processes affect our ability to determine this structure?

B. What fragments originated from the same protoplanetary body, and what was the original distribution of those parent bodies?
  • What asteroid fragments are associated dynamically, suggesting a common origin?
  • What objects are compositionally linked?

C. What are the early steps in planet formation and evolution?
  • What are the compositions and structures of surviving protoplanets?
  • What does their cratering record combined with the cratering record of younger surfaces reveal about the primordial size distribution of objects in the asteroid belt?
  • What do meteorites tell us about formation and evolution processes of these bodies? How well do they sample the asteroid belt?

III. HOW CAN THE KEY SCIENCE QUESTIONS BE ADDRESSED?

DETERMINING THE PRIMORDIAL COMPOSITION GRADIENT

This requires acquiring data on the composition and physical properties of a massive number of asteroids. Some information (albedos and diameters) may be acquired for a very large sample of asteroids over a relatively short period of time from an orbiting spacebased facility, but other information such as spectral reflectance, lightcurves and phase curves, radar determined properties and morphology, are necessarily acquired one object at a time and requires a sustained program to support such observations.

Relating observations to compositions requires an understanding of how spectra relate to specific minerologies and how surface scattering properties effect that relationship. In addition, the effects of surface impacts and space weathering over time must be understood. This requires laboratory studies of meteorites and terrestrial analogs of other interplanetary materials.

Leveraging composition from spectra may be limited when the bulk composition of an asteroid are not optically active minerals or their spectra is masked (e.g., by low-albedo carbonaceous material). Unfortunately, not all asteroid compositions are thought to be represented in meteorite collections. This situation might be improved with samples from dozens of targetted asteroids, but this is not practical in the next decade.

Even given a detailed information on the modern compositional structure of the asteroid belt, inferring its primordial structure requires an understanding of the various dynamical processes acting on asteroid orbits, such as gravitational perturbations, chaotic scattering, Yarkovsky effects, collisions, etc. It also requires an understanding of collision mechanics and the preferential erosion of some compositions with their associated physical properties from the modern population of objects.

Groundbased Observations

An important part of gathering information needed to address the key questions above is the brute force observations of many thousands of main belt asteroids. Fundamental observations include low-resolution spectroscopy and lightcurve studies.

How many asteroids are there for which spectra can be obtained using low-resolution systems on telescopes of different apertures? [I am thinking of figures showing diameter vs au for different albedos with curves indicating the limits of different aperture systems - what would it take to get everything having a diameter greater than 100, 50, 10, 5, 1 km?]

Q: What programmatic resources (x number of telescopes operating y nights a year at a funding level of z for w years) would be required to obtain spectra of all asteroids above a given diameter?

Q: To what extent has the job been done?

Q: Previous 3 questions applied to lightcurves.

Even modest aperture telescopes are capable of doing lightcurve studies of tens of thousands of asteroids. This suggests an opportunity for small institutions to engage in such studies, perhaps leveraging basic science education opportunities for students while contributing to very basic knowledge about asteroids.

Q: What laboratory programs are needed to support this effort? Meteorites are being collected all the time. Is there a an ongoing program(s) which generate ongoing information useful to asteroid spectroscopy?

Q: Radar can provide information surface properties, reflectivity, spin state, and morphology. To what extent can radar be used to probe the properties of main belt asteroids? For current facilities, what is the diameter/distance (for a given reflectivity) limits for generating morphology models and determining radar reflectance properties? How many asteroids do these include? What programmatic resources would be required to obtain properties/morpholodies for them?


Q: What upgrades to radar facilities are planned or desired for the next decade and how would such upgrades impact the above question?

Arecibo has been recently upgraded and should reach its nominal sensitivity perhaps within a year. Support for personnel at Arecibo and for asteroid radar astronomers could certainly be better.

Q: An orbiting IRAS-like facility (with perhaps a larger aperture and modern detectors) in a polar orbit could detect what fraction of main-belt asteroids above what diameter at a given heliocentric distance? Since Earth's angular motion is greater than that of MBAs, a sweep of all ecliptic longitudes at 90 degrees solar elongation over the course of a year could sample almost all detectible MBAs. A visible light channel would be needed (having no thermal component) to ascertain albedos and diameters. How would pointing mode observations (e.g., to map out collisional debris structures or comet dust distributions) impact the completeness of the survey? Would it make more sense to accumulate all pointed observations to the beginning and end of the mission?

Q: What is the status of programs supporting laboratory studies of meteorites and materials applicable to the interpretation of remote sensing observations of asteroids?

ADDRESSING B

Identify asteroids having discernable dynamical relationships (families), determine their physical and spectral properties and see if a common origin makes cosmochemical sense.

Identify asteroids more remote dynamically that may be geochemically related. This does not necessarily mean compositionally identical, but would embrace metallic core fragments with asteroids consisting of mantle material.

Identify meteorites associated with specific asteroids and asteroid families.

The mechanics of collisional fragmentation and disruption must be understood.

Flybys of many members of a particular family would reveal cratering records which could be used to determine relative (absolute?) ages of family members, identifying multigenerational collisional fragementation histories. Flybys can also determine mass densities and help determine whether they are primordial or arise from collisional history.

Q: Can cratering counts be used to date asteroid surfaces in years before present?

ADDRESSING C

This requires an Understanding of compositional variations within protoplanetary objects and what it says about the planet formation process and early protoplanetary evolution (e.g., due to heating). This can be accomplished by studying individual protoplanets [rendezvous missions] and by piecing protoplanets back together from their identifiable fragments.

Multiple sample returns from family fragments showing spectral variations may provide important detailed geochemical information about the formation and internal evolution of the parent body prior to disruption. This might be accomplished to a more limited extent by multiple rendezvous to family fragments.

Q: Do we know anything about the uniformity of the impactor population throughout the asteroid belt as a function of time? To address this, question we might have to launch a flotilla of flyby missions to a range of asteroid sizes (ages) and heliocentric distances.

Q: Can meteorites be grouped by potential origin from common protoplanetary parent bodies?

IV. MAINTAINING CAPABILITIES FOR THE FUTURE

The Key Scientific Questions for main belt asteroid studies are not going to be conclusively answered over the next decade. To ensure that the capacity to address these questions is maintained, requires strong and steady support for the underlying R&A programs which serve both to address these question directly while creating the necessary basis for future asteroid missions.

V. DATA ARCHIVE AND ACCESS ISSUES

Data associated with asteroid studies is archived by the NASA Planetary Data System. The data is peer reviewed as part of the ingestion process to ensure quality, consistency, and adequate documentation. To maximize usefulness of this data requires the creation of an interface which would allow researchers to identify desired data. This might include a listing of all holdings regarding a particular object, or the identification of those objects meeting SQL criterion across all asteroid data sets.

Challenges:

  • All groundbased as well as spacebased data should be captured.
  • Relevant laboratory data should be captured
  • All archive products should be subjectable to query

    VI. UTILIZATION POTENTIAL

    The pursuit of basic science in the main asteroid belt may provide information that is useful to the utilization of asteroids in near-earth orbits to support future human space activity. For instance, the efficiency at which asteroids of given sizes from given (compositional) regions are removed from the main belt into near-earth orbits is useful as is the growth and physical properties of regolith of carbonaceous to metallic objects. See Jones et al. (The Next Giant Leap: Human Exploration and Utilization of NEOs).

    VIII. ADVANCED TECHNOLOGY

    The continued improvement of solar panels and electric propusion thrusters create the capability for multiple rendesvous and flyby missions that are not practical with chemical-based propulsion systems.

    For multiple flyby missions, solar sail technology may be ideal.

    IX. EDUCATION AND PUBLIC OUTREACH

    The advent of remote observational facilities coupled with the large numbers of asteroids, make asteroids a wonderful target for seasonally independent education. Programs could be designed in part to acquire data needed to address the key scientific questions - such as spectra and lightcurves. For instance, a student (high school or college) could acquire a spectrum or lightcurve on an asteroid (most might have been previously unobserved), and then fit a simple spectral model (2 or 3 component) to the spectrum, or triaxial ellipsoid to the lightcurve. By comparing the final result to the data, students could learn that science is an iterative process - and that there are no final answers, just better understanding.

    [My criticism of most "education and public outreach" is that it boils down to not much more than PR and often leaves students and the public with the misperception of science as a series of marvelous discoveries that explain all.]

    X. RECOMMENDATIONS (?)

    2003-2013

    (1) Asteroid missions addressing the key science questions cannot be planned nor provide a maximum return without a credible and continuing investment in earthbased and spacebased remote observation programs and theory programs. These programs have suffered at NASA in recent years as funding has shrunk. [detail program areas, including radar]

    (2) IRAS2 - to achieve albedos and diameters for all main belt asteroids having D>1 km through the Trojan population. Must have non-thermal visible channel, may limit thermal channels to 12 and 25 microns, simplifying the experiment relative to IRAS..

    (3) Protoplanet rendezvous missions

    (4) Technology development for large number of multiple asteroid flybys using microsatellites and possible advanced propulsion systems such as solar sails that would allow for multiple flybys per spacecraft.

    (5) Technology development for long-lived multiple asteroid rendesvous missions

    (6) Preparation for beyond. Trojan rendezvous, asteroid family rendesvous, multiple asteroid sample return.

    // end //

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