Another science case now under study is low-resolution spectroscopy of NEO's to begin to characterize their composition. Presently, there are about a dozen "classes" of asteroids. However, less than half of those are represented in the NEO's. This is probably due to the way asteroids in the Main Belt migrate into Earth-crossing orbits.
We have "samples" of the NEO's in the form of
meteorites so that we can, to first order, determine
the composition of these asteroids based on the composition
of the meteorites. In detail, there are still some
differences that are thought to be due to "space weathering"
of the surfaces of the asteroids. Compositionally, most NEO's
are stony with a few being metallic.
The following figures from the books Asteroids
(1979, T. Gehrels, ed., The University of Arizona Press,
1181 p.) and Hazards due to Comets & Asteroids
(1994, T. Gehrels, ed., The University of Arizona Press,
1300 p.) illustrate some of the issues Project ASTEROID
will pursue on the topic of asteroid composition.
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This figure from the paper by John L. Remo, "Classifying and Modeling NEO Material Properties and Interactions" in Hazards, p. 561, shows a comparison of reflectance spectra of asteroid and meteroid types that may be interpreted (according to Remo) as associating some such types. |
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This figure from Remo's paper (p. 560) shows individual absorption bands of solar reflectance spectra for feldspar, olivine, and pyroxene minerals and iron-nickel metal as a function of wavelength. Remo states "Meteroritic material and presumably related asteroid materials composed of a mixture of these materials is (ideally) likely to to possess composite spectra of these curves." Such spectra from asteroids would indicate only the composition of an optically thick surface layer. Note that these curves cover the entire optical and J-H-K bands of the near infrared, while Project ASTEROID will limit itself to optical spectroscopy. Note that by simply looking at the tilt of the spectra in the 0.4 micron to 1.0 micron region, a first-order reading of composition may be obtained. |
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This figure from the paper by C. R. Chapman, "The Asteroids: Nature, Interrelations, Origins, and Evolution" in Asteroids, p. 36, shows mean spectra for S-like asteroids as a function of wavelength. The mean spectrum of 17 asteroids with a < 2.4 AU is at the top. The bottom curve shows the mean spectrum of 51 asteroids with a > 2.6 AU. This indicates that asteroids of a certain type (and therefore, composition) tend to have the same spectra, which is critical to our investigations. |
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This figure from Chapman's paper (p. 37) shows the schematic inter-relationship among pyroxene, olivine, and metal content of asteroids by volume percent. In this figure, Chapman notes that spectra are from a paper by Gaffey spanning the range from 0.3 to 2.5 um (tick marks each 0.5 um). Chapman states "Two components in each mixture contribute roughly equal spectral traits. Also, the third component is present in each mixture in volumetric amounts of 5 to 10%. Note the spectral dominance of pyroxene; its features are prominent even though it is a minority constituent of all three mixtures. Modifications of the 0.9 um pyroxine band due to olivine are: A, depression near 1.2 um; B, shift of band center to longer wavelength. Modifications of the pyroxene spectrum by metal are: C, high 2 um reflectance (also could be due to olivine); D, higher ultraviolet reflectance; E, shallower absorption band depth. A mixture of olivine and pyroxene makes the 2 um band, F, shallower than the combined band near 1 um. |
While recent studies of asteroids can obtain high resolution (spectroscopic R > 100) visible spectra of asteroids, the NEO's that we are likely to be observing are too faint for such detailed study with a 2-m class telescope. We are therefore limited to much lower resolution spectra, say R = 10. Fortunately, the original spectral classification of asteroids that still does well in distinguishing among the classes was done with 8 filters. Therefore, a spectroscopic resolution R = 10 should be sufficient to yield reasonable compositional information. Given spectral slopes (spectrally flat to red) and absorption features (due primarily to olivine, pyroxene, and feldspar), an SNR of about 10 is sufficient to allow classification of these faint asteroids.
However, some asteroids may be too faint to allow us to obtain an SNR of 10 with a spectroscopic resolution R of 10. In those cases, we will bin the data to lower resolution. At the very least, this will give us spectral slopes if we can still attain the minimum SNR of 10. This will still allow us to extract some compositional information from the data.
Last modified: January 3, 2008.