Photometric Studies of Eros from NEAR Data

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Photometric Studies of Eros from NEAR Data

• Jianyang Li
• Mike AHearn
• Lucy McFadden

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Disk-integrated Photometry Is Not Enough

• Non-zero lightcurve amplitudes
• Cross-section changing
• Domingue et al. 2002
• Whole-disk sense
• Only NEAR images with whole disk

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Forward Simulation

• Hapkes parameters (Domingue et al. 2002)
• Eross shape model (Thomas et al. 2002)
• Compute theoretical lightcurves

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Fit Ground Based Data

• Lightcurve maxima
• Assume the maximum cross-section 333 km2
• RMS 0.064, 0.5 of the average lightcurve maxima

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NEAR Data

• Disk-resolved
• 550nm (F1 filter)
• Images at S/C range about 100 km, resolution
  10m/pix
  
• Eross disk fill 70 image frame or more

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Fitting Scheme

• ? log ? / ? log w
• (? ? / ? w )(w / ?)
• Large quantity means important data
• Published Eross photometric parameters (Domingue
  et al. 2002)

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Our Scheme

• Fit w and ? using MSI images around 60? phase
  angles
  
• Fit g using all available MSI images (60?

MSI between 55? and 65?
All MSI data
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Fitting NEAR MSI Data

• Assume B01.0, h0.022
• RMS error 0.0032, 6 of average
• 10 error estimated from scattering of residual
• Caused by albedo variation or surface feature?

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Results and Comparison
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Albedo w

• Single-scattering albedo
• Smaller than before
• Comparable with simulations
• Close to the average value of S-type asteroids
• Consistent with spectrum

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Asymmetry Factor g (-0.29 vs. -0.25)
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Opposition Effect

• Lack of data near opposition
• NEAR MSI data are between 50? and 120?
• Ground-based data only in fitting opposition
  parameters
  
• In this work, B0 was allowed to exceed 1
• B01.38, h0.015

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Roughness Parameter ?

• Image footprint size comparable with Clark et al.
  (2002), 2.5-5 km/spectrum
  
• Domingue et al. (2002) used whole disk averaged
  
  

• A local parameter and sensitive to scale size
  (Shepard 2001)
  
• Different slope distributions at different scale
  sizes

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Summary

• Photometric analysis using NEAR MSI 550nm images,
  ground-based lightcurves, and theoretical
  simulations
  
• Five Hapkes parameters were found
• Geometric albedo 0.25, Bond albedo 0.098
• Smaller than previous values
• Move Eros closer to the values of average S-type
• Consistent with Eros spectrum