Understanding and Protecting Our Home Planet

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New Constraints from STEREO on the Population of
Vulcanoids Interior to Mercury

S. A. Stern1, D. D. Durda1,2, A. Steffl2, D.
Hassler2, and N. Cunningham3 1NASA Headquarters,
2SwRI, 3U. Nebraska
Presentation 50.04
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The Vulcanoid Population
4-12 solar elongation

• Intrinsically interesting new class of objects
• Sample of condensed material from the early inner
  solar system
• Relevance to Mercurys cratering record and
  chronology

Origin/sources of Vulcanoid material

• Left over planetesimals high-temperature end of
  condensation sequence
• Debris from giant impact that might have stripped
  away Mercurys rocky mantle

Removal/loss of Vulcanoid material

• Evaporation
• P-R drag, Yarkovsky Effect
• Collisions
• Gravitational perturbations

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Previous Searches

• Ground-based
• Perrine (1902, 1906, 1909)
• Campbell and Trumpler (1923)
• Courten et al. (1976a,b)
• Leake et al. (1987)
• Campins et al. (1996)

• High-Altitude Airborne
• Durda and Stern (2001-2002)

• Spacecraft (SOHO)
• Durda et al. (2000) Most constraining previous
  search No Vulcanoids brighter than V 8.5
• Schumacher and Gay (2001)

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Solar TErrestrial RElations Observatory
The Heliospheric Imager (HI) instrument aboard
the NASA STEREO spacecraft, launched on 25
October 25 2006, provides a new and sensitive
opportunity to explore the inner heliosphere from
the Suns inner corona and the region of space
near the Sun.
STEREO

• HI-1 Instrument
• Vulcanoid zone lies fully within HI-1 field of
  view
• Ability to observe faint stellar sources (40
  times more sensitive than previous search with
  SOHO)
• 0.6 arcmin pixels
• Parallax due to spacecraft orbital motion moves
  Vulcanoids 2.5 arcsec/min relative to background
  stars

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HI Image Processing

• Remove hot pixels
• Top three pixels discarded from each image set
• Calculate median of remaining pixel values
• Subtract resulting median image from individual
  images to remove fixed pattern noise

• Remove background gradient
• At each pixel location calculate average of all
  pixel values in circular aperture between 5 and
  12 pixels from central point
• Subtract resulting image from individual images
  to remove gradient

• Co-register and blink
• Use 10 stars common to all images in animation
  sequence as registration fiducials
• Determine rotation, offset, and scaling necessary
  to align individual images with reference image

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HI Image Processing

• Remove hot pixels
• Top three pixels discarded from each image set
• Calculate median of remaining pixel values
• Subtract resulting median image from individual
  images to remove fixed pattern noise

• Remove background gradient
• At each pixel location calculate average of all
  pixel values in circular aperture between 5 and
  12 pixels from central point
• Subtract resulting image from individual images
  to remove gradient

• Co-register and blink
• Use 10 stars common to all images in animation
  sequence as registration fiducials
• Determine rotation, offset, and scaling necessary
  to align individual images with reference image

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Search Results
stereo_a_hi1_feb7.mov
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Search Results
We examined five 48-hour sequences of images,
spaced about 10 days apart (24 processed HI-1
images per sequence). Moving were objects
located through visual examination of the five
movie sequences and identified using commercial
astronomical software and search tools on the MPC
web site. Main-belt asteroids as faint as V ?
13.5 identified. Objects discovered during the
search

• Planets (Mercury, Venus, Uranus, Neptune)
• Asteroids (1 Ceres, 10 Hygiea, 29 Amphitrite, 241
  Germania, 349 Dembowska, 385 Ilmatar, 444 Gyptis,
  660 Crescentia, 678 Fredegundis)
• Comet (C/2006 M4 SWAN)

Average magnitude limit across search field V ?
12.5. Assuming a Mercury-like albedo and phase
function, this translates to D ? 6km
Vulcanoids. No Vulcanoids found.
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Extras