Mario Juri´c Robert Lupton Princeton Universityfaculty.washington.edu/ivezic/talks/Hayden.pdf ·...
Transcript of Mario Juri´c Robert Lupton Princeton Universityfaculty.washington.edu/ivezic/talks/Hayden.pdf ·...
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SDSS Asteroid Observations
Zeljko Ivezic
Mario Juric
Robert Lupton
Princeton University
American Museum of Natural History, New York, May 3 2003
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Asteroids as seen from spacecrafts
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What is the significance of ground based asteroidstudies in an era when spacecrafts can obtain such
breathtaking photographs?
The answer is simple; the SDSS asteroid observations provide
• A sample 30,000 times larger than the one shown in previous
figure
• Five-color images, rather than black-and-white images
• Sensitivity to detect asteroids smaller than the smallest craters
visible on the four objects in previous figure
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Sloan Digital Sky Survey
• Imaging Survey
– 10,000 deg2
– 5 bands (ugriz: UV-IR), 0.02 mag photometric accuracy
– < 0.1 arcsec astrometric accuracy
– 100,000,000 stars and 100,000,000 galaxies
• Spectroscopic Survey
– 1,000,000 galaxies
– 100,000 quasars
– 100,000 stars
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A 2.2’ × 1.9’ piece of sky (0.2s)
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SDSS Asteroid Observations
Moving objects in Solar System can be efficiently detected out to
∼ 20 AU even in a single scan: 5 minutes between the exposures
in the r and g bands (algorithm: Lupton et al., 2002).
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Asteroids move during 5 minutes and thus appear to have pecu-
liar colors.
The images map the i-r-g filters to RGB. The data is taken in
the order riuzg, i.e. GR··B
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SDSS Asteroid Observations
• Moving objects must be efficiently found to prevent the con-
tamination of quasar candidates (and other objects with non-
stellar colors)
• Detected as moving objects with a baseline of only 5 minutes
• The sample completeness is 90%, with a contamination of 3%
• The velocity errors 2-10%, sufficient for recovery within a few
weeks
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Major Asteroid Surveys:
• Mc Donald Survey (Kuiper et al. 1958): 1000 asteroids
(m<16.5, 1,200 deg2)
• Palomar-Leiden Survey (van Houten et al. 1970): 2000 aster-
oids (m<20, 200 deg2)
• Spacewatch (Gehrels et al. 1991): 60,000 asteroids (m<21,
4,000 deg2)
• SDSS: ∼140,000 asteroids (m<21.5, 6,000 deg2) in five bands
SDSS measured ∼100 times more asteroid colors than all pre-
vious surveys together (the largest previous multi-color survey:
ECAS, Zellner et al. 1985, with 600 objects)
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Main SDSS Asteroid Results
• A measurement of the main-belt asteroid size distribution to
a significantly smaller size limit (< 1 km) than possible before
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SDSS Sensitivity for Detecting Moving Objects
SDSS can detect a 100-mile object beyond Neptune, and a 100-
yard object in the main asteroid belt
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Main SDSS Asteroid Results
• A measurement of the main-belt asteroid size distribution to
a significantly smaller size limit (< 1 km) than possible before
• Discovery of a break in the size distribution at D ∼ 5 km
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COMPARISON OF ASTEROID SIZE DISTR� IBUTION: OBSERVATIONS AND MODELS
The asteroid size distribution (Davis 2002, in Asteroids III).
SDSS results:1) Extended the observed range to ∼300m2) Detected the second break at ∼5 km
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Main SDSS Asteroid Results
• A measurement of the main-belt asteroid size distribution to
a significantly smaller size limit (< 1 km) than possible before
• Discovery of a break in the size distribution at D ∼ 5 km
• A smaller number of asteroids compared to previous work:
the number of asteroids with diameters larger than 1 km is
about 0.75 million
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1 101 10
The impact rate for D>1 km: once in a million years
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Main SDSS Asteroid Results
• A measurement of the main-belt asteroid size distribution to
a significantly smaller size limit (< 1 km) than possible before
• Discovery of a break in the size distribution at D ∼ 5 km
• A smaller number of asteroids compared to previous work:
the number of asteroids with diameters larger than 1 km is
about 0.75 million
• The distribution of main-belt asteroids in 4-dimensional SDSS
color space is strongly bimodal (rocky S-type and carbona-
ceous C type asteroids)
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The colors of a sample of known asteroids observed by SDSS.
The colors depend on asteroid surface reflectivity.
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Main SDSS Asteroid Results
• A measurement of the main-belt asteroid size distribution toa significantly smaller size limit (< 1 km) than possible before
• Discovery of a break in the size distribution at D ∼ 5 km
• A smaller number of asteroids compared to previous work:the number of asteroids with diameters larger than 1 km isabout 0.75 million
• The distribution of main-belt asteroids in 4-dimensional SDSScolor space is strongly bimodal (rocky S-type and carbona-ceous C type asteroids)
• A bimodality is also seen in the heliocentric distribution ofasteroids: the inner belt is dominated by S type asteroidscentered at R ∼ 2.8 AU, while C type asteroids, centered atR ∼ 3.2 AU, dominate the outer belt.
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How do we know this? We cannot estimateorbits from our data.
• We cannot estimate orbits, but we can get (crude) distances
from the apparent motion of our asteroids
• We can use catalogues of known asteroids, and find them in
SDSS imaging data.
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Use these colors to color-code symbols and plot the distribution
of orbital elements...
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The semi-major axis v. (proper) inclination of a sample of known
asteroids detected by SDSS
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The semi-major axis v. (proper) inclination of a sample of known
asteroids detected by SDSS
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The osculating inclination vs. semi-major axis diagram.
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Although optimized for Extragalactic science,
SDSS is significantly contributing to
Solar System science
... and the data are public:
SDSS Moving Object Catalog available
at
www.sdss.org
American Museum of Natural History, New York, May 3 2003
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Stars (and galaxies) lie in well-defined parts of colorn space.
When searching for quasars, we are interested in the ‘outliers’,
the ones that don’t behave.
All the stars detected in about 2.5 square degrees of sky;
yellow-red color v. red-infrared color
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951
Gaspra (top, Galileo), Deimos and Phobos (bottom, Viking)
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Ida
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A fast-moving
asteroid.
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