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Near-Earth Objects in the Sloan Digital Sky Survey

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Near-Earth Objects in the Sloan Digital Sky Survey Near-Earth Objects in the Sloan Digital Sky Survey S. Kent Nov 27, 2009 Steve Kent, Tom Quinn, Gil Holder, Mark Schaffer, Alex Szalay, Jim Gray, Zeljko Ivezic Outline I. Near-Earth Objects II. Brief Description of SDSS imaging survey III. NEO population IV. NEO Composition and origin Particle to Astro Translation Guide Particle Physics Term Astrophysics Term Beam Line Telescope Monte Carlo Simulation Calorimetry Photometry Energy histogram Spectrum Pedestal Bias Trigger table Target Selection Algorithm Tracking Astrometry Silicon Pixel Detector Silicon Pixel Detector Charged particle track Signal Noise I. NEO Basic Information 1. Definition: NEO = Near Earth Object (asteroid or comet). Perihelion q < 1.3 AU. ~7000 known. 2. PHA = Potentially Hazardous Asteroid ( big & might hit the earth). 997 Known H=13 10 km diam - Dinosaur extinction H=18 1 km diam - Goal for completeness (H is apparent mag of asteroid viewed face-on from sun at a distance of 1 AU) Distribution of Main Belt Asteroids Orbits of NEO's Typical NEA Orbit Programs to find NEOs 1. LINEAR (Lincoln Near Earth Asteroid Research) MIT/Lincoln Lab Telescope near Socorro, NM 2. NEAT ( Near Earth Asteroid Tracking) JPL Telescopes in Hawaii / Palomar (CA) 3. LONEOS (Lowell Observatory) Lowell, AZ 4. Spacewatch (Steward Observatory) Kitt Peak 5. Catalina Sky Survey Steward Observatory; ANU 3 telescopes (Catalina Mtns; Siding Springs) SpaceGuard - Goal is to find 90% of all NEAs D > 1 km (H~18) Aside ... The “measure” of a NEO ● Absolute Magnitude H ● Diameter D ● Mass M ● Energy E (Megatons) ● Crater diameter ● Inter-relationships depend on – Density – Albedo – Impact target (Moon v. Earth) – Impactor physics Puzzles Regarding NEOs ● Origin – Kirkwood gaps (Resonance with Jupiter) ● Collisions & scattering? – Inner asteroid belt (Multiple resonances with Jupiter & Saturn) ● Colors and composition – NEOs do NOT have same color or spectral energy distribution as Main Belt asteroids ● Space weathering? ● Luminosity Function – Excess of low luminosity asteroids? II. THE SLOAN DIGITAL SKY SURVEY AS A NEO DETECTOR (SDSS I and II; not III) Participating Institutions ● American Museum of Natural History ● Korean Scientist Group ● Astrophysical Institute Potsdam ● LAMOST ● University of Basel ● Los Alamos National Laboratory ● Cambridge University ● Max-Planck-Institute for Astronomy/Heidelberg ● Case Western Reserve University ● Max-Planck-Institute/Garching ● University of Chicago ● New Mexico State University ● Drexel University ● Ohio State University ● Fermi National Accelerator Laboratory ● University of Pittsburgh ● Institute for Advanced Study ● University of Portsmouth ● Japanese Participation Group ● Princeton University ● Johns Hopkins University ● US Naval Observatory ● Joint Institute for Nuclear Astrophysics ● University of Washington ● Kavli Institute for Particle Astrophysics and Cosmology Stanford/SLAC ● External Participants Funding Agencies ● Alfred P. Sloan Foundation ● Participating Institutions ● NASA ● National Science Foundation ● Department of Energy ● Japanese Monbukagakusho ● Max Planck Society ● Higher Education Council for England ● Management: Astrophysical Research Consortium Instrumentation 2.5 M Telescope 149 Mpix Camera U. of Washington Plug plate designs Fermilab Plug plates Data Tapes Apache Point Obs. IMAGING SURVEY North 54 sec per filter 5.4 min per 5 filters East (=>NEOs rotate and are irregular) Two interleaved scans (run 259, 273) taken on succesive nights, Nov 1998 2.5 degrees wide Color coding: g' r' i' (3 images taken in succession over 5 minutes) Stripe Layout 26,000 sq. deg. (including repeats) that are “good” Benefit of New Mexico R-5107B Restricted Airspace White Sands Missile Range ==> Very few plane trails SDSS Transient Objects Comet High earth orbit satellite Dalcanton Meteor (ASTRON) Low earth orbit (red) and geosync (blue) satellites ?????? Main-belt asteroid Near-earth object Solar System Objects in SDSS ● Slow-moving point-source objects – Proper motion less than 1 deg/day – All detections measured, linked by photo, and included in object catalog. – Most are main belt asteroids – 106 are known NEOs ● Fast moving extended-source objects – Proper motion greater than 1.2 deg/day – Detections included in object catalog but not linked by photo or well measured – All are NEOs ● Comets III. Finding fast NEOs in SDSS 1. Classified as GALAXY by photo 2. Proper motion > 1.2 deg/day 3. m(r') <~ 21 4. Match independent object detections based on orientation, angular velocity. 114 good detections (60% of imaging data) => Only one with known orbit (2007 WD5) Magnitude vs. Angular Velocity Elongation vs. Absolute Mag. H=25 H=18 H=25 θ Earth Asteroid Absolute Magnitude: H = mag. of asteroid illuminated face-on by sun at distance Sun of 1 AU. Cumulative Distribution vs. Elongation from Opposition Modeling SDSS NEO Population ● Assume single power law (H=21 to 26) – log N = kH + const. – Solve for k, const. ● Phase angle parameter (G=0.15) ● Orbit distribution – Use JPL catalog of 1700 known NEOs with H<19 to define distribution in a, e, i. ● Simulate SDSS scanning pattern; apply selection criteria. ● Constraints: Three observables. Apparent Mag. Distribution Proper Motion Distribution Solar Elongation Distribution Results for 114 SDSS Detections Distance D = 0.06 to 0.14 AU Absolute Mag H ~ 21-26 (20 to 250 m diameter) Total number to H=26 1 million Power law slope k 0.55 Earth collision rate @ 5 Megatons 4,000 years Colors Errors too large to be of much use Earth Collision Rate Examples of SDSS NEO-type event Meteor Crater Arizona 50,000 B.C. Mount Bravo St. Helens Nuclear 1980 Test 1954 Tunguska Anomaly ● 1908 Impact near Tunguska River, Siberia ● Impact energy 5-20 Megatons ● 100 years ago - why so recent? – MTBI for 5 Megatons is – 4000 years IV. Composition and Origin of NEOs SDSS-Detected Resonances ν 3:1 5:2 2:1 6 Trojans Hungarias Outer Belt Inner Belt Mid Belt Mars NEOs Crossers Colors v. Taxonomy ● Colors: Easy to measure – Coarse indicator of composition – SDSS Moving Object Catalog (MOC) - 100,000 objects ● Taxonomy : based on spectroscopy – Empirical, but presumption is that it is tied to chemical composition of asteroids ● Many taxonomic systems - most complete is MIT SMASS system (Small Mainbelt Asteroid Spectroscopic Survey) – Based on observations of 1600 main-belt asteroids and 400 NEO – Optical and (some) NIR spectra – 595 objects have SMASS types and SDSS colors SDSS Colors (Main-belt asteroids) a * z - S i C (Ivezic et al. 2001) a * (Parker et al. 2008) Asteroid Families (main belt) All Families Only n o i t a n i l c n I Semi-Major Axis (Parker et al. 2008) Taxonomy from Spectra - SMASS system Common g r i z Rare (Binzel et al. 2001) Chondrite Conundrum ● Ordinary chondrites are most common meteorite on Earth. ● Taxonomically similar to Q and O complexes, which are rare in main-belt asteroids ● Q and O, however are more common in NEOs. ● Why? – Space weathering? – Luminosity effect? SMASS Spectra converted to synthetic SDSS Colors SMASS Main Belt Asteroids SMASS NEOs Classifications from SDSS colors ~80% reliability Mainbelt Complexes (SDSS data) X S V S C Origin of NEOs (Bottke et al. 2002) ● Asteroids and comets that evolve dynamically from several sources – Mars crossers (27%) ν resonance (37%) – 6 – 3:1 resonance (20%) – Outer belt (microresonances) (10%) – Jupiter Family Comets (6%) ● Collisional fragmentation followed by migration in semi-major axis via Yarkovsky effect ● Chaotic orbits in resonances - few million year migration timescale. Example of Yarkovsky Drag (Hungarias) Small objects drift further in radius than large objects “Simulation” of NEO Population ● Use Bottke et al. (2002) model of origin ● Select sample of main-belt SDSS asteroids near each source – E.G. 3:1 resonance; select 2.49 < a < 2.51 – Use Trojan asteroid population (a ~ 5 AU) to represent JFC population ● Combine in appropriate proportions ● Predict color and taxnometric distributions. Complications ● SDSS photometry has 5% intrinsic scatter compared to synthetic SMASS photometry ● Color-magnitude effects - faint objects are bluer (SMASS) (SDSS) Simulation v. SMASS data Complex Simulation Bright NEO/Dithered Faint NEO/Dithered C 0.13 0.09 0.11 X 0.07 0.09 0.17 D 0.09 0.01 0.04 S 0.43 0.32 0.29 Q 0.13 0.24 0.25 O 0.01 0.04 0.05 V 0.08 0.09 0.05 A 0.03 0.09 0.02 R 0.02 0.03 0.02 Medians a* 0.10 0.09 0.04 g-r 0.65 0.65 0.60 r-i 0.19 0.19 0.18 i-z -0.02 -0.04 -0.01 slope 0.32 0.26 0.22 Results of Simulation ● Average color distribution is reproduced, but ● Overpredict S types and underpredict Q and O types => chondrite dilemma remains. – NEO progenitor population still not properly isolated in MC/MB samples ==> ● rapid dynamical evolution timescale? ● space weathering? ● Overpredict D types (but these are dominated by include of Trojan asteroids to represent the JFC population, which may be wrong). ● Curiosity: Most NEO X types are faint - would not see in SDSS Mainbelt sample. Summary ● SDSS provides useful information on NEOs – Luminosity function of small NEOs matches low- end of prior measurements – SDSS MOC provides color atlas of Main Belt asteroids for matching to NEO population; importance of color-magnitude effects. ● Main lesson from SDSS: – It is possible to be 3x over budget, 5 years behind schedule, fail to meet target science goals, and still be considered a spectacular success..
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