Properties of the NEO Population: The ACM 2005 View Richard P. Binzel (MIT) Dmitrij Lupishko ( Kharkov Observatory)

Near-Earth Objects (NEOs) Asteroids or comets with perihelia ≤ 1.3 AU.

Near-Earth “Objects”

NEO Spectral Measurements Lazzarin et al. “Survey in the visible and near-IR (SINEOS)” at ESO and La Palma. Lazzaro et al. “Small Solar System Objects Spectro- scopic Survey (S 3 OS 2 )” at ESO. Dandy et al. BVRIZ colors at La Palma. Vernazza et al. Visible + near-IR. La Palma + IRTF. MIT-Hawaii-IRTF Near-IR (IRTF) public domain data. Available: smass.mit.edu ALSO… P7.2 DeLeon; P7.5 Marchi; P7.10 Vernazza; …

NEO Albedo Measurements Delbo et al. (2003) NASA Keck Wolters et al. (2004) UKIRT Fernandez et al. (2005) Keck + UH telescopes

Keeping up the pace DiscoveriesSpectraAlbedos

Overall Properties: NEO population shows the full diversity of the main-belt asteroids. NEO Population

2001 XR31: Possible R- or V-type (Marchi et al. 2004) Additional V-types (Marchi et al. 2005) Examples of Unusual Types

(33342) 1998 WT24 E-type with very low P max (Kiselev et al. 2004)

Comparisons to Meteorites Ordinary Chondrites [Q-type NEOs] (Angeli & Lazzaro 2002) (Lazzarin et al. 2005) Carbonaceous Chondrites (Lazzarin et al. 2005) Aubrites (Clark et al. 2004) Also Gaffey et al. (1992), Burbine et al. (2002), Gaffey & Kelly (2004)

The Ordinary Chondrite Problem The most common class of meteorites (OCs) does not match the most common class of asteroids (S-types). Ordinary Chondrites ~80% Carbonaceous Chondrites Irons Achondrites Stony-Irons NEO Statistics S: 44% Q: 23% X: 14% C: 10%

Long debated hypothesis... (Wetherill & Chapman 1988; Chapman 1996; Beth Clark et al. Asteroids III) Ordinary chondrite source bodies disguised by “space weathering” to look like S-asteroids.

X-ray spectrometer at Eros provides in situ link between S-asteroids and ordinary chondrite meteorites. (Trombka et al. 2000) NEAR

Progress in Laboratory Explanation for Space Weathering Hapke et al. (1975); Pieters et al. (2000) Progress by Many Moroz et al. (1996; 2000) Sasaki et al. (2001, 2003) Kurahashi et al. (2002) Strazzula et al. (2005)

1. Transition from ordinary chondrites to S-asteroids should be continuous. Observable Side Effects 2. Transition dependent on surface age. –Young surfaces should look like ordinary chondrites. –Older surfaces should look like “S-asteroids”

Continuous Transition S-asteroid Q-asteroid MIT - Hawaii - IRTF NEO Reconnaissance Data available: smass.mit.edu

Continuous Transition (Lazzarin et al. 2005) Larger “More red” Size Dependence (Dandy et al. 2003) Smaller “Less red”

Spectral transition is size dependent. Transition “complete” at 5 km size. Q S 5 km Main belt average (S-types) (ordinary chondrites) Running Box Average Q- and S-types What are the relative roles of weathering vs. regolith retention? Binzel et al. (2004) Smaller sizes = “younger, fresher” surfaces. Larger sizes = older “weathered” surfaces. Cheng (2004) Collision fragments Primordial survivors

Albedo Dependence Trend toward brighter “fresher” surfaces at smaller sizes. (Delbo et al. 2003) NEOs Main Belt Harris-DLR (2005)

Near-Earth “Objects”

Major Advances: Physical measurements of comet nuclei and “asteroids” in comet-like orbits. Abell et al. (2005)Fernandez et al. (2001, 2005) Jewitt (2004, Comets II) ALSO… P3.1 Abell; P3.9 Lamy; P14.1Alvarez-Candel; P14.8 DeMeo; P14.19 Lowry;

Tell “Tail” Signs for Low Albedo NEOs Standard Thermal Model (Lebofsky & Spencer 1989) shows low albedo asteroids in near-Earth space are warm enough to emit in the near-IR. First application using IRTF SpeX (2.5  m) to NEOs (1998 ST27) reported by Abell (ACM 2002; Thesis 2003). Model templates for application to NEOs by Rivkin et al. (2005).

Determining the Fraction of “Dead Comets” in NEO Space Criterion #1. Have orbits similar to Jupiter-family comets: T j < 3. Must debias NEO discovery statistics for low albedo objects. Fernandez et al. (2001)

Debiased NEO Population (Stuart 2003 Ph.D. Thesis; Stuart & Binzel 2004) KEY RESULT: 30% of the total NEO population resides in orbits having T<3.

(Fernandez et al set threshold < 0.075) Or Have “C-, D-, P-type” spectra (as a proxy for low albedo). -->Fernandez et al. (2005): 53 +/- 9 % -->Binzel et al. (2004): 50 +/- 10% Criterion #1: Debiased T<3 population Criterion #2: Also have comet-like albedo. Determining the Fraction of “Dead Comets” in NEO Space T<3 NEOs

Criterion #1: Debiased T<3 population = 30% of all NEOs Criterion #2: 50% of T<3 NEOs have “comet-like” albedos/colors Results for the Fraction of “Dead Comets” in NEO Space RESULT: 0.50 x 0.30 = /- 5% of the total NEO population are extinct comet candidates.

CONCLUSIONS Space weathering has come of age. –S asteroid connection to ordinary chondrites. –Transition at 5 km may be key to unraveling weathering vs. regolith particle size/retention. T<3 population has a distinct character. –Bias corrected: 30% of all NEOs reside T< /- 5% of the total NEO population are “dead comet” candidates. Ongoing & future challenge: Further scrutiny of comet candidates.

FIN

Properties of the Near-Earth Object Population Richard P. Binzel (MIT) Dmitrij Lupishko ( Kharkov Observatory)