{ Dedicated to Doug ReVelle }

1. {Dedicated to Doug ReVelle} Synergy Between Asteroid Astronomy, Bolide Observations, and Meteorite Research Clark R. Chapman Southwest Research Institute, Boulder, Colorado, USA and Alan W. Harris Space Science Institute, La Canada, California, USA Pat Rawlings, SAIC Session: “Asteroids and Meteorites” Meteoroids 2010 24 May 2010 Breckenridge, Colorado, USA

2. Three Different Specialties… as of many decades ago • Asteroids: points-of-light moving in the sky: orbits, rough sizes, non-spherical shapes indicated by lightcurves. “Vermin of the skies.” • Meteors: flashes in the night-time sky, often occurring in showers, occasionally brilliant • Meteorites: rocks from the heavens, analyzed in laboratories for structure, mineralogy, composition, age, etc. It was understood that they were related, but they were studied by different specialists, from astronomers to geologists and chemists, by different techniques, from telescopes to microscopes, reported in different meetings and little observational or measured data related them to one another.

3. Modern Advances… • Asteroids: Studied up-close by spacecraft, radar, spectral reflectance, HST & Spitzer, telescopic searches find NEAs just a few meters across • Meteors: Studied by radar, spectros-copy, networks of CCD monitoring, impact flashes on the Moon, IDPs • Meteorites: Powerful/sophisticated lab techniques, Antarctic collection, meteorites on Mars

4. The Merging of Asteroid Science and Meteor Science Ceplecha (1996) • As late as 1990, the smallest NEA found with telescopes was ~200 m diameter. • There was a gap from the largest photographic meteors of over 5 orders of mag. in mass. • The Spaceguard Survey now samples NEAs down to ~1 m diameter, overlapping meteor/bolide observations. • Graph from Ceplecha (recently deceased) Near-Earth Asteroids: 1990 Photographic Meteors Near-Earth Asteroids: 2009

5. Catalina Sky Survey 2008 TC3: Merger of Asteroids/Meteors/Meteorites Discovery and short-term warning enables: (a) pre-impact telescopic observations, (b) bolide observations, and (c) meteorite recovery • 2008 TC3 was the first Near Earth Asteroid ever discovered (Catalina Sky Survey, 7 Oct. 2008) that was then predicted, for sure, to strike the Earth. It was then observed telescopically for lightcurve and spectral properties. • 19 hours after discovery, the predicted impact occurred and was recorded, and many resulting paired meteorites (the unusual Almahata Sitta ureilite) were later collected on the ground. • This first-ever event was not a fluke: we must expect future predictions of meteorite strikes, from the existing Spaceguard Survey, without even waiting for the “next generation” surveys. (Short-term warning capability: never evaluated!) • The most likely warning of an actual hazardous NEA impact will be one of these “final plungers,” providing hours to weeks of warning to evacuate. TC3 asteroid moving (W. Boschin, TNG) TC3 atmospheric train (M. Mahir) Almahata Sitta fragment on the ground in Sudan (P. Jenniskens)

6. Searching for Final Plungers Chapman & Harris (2009) • Red box indicates general range surveyed by Spaceguard telescopes. • We used known Potentially Hazardous Asteroids (PHAs) to simulate the direction impactors come from: 53% are within survey boundaries, in the general anti-solar direction, concentrated within the well-covered red oval. • Maximum warning time before impact depends on NEA size and on survey limiting magnitude. 25 m NEOs are seen more than a week before they hit by the current Catalina Sky Survey; TC3s can be seen about 2 days out. • Next generation surveys “could” find most night-time 25 m NEOs 40 days out, most TC3s a week out. But will they? (Current cadence design says no.)

7. Searching for Final Plungers Chapman & Harris (2009) • Red box indicates general region surveyed by Spaceguard telescopes. • We used known Potentially Hazardous Asteroids (PHAs) to simulate the direction impactors come from: 53% are within survey boundaries, in the general anti-solar direction, concentrated within the well-covered red oval. • Maximum warning time before impact depends on NEA size and on survey limiting magnitude. 25 m NEOs are seen more than a week before they hit by the current Catalina Sky Survey; TC3s can be seen about 2 days out. • Next generation surveys “could” find most night-time 25 m NEOs 40 days out, most TC3s a week out. But will they? (Current cadence design says no.) Next generation Now

8. Importance of Linking Asteroids, Meteors, and Meteorites TC3 Reflectance Spectrum: Wm. Herschel Telescope(Fitzsimmons, Hsieh, Duddy & Ramsay) • A major goal of meteoritics is to determine the origin and nature of meteorite parent bodies; asteroids are those bodies, or fragments of them. • Linking bolides and recovered meteorites to specific asteroids has been difficult. • Orbits derived from fireball photos for recovered meteorites are imprecise and would not identify the parent body anyway because asteroid orbits evolve • Comparisons of meteorite and asteroid reflectance spectra are (unfortunately) highly non-unique • Calibration of telescopic databases for asteroids (either by in situ spacecraft missions to asteroids, or by cheaper identifications like TC3) can powerfully link meteorites to parent asteroids. • Inferences from bolide data can be calibrated by linking to the specific meteorite and asteroid. • We can compare properties of recovered meteorites and observed bolide phenomena with the taxonomic class, interpreted mineralogy, spin, size, shape, precise orbit, etc. for the specific asteroid that was pbserved before it impacted. TC3 Lightcurve (Clay Center Observatory) There is a potential revolution in asteroid science, meteor science, and meteoritics: predictions enable pre-impact observations of the asteroid and bolide that produce recovered meteorites.

9. Example: Rosetta and (21) Lutetia Lutetia/meteorite spectral comparisons • Rosetta flies by 100 km Lutetia in July • Arguments abound about meteorite analog/s for this M(W)-type asteroid • “M” is mnemonic for “metal” but Rivkin (2000) showed that a subset of M’s have a 3μm hydration band (‘Wet’) • Also, I suggested (1970s) that M-like spectra might be enstatite chondrites • But Lutetia was selected as flyby target because of arguments that it may be a carbonaceous chondrite • Relevant data include polarization, visible and radar albedos, thermal IR emission spectra, UV/visible/near-IR reflectance spectra, mass+shape → bulk density • Truth table → “wet” enstatite chondrite • Rosetta may yield ambiguous results: We need a TC3-like-event for an M(W)! Barucci et al. (2005) Vernazza et al. (2009)

10. Origins of Meteoroids that Yield Bolides and Meteorites • Classical/cartoon model: chips from solid rocky asteroids. • 1990s model: meteoroids dislodged by cratering events and catastrophic disruptions on “rubble pile” asteroids, drift by Yarkovsky Effect into orbital resonances, and are thereby converted into Earth-crossing orbits. • Very recent alternative (or additional) modes: landslides and equatorial escape after spin-up of “rubble pile” near-Earth asteroids by YORP… or by planetary tides • Scheeres et al. (2010) propose that microgravity NEAs behave with physics governing microscopic dust aggregates

11. Once Upon a Time: Collisions…Now it’s just Sunlight! (And Tides) Yarkovsky Effects are due to sunlight shining on asteroids: light is directly absorbed but re-radiated in the thermal infrared asymmetrically. This pushes or pulls an asteroid in its orbit or produces a “windmill” effect on its spin. • Interasteroidal collisions (both catastrophic disruptions and frequent, small cratering events) seemed to explain everything that happened to asteroids after early accretion and thermal processing: size distribution, spin rates and axis tilts, liberation and delivery of smaller asteroids and meteorite fragments into resonances, asteroid satellite formation, regolith properties, etc. • Yarkovsky Effect (reintroduced for 3rd time in 20th century by D. Rubincam in 1980s) shown by Farinella, Vokrouhlicky, Bottke and others to cause meteoroids from anywhere in inner half of main asteroid belt to drift into resonances, which deliver them to Earth. • YORP Effect (resurrected from mid-20th century by D. Rubincam in 1998) shown to be the major process shaping the axial tilts and spin rates of smaller asteroids. [Radzievskii 1954: “A mechanism for the disintegration of asteroids and meteorites.”] • These two Yarkovsky Effects may dominate the physical and dynamical behavior of smaller asteroids.

12. YORP Spin-Up, Binary Formation, and Mass Shedding…and Tides… Ostro et al. (2006) Gravitational slope on KW4-α • Arecibo radar data on NEA 66391 (1999 KW4; Ostro et al.), and analyses/modeling by Scheeres, Fahnestock, Walsh, Michel, Richardson, et al. open a new paradigm for the evolution of small rubble piles: • Asymmetric solar radiation spins some of them up, so mass moves to zero-G equatorial ridge, shedding mass, forming satellite/s, escape or reimpact of satellites, and escape of meteoroids into interplanetary space. • ~1/3 of NEAs are binaries, or have satellites or contact-binary shapes, implying a common evolutionary track. An NEA may undergo generations of satellite formation during its dynamical life in the inner solar system. • No modeling has yet been done on meteoroid production rates, but this could be a major source of meteorites. CRE ages may reflect such surficial landslide processes rather than impact-churned regolith processes. Tidal Mass-Shedding Following a suggestion by Nesvorny et al., Binzel et al. (2010) show that tidal encounters with Earth “freshen” NEA surfaces. K. Walsh, P. Michel & D. Richardson (2008)

13. Conclusion: Merged, Interdisciplinary Research ASTEROIDS METEORS/IDPs METEORITES

14. Short-Term Warnings: Spaceguard Survey does Better than We Thought! • Was it a miracle that telescopes saw what was plausibly the largest NEA to impact Earth in 2008? No! Capability to see “final plungers” was overlooked. • Analyses in the 1990s of the “Spaceguard Survey” only considered cataloging of near-Earth asteroids (NEAs); short-term warning was evaluated only for rare comets. • Thus it was thought that there was only a tiny chance that a dangerous inbound 30-m NEA would be seen, let alone a 3-m “TC3”. • Short-term hazard warning was evaluated (NASA SDT 2003) for the “next generation” surveys, but not for small NEAs and meteorite recovery. “Consider a 30–40-m office-building-sized object striking at 100 times the speed of a jetliner…. Even with the proposed augmented Spaceguard Survey, it is unlikely that such a small object would be discovered in advance; impact would occur without warning.” – C. Chapman, EPSL (2004). “a short lead time for an NEO is extremely unlikely – we can expect either decades of warning or none at all” – Morrison, Harris, Sommer, Chapman & Carusi (“Asteroids III” 2002)