Clark R. Chapman Southwest Research Inst. Boulder, Colorado

1. Introductory Briefing: Spacecraft Results about NEO Properties Relevant to the Impact Hazard Clark R. Chapman Southwest Research Inst. Boulder, Colorado NASA Workshop: “Near-Earth Object Detection, Characterization, and Threat Mitigation” Vail, Colorado 14:00 Monday, 26 June 2006 Gary Emerson

2. Relevance to the Hazard… • An NEO’s properties affect the magnitude and nature of the damage done if it were to impact the Earth • Mass (kinetic energy) is the overwhelming factor • Composition, density, binarity matter only a little (a 30 m iron body would hit, a stony body would explode above) • NEO properties dramatically affect almost all technological approaches to deflection/destruction • How can devices be attached? • How will it respond to ablation, impact, or explosion? • NEO properties affect the 3 R’s in threat mitigation (if deflection is impossible or fails): Readiness/Response/Recovery • People may fear comet tails, non-existent radioactivity, unlikely “after-impacts” • Responders need to know what phenomena will precede or follow an impact, based on the nature of the impacting object

3. Galileo Observes SL9 Impacts • The only observation of a major impact in the Solar System was of Comet Shoemaker-Levy 9’s 1994 impacts into Jupiter. • These images and lightcurves of the bolides and subsequent fireballs for impacts of 3 comet fragments were obtained by Galileo’s camera. • The original comet was very under-dense and fragile (it was torn into 20 pieces by Jupiter’s tidal forces). • Nevertheless, the impacts were stupendous; there likely would be civilization-threatening consequences if such an impact were to happen on Earth. (K.E. matters!)

4. There are Many Ground-based Ways to Learn About NEO Properties… Stellar occultation Delay-Doppler Radar Lightcurves Adaptive Optics

5. …but Spacecraft have Many Advantages • On the “human scale,” (sub-millimeter-to-meter; e.g. necessary to attach a device to an NEO), ground-based data only provide indirect, statistical info. • By flying nearby, orbiting, or landing, spacecraft can obtain extremely high resolution data. • By landing and physically interacting with the surface, a spacecraft can make measurements highly relevant to future deflection operational requirements.

6. Spacecraft Missions to NEOs or their Analogs • Galileo: Gaspra, Ida, SL9 impacts • NEAR-Shoemaker: Mathilde, Eros • Stardust: Comet Wild 2 • Deep Impact: Comet Tempel 1 • Hayabusa (MUSES C): Itokawa • Others: • Low-resolution images of asteroids and comets by many spacecraft • Studies of Martian moons Phobos and Deimos…but they are poor NEO analogs Comet Borrelly

7. Martian Satellites as (Inexact) Asteroid Analogs Phobos & Deimos: trapped orbiting Mars, broken up and reaccreted

8. Gaspra: First Asteroid Imaged by a Spacecraft October 29th, 1991: Galileo spacecraft takes pictures of this 18 x 9 km main-belt asteroid. It is angular (“faceted”) with many small craters, few big ones, and hints of “grooves” Spectra, few craters hint at strong “stony-iron” composition

9. Galileo’s even Closer Study of an even Larger Asteroid: Ida • August 28, 1993 • Moon-like craters • Subtle colors (en-hanced here) show material ejected and distributed around Ida from a large, recent crater

10. Ida Looks Much Like the Moon (and Eros)... …at scales of 100s of meters and larger, but it may be just like Eros (and not like the Moon) at human scales

11. “Space-weathering” Trends: Ida is an Ordinary Chondrite • Ida is not spectrally uniform. • Extreme spectrum for bright (blue) fresh crater is in direction of L-type ordinary chondrite (OC) meteorites. • Evidently Ida is an OC, but micrometeorite impacts and solar wind have darkened and reddened its surface… except where occasional-ly refreshened.

12. Ida’s Moonlet, “Dactyl” • Dactyl: 1.6 km (1 mile) diameter • 1st asteroidal moon ever discovered (by Ann Harch) • Rather spherical shape • Saturated with craters ranging in size from this building to a football stadium • Do you see a crater chain? • Broken-up, reaccreted? Dactyl is a poor analog for a small NEO…but it is part of an asteroid system. NEO satellites would have to be deflected, too. “Ida-shine”

13. NEAR Shoemaker is Built, Launched, Flies by Mathilde • Mathilde is a large, carbonaceous (C-type) main-belt asteroid, the only C-type yet studied by spacecraft • C-types dominate in the main belt, are common among NEOs • Spatial resolution is not good, but dominance by huge craters is obvious (ascribed to response of fluffy material to impact) • Bulk density of Mathilde is low: ~1.3 g/cc (micro- or macro-porosity?)

14. On to the NEO Target, Eros Himeros and Psyche craters are large compared with the radius of Eros, but its elongated shape is not primarily shaped by cratering impacts.

15. Unexpected Small-Scale Geology of Eros • Flat ponds and “beaches” • Small craters absent; boulders dominate

16. Closing in on Eros… Sample of results from laser altimeter • Location of final imaging sequence on rim of largest crater, Himeros • NEAR Shoemaker apparently lands in a flat “pond” inside an 80-meter wide crater • Spacecraft survives, transmits data, but not designed for in situ investigations

17. Fifth Last Image (largest boulders are 3 meters across)

18. Final Four Frames

19. Closest Image of Eros

20. Eros is NOT Like the Moon! These pictures are at roughly the same scale, one a slant view, the other from directly above. Eros has rocks. The Moon has craters.

21. Eros/MoonComparisons The Surface of Eros is NOT like the Lunar Regolith! • Ejecta are widespread on Eros, much lost to space, few generations of churning • Lunar ejecta are repeatedly churned in situ; soil becomes very mature • Rocks (ejecta blocks from far-away large impacts and exhumed from below) remain in place, cover the surface of Eros • Lunar rocks are fragmented and eroded; surface is covered by craters • Flat, pond-like deposits (of fines) are common in depressions -- few rocks or craters • Electrostaticly levitated dust on Moon does not form ponds, at least not commonly

22. Eros Composition: Ordinary Chondrite (most common meteorite type) • Near-infrared spectra show S-type absorption bands consistent with L or LL ordinary chondrite (OC) • X-ray spectra compatible with OC and nothing else • Sulfur is low, probably because of space-weathering of this volatile element

23. Asteroid Surfaces Compared, Prior to Hayabusa (c. 2005) • GASPRA Big craters absent (except “facets”?); small craters undersaturated. Young and/or made of strong metal, not rock. • IDA Saturated with large craters. Old, lunar-like megaregolith (2-chunk rubble pile?); small-scale surface like Eros. Anchor to what? • MATHILDE Supersaturated by giant craters (small scales unknown). Low-density materials and/or voids, perhaps compressible or loosely bound. Analogs: mud, sand, styrofoam? • EROS Shattered shard, only source of data at hi-res scales. Amazing! Surface character still not understood. Anchor to what?

24. Hayabusa Explores Itokawa (late 2005, published in Science, 2 June 2006) • Japanese spacecraft station-keeps with ~300 m S-type NEO • Lander/hopper is “lost in space” • Anomalies in attempts to touch down and collect samples • Delayed return of minimal sample still possible in 2010 • Most detailed ever study of an asteroid from very close up • Main conclusion: “rubble pile” of ordinary chondritic composition

25. “Rubble Piles”: A 29-Year-Old Theory Now Confirmed! Most collisions impart inadequate kinetic energy to disperse the fragments from their mutual gravitational field, so they fall back together into a strengthless “pile of rubble”... perhaps many generations until a catastrophic, disruptive, family- forming collision occurs.

26. Itokawa • Itokawa is about the size of Apophis (which may hit the Earth in 2036) • It is covered with boulders of many sizes, has almost no craters, has smooth “seas” • The composition is rocky (OC, with density ~3.5) but the bulk density is ~1.9, indicating 40% porosity: a “rubble pile” • It may be a “contact binary” (“body” & “head”)

27. Contact Binary? The “Neck” 50 m 50 m 50 m 50 m 20 m

28. Geophysical Attributes • Left: Relative gravitational and rotational potential (top=N hemisphere, bottom=S hemisphere) • Right: Slopes on Itokawa (N & S same as for potential)

29. Composition of Itokawa: LL-Type Ordinary Chondrite • Left: Infrared absorption bands plot in LL OC field • Right: X-ray fluorescence spectra plot closest to LL (or L) OC field

30. Itokawa: Very Close Up!

31. Itokawa and Eros Compared at Centimeter Scale • Highest resolution images: Itokawa Muses Sea (left, and at similar resolution to Eros, upper right) compared with Eros pond (lower right), all at same scale… Are they the same? For sure, Itokawa’s “seas” are covered by gravel, not dust! 7 mm per pixel; this scale bar = 1 m

32. Mechanical/Thermal Properties of Itokawa’s Surface • Hayabusa bounced twice (being in contact with the surface for a few seconds) then “landed” for half-an-hour – not really as planned. • The temperature (hence thermal inertia) was also measured. • Measurements are consistent with the images: the surface behaved neither like hard rock nor like fine, powdery, lunar-like regolith, but in between. • The surface of the smooth “seas” is coarse-grained, gravelly regolith.

33. Shapes and Topography of Comet Nuclei Comet Halley Images NOT to same scale • Comets range from roundish to elongated • Topography: craters, smooth regions, plateaus, ridges… • Below: Hyperion, a moon, not a comet Comet Tempel 1 Hyperion: a satellite of Saturn, not (now) a comet Comet Wild 2 Comet Borrelly

34. Tempel 1 Up Close: What Does It Mean?

35. Properties of Tempel 1 • The bulk density is estimated to be 0.6 g/cc • The particulates are estimated to be very fine, talcum-powder-like, in a layer tens of meters deep with virtually no strength • Low thermal inertia • Topography shows plains, exhumed features, scarps, impact craters…nucleus appears to be layered • The final impact crater could not be seen due to the unexpected brilliance and obscuration of the ejecta plume • Comet’s surface is locally active, with frequent natural outbursts Looking back on the ejecta plume. How would we anchor to an object like this? It is fine powder; there is essentially no gravity. It would be a real challenge!

36. Shapes of Comet Nuclei & Asteroids Gaspra Images NOT to same scale Kleopatra Tempel 1 Wild 2 Mathilde

37. Surfaces and Interiors: Aspects Relevant for Deflection Issues • There is much of scientific interest about NEO shapes and surfaces, but here I concentrate on aspects relevant to deflection and other human-scale interactions with them. • Are surfaces solid so we could anchor to them? Not usually • Are regoliths deep or thin,patchy or ubiquitous? Moon is bad analog • Are surfaces covered with huge boulders? Dusty? Often ‘yes’ & ‘yes’ • How reliably do remote-sensing observations and pictures tell us about interior properties? Better than nothing, but very poorly • Are NEO surfaces likely to be similar or the same, or must we study the particular body to be deflected? Studying diversity helps us prepare for the unexpected, but we must study the particular body, if possible • Given the diverse appearances, is our knowledge gleaned from Eros, Itokawa, et al. relevant to other NEOs? Itokawa is more relevant than Eros, which is a much larger body • What forces would de-stabilize rubble-pile interiors? Needs study • How well are satellites bound? Would they be a danger? Needs study We must study: (a) diversity of surfaces; (b) interiors

38. Conclusions… • NEO properties are mostly irrelevant for physical and environmental consequences, except for size…it’s the explosive energy that counts. • For deflection requiring physical interaction with the surface, there is an enormous diversity of NEO surface properties, material types, and internal configurations…not to mention possible satellites. And individual bodies are heterogeneous in some attributes.