Investigating the Near-Earth Object Population William Bottke Southwest Research Institute William Bottke Southwest Research Institute

When people think about “asteroids”, they mostly do not picture this:

They think of this…

Or this… “Killer Asteroid,” National Geographic Television, 2004

Hazards From Asteroid Impacts Leonid Meteor Shower Peekskill Meteorite Tunguska, 1908 Jupiter Impacts 1994 Smallest Most Frequent Huge Extremely Rare Chicxulub, 65 Million Years Ago

Environmental Consequences of Small Impacts

A Better Way to Think About Asteroids is This…

Meteorites are from Asteroids Meteorites are hand-samples of asteroids (and possibly comets) that have survived passage through our atmosphere to reach Earth. Meteorites are hand-samples of asteroids (and possibly comets) that have survived passage through our atmosphere to reach Earth. Properly analyzed, these objects can tell us about planet formation processes and evolution in the solar nebula. Properly analyzed, these objects can tell us about planet formation processes and evolution in the solar nebula. Ordinary Chondrite Iron Meteorite

Fossils of Formation ~10 6 objects with diameters D > 1 km between Mars and Jupiter ~10 6 objects with diameters D > 1 km between Mars and Jupiter

NEOs are the “Middlemen” Between Meteorites and Main Belt Asteroids Jupiter Family Comets Asteroid Belt Near Earth Objects NEOs Meteorites

Some NEO Science Questions  NEO orbit and size distributions? Compositions? Internal structures?  How are NEOs replenished over time? By what populations?  How do NEOs physically evolve? What does this tell us about planetesimal evolution?  How are NEOs connected to meteorites? Where did their parent bodies originate?  Did NEO deliver water to the Earth?  How has the NEO impact rate on the terrestrial planets changed with time? To answer these questions, we need the most accurate model of the NEO population possible. Itokawa Eros Geographos

How Do Near-Earth Asteroids Get Here? (Part 1) Jupiter Family Comets Asteroid Belt Near Earth Objects Asteroid Collision

Collisions in the Asteroid Belt Asteroids strike one another and create ejecta. Asteroids strike one another and create ejecta. Most fragments are ejected at low velocities (V < 100 m/s). Most fragments are ejected at low velocities (V < 100 m/s). Sample references: Benz and Asphaug (1999); Michel et al. (2001); Durda et al. (2004)

How Do Near-Earth Asteroids Get Here? (Part 2) Jupiter Family Comets Asteroid Belt Near Earth Objects Dynamical Escape Asteroid Collision

Koronis family Bottke et al. (2001) Observed Observed Model Model Yarkovsky Effect Allows Fragments to Reach “Escape Hatches” Distance From Sun (Further)(Closer) Elongation of Orbit Very Circular Very Elongated

What Happens to Them Along the Way? En route to the inner solar system (and Earth), lots of strange things can happen to NEOs…

Far view Far view Close up Close up Asteroid Collisions Create Fragments! The fragments can be very different from one another, implying different origins and evolution histories. The fragments can be very different from one another, implying different origins and evolution histories.

Sunlight Causes Fragments to Spin Up! Spin-Up and Disruption of a Contact Binary by YORP Walsh and Richardson (2007) Absorbed and reemitted sunlight can change asteroid’s spin rate! Absorbed and reemitted sunlight can change asteroid’s spin rate! This effect can produce strange shapes and even binary asteroids! This effect can produce strange shapes and even binary asteroids!

Planetary Encounters Can Pull Them Apart! Disruption of an NEO During Close Pass with Earth Gravitational (tidal) forces from Earth can pull the asteroid apart! Gravitational (tidal) forces from Earth can pull the asteroid apart! These effects can also produce asteroid satellites! These effects can also produce asteroid satellites! Richardson, Bottke, and Love (1998); Walsh and Richardson (2006)

How Can Arecibo Radar Help Us Obtain the Properties and Geologic Context of NEOs? Direct determination of how sunlight (Yarkovsky and YORP thermal effects) affects evolution of NEOs: Direct determination of how sunlight (Yarkovsky and YORP thermal effects) affects evolution of NEOs:  This helps us understand the timing of asteroid breakups, the evolution of the fragments over time, and where they will go in the near future. NEO physical properties NEO physical properties – Precise shapes and spin rates for NEOs. – Detection of NEOs with satellites. – NEO crater histories and surface properties. – NEO orbital/composition distributions  These parameters provide critical clues about NEO origins and evolution.

Exploring Near Earth Objects Another way to determine the context of meteorites is to use spacecraft to return samples from an NEO. Another way to determine the context of meteorites is to use spacecraft to return samples from an NEO. OSIRIS mission to return samples from “1999 RQ36”.

Radar Provided Unparalleled Knowledge of 1999 RQ36 Radar provided RQ36’s approximate shape, size, spin properties, surface properties, and precise orbit, all which were critical for mission planning. Radar provided RQ36’s approximate shape, size, spin properties, surface properties, and precise orbit, all which were critical for mission planning.

NEOs are the “Middlemen” Between Meteorites and Main Belt Asteroids Rocks and sediments in a riverbed yield information on the types of material found upstream. Rocks and sediments in a riverbed yield information on the types of material found upstream.

NEOs are the “Middlemen” Between Meteorites and Main Belt Asteroids Rocks and sediments in a riverbed yield information on the types of material found upstream. Rocks and sediments in a riverbed yield information on the types of material found upstream. NEOs and meteorites tell us about the nature and evolution of bodies in the main asteroid and Kuiper belts. NEOs and meteorites tell us about the nature and evolution of bodies in the main asteroid and Kuiper belts. ItokawaEros