Transcript NEO_Radar

Investigating the Near-Earth Object
Population
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
Huge
Extremely
Rare
Smallest
Most
Frequent
Leonid Meteor
Shower
Tunguska,
1908
Peekskill
Meteorite
Jupiter
Impacts
1994
Chicxulub,
65 Million Years Ago
Environmental Consequences
of Small Impacts
A Better Way to Think
About Asteroids is This…
Meteorites are from Asteroids
Ordinary Chondrite
Iron Meteorite
 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.
Fossils of Formation
~106 objects with diameters D > 1 km between Mars and Jupiter
NEOs are the “Middlemen” Between
Meteorites and Main Belt Asteroids
Asteroid Belt
Near Earth
Objects
NEOs
Jupiter Family Comets
Meteorites
Some NEO Science Questions
Itokawa
 NEO orbit and size distributions?
Compositions? Internal structures?
 How are NEOs replenished over
time? By what populations?
 How do NEOs physically evolve?
Eros
Geographos
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.
How Do Near-Earth Asteroids Get Here?
(Part 1)
Asteroid Belt
Asteroid Collision
Near Earth
Objects
Jupiter Family Comets
Collisions in the Asteroid Belt
 Asteroids strike
one another and
create ejecta.
 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)
Asteroid Belt
Asteroid Collision
Near Earth
Objects
Dynamical Escape
Jupiter Family Comets
Yarkovsky Effect Allows Fragments to
Reach “Escape Hatches”
Very
Elongated
Koronis family
Elongation
of Orbit
• Observed
• Model
Very
Circular
(Closer)
Distance From Sun
(Further)
Bottke et al. (2001)
What Happens to Them Along the Way?
En route to the inner solar system (and Earth),
lots of strange things can happen to NEOs…
Asteroid Collisions Create Fragments!
 Far view
 Close up
 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
 Absorbed and reemitted
sunlight can change
asteroid’s spin rate!
 This effect can produce
strange shapes and
even binary asteroids!
Walsh and Richardson (2007)
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!
 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:
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
– 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
OSIRIS mission to
return samples
from “1999 RQ36”.
 Another way to determine the context of meteorites is to use
spacecraft to return samples from an NEO.
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.
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.
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.
 NEOs and meteorites tell us
about the nature and evolution
of bodies in the main asteroid
and Kuiper belts.
Eros
Itokawa