[meteorite-list] Astronomers Establish the Strength of High-Inclination Asteroids

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 5 Nov 2013 10:03:34 -0800 (PST)
Message-ID: <201311051803.rA5I3YKS009468_at_zagami.jpl.nasa.gov>

http://www.naoj.org/Pressrelease/2013/11/04/index.html

Astronomers Establish the Strength of High-Inclination Asteroids
Subaru Telescope Press Release
November 04, 2013

A team of astronomers from the National Astronomical Observatory of Japan
and the University of Hyogo used the Subaru Prime Focus Camera (Suprime-Cam)
mounted on the Subaru Telescope to observe faint asteroids with highly
inclined orbits. They found that a smaller fraction of tiny bodies occur
among high-inclination asteroids than those near the ecliptic plane. This
means that large asteroids in high-velocity collisions between asteroids
probably have a greater increase of strength in resisting disruption than
those in the present mean-velocity collisions. Clarification of the relationship
between collisional velocity and asteroids' disruptive strength is helpful
in understanding the collisional evolution of asteroids in the early Solar
System.

Asteroids, small rocky or metallic objects that mostly orbit in the zone
between Mars and Jupiter (i.e., the main asteroid belt or MAB), continuously
collide with one another after their formation. A so-called "catastrophic
collision", when objects suddenly hit each other with great force and
incur significant damage, alters asteroids, the fragments of which become
newly-born asteroids. Collisional evolution refers to changes in the size
and number of asteroids as collisions repeat over time. Asteroids of a
certain size decrease, because larger-bodied objects may fragment after
catastrophic collisions. The primary factor controlling the balance between
the increase and decrease in the size and number of asteroids through
continuous collisions is the asteroid's material strength against impacts.
The strength of asteroids larger than about 100 meters in diameter increases
with size, because gravity holds such larger objects together in a process
called "gravitational binding." The population distribution of asteroids
results from how much their strength against collisions increases in relation
to their size. Therefore, measurements of their population distribution
indicate properties of asteroids' strength and provide information necessary
for investigating the collisional history of the asteroid belt.

Previous observations of asteroids' population distribution supply the
data to model their collisional evolution. However, astronomers know very
little about the early collisional evolution in the main asteroid belt,
because newborn Jupiter scattered the orbits of asteroids and sped up
their relative velocities so that they were colliding with each other
at a much faster rate than at present. How, then, can astronomers learn
more about the strength properties of asteroids that collide at such high
velocities and are not orbiting in similar, nearly circular orbits along
the ecliptic plane, i.e., a reference plane based on Earth's orbit projected
in all directions?

To address this question, the current team of astronomers focused their
research on the population distribution of asteroids with highly inclined
orbits, because their collisional velocities are significantly high and
can provide information about their strength properties under high-velocity
collisions. No previous observations have measured the population distribution
of high-inclination asteroids in the desirable range of several hundred
meters to several kilometers. Therefore, the team decided to use Suprime-Cam
mounted on the 8.2-m Subaru Telescope to conduct an optical wide-field
observation of small, main-belt asteroids with high inclinations. Suprime-Cam's
position at prime focus combines with Subaru's large primary mirror to
provide a particularly wide field of view, which is ideal for targeting
such faint and sparsely-populated asteroids. To observe a sufficient number
of these objects within a limited amount of time, the team developed a
new, efficient asteroid detection technique (Figure 1) and decided to
survey a sky area at high ecliptic latitudes, where high-inclination asteroids
are likely to be located.

[Figure 1]
Figure 1: (Left and Center) Optical images from the Subaru Telescope's
Suprime-Cam, obtained in the same field at 20-minute intervals on August
24, 2008. (Right) Processed image from the two optical images. Only the
moving asteroid remains after the background stars and galaxies were masked.
It appears to be elongated, because it moved during the 4-minute exposures.
(Credit: NAOJ)

During their two nights of observations, they detected 441 moving objects,
about 380 of which are main-belt asteroids. Since the diameters of these
asteroids are too small to measure directly, the team calculated their
size from their estimated orbits and brightness and found that the detected
asteroids have diameters ranging from about 700 meters to 6 kilometers.
Almost half of the asteroids have diameters smaller than 1 kilometer with
inclinations higher than 15 degrees.

Figure 2 shows the population distribution obtained from the asteroid
sample. The slope of the distribution changes sharply when the asteroids
are about one kilometer in diameter--the same pattern that a previous
study confirmed in asteroids near the ecliptic plane. A careful comparison
of the population distribution of asteroids with diameters ranging from
600 meters to 5 kilometers with that of asteroids near the ecliptic plane
revealed that high-inclination asteroids have a smaller proportion of
small to large objects (i.e., a shallower population distribution). This
finding indicates that high-velocity collisions accelerate the rate of
increase in the strength of asteroids according to their size, i.e., properties
of asteroid disruptive strength depend on collisional velocity. In terms
of collisional evolution, the results of this study indicate that in the
early Solar System when Jupiter's birth triggered collisions of asteroids
at higher velocities than now, large asteroids were more resistant to
disruption and had longer lifespans than those in typical, present-day
collisions. The team plans to use Hyper Suprime-Cam (HSC), Subaru Telescope's
powerful new prime-focus camera (Hyper Suprime-Cam Ushers in a New Era
of Observational Astronomy), to conduct large-scale surveys to further
investigate the dynamical/collisional evolution of various small-bodied
populations in the Solar System.

[Figure 2]
Figure 2: Relationship between the diameter and cumulative number of bodies
larger than the size obtained from the observed asteroids. The orange
dotted line shows the detection limit for asteroids. The red circles show
the diameter range used for evaluation of the distribution slope, which
indicates the asteroid population. The crosses show the excluded range.
The slope changes at one kilometer, a shift that asteroids near the ecliptic
plane also show. The blue and green lines show the estimated slope of
asteroids in diameter ranges smaller and larger than one kilometer, respectively.
The former provides a basis for comparison with that of asteroids smaller
than one kilometer near the ecliptic plane. A difference between the slopes
of near and far ecliptic populations indicates that the collisional evolutions
were different. (Credit: NAOJ)


Reference

Terai, T., Takahashi, J., and Itoh, Y. 2013 "High Ecliptic Latitude Survey
for Small Main-Belt Asteroids", Astronomical Journal, Volume 146, Issue 5.
Received on Tue 05 Nov 2013 01:03:34 PM PST