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

[Ron Baalke]

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.