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.