Laboratory simulations for some characteristics of asteroid (25143) Itokawa

Seikei Astronomy and Meteorology Club

Seikei High School

Kichijoji-kitamachi 3-10-13, Musashino. Tokyo 180-8633

1. Introduction

The spacecraft “Hayabusa (Falcon) = Muses C” is a sample return mission to the asteroid (25143) Itokawa. Hayabusa reached the target on Sept. through Dec. 2005, and measured the surface topography, mass, gravity and the other features. These data reveal that Itokawa is rubble-pile asteroid and the global Itokawa as “sea otter” like shape, and has two distinct parts such as rough terrain and smooth one. JAXA made 1/2000 precise scale model of Itokawa using the Hayabusa data.

We tried some laboratory simulations about Itokawa on relation with Hayabusa mission.

2. Experiment 1; Laboratory Lightcurve

1)Purpose

As a preliminary examination about Itokawa, some lightcurve of the asteroid was measured by ground surface observation to clarified rotation cycle and axis (e.g.; Kaasalainen et al and 2003). The shape of Itokawa which is obtained by Hayabusa mission, is so complicated rather than prospected shape by preliminary research. We measure refractance of this Itokawa precise model using cooled CCD camera to examine what the asteroid precise topology influent upon light curve.

2)Method

We placed the model on turn table in the dark room, and put the parallel rays of light source besides the model. The phase angle of the light source changed into 45 degrees, 90 degrees, 135 degrees from the line on the camera through the model to recreate the position between Sun and the asteroid. The model was turned by 20 degrees interval about X-axis, Y-axis, Z-axis, then, 36 pieces images were taken for two laps of rotation. We measured brightness of reflected light for pictorial data using image processing software. The real rotation axis is near Z, however, we can examine the other fictional situation on laboratory simulation.

3)Results

The obtained light curve about Z-axis is similar to the ground photometry results from 2000 to 2001 in National Astronomical Observatory of Japan etc, however, the top value and width of first peak is different from the second one. And, the difference between the maximum and minimum value of the curve are changed by the position of the light source. The asymmetric shape of light curve is caused that the Itokawa has not rotational symmetry. Comparison among ground photometry and our experiment results show that the ground photometry has influence of light by the atmosphere, and/or narrow apature effect on CCD. More detailed ground photometry will detect asymmetric light curve such as our experimental results.

3.Experiment 2; Seismic Segregation of Small Asteroids

1)Purpose

The other experiment is to make the landform feature of Itokawa. Surface of the Itokawa was divided into rough terrain and smooth one. The former was covered with numerous boulders, and the latter was not done so. Plausible process is the seismic segregation in boulders and regolith induced by impacts. Such segregation would control surface morphology of small asteroids (Izenberg and Barnouin-Jha, 2006), and would bring similar features of the dichotomy of Itokawa.

2)Experiment

Here we demonstrate experimental simulations of the seismic segregation on boomerang-shaped slopes likely to Itokawa with vibration exciters. Peridotite sand (3.2g/cm3) was adopted as regolith simulant. We crush peridotite (Kurouchi-yama, Gunma Pref.).

3)Results

This simulation showed that particles allocation in size controlled surface features. A higher contents of smaller grains and gentle slope tend to bring the similar dichotomy of surface.

Reference

[1] N. R. Izenberg and O. S. Barnouin-Jha, (2006), Laboratory Simulations of Surface Effects on Low Gravity Bodies, 37th Annual Lunar and Planetary Science Conference, abstract no.2017.

[2] M. Kaasalainen et al.,(2003), CCD photometry and model of MUSES-C target (25143) 1998 SF36, Astronomy & Astrophysics, 205, 29 - 32.