Laboratory simulations for some characteristics of asteroid (25143) Itokawa

Seikei Astronomy and Meteorology Club

Hayabusa and Itokawa mission

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.

Hayabusa mission

Itokawa 1/2000 scale model

Basic data of Itokawa・ Size : 535, 294, and 209 m each・ Type : S type, as low iron chondritic・ Bulk Density : 1.9 ± 0.13 g/cm3

・ Bulk porocity : 40% -> Rubble-pile asteroid

Experiment 1: Laboratory Lightcurve

To request the rotating period etc. before the inquiry by the falcon, the light curve of Itokawa has been decided by photometry by the ground observation. (Kaasalainen et al and 2004, etc.)

　 Geographical features in the asteroid might influent to the light curve what .

Experimental Method

Apparatus

We placed the model on turn table in the dark room, and put light source besides the model.

The model was turned by 20 degrees interval about X-axis, Y-axis, Z-axis, then, 36 pieces images weretaken for two laps of rotation.

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.

We measured brightness of reflected light for pictorial data using image processing software.

Consideration Z 　 90軸 度

0

500000

1000000

1500000

2000000

2500000

3000000

3500000

4000000

4500000

0 2 4 6 8 10 12 14 16 18 20

(×20 )角度 度

明る

さ

1周目2周目

The obtained light curve about Z-axis is similar to the ground photometry results from 2000 to 2001 in National Astronomical Observatory of Japan.

The top value of first peak is different from the second one. 　→It is caused that the Itokawa has not rotational symmetry.

Experiment 2: Seismic Segregation Process We make Itokawa with a massage device.

Seismic segregation process on the asteroids

The other experiment is to make the landform feature of 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, and would bring similar features of the dichotomy of Itokawa.

Experiment We reproduce shape of Itokawa by an analog experiment.

・ Simulant Peridotite sand(3.2g/cm3)was adopted as re

golith simulant. (We crush peridotite (Kurouchi-yama Gu

nma Pref.)

・ Landform Model V shaped foam polystyrene as Itokawa

Muses Sea

・ Vibrator Handy massarger

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. We have a plan of same experiment on the condition of minor gravity field.

Comparison of Itokawa and results

↑Photograph of ItokawaThe　image　which　assigned　massage　device　1　to　smallest　sample　E 　 15　°　for　37.2　seconds

We could re-create the Itokawa landform!