- Part 2: Simulation Update upon Arrival of Itokawa (September 2005) -
(clicking on the figures will start the movie).
The asteroid probe MUSES-C (Hayabusa), launched on May 9th 2003, succeeded in reaching asteroid Itokawa on September 12th 2005, after interplanetary cruise for about 2 years and 4 months. You can find some pictures of the asteroid taken from a distance of about 20 km. At the end of November 2005 Hayabusa will try to collect samples from the asteroid surface performing two touch down maneuvers.
From the pictures taken by Hayabusa it is possible to see that the potato-shape asteroid has dimensions approximately of 540m x 270m x 210m. These pictures and successive further scientific observations will help in having a better idea about the surface composition and geometry and in deciding the spots where to try the touch downs.
Since the surface characteristics are not known, several simulations have been carried out in our laboratory using different surface roughness, inclination in the contact area, and coefficient of friction. Four new animations are shown below.
“Fractals” have been used to represent the asteroid surface, since they are well known as being a very good mathematical representation of natural topography. In our simulations we used two types of fractals, one relatively smooth and one more rough.
Examples of the complete touch down maneuver are shown below for both types of surfaces (clicking on the figures will start the movie).
Left: Example with smooth surface. Right: Example with rough surface.
The contact between the probe and the surface is carried out using a 1m long horn. This element is basically an elastic spring, to better adapt to the roughness of the surface. Numerical simulations and experimentations have shown how the structure can actually perform very well on such type of surface.
The contact is detected by measuring the deformation of the end tip of the horn (using a laser finder). As soon as the contact is detected, a bullet-like projectile is fired from the top of the horn. The high velocity impact between the projectile and the surface should generate a few grams of fragmented material from the asteroid, material that will be collected by the horn. The contact time has been estimated to be between 1 and 2 seconds in duration.
Furthermore, at the contact detection time, four of gas jet-thrusters installed under the probe main body are fired to generate the thrust necessary to lift off Hayabusa from the asteroid surface.
Numerical simulations have been carried out in particularly to verify the attitude of the probe during the touch down. The probe will tend to tilt because of the inclination of the surface in the contact area and the off-axial inertia distribution of the probe. This change in attitude depends also from the approaching velocity, controlled to be not more than 10 cm/s along the vertical direction and 8 cm/s along the horizontal direction (these are meant as relative velocities, i.e. between the probe and the asteroid).
But because the thrusters will be fired as soon as the contact is detected, the possibility that the probe will tip over and result in a dangerous situation should be low. Finally, during the ascent phase, a control system will switch on/off the thrusters to perform a control on the probe attitude.
The mission has been also complicated by some technical problems. On October 2nd Hayabusa lost control of a second reaction wheel, after the failure of a first one in July. These reaction wheels are used to fine-adjust the attitude of the probe when necessary. At the present, the attitude control is performed using the remaining reaction wheel while thrusters are used to compensate for the broken ones. And particularly, for the ascending maneuver after the touchdown gas-jet thrusters are mainly used, therefore the problems of reaction wheels will not directly jeopardize the touchdown and liftoff maneuvers.
More specific and detailed information about Hayabusa guidance and control system can be found in the August 2005 edition of the Journal of Japan Society for Aeronautical And Space Sciences （Vol. 53, No. 619, pp. 237-248, in Japanese）.
During a proximity operation, candidate touch down areas were identified. On November 1st 2005, two candidate areas are announced (the first one is shown in the next figure, point A).
Click here for a detailed report on the proximity operations performed by Hayabusa.
On November 9th 2005 Hayabusa approached within 70 meters from Itokawa, showing better details of the asteroid surface.
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Images and animations in this page can be freely used. But please acknowledge to Dr. Yoshida when used in news or publication medias. If you want an high definition version of the animations (avi), please contact here.