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Hayabusa's Scientific and Engineering Achievements
during Proximity Operations around Itokawa

Hayabusa arrived at its exploration target, near Earth asteroid Itokawa, on September 12th of this year after having been propelled there via ion engines and an Earth swing-by. Since then, it has successfully performed orbital maneuvers, precisely keeping its position relative to Itokawa. The Hayabusa project team has made many discoveries while carrying out their ambitious scientific observations of Itokawa. This release summarizes and reports the major scientific and engineering achievements in advance of Hayabusa's unprecedented and historic descent to the surface of Itokawa for sample collection middle to later this month.

Hayabusa is a technology demonstration spacecraft focusing on key technologies that are required for future large-scale sample and return missions, yet is also making new scientific observations and discoveries. The technology demonstration component of the mission consists of five goals: ion engine propulsion in interplanetary cruise, ion engine propulsion in combination with an Earth gravity assist, autonomous guidance and navigation using optical measurements, collection of surface samples in an ultra-low gravity environment and the direct recovery of these samples on the ground after its return from interplanetary flight. To date the Hayabusa project has accomplished these demonstrations up through the third goal. Specifically, at the time of arrival at Itokawa, Hayabusa had driven its proprietary new ion engines for 26,000 hours, including their operation during an Earth flyby. It has also perfectly completed a period of hybrid optical navigation (Fig. 1) followed by precise guidance and navigation of the spacecraft during its station keeping period around Itokawa. (Fig. 2) These engineering achievements are the primary mission of Hayabusa and their successful completion is a great achievement.

The deep-space exploration technologies that the world's space agencies are pursuing consist of three major elements: high efficiency electric propulsion for cruise, rendezvous with target destinations and round-trip flights back to the Earth. As of this time Hayabusa has accomplished the first and second of these elements, leading the way for the space exploration agencies of the world. Furthermore, robotic sample collection and return from an extra terrestrial object has not been executed before, and is not currently planned, except for Hayabusa, which will attempt to gather a bulk sample from Itokawa. Hayabusa's success clearly shows that Japan's deep space exploration technology has reached the level of the world's most developed space agencies, and that Japan is now in a leadership position in some select engineering fields. Thus Hayabusa opens a new era in solar system exploration.

For the scientific aspects of the mission, Hayabusa carries four instruments that have performed successful observations to date: AMICA, a visible imager with multi-band filters has exposed 1,500 images amounting to almost 1 GB of data, NIRS, a near infrared spectrometer has taken 75,000 measurements distributed globally over the body, LIDAR, a laser altimeter has accumulated 1.4 million measurements globally, and XRS, a X-ray spectrometer has received and integrated its signal for 700 hours. In addition to these, spacecraft tracking data has been used to measure properties of the asteroid as well. These unprecedented scientific measurements are briefly described and reported in what follows.

(A) Morphological and geological discoveries about Itokawa: The a priori theoretical assumption that small near-Earth asteroids should have geologically homogeneous features was completely overturned by the observation of a wide variety of surface features and types at Itokawa. The surface is covered with huge boulders and, for the first time, naked surfaces not covered with regolith have been exposed. (Fig. 3, 4, 5, 6, 7) Previously visited asteroids were covered with thick regolith, thus Itokawa's surface is like nothing that has seen before, which is quite fortunate for the Hayabusa mission. The opportunity to observe the true asteroid surface, which is usually concealed from view, advances our understanding of spectroscopic observations of asteroids taken from Earth, and allows us to expand our knowledge of near Earth asteroids.

(B) Taking advantage of the observations made with the onboard instruments, sufficiently detailed information about the sampling sites has been obtained, and the relation between the potential samples and the spectroscopic data has been correctly correlated. As a technology demonstration mission, Hayabusa has already finished the preliminary steps towards the primary sample and return goal. (Fig. 8) These samples will provide important scientific clues concerning the puzzlingly inconsistent correlations between S-type asteroids and ordinary chondrites, and lead to an improved understanding of the space weathering effect, which may clarify our understanding of the early solar system and Earth.

(C) Combinations of the Itokawa images along with spacecraft navigation information has enabled shape and gravity models to be numerically defined. The science team has started to study and identify the special mechanisms that can move boulders and regolith in the ultra-low gravity environment associated with small objects. (Fig. 9) The gravity and slope information and estimates of the density of boulders and regolith distribution on the surface, combined with comparisons with meteorites, will advance our interpretation and understanding of asteroid planetology.

(D) Using the laser altimeter and optical navigation camera, along with range and range-rate measurements from ground-tracking stations, have led to a successful mass and density estimate for Itokawa. The density has been estimated to be 2.3 +/-0.3 gram/cc, which is a little lower than that measured for rocks on the ground or for other S-type asteroids measured to date. This may indicate that there is substantial porosity for this body, and forces conventional views of these small objects to be changed drastically. When the samples are successfully returned and recovered, the actual porosity will be clarified and our knowledge of how the Earth relates to meteorites will be greatly improved.

The exploration of small solar system bodies contributes to an improved understanding of the Earth itself, as well as to a more comprehensive interpretation of the constituents and potential resources that these celestial objects contain. The scientific discoveries reported here will redefine scientific notions and views of asteroids from the pre-Hayabusa era, and are a remarkable accomplishment that Japan has contributed to planetary exploration.

In view of the scientific results described above, JAXA has determined the landing/sampling sites candidates and the descent target point for rehearsal, along with their planned dates and times.
The landing/sampling sites must be free of obstacles and smooth enough to ensure safety, a top priority, while at the same time the surface inclination and the ground station coverage for Hayabusa must be taken into account. Taking these issues into consideration, the candidate sites and schedule were determined. (Fig. 10)
The first site candidate is the regolith expanse in the middle of Itokawa, known as the MUSES-SEA area (Fig. 11), and the second candidate site is the Woomera desert (Fig. 12)at the tip end of Itokawa, where the terrain is broad and flat. The rehearsal target is the area located close to the spin axis, a little east of the first site. The date and time of the planned events (Japan Standard Time) are listed below.

  1. Rehearsal Descent November, 4th, 14 o'clock,
  2. 1st Touch-down November, 12th, 15 o'clock,
  3. 2nd Touch-down November, 25th, 15 o'clock.

The purpose of the rehearsal descent is, first of all, to make sure that the proximity laser range finder works as intended, as its function has not been calibrated during cruise. The second purpose is to confirm whether the target marker image can be extracted against the asteroid surface, using onboard image processing that illuminates it using flash lamps onboard the spacecraft. The third purpose is to deploy and place the hopping robot MINERVA on the asteroid surface. Deploying MINERVA conflicts with the touch-down sequence, so it will be separated in advance of the sampling runs. The touch down sequence is briefly described in Fig. 13.

In conjunction with this very big challenge, JAXA is also starting a nation-wide campaign called 'You Name the Landing Site'. The names assigned to the sites may not be officially registered by the International Astronomy Union (IAU) as the sites are very small. However, JAXA, as a finder, declares that the sites will be given those selected names. The application page is
https://ssl.tksc.jaxa.jp/hayabusa/
and will be open until 17:00 on November 30th. The application form there is available from early November. The actual naming will occur after the completion of the Hayabusa proximity observation period, in early December.


* Note: The data, including images, in this release are not calibrated and are not suitable for scientific investigations unless JAXA and the joint Science Team of Hayabusa validates them. Whenever the images are used the citation should read 'ISAS/JAXA'.

fig.1

Fig. 1 Optical Navigation

fig.2

Fig. 2 Station Keeping Operation

fig.3

Fig. 3 Itokawa +90 deg Surface

fig.4

Fig. 4 Itokawa +270 deg Surface

fig.5

Fig. 5 Itokawa, its 0 deg Surface from north, Boulders Plane

fig.6

Fig. 6 Itokawa, its +180 deg Surface with a Huge Boulder and Woomera Desert Basin

fig.7

Fig. 7 Reclaimed Craters, Regolith Expanse near Polar region

fig.8

Fig. 8 Near Infrared Spectrometer Information

fig.9

Fig. 9 Examples of Gravity Map and Slope Map

fig.10

Fig. 10 Landing/Sampling Sites Candidates A & B

fig.11

Fig. 11 MUSES-SEA Area, Site candidate-A. Mountains, Boulders and Dimples, Craters.

fig.12

Fig. 12 Woomera Desert Basin, Site candidate-B, A huge Boulder is seen.

fig.13

Fig. 13 Touching-Down Sequence

November 2, 2005