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Hayabusafs Scientific and Engineering Achievements during Proximity Operations around
Itokawa.

 

 

Hayabusaarrived at its exploration target, near Earth asteroid Itokawa, on September 12th
of this year after having been propelled there via ion engines andan 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fs unprecedented
and historic descent to the surfaceof Itokawa for sample collection middle to later this
month.

Hayabusa is a technology demonstration spacecraftfocusing on key technologies that are
required for future large-scale sample and return missions, yet is also making newscientific
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 returnfrom 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 proprie
tary 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 duringits 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fsspace 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f
ssuccess clearly shows that Japanfs deep space exploration technology has reached the
level of the worldfsmost developed space agencies, and that Japan is now in a leadership
position in some select engineeringfields. Thus Hayabusa opens a newera in solar system
exploration.

For the scientific aspects of the mission, Hayabusa carries four instruments that have
performed successfulobservations 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
thatsmall near-Earth asteroidsshould 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 visitedasteroidswere covered with
thick regolith, thus Itokawafs 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 isusually concealed from view, advancesour understanding of spectroscopic observations
of asteroids taken from Earth, and allows us toexpand 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 puzzlinglyinconsistentcorrelations between S-type asteroids and ordinary chondrites, and
lead to an improved understanding of the space weathering effect, which may clarify our
understandingof the early solar system and Earth.

(C) Combinationsofthe Itokawa imagesalong 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 moveboulders and regolith in the ultra-low gravity
environment associatedwith 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 plane
tology.

(D) Using the laser altimeter and optical navigation camera, along with range and range-rate
measurements fromground-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 indicatethat there issubstantialporosityfor this body, andforces 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 relatesto meteorites will be greatly improved.

The exploration of small solar system bodies contributes to an improved understanding of the
Earthitself, as well as to a more comprehensive interpretation of the constituents and potential
resources that these celestial objects contain. The scientific discoveries reported here willredefinescientificnotionsandviewsof asteroids from the pre-Hayabusa era, and are a
remarkable accomplishment that Japanhas 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 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 DescentNovember, 4th, 14 ofclock,
2.1st Touch-downNovember, 12th,15 ofclock,
3. 2nd Touch-downNovember, 25th,15 ofclock.

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eYouNametheLandingSitef. 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 availablefrom
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 eISAS/JAXAf.

 

 
 

Fig. 1 Optical Navigation


Fig. 2 Station Keeping Operation


Fig. 3 Itokawa +90 deg Surface


Fig. 4 Itokawa +270 deg Surface


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


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


Fig. 7 Reclaimed Craters, Regolith Expanse near Polar region


Fig. 8 Near Infrared Spectrometer Information


Fig. 9 Examples of Gravity Map and Slope Map


Fig. 10 Landing/Sampling Sites Candidates A & B


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


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


Fig. 13 Touching-Down Sequence




 
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