[meteorite-list] Hayabusa Landed on and Took Off from Itokawa successfully

[Ron Baalke]

http://www.isas.jaxa.jp/e/snews/2005/1124_hayabusa.shtml

Hayabusa Landed on and Took Off from Itokawa successfully 
Detailed Analysis Revealed 
JAXA
November 24, 2005

Hayabusa attempted its first soft-landing on Itokawa for the purpose of touch 
down and sample collection on November 20-21, 2005. Below is the data 
information with the related advance report on its status.

Hayabusa started descending at 9:00pm on Nov. 19th, 2005 (JST) from 1km in 
altitude. The guidance and navigation during the process of approach was 
operated normally, and at 4:33am on Nov. 20th, the last approach of vertical 
descent was commanded from ground, of which soft-landing was successfully 
achieved almost on the designated landing site of the surface. Deviation from 
the target point is now under investigation but presumed within a margin of 30m. 
The approaching trajectories in the quasi-inertial coordinate system and 
Itokawa-fixed coordinate system are shown in Data-1. Information on the altitude 
and its rate during the descent as measured by Doppler data is shown in Data-2. 

The velocity at the time of starting descent was 12cm/sec. At the altitude 54m 
at 5:28am, wire-cutting of target marker was commanded, after which, at 5:30am 
at altitude 40m, the spacecraft autonomously reduced its own speed by 9cm/sec to 
have substantially separated the target marker. It means that Hayabusa's speed 
became 3 cm/sec. Separation and freefall of the marker was confirmed from the 
image (Data-3) as well as from descending velocity of the spacecraft at the time 
of reducing the speed. The marker is presumed to have landed on southwest (upper 
right on the image) of MUSES Sea. 

Hayabusa then switched its range measurement from Laser Altimeter (LIDAR) to Laser 
Range Finder (LRF) at the altitude 35m and moved to hovering by reducing descending 
speed to zero at 25m above the surface, below where Hayabusa, at 5:40am at altitude 
17m, let itself to freefall, functioning itself to the attitude control mode 
adjustable to the shapes of the asteroid surface. At this point, the spacecraft 
autonomously stopped telemetry transmission to the earth (as scheduled) to have 
changed to transmission with beacon mode more efficient for Doppler measurement by 
switching to low gain antenna (LGA) coverable larger area. 

Since then, checking of the onboard instruments was not possible on a real time 
basis (as scheduled), but as a result of analyzing the data recorded onboard and 
sent back to the earth in the past two days, Hayabusa seemed to have autonomously 
judged to abort descending and attempted emergency ascent because its Fan Beam 
sensors for obstacle checking detected some kind of catch-light. Allowable margin 
is set for Hayabusa for its attitude control, in the case the spacecraft takes off 
the ground by accelerating the velocity on its own. Under such circumstances, the 
spacecraft's attitude was out of the margin, because of which continuing of safe 
descent was consequently chosen. As a result, Hayabusa did not activate its Touch 
Down Sensor function. 

At the timepoint of Nov. 21, Hayabusa was judged not to have landed on the surface. 
According to the replayed data, however, it was confirmed that Hayabusa stayed on 
Itokawa by keeping contact with the surface for about 30 minutes after having softly 
bounced twice before settling. This can be verified by the data history of LRF and 
also by attitude control record (Data-4).

This phenomenon took place during switching interval from Deep Space Network (DSN) 
of NASA to Usuda Deep Space Center, because of which the incident was not detected 
by ground Doppler measurement. The descending speed at the time of bouncing twice 
was 10cm/sec. respectively. Serious damage to the spacecraft has not been found yet 
except heating sensor that may need checking in some part of its instrument.

Hayabusa kept steady contacting with the surface until signaled from ground to make 
emergency takeoff at 6:58am (JST). The Touch Down Sensor supposed to function for 
sampling did not work because of the reason above stated, for which reason firing 
of projector was not implemented in spite of the fact that the spacecraft actually 
made landing. The attitude at landing is so presumed that the both bottom ends of 
+X axis of sampler horn and either the spacecraft or tip end of the solar panels was 
in contact with the surface. Hayabusa became the world-first spacecraft that took 
off from the asteroid. Really speaking, it is the world-first departure from an 
celestial body except the moon. 

After departure from the asteroid by ground command, Hayabusa moved into safe mode 
due to the unsteady communication line and the conflict with onboard controlling 
and computing priority. The comeback from safety mode to normal 3-axis control mode 
needed full two days of Nov. 21 and 22. Owing to this reason, replaying of the data 
recorded on 20th is still midway, which means the possibility to reveal much more 
new information through further analysis of the data. As of now, the detailed image 
of the landing site to know its exact location has not been processed yet. Hayabusa 
is now on the way to fly over to the position to enable landing and sampling sequence 
again. It's not certain yet if or not descent operation will be able to carry out 
from the night of Nov. 25 (JST). We will announce our schedule in the evening of 
Nov. 24. 

Descending and landing operation will all depend upon availability of DSN of NASA. 
We would like to express our sincere gratitude for cooperation of NASA for tracking 
networks including backup stations. 

(Data-1) Approach to Itokawa and descending trajectory 

Figures below indicate approaching trajectory of Hayabusa at descending and landing 
on Nov. 20th. Fig. 1a describes the trajectory in quasi-inertial coordinate system 
with z-axis (bottom of fig.) directed toward the earth. Fig. 1b describes the 
trajectory as against the Itokawa-fixed coordinate system. The trajectory plan was 
altered according to the occasion during its operation but it is clear from the 
figure that actual flight route was very close to the one planned in advance. 


Fig. 1a: Actual descending trajectory as compared to the scheduled plan.
(Quasi-inertial coordinate system) 

Fig. 1b: Actual descending trajectory as compared to the scheduled plan.
(Itokawa-fixed coordinate system)

Fig. 1c is to comply with fig. 1a to show actual trajectory overlapped on alternated 
trajectory plan subject to changes from time to time according to the occasional 
situation. Each dot indicates the location of the spacecraft presumed on ground 
from the surface shapes by processing the compressed image data occasionally. 
Figures show that guidance was carried out almost according to the scheduled 
trajectory. 

Fig. 1c: Navigation and guidance (Quasi-inertial system)

>From further up in altitude, the dotted locations presumed from the surface shapes 
vary with discrepancy but from below 1km sufficiently reliable information is 
obtained. The figure shows that the spacecraft was precisely guided according to 
re-scheduled trajectory plan. 

(Data 2): Data history of descending altitudes to Itokawa and its descending rate. 

Fig. 2a is the Doppler velocity history measured at Usuda and DSN stations, which 
roughly indicates the descending velocity of Hayabusa to Itokawa. The figure shows 
that the velocity of Hayabusa at the start of vertical descent was about 12cm/sec. 
and that the spacecraft reduced its speed autonomously controlling the velocity 
accelerated by the gravity of the asteroid.

Fig. 2b shows the updated altitude information at the right timing that was 
presumed from the surface conditions by integrating Doppler velocity information. 
The figure indicates the approximate altitude from the center of the asteroid mass. 
The dotted green line in the figure indicates the altitudes from the surface of 
ITOKAWA measured by laser altimeter. We can roughly understand the situation of 
each event at the time of happening by referring to both data of laser altimeter 
and Doppler velocity information.

Fig. 2a: Doppler measurement during descent of Hayabusa

Fig. 2b: Altitude history of Hayabusa during descent
(or distance history from the center of the asteroid mass).

The increase in Doppler velocity at 5:40am (JST) (21:40 world time) is because of 
landing on the surface of Itokawa as further explained below. From then on, 
tracking was switched to Usuda station, because of which we could not obtain 
Doppler velocity information for a while but the movement of the spacecraft was 
partly known from LRF, of which data has been partly analyzed as to the later 
movement of the spacecraft. 

(Data 3) Target marker with 880,000 names separated from Hayabusa and tracking 
from aboard. 

The target marker was released from the spacecraft at the relative velocity of 
9cm/sec. The delivery location is southwest of (right under in fig. 3) MUSES Sea. 
The target marker was designed to reduce bouncing rate by appropriately filling up 
the inside of aluminum sphere with fine pellets made of high-polymer materials to 
induce multiple collisions inside to increase consumption of energy. The marker was 
developed through repeated tests conducted on ground as well as in a non-gravity 
vacuum tube to prove its low repulsion.

Fig. 3: Target marker with 880,000 names separated from Hayabusa 
(left: imaged at 32m high at 5:33am JST) 

(Data 4) Data history of LRF

Fig. 4a indicates the measured data of onboard LRF 

Fig. 4a: Data history of LRF at landing (unit; m)

Fig. 4b Discrepancy data history of attitude control at landing (Y axis only unit 
degree)

According to the distance information data, after self-adjusting movement toward 
the surface at altitude 17m, moving slightly upward was observed, after which 
Hayabusa kept freefall by gravity as scheduled to have landed on the surface at 
almost zero altitude around 6:10 (JST), which was followed by the second bouncing 
to have eventually kept the altitude almost at zero for time lapse of about 30 
minutes.

Fig. 4c: Data history of accumulated time of injecting chemical engine at landing 
(unit second)

Figure 4c indicates data history of accumulated time of chemical engine thrusting. 
>From the data, we know that (1) at around 20:40 (WT), 5:40am (JST), (2) around 21:10, 
6:10am and (3) around 21:30, 6:30am a big torque force worked on the spacecraft for 
which balancing chemical engine was forced to operate to compensate it. The above 
(1) was to comply with topographical alignment but (2) and (3) were caused by 
landing as known from data history of LRF. 

>From the data information that chemical engine repeated injection at constant 
frequency toward the specific direction for 30 minutes from 21:40 (WT), 6:40am 
(JST) to 22:10, 7:10am and measured distance by LRF was small enough and also the 
attitude was kept at steady angle, we know for sure that Hayabusa steadily 
maintained the attitude from the time of landing. As chemical engine was 
programmed to inject 15milli second per a second, when once landed, it could not 
overpower the gravity for taking off and changing attitude but jut kept injecting 
as long as allowed. 

Combined interpretation of fig. 2a and 2b tells us that Hayabusa was standing on 
the surface still with the attitude kept as it landed. From the data information, 
its attitude shifting was verified as in accordance with the rotation of the 
asteroid, and it is presumed that the attitude was kept steady as at landing with 
the side of ion engine facing approximately eastward of Itokawa (leftward in the 
fig. 3a).