Analysis of Autonomous Rendezvous Docking and Sample Transfer Technologyfor a Space Probe in the Jupiter Trojan Region

Yuki TAKAO1), Shigeo KAWASAKI2), Thoshihiro CHUJO1), Shota KIKUCHI1),Kazuaki IKEMOTO1), Satoshi KITAO3), Hideki KATO2), Osamu MORI2)

1)The University of Tokyo, Tokyo, Japan2)Japan Aerospace Exploration Agency, Sagamihara, Japan

3)Aoyama Gakuin University, Sagamihara, Japan

In the exploration mission of Jupiter Trojans by a solar power sail which is planned at present in JAXA,sampling from the asteroid and sample return by use of the lander spacecraft are considered. Communicationdelay in the Jupiter Trojan region is however so large that the remote manipulation from the ground station isalmost impossible. This study analyzes the technology of rendezvous docking with autonomous navigation andguidance, sample transfer technology from the lander to the motherspacecraft, and evaluates the feasibility of thesystem.

1 Introduction

Japan Aerospace Exploration Agency (JAXA) is cur-

rently planning to explore the Jupiter Trojan asteroids by

use of a solar power sail. Solar power sail is an extended

concept of solar sails, whose sail membrane is equipped

with thin-film solar cells. Owing to the large area of

the sail, a solar power sail can generate large amount of

electric power even in the distant region from the sun.

JAXA launched the small solar power sail demonstrator

IKAROS in 2010, and succeeded to demonstrate the tech-

nology of both solar sail and solar power sail[1]. Figure

shows the concept of solar power sail, and the appearance

of IKAROS.

Utilizing a solar power sail makes it possible to drive

ion engines in the outer regions. This achieves the hybrid

propulsion system of ion engines and solar sailing, which

can be widely applied in the future deep space missions.

JAXA has proposed to explore the Jupiter Trojan region

with the next solar power sail for the first time in the

world[2]. In this mission, the sail is fully equipped with

thin-film solar cells, and the large ∆V is acquired by ion

engines in combination with the Earth and Jupiter grav-

ity assist. Figure 2 shows the concept of the next solar

power sail mission.

One of purposes of the mission is to collect samples

from the surface of the asteroid, and takes them back to

Earth (sample return). Since it is difficult for the solar

power sail, which is spin stabilized with the large sail,

to land on the asteroid, the small lander is used for the

sampling (Fig. 3). The lander is separated from the

solar power sail mother spacecraft (MSC), lands on the

asteroid, and collects samples from the asteroid. Then,

it returns to and docks with the MSC. This rendezvous

and docking sequence must be performed autonomously

because of the large communication delay in the Jupiter

Trojan region.

The Japanese ETS-VII mission successfully demon-

strated the autonomous rendezvous docking in 1998 for

the first time in the world[3]. ETS-VII consisted of two

Fig.1 Concept of solar power sail, and appearance

of the small solar power sail demonstrator IKAROS.

satellites, the chaser and the target. They utilized the

global positioning system (GPS), a rendezvous laser

radar (RVR), and a camera-type proximity sensor (PXS)

for their navigation. However, the GPS cannot be used

in the Jupiter Trojan region in case of the solar power

sail mission. In addition, the target spacecraft in the

next solar power sail mission (i. e. solar power sail MSC)

is basically non-cooperative because it is spin-stabilized

with the large amount of moment of inertia, while the

target of ETS-VII could perform the attitude control for

the rendezvous docking. Also, the lander must transfer

the collected samples to the re-entry capsule which

belongs to the MSC.

This paper presents the newly developed rendezvous

docking and sample transfer system. This method can

be widely applied to the future asteroid exploration mis-

sions.

1

Fig.2 Concept of Jupiter Trojan asteroid explo-

ration mission by the next solar power sail. Size of

the sail is about 50m × 50m.

Fig.3 Design of the lander for the Jupiter Trojan

exploration mission.

2 Sample Return Scenario

Figure 4 shows the mission sequence related with sam-

ple return. The solar power sail (MSC) stays at the home

position (HP), whose altitude is 250km from the asteroid,

in the ordinary operation. When the lander-separation

operation starts, the MSC descents to 1km altitude to

separate the lander. After the separation, the lander

starts its descent, sampling, and in-situ analysis. The

MSC simultaneously ascends to 50km altitude (docking

altitude) and keeps that position until the RVD is com-

pleted. After finishing the in-situ analysis of the samples,

the lander starts ascending toward the MSC, and per-

forms the RVD. Then, the samples inside the lander are

transferred to the re-entry capsule of the MSC. Finally,

the lander is again separated from the MSC to reduce the

system weight in the return path to Earth.

In the following sections, detailed ways of the ren-

dezvous docking and sample transfer are described.

3 Rendezvous Docking

Figure 5 shows the sequence of the rendezvous proposed

in this study. The lander and the MSC perform naviga-

tion by use of RF sensors, which are described later, after

Altitude

(Not to scale)

Time

(Not to scale)

HP:

250km

H3:

50km

1km

Fig.4 Mission sequence related with sample return.

Fig.5 Sequence of the rendezvous docking.

the takeoff. When the lander reaches the relative distance

of 1km from the MSC, the lander switches the navigation

sensor to the optical navigation camera (ONC). When

the lander reaches the relative distance of about 2m, it

docks with the MSC by use of a method called berthing.

Finally, the lander transfers the samples to the re-entry

capsule.

3.1 Rendezvous

3.1.1 RF sensor

Radio frequency (RF) sensor utilizes the character-

istics of an active integrated phased array antenna

(AIPAA) to determine the relative relations between the

two antennas[4, 5]. This sensor is to make it possible to

estimate the position and attitude without conventional

ones such as GPS.

Phased array antenna is an aggregate of small an-

tennas: phases of their electromagnetic radiations can

be electronically controlled by phase shifters to control

the beam direction with high accuracy. The AIPAA is

the one with amplifiers added to its elements and phase

shifters. Figure 6 shows the appearance of the AIPAA.

The RF sensor in this mission not only allows the two

way communication by the AIPAA, but has ranging func-

tion, Doppler shift measurement function, and retrodirec-

tive function. The retrodirective function can measure

the direction from which the pilot signal comes, and re-

turn the signal back to that direction. The principle of

the function is that the direction of the pilot signal is cal-

culated from the difference of phase of the signal received

by each adjacent antenna element. This makes it possi-

2

10~15cm

Fig.6 Appearance of the RF sensor

ble for the lander and the MSC to know the direction in

which they exist each other.

RF sensors can be used from the takeoff to the relative

distance of 1km because their accuracy deteriorates in

the closer region.

3.1.2 Optical navigation

From the relative distance of 1km through 100m, the

ONC captures the whole image of the sail of the MSC.

The relative position of the lander to the MSC is calcu-

lated from the size of the sail photographed by the ONC.

The attitude of the lander is calculated by use of inertial

reference unit (IRU) through the whole sequence.

When the lander reaches the relative distance of 100m,

the ONC captures LED markers attached on the bottom

side of the main body of the MSC because the ONC can-

not capture the whole image of the sail at the distance.

The relative position is calculated from the distribution

of the markers in the image.

When the lander reaches the relative distance of about

2m, it stops ascending and performs docking by use of

the berthing method.

3.2 Docking

3.2.1 Berthing method

Berthing originally means to bring a vessel to a berth,

for example by use of something like ropes. In the pro-

posed method, the lander uses an extension boom (Fig.

8) instead of ropes. The extension boom is a telescopic

rolled-up boom which is made of tri-axial woven fabric

CFRP[6, 7]. On the other hand, the MSC has a holding

space for the lander, the top of which is tapered.

As shown in Fig. 7, the lander extends the extension

boom as the first step of the berthing method. Next, the

lander approaches toward the MSC to insert the boom

into the holding space. Due to the tapered shape of the

holding space, the tip of the boom is guided to the con-

nection part of the MSC regardless of some errors of guid-

ance, navigation and control. After the connection of the

boom with the MSC, the lander revolves the boom to

complete the docking.

Fig.7 Sequence of the berthing method.

Fig.8 Extension boom[7].

The tip of the boom is connected with the MSC by the

electromagnetic force between the magnetic substance on

the boom and the electromagnet on the connection part.

This is to make the second separation of the lander easier

compared to the use of latching structures. In addition,

the connection part is designed to be rotation free to

avoid the torsion of the boom, which may be caused by

the relative spin between the lander and the MSC after

the connection.

The lander must be fixed to the MSC, which is a

spinning object, to secure the sample transfer path.

To achieve this, the lander has protrusions on its side

while the MSC has the triangle-shaped grooves on its

holding space (Fig. 9). First, the lander roughly adjusts

its rotation to the MSC by use of the ONC. Next, the

lander revolves its boom. The protrusions of the lander

is then guided along the grooves of the MSC, and the

relative rotation is finally fixed.

3.2.2 Simulation

The feasibility of the proposed berthing method is ver-

ified by the numerical simulation. The lander first stays

at the initial position; 2m beneath the MSC with cer-

tain errors. Then, the lander starts ascending toward the

MSC by one ∆V with errors. The collision of the tip of

the boom and the wall of the holding space is modeled

by the spring-damper system. The reaction force and the

3

Protrusions

Grooves

Fig.9 Structures to fix the relative rotation between

the lander and the MSC.

Table.1 Conditions for the berthing simulation.

extension boom

length 1.5 m

diameter 40 mm

Young’s modulus 10 GPa

buckling point 15 MPa

mass 95 kg

lander diameter 650 mm

height 630 mm

frictional force are given as follows.

Fr = −kx− cx (1)

Ff = µFr (2)

where x is the displacement (depression) of the wall, k

is the spring constant, c is the damping coefficient, µ

is the coefficient of friction.The collision characteristics

such as k, c, and µ are measured by the experiment. The

conditions for judging the berthing to be successful are

as follows.

• The tip of the boom reaches the connection part

and is captured by the electromagnet.

• The extension boom does not buckle through the

sequence.

Materials of the boom such as elasticity, buckling point,

are measured by the strength test. These conditions for

the simulation are summarized in Table. 1.

The berthing motion is simulated multiple times with

various initial states and errors of position, velocity and

attitude of the lander. As a result, requirements for the

successful docking are obtained as shown in Table 2. Fig-

ure 10 shows one example of the successful trials.

Table.2 Requirements for the berthing obtained by

the simulation.

initial state errorposition: 15 cm

attitude: 1 deg

∆Vrequirement: 8 cm/s

permissible error: 1 cm/s

taper angle 30◦

(a) Initial

(b) Final

Fig.10 Verification of the berthing method by the

multi-body simulation. The red line Describes the

path the tip of the boom followed.

4 Sample Transfer

The lander transfers samples to the MSC after the ren-

dezvous docking. At the sampling, samples are contained

in a container called sample catcher via induction path-

way. When the rendezvous docking is completed, the

lander pushes up this sample catcher toward the re-entry

capsule by use of an extension mast (Fig. 11, 12).

The feasibility of the sample transfer method is verified

by experiments. First, we confirmed that the required

amount of samples are contained in the sample catcher.

In this experiment, collected samples are blown through

the sampler hone, callow cell and induction path way.

Then, mass of samples contained in the sample catcher

is measured.

Next, an experiment is conducted which simulates the

sample catcher transfer (Fig. 13). As a result, the sample

catcher is confirmed to be successfully transferred to the

capsule regardless of friction.

4

Sampler

Callow cell Induction pathway

Ablator

Extension mast

Sample catcher

Samples

Capsule

MSC

Lander

Fig.11 Schematic chart of the sample transfer.

Fig.12 Extension mast used for the sample transfer.

Fig.13 An experiment of the sample catcher trans-

fer.

5 Conclusion

An autonomous rendezvous docking and sample trans-

fer strategy for the Jupiter Trojan exploration mission

is proposed. In this mission, the small lander utilizes

newly developed RF sensors and berthing method. This

rendezvous docking technique allows a completely au-

tonomous operation. Also, the sample transfer method

using inflatable extension mast is an unprecedented

method.

This paper presented the operation scenario, and con-

firmed the effectiveness of the system. The use of small

lander is considered to be crucial in future deep space

exploration mission since it is unrealistic that a mother

spacecraft lands on and explores a large, heavy celestial

bodies. This study gives an effective insight to such mis-

sions.

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