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Planet-A mission to Halley
1—95, 1983 ~ ~ ~ Vo~ .2, No. 12, pp.9 I’~-~L~~’d in (:i~iL Ifti tam. Al I ri ght s rcs~rv~d.
.;
0273-U77/83/12009105$03.OO/O Coi~’rigIit © COSPAR
PLANET-A MISSION TO HALLEY K. 1-Lirao The Institute ofSpace and Astronautical Science, Tokyo, Japan
ABSTRACT
Looking at the chance of the next apparition of the Halley cc.r~et in 1986, ISAS decided to send a first Japasanese interplanetary spacecraft for the study of cometary hydrogen coma and solar wind The Planet—A spacecraft which carries ~TV inagirs camer~and solar bind plasma aralyser will be laurched in August l~85and flyby ti e Ha~le, comet ar early March 1986 ~n.1tn the distance ~f several million kilometers from the comet nucleus. This mission is not only self—consistent but collaborative with other space nission as well as earth—bound observa— ticns. In the present paper, the Planet—A mission to Halley is described with brief explanation of the spacecraft. INTRODUCTION The Halley comet, a brilliant predictable period comet, will visit tbe inner solar system with its perihelion on 9 March 1986 The last visit of tl-’e comet was in 1910 and the comet revealed its marvellous show between the sun and the earth. It was studied by modern telescopes from the ground and various new discoveries were uaae by these observations in the ~isible tange In the next apparition, the perihelion will occur at the reverse side of the sun respective to the earth Therefore ground based observation is rather inappropriate especially near the perihelion passage wi-en the acti~ityis almost highest However, there exists a modern technique like spaceprobe which makes it possible to observe the comet in an adequate location or even in a very near vic~.naty It is a Quite exciting and important misstor to send a sciertific probe for the investigation of cometary science The first observation of a comet from space was carried Out by 010-2 in the vacuum ultraviolet range aimed at the comet Tago—Sato—Kosaka El] ~ large hydrogen coma was identified by the Lyman alpha line spectrum and also other lines were obtaineo indicating the existence of several molecules and radicals Successive1~the comet Bennet appeared and ~as extensively observed by OGO—5 and sounding rockets The widely and regi..larly distributed hydrogen coma was observed by these observations [21 In 1973, the comet Kohoutek was observed with ultraviolet cameras by a sounding rocket and obta_red ver) vice pictures of the hydrogen coma [3] These results were analysed and several models have been irtroduced [4] The evaporation rate of H 20 from the surface of comet ‘aucleus can be inferred from an image of hydrogen coma. Other LW spectrums were also observed by these space vehicles and chemistry in the atmosphere of comets have been much clarified. In the next apparition, several spacecrafts will be sent to the Halley comet. ESA will send the Giotto spacecraft to encounter to the Halley’s nucleus with distance of around 500 km [5]. Giotto’s main objectives are an imaging of the Comet nuclues and a chemistry of the inner coma, etc. USSR will send two spacecraft (VEGA) by Venus swingby to encounter with the Halley comet [6] Its main ob3~ctives are complementary to those of Giotto its approach distance being of the order of 10 Km. ISIS decided also to send a spacecraft called Planet—A to flyby the Halley comet. It will be launched in the middle of August 1985 by a new launcher called M—3S11. Limited by its ts hydrogen coma during of days before and after its flyby launching capability, spacecraft weighsseveral around tens 140 Km and carries VUV TV camera for and the imaging wind solar of conet analyser for the study of energy spectrum and pitch angle distribution of solar wind particles during the mission of spacecraft. Another spacecraft MS—T5, which will be launched in early January 1985, will be located about 0.1 AU apart from the Planet—A when the latter approaches the comet. The MS—T5 carries plasma wave analysers, solar wind proton detector and magnetometer. These two spacecrafts are included in the Planet—A program.
K.
II
SClE~TlllC UBJI:CflVL~ 1-s aescrihec above, the flalicy comet can not be observed conpietel” ti-rough ~ts perii-.eioi’ ~ passage period from either grcun~ or earth orbiting satellite because the periheJion passage occurs at the reverse side of the sun respective to the earth. One of the nair objectives of the ~TV imaging of th~ J’iauct—A is to observe the formation and decaying processes of the hydrogen coma of the comet continuously. By this observation, the change of tile mechanism. of hydrogen and tne interaction between hydrogen coma and solar wind can he studied. There are several iaecbanisra of formation of hydrcgcr. from the main abundance of water which evapolated from the “dirty snow ball’ of Whipple [7]. The velocity of expanding hydrogen atoms is strongly dependent on the formation mechanism of the atom. Direct photo—dissociation of water resusits in the ato~ic velocity of 20 Km/a, but the atoms formed by the dissociative recombination cf IL,O with electrons or by the photo—dissociation of OH radicals have a velocity of 8 Km/s. Kelier [4] summarized several models of cometary hydrogen coma by taking these cifferent velocities into consideration to interprete observed results. The long term observation of the hydrogen coma of the Halley comet by Planet—A may be quite important for this objective. The simultaneous observaCions with Ciotto and VEGA are particlularly useful for the cometary science, because these spacecrafts observe different parts of the comet. The interaction between comet and solar wind is also an interesting problem. quite remarkably in the comet tail. It is also considered to exist in the region. The solar wind may affect the coma dynamically and electro—dynamically turbulent motion in the hydrogen coma. The Planet—A spacecraft has a solar analyzer and MS—T5 is equipped with instruments for solar wind measurement.
Itis cbsc-rved cometary coma and may excite wind particle
ORBIT As the Planet—A’s primary scientific objective is the continuous imaging of the hydrogen coma of the Halley—comet during several tens days before and after the cometary perihelion passage, it is not neccesary to achieve very close encounter with the comet. Due to the light weight of the spacecraft which is caused by the limit of capability of the launcher, the spacecraft has no armour or dust shield to protect it from cometary dust. It is not yet decided definitely how close the spacecraft should flyby from the ccomet nucleus, although several million kilometers nay be selected. As a result of preliminary studies, two launch windows were found, one in early 1985 and the other in the sumser of 1985. Therefore it is scheduled to launched Planet—A in August 1985 and MS—T5 in Jananuary 1985. ?~S—T5is a test spacecraft to confirm the capability of the launcher as well as all new techniques that are to be used in the interplanetary mission for the first time. However, some solar wind measurement are also to be carried out by the HS—T5 which will arrive at the location of about 0.1 AU separated from the Planet—A when the latter makes the nearest encounter with the Halley
comet.
Although
close
encounter
is not necessarily
required
for the
Planet—A,
an
encounter trajectory has been selected as nominal at the present. Two orbital parameters of Planet—A and MS—T5 are shown in Table 1 and heliocentric orbits of both spacecraft are shown in Fig. 1. The launching of these two spacecrafts is made by direct injection without using parking orbit to save the weight of the attitude control system which is required to reorient the kick stage on the parking orbit. Therefore, the spin—stabilized third stage and kick stage will be fired serially near the apogee of the second trajectory.The trajectory determination will he carried out by mear.s of range and range—rate technique by using a 10 M~ dish antenna at the launching range on the launch day and by a 64 M~ dish antenna at Nagano
PLANET—A Launch Date
Arrival Date a
(1(a)
e
MS-T5
August 14, 1985 March
December 31, 1984
8, 1986
127,252,082.
136,235,370.
0.193
0.102
i D
(deg) (deg)
0.699 140.93
1.314 280.11
~
(deg)
147.15
244.67
January C
2
10,
1986
8.84
May 1, 1985 6.14
3 (Km/s)
TAbLF 1 Nominal Orbital
Parameters
of PLANET—A and
VS—15
Planet—A Nission
to Hal Icy
PLANET—A
93
~S-T5
Launch Date August 14. 1985 Arrival Date March 8. 1986 Cj 8.84 km2/sec2
Launch Date 2/sec2 December 31. 1984 C 3 = 6.14 kIn
~ Fig. 1
Heliocentric
Orbit of PLANET—A and MS—T5
prefecture on successive days. On the third day, a midcourse correction will be carried out to achieve the planned orbit. By using the nominal trajectory the distance between the Halley comet and the Planet—A is computed and shown in Fig. 2. SPACECRAFT
60
The spacecraft is spin—stabilized one with two stable spinning rates of 5 rpm and 0.2 rpm. It is fundamentally a cylindrical shape with diameter of 1.4 meters and height of 0.7 meter. On the top plate, a high gain despinming antenna of off—set parabola type, two reaction control thrusters complexes, and a small housing of guide mirror for VUV imaging are mounted. To the bottom plate, medium gain and low gain antennae are attached. The total height including above is about 2.45 meters, and total weight is around 140 Kg. Outside of cylindrical body are mounted solar cells which generate an electric power of about 70 W at 1 AU distance. Planet—A spacecraft is composed of power, communication and antennae, data processing, control, structurual thermal control subsystems and scientific instruments. In Fig. 3, the Planet—A spacecraft is shown. The }IS—T5 spacecraft is almost identical with the Planet—A ex— cept its scientific instruments including a boom for inagntometer sensor and two boom antennae for the plasma wave analyser. Some subsystems are described in the following.
500 400
0 30~ 201
ioc c
i
I
‘85 8/14 9/I
Fig. 2
11/1
10/1
12/1
•86 I/I
2/1
4/1
Distance between spacecraft and the comet Halley, 1: Planet—A 2: HS—T5
High Cain Antenna (Mechanical despinning antenna)
vvv
Camera
-
Guiding Mirror Housing
Ilydrazine Actuators ‘
-
~---:: to,., Gain Antenna Medium Gain Antenna Solar Cells
Communication subsystem. Planet— A communication subsystem consists of telemetry transmitter, command receiver, transponder an? antennae. All communications
3/1’
Fig. 3
Planet—A Spacecraft
94
K. l[irao
are carried
Out
frequencies
are
by four S—band frequencies for Planet—A and MS—T5 spacecraft. All four compatible with NASA’s deep space frequencies. The high gain antenna of
offset parabolic type is mechanically despun to point to the ground station with 64 is dish. Gain of onboard antenna is about 23 db and 21.5 db for transmitting and receiving, respectively. The transmitting power of onboard transmitter can be changed to 1 or 5 watts by command. A data transmission rate of down link is 1024 bps or 64 bps which depends on the distance between spacecraft and the earth. For the up link a command signal is sent by a bit rate of 16 bps with 512 Hz subcarrier. The ranging is carried Out by means of combination of transponder, receiver and transmitter by RN code with 20 code components. Three antennae of high, medium and low gain are mounted on the spacecraft. A low gain antenna is a turnstyle one attached to the aft plate and used when the spacecraft is located near the earth after launching. A nedium gain antenna is a three—element colinear array with its gain of 6 db. This is also attached to the aft plate of the spacecraft and is used for the backup of a high gain antenna. A high gain antenna has an offset—parabolic reflector. The diameter is 80 cm and the focal length is 40 cm. This antenna is driven by a despinning motor mechanism to face it to the earth on the spinning spaccraft. It is, of course, used for telemetry transmission, command reception and ranging during the mission. Data Processing subsystem. As the brain of the spacecraft, a data processing subsystem is mounted. It generates all kind of timing and control pulses used in the spacecraft and supplies them to respective subsystems and units. It also receive all signals to be transmitted from onboard instruments and multiplexes them into a frame format combined with frame synchronizing patterns and clock signal. The multiplexed signal is sent to a transmitter through a PCH modulator or to a magnetic bubble data recorder. Command signals sent from a ground station is also decoded and supplied to respective instruments through the data processing subsystem. Programmed commands are stored in the subsystem and distributed to instruments automatically. Attitude and velocity control subsystem. Attitude and velocity controls of the spacecraft are carried out by this subsystem. It consists of attitude sensors (solar sensors and star mapper), bias momentum wheel and monoplopellant gas jets. After injection, the spacecraft is required to be attitude—controlled to stabilize its spin axis perpendicularly to the ecliptic plane as soon as possible for its cruising into the interplanetary space. It is also neccessary to make midcourse corrections in velocity and to keep the spacecraft’s attitude constant. It is required to change the spinning rate correspoding with imaging and non— imaging status. The perpendicularity of the spinning axis of the spacecraft respective to the eliptic plane is verified by the correct values of the output of both solar sensor and star napper. The sun sensor is also used to obtain a spin synchronized pulse train and azimuthal direction of the sun to control the solar shutter of TV camera or to determine imagir.g time and jet thrusting time for attitude control. The star mapper is now planned to catch the Canopus (ct—carinae) as the main target for attitude determination in cruising phase. But as the mapper is designed to detect the star of visual magnitude of 1.5, it may catch some other stars in midcourse correction phase. A momentum wheel whose nominal stored angular momentum is 20N—m—sec at 3300 rpm wheel speed is used in the spacecraft. It will be actuated during the imaging operation of VUV camera to stabilize the spacecraft with 0.2 rpm spinning. As the actuator of attitude and spin rate controls and trajectory correction, six hydrogen propellant gas thrusters are mounted on the spacecraft. Hydrogen fuel is stored in two spherical—shaped titanium tanks pressurized with nitrogen gas. Scientific instruments on Planet—A spacecraft. The Planet—A spacecraft carries two scientific intruments, vacuum ultra—violet TV camera and solar wind particle analyser. The vacuum ultra—violet TV camera takes the picture of hydrogen coma of the Halley comet with a field of view of 2.5°x 2.5°. The wave length region is limited by the material for photo—electric conversion which is not yet difinite. Two—dimensional CCD is used for the last detector. To avoid the blur of image by spinning motion of the spcecraft during exposure, a special technique called spin synchronized charge shift is used on CCD. A detailed explanation is given in another article in the same volume [8]. It weighs 5.8 Kg and consumes 5.3 watts. The solar wind particle analyser measures three—dimensional distribution of the solar wind plasma within ±30° to the ecliptic plane. The analyser consists of two charged particle detectors for electrons and ions, in the energy range from 30 eV up to 16 keV in 96 steps with exponential energy sweep. Each detector is a 270°spherical electrostatic analyser with a micro—channel plate. The view direction is perpendicular to the spin axis of the spacecraft, with a view field of ±30°in parallel and ±2.5° in perpendicular directions to the spin axis respectively. The anode of nultichannel plate is divided into five to measure a 5—points angular distribution in the parallel—to—axis plane with a resolution of 12°. In the perpendicular—to—axis plane, the angular distribution is obtained by the spinning with non—uniform resolution of 5.625° interval within ±22.5~ centered to sun—spacecraft direction and 22.5°interval in other direction. As every four energy steps are scanned in one spin, 24 spins are necessary to complete all 96 steps. The analyser is operated only during a high spin rate interval. Data obtained during one spin is temporarily stored in a buffer memory and sent out during the next energy sweep. It weights 4.8 Kg and consumes 5.2 vatts.
Planet—A Mission to Halley
ScientificlnstrumefltS on NS—T5 ~pacecraft As the MS-T5 specraft is complemental with the Planet—A spacecraft in our Halley program as above described, some descriptions about scientific instruments onboard MS—T5 should be made. The first instrument is a plasma wave probe to detect the plasma wave instability at local plasma frequency and electon cyclotron harmonic wave emission in a frequency range from 70 Hz to 200 KHz. The principal purpose of this experiment is to clarify the spectra of the plasma waves combined with the magnetic field and solar wind data, that control the effective viscosity and the effective collisions of the interplanetary space plasma. The electric field component is observed in a frequency range from 3 KHz to 200 KHz with the bandwidth of 1 KHz. A short dipole antenna with length of about 10 a from tip to tip is used. The magnetic field component is measured in a frequency range from 70 Hz to 3 KHz with the bandwidth of f~f/fO.l5. A search coil is used to pick up the oscillating magntic field abo~Je the threshold of 5 pT. Total weight is 5.2 Kg and it consumes 4 watts. As the second instrument, a multigrid retarding potential trap is used to measure solar wind ion flux and its bulk velocity and temperature. Sweep range of the retarding voltage is from 300 V to 2 Ky. It weighs 1.7 Kg and consume 3.1 watts. The third instrument is a ring—core type fluxgate magnetometer to measure both microscopic and macroscopic structures of the interplanetary magnetic field. These measurement are quite useful to interprete the geometrical feature of the comet. The sensor consists of three orthogonally fixed ring—cores on the tip of the boom. The dynamic range and the noise level are 64 nT and less than 30 pT, respectively. The resolution is 62.5 pT/bit and 250 pT/bit for far space and near space, respectively. It weighs 5.2 Kg and consumes 4.4 watts. CONCLUSION REMARKS As the first Japanese interplanetary mission, the VUV imaging of the hydrogen coma of the Halley Comet as well as the measurements of solar wind parameters are selected. Although this mission selected as an self—consistent mission is for cometary science, it is also believed to contribute to the cometary science much more by collaborating and being coordinated with other missions, Giotto and VEGA, as well as earth—orbiting and ground based observations To perform the mission ISAS Is now undertaking the improvement of launching vehicle, construction of the deep space ground station with a 64 a parabolic dish and fabrication of a proto—model for Planet—A/NS—T5 spacecraft which is now described in this paper. These three main undertakings are proceeding on schedule. REFERENCES
1. 2 3 4. 5 6. 7.
A.D, Code, in NASA SP—3l0 (1972) 109 J L Bertaux in C K Acad Sci Paris 270 (1970) 1581 C B Opal in Science 185 (1974) 702 ILU. Keller, Space Sd. Rev. 18 (1976) 641. CIOTTO, Comet Halley Flyby, ESA SCI(80) 4 (1980) R.Z. Sagdev, in IA~”8lJep~rt,(1981). F.L. ‘lhipple, Ap. J., lU (1950) 375. 8. K. liirao, in : this volume.
.;
0273-U77/83/12009105$03.OO/O Coi~’rigIit © COSPAR
PLANET-A MISSION TO HALLEY K. 1-Lirao The Institute ofSpace and Astronautical Science, Tokyo, Japan
ABSTRACT
Looking at the chance of the next apparition of the Halley cc.r~et in 1986, ISAS decided to send a first Japasanese interplanetary spacecraft for the study of cometary hydrogen coma and solar wind The Planet—A spacecraft which carries ~TV inagirs camer~and solar bind plasma aralyser will be laurched in August l~85and flyby ti e Ha~le, comet ar early March 1986 ~n.1tn the distance ~f several million kilometers from the comet nucleus. This mission is not only self—consistent but collaborative with other space nission as well as earth—bound observa— ticns. In the present paper, the Planet—A mission to Halley is described with brief explanation of the spacecraft. INTRODUCTION The Halley comet, a brilliant predictable period comet, will visit tbe inner solar system with its perihelion on 9 March 1986 The last visit of tl-’e comet was in 1910 and the comet revealed its marvellous show between the sun and the earth. It was studied by modern telescopes from the ground and various new discoveries were uaae by these observations in the ~isible tange In the next apparition, the perihelion will occur at the reverse side of the sun respective to the earth Therefore ground based observation is rather inappropriate especially near the perihelion passage wi-en the acti~ityis almost highest However, there exists a modern technique like spaceprobe which makes it possible to observe the comet in an adequate location or even in a very near vic~.naty It is a Quite exciting and important misstor to send a sciertific probe for the investigation of cometary science The first observation of a comet from space was carried Out by 010-2 in the vacuum ultraviolet range aimed at the comet Tago—Sato—Kosaka El] ~ large hydrogen coma was identified by the Lyman alpha line spectrum and also other lines were obtaineo indicating the existence of several molecules and radicals Successive1~the comet Bennet appeared and ~as extensively observed by OGO—5 and sounding rockets The widely and regi..larly distributed hydrogen coma was observed by these observations [21 In 1973, the comet Kohoutek was observed with ultraviolet cameras by a sounding rocket and obta_red ver) vice pictures of the hydrogen coma [3] These results were analysed and several models have been irtroduced [4] The evaporation rate of H 20 from the surface of comet ‘aucleus can be inferred from an image of hydrogen coma. Other LW spectrums were also observed by these space vehicles and chemistry in the atmosphere of comets have been much clarified. In the next apparition, several spacecrafts will be sent to the Halley comet. ESA will send the Giotto spacecraft to encounter to the Halley’s nucleus with distance of around 500 km [5]. Giotto’s main objectives are an imaging of the Comet nuclues and a chemistry of the inner coma, etc. USSR will send two spacecraft (VEGA) by Venus swingby to encounter with the Halley comet [6] Its main ob3~ctives are complementary to those of Giotto its approach distance being of the order of 10 Km. ISIS decided also to send a spacecraft called Planet—A to flyby the Halley comet. It will be launched in the middle of August 1985 by a new launcher called M—3S11. Limited by its ts hydrogen coma during of days before and after its flyby launching capability, spacecraft weighsseveral around tens 140 Km and carries VUV TV camera for and the imaging wind solar of conet analyser for the study of energy spectrum and pitch angle distribution of solar wind particles during the mission of spacecraft. Another spacecraft MS—T5, which will be launched in early January 1985, will be located about 0.1 AU apart from the Planet—A when the latter approaches the comet. The MS—T5 carries plasma wave analysers, solar wind proton detector and magnetometer. These two spacecrafts are included in the Planet—A program.
K.
II
SClE~TlllC UBJI:CflVL~ 1-s aescrihec above, the flalicy comet can not be observed conpietel” ti-rough ~ts perii-.eioi’ ~ passage period from either grcun~ or earth orbiting satellite because the periheJion passage occurs at the reverse side of the sun respective to the earth. One of the nair objectives of the ~TV imaging of th~ J’iauct—A is to observe the formation and decaying processes of the hydrogen coma of the comet continuously. By this observation, the change of tile mechanism. of hydrogen and tne interaction between hydrogen coma and solar wind can he studied. There are several iaecbanisra of formation of hydrcgcr. from the main abundance of water which evapolated from the “dirty snow ball’ of Whipple [7]. The velocity of expanding hydrogen atoms is strongly dependent on the formation mechanism of the atom. Direct photo—dissociation of water resusits in the ato~ic velocity of 20 Km/a, but the atoms formed by the dissociative recombination cf IL,O with electrons or by the photo—dissociation of OH radicals have a velocity of 8 Km/s. Kelier [4] summarized several models of cometary hydrogen coma by taking these cifferent velocities into consideration to interprete observed results. The long term observation of the hydrogen coma of the Halley comet by Planet—A may be quite important for this objective. The simultaneous observaCions with Ciotto and VEGA are particlularly useful for the cometary science, because these spacecrafts observe different parts of the comet. The interaction between comet and solar wind is also an interesting problem. quite remarkably in the comet tail. It is also considered to exist in the region. The solar wind may affect the coma dynamically and electro—dynamically turbulent motion in the hydrogen coma. The Planet—A spacecraft has a solar analyzer and MS—T5 is equipped with instruments for solar wind measurement.
Itis cbsc-rved cometary coma and may excite wind particle
ORBIT As the Planet—A’s primary scientific objective is the continuous imaging of the hydrogen coma of the Halley—comet during several tens days before and after the cometary perihelion passage, it is not neccesary to achieve very close encounter with the comet. Due to the light weight of the spacecraft which is caused by the limit of capability of the launcher, the spacecraft has no armour or dust shield to protect it from cometary dust. It is not yet decided definitely how close the spacecraft should flyby from the ccomet nucleus, although several million kilometers nay be selected. As a result of preliminary studies, two launch windows were found, one in early 1985 and the other in the sumser of 1985. Therefore it is scheduled to launched Planet—A in August 1985 and MS—T5 in Jananuary 1985. ?~S—T5is a test spacecraft to confirm the capability of the launcher as well as all new techniques that are to be used in the interplanetary mission for the first time. However, some solar wind measurement are also to be carried out by the HS—T5 which will arrive at the location of about 0.1 AU separated from the Planet—A when the latter makes the nearest encounter with the Halley
comet.
Although
close
encounter
is not necessarily
required
for the
Planet—A,
an
encounter trajectory has been selected as nominal at the present. Two orbital parameters of Planet—A and MS—T5 are shown in Table 1 and heliocentric orbits of both spacecraft are shown in Fig. 1. The launching of these two spacecrafts is made by direct injection without using parking orbit to save the weight of the attitude control system which is required to reorient the kick stage on the parking orbit. Therefore, the spin—stabilized third stage and kick stage will be fired serially near the apogee of the second trajectory.The trajectory determination will he carried out by mear.s of range and range—rate technique by using a 10 M~ dish antenna at the launching range on the launch day and by a 64 M~ dish antenna at Nagano
PLANET—A Launch Date
Arrival Date a
(1(a)
e
MS-T5
August 14, 1985 March
December 31, 1984
8, 1986
127,252,082.
136,235,370.
0.193
0.102
i D
(deg) (deg)
0.699 140.93
1.314 280.11
~
(deg)
147.15
244.67
January C
2
10,
1986
8.84
May 1, 1985 6.14
3 (Km/s)
TAbLF 1 Nominal Orbital
Parameters
of PLANET—A and
VS—15
Planet—A Nission
to Hal Icy
PLANET—A
93
~S-T5
Launch Date August 14. 1985 Arrival Date March 8. 1986 Cj 8.84 km2/sec2
Launch Date 2/sec2 December 31. 1984 C 3 = 6.14 kIn
~ Fig. 1
Heliocentric
Orbit of PLANET—A and MS—T5
prefecture on successive days. On the third day, a midcourse correction will be carried out to achieve the planned orbit. By using the nominal trajectory the distance between the Halley comet and the Planet—A is computed and shown in Fig. 2. SPACECRAFT
60
The spacecraft is spin—stabilized one with two stable spinning rates of 5 rpm and 0.2 rpm. It is fundamentally a cylindrical shape with diameter of 1.4 meters and height of 0.7 meter. On the top plate, a high gain despinming antenna of off—set parabola type, two reaction control thrusters complexes, and a small housing of guide mirror for VUV imaging are mounted. To the bottom plate, medium gain and low gain antennae are attached. The total height including above is about 2.45 meters, and total weight is around 140 Kg. Outside of cylindrical body are mounted solar cells which generate an electric power of about 70 W at 1 AU distance. Planet—A spacecraft is composed of power, communication and antennae, data processing, control, structurual thermal control subsystems and scientific instruments. In Fig. 3, the Planet—A spacecraft is shown. The }IS—T5 spacecraft is almost identical with the Planet—A ex— cept its scientific instruments including a boom for inagntometer sensor and two boom antennae for the plasma wave analyser. Some subsystems are described in the following.
500 400
0 30~ 201
ioc c
i
I
‘85 8/14 9/I
Fig. 2
11/1
10/1
12/1
•86 I/I
2/1
4/1
Distance between spacecraft and the comet Halley, 1: Planet—A 2: HS—T5
High Cain Antenna (Mechanical despinning antenna)
vvv
Camera
-
Guiding Mirror Housing
Ilydrazine Actuators ‘
-
~---:: to,., Gain Antenna Medium Gain Antenna Solar Cells
Communication subsystem. Planet— A communication subsystem consists of telemetry transmitter, command receiver, transponder an? antennae. All communications
3/1’
Fig. 3
Planet—A Spacecraft
94
K. l[irao
are carried
Out
frequencies
are
by four S—band frequencies for Planet—A and MS—T5 spacecraft. All four compatible with NASA’s deep space frequencies. The high gain antenna of
offset parabolic type is mechanically despun to point to the ground station with 64 is dish. Gain of onboard antenna is about 23 db and 21.5 db for transmitting and receiving, respectively. The transmitting power of onboard transmitter can be changed to 1 or 5 watts by command. A data transmission rate of down link is 1024 bps or 64 bps which depends on the distance between spacecraft and the earth. For the up link a command signal is sent by a bit rate of 16 bps with 512 Hz subcarrier. The ranging is carried Out by means of combination of transponder, receiver and transmitter by RN code with 20 code components. Three antennae of high, medium and low gain are mounted on the spacecraft. A low gain antenna is a turnstyle one attached to the aft plate and used when the spacecraft is located near the earth after launching. A nedium gain antenna is a three—element colinear array with its gain of 6 db. This is also attached to the aft plate of the spacecraft and is used for the backup of a high gain antenna. A high gain antenna has an offset—parabolic reflector. The diameter is 80 cm and the focal length is 40 cm. This antenna is driven by a despinning motor mechanism to face it to the earth on the spinning spaccraft. It is, of course, used for telemetry transmission, command reception and ranging during the mission. Data Processing subsystem. As the brain of the spacecraft, a data processing subsystem is mounted. It generates all kind of timing and control pulses used in the spacecraft and supplies them to respective subsystems and units. It also receive all signals to be transmitted from onboard instruments and multiplexes them into a frame format combined with frame synchronizing patterns and clock signal. The multiplexed signal is sent to a transmitter through a PCH modulator or to a magnetic bubble data recorder. Command signals sent from a ground station is also decoded and supplied to respective instruments through the data processing subsystem. Programmed commands are stored in the subsystem and distributed to instruments automatically. Attitude and velocity control subsystem. Attitude and velocity controls of the spacecraft are carried out by this subsystem. It consists of attitude sensors (solar sensors and star mapper), bias momentum wheel and monoplopellant gas jets. After injection, the spacecraft is required to be attitude—controlled to stabilize its spin axis perpendicularly to the ecliptic plane as soon as possible for its cruising into the interplanetary space. It is also neccessary to make midcourse corrections in velocity and to keep the spacecraft’s attitude constant. It is required to change the spinning rate correspoding with imaging and non— imaging status. The perpendicularity of the spinning axis of the spacecraft respective to the eliptic plane is verified by the correct values of the output of both solar sensor and star napper. The sun sensor is also used to obtain a spin synchronized pulse train and azimuthal direction of the sun to control the solar shutter of TV camera or to determine imagir.g time and jet thrusting time for attitude control. The star mapper is now planned to catch the Canopus (ct—carinae) as the main target for attitude determination in cruising phase. But as the mapper is designed to detect the star of visual magnitude of 1.5, it may catch some other stars in midcourse correction phase. A momentum wheel whose nominal stored angular momentum is 20N—m—sec at 3300 rpm wheel speed is used in the spacecraft. It will be actuated during the imaging operation of VUV camera to stabilize the spacecraft with 0.2 rpm spinning. As the actuator of attitude and spin rate controls and trajectory correction, six hydrogen propellant gas thrusters are mounted on the spacecraft. Hydrogen fuel is stored in two spherical—shaped titanium tanks pressurized with nitrogen gas. Scientific instruments on Planet—A spacecraft. The Planet—A spacecraft carries two scientific intruments, vacuum ultra—violet TV camera and solar wind particle analyser. The vacuum ultra—violet TV camera takes the picture of hydrogen coma of the Halley comet with a field of view of 2.5°x 2.5°. The wave length region is limited by the material for photo—electric conversion which is not yet difinite. Two—dimensional CCD is used for the last detector. To avoid the blur of image by spinning motion of the spcecraft during exposure, a special technique called spin synchronized charge shift is used on CCD. A detailed explanation is given in another article in the same volume [8]. It weighs 5.8 Kg and consumes 5.3 watts. The solar wind particle analyser measures three—dimensional distribution of the solar wind plasma within ±30° to the ecliptic plane. The analyser consists of two charged particle detectors for electrons and ions, in the energy range from 30 eV up to 16 keV in 96 steps with exponential energy sweep. Each detector is a 270°spherical electrostatic analyser with a micro—channel plate. The view direction is perpendicular to the spin axis of the spacecraft, with a view field of ±30°in parallel and ±2.5° in perpendicular directions to the spin axis respectively. The anode of nultichannel plate is divided into five to measure a 5—points angular distribution in the parallel—to—axis plane with a resolution of 12°. In the perpendicular—to—axis plane, the angular distribution is obtained by the spinning with non—uniform resolution of 5.625° interval within ±22.5~ centered to sun—spacecraft direction and 22.5°interval in other direction. As every four energy steps are scanned in one spin, 24 spins are necessary to complete all 96 steps. The analyser is operated only during a high spin rate interval. Data obtained during one spin is temporarily stored in a buffer memory and sent out during the next energy sweep. It weights 4.8 Kg and consumes 5.2 vatts.
Planet—A Mission to Halley
ScientificlnstrumefltS on NS—T5 ~pacecraft As the MS-T5 specraft is complemental with the Planet—A spacecraft in our Halley program as above described, some descriptions about scientific instruments onboard MS—T5 should be made. The first instrument is a plasma wave probe to detect the plasma wave instability at local plasma frequency and electon cyclotron harmonic wave emission in a frequency range from 70 Hz to 200 KHz. The principal purpose of this experiment is to clarify the spectra of the plasma waves combined with the magnetic field and solar wind data, that control the effective viscosity and the effective collisions of the interplanetary space plasma. The electric field component is observed in a frequency range from 3 KHz to 200 KHz with the bandwidth of 1 KHz. A short dipole antenna with length of about 10 a from tip to tip is used. The magnetic field component is measured in a frequency range from 70 Hz to 3 KHz with the bandwidth of f~f/fO.l5. A search coil is used to pick up the oscillating magntic field abo~Je the threshold of 5 pT. Total weight is 5.2 Kg and it consumes 4 watts. As the second instrument, a multigrid retarding potential trap is used to measure solar wind ion flux and its bulk velocity and temperature. Sweep range of the retarding voltage is from 300 V to 2 Ky. It weighs 1.7 Kg and consume 3.1 watts. The third instrument is a ring—core type fluxgate magnetometer to measure both microscopic and macroscopic structures of the interplanetary magnetic field. These measurement are quite useful to interprete the geometrical feature of the comet. The sensor consists of three orthogonally fixed ring—cores on the tip of the boom. The dynamic range and the noise level are 64 nT and less than 30 pT, respectively. The resolution is 62.5 pT/bit and 250 pT/bit for far space and near space, respectively. It weighs 5.2 Kg and consumes 4.4 watts. CONCLUSION REMARKS As the first Japanese interplanetary mission, the VUV imaging of the hydrogen coma of the Halley Comet as well as the measurements of solar wind parameters are selected. Although this mission selected as an self—consistent mission is for cometary science, it is also believed to contribute to the cometary science much more by collaborating and being coordinated with other missions, Giotto and VEGA, as well as earth—orbiting and ground based observations To perform the mission ISAS Is now undertaking the improvement of launching vehicle, construction of the deep space ground station with a 64 a parabolic dish and fabrication of a proto—model for Planet—A/NS—T5 spacecraft which is now described in this paper. These three main undertakings are proceeding on schedule. REFERENCES
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