- Email: [email protected]
Chandrayaan-1 mission to the Moon
Acta Astronautica 63 (2008) 1215 – 1220 www.elsevier.com/locate/actaastro
Chandrayaan-1 mission to the Moon Jitendra Nath Goswamia,∗ , Mylswamy Annaduraib a Planetary and Geosciences Division, Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India b ISRO Satellite Center, Bangalore 560017, India
Received 5 December 2007; received in revised form 20 March 2008; accepted 6 May 2008 Available online 1 July 2008
Abstract Chandrayaan-1 is the first Indian planetary exploration mission that will perform remote sensing observation of the Moon to further our understanding about its origin and evolution. Hyper-spectral studies in the 0.4–3 m region using three different imaging spectrometers, coupled with a low energy X-ray spectrometer, a sub-keV atom analyzer, a 3D terrain mapping camera and a laser ranging instrument will provide data on mineralogical and chemical composition and topography of the lunar surface at high spatial resolution. A low energy gamma ray spectrometer and a miniature imaging radar will investigate volatile transport on lunar surface and possible presence of water ice in the polar region. A radiation dose monitor will provide an estimation of energetic particle flux en route to the Moon as well as in lunar orbit. An impact probe carrying a mass spectrometer will also be a part of the spacecraft. The 1 ton class spacecraft will be launched by using a variant of flight proven indigenous Polar Satellite Launch Vehicle (PSLV-XL). The spacecraft will be finally placed in a 100 km circular polar orbit around the Moon with a planned mission life of two years. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction The current decade has seen a revival in the field of planetary exploration with several new initiatives to explore the various solar system objects. In particular, a renewed effort to explore the Moon has started in 2003 with the Smart-1 mission of ESA that will be followed by Japanese, Chinese, Indian and American missions to Moon during the next two years. A proposal for a mission to the Moon was put forward by Indian planetary scientists in the late nineties and the Indian Space Research Organization (ISRO) initiated spadework for such a mission in 2001. As a part of this new
∗ Corresponding author. Fax: +91 79 6301502.
E-mail addresses: [email protected] (J.N. Goswami), [email protected] (M. Annadurai). 0094-5765/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2008.05.013
initiative to have dedicated planetary science missions, the Chandrayaan program was conceived. Chandrayaan-1, the first such mission is now in an advanced stage of preparation and will be launched in 2008. This paper describes the scientific objectives, the various payloads, the mission details, observational plans and some project management aspects of the Chandrayaan-1 mission. 2. Scientific objectives and payloads The primary scientific objective of the Chandrayaan1 mission is to further our understanding of the origin and evolution of the Moon based on high resolution selenological and chemical mapping of the Moon. A suite of baseline payloads, identified to meet this scientific objective, include a terrain mapping camera (TMC), a hyper-spectral imager (HySI), a low energy
1216
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X-ray spectrometer (LEX), a high energy X– ray spectrometer (HEX) and a lunar laser ranging instrument (LLRI). These payloads, developed at the Space Application Center, Ahmedabad, the ISRO Satellite Center, Bangalore, Physical Research Laboratory, Ahmedabad, and the Laboratory for Electro-Optics System, Bangalore, will provide simultaneous mineralogical, chemical and photo-geological mapping of the lunar surface at resolutions better than previous and currently planned lunar missions. They will allow (i) direct estimation of lunar surface concentration of the elements Mg, Al, Si, Ca, Ti and Fe with high spatial resolution ( 30 km), (ii) high resolution (∼100 m) UV–VIS–NIR mapping of the lunar surface to identify abundances of various lunar minerals, (iii) high resolution 3D mapping of the lunar surface and (iv) nature of volatile transport on Moon, particularly to colder lunar polar regions. An impact probe carrying mass spectrometer, video camera and a radar altimeter, developed at the Vikram Sarabhai Space Center and the Space Physics Laboratory, Thiruvananthapuram, will complement these baseline payloads and will be a technological fore runner for future proposed lunar landing missions. ISRO also offered opportunity to the international scientific community to participate in Chandrayaan-1 mission and several payloads that complement and supplement the basic objectives of the Chandrayaan-1 mission have been selected for inclusion in this mission. These are: a miniature imaging radar instrument (Mini-SAR) to explore the polar regions of the Moon to look for possible presence of water ice, two infrared spectrometers (SIR-2 and Moon mineralogy mapper: MMM) for extending the range beyond that of the HySI, a low-energy X-ray spectrometer (CIXS) for high resolution chemical mapping, a sub-keV atom reflecting analyzer (SARA) for detection of solar wind sputtered low energy neutral atoms as well as studies of lunar surface magnetic anomalies and a radiation dose monitor (RADOM) for monitoring energetic particle flux in the lunar environment. Three of the payloads, SIR-2, CIXS and SARA, developed at the Max-Planck Institute, Lindau, Rutherford Appleton Laboratory (RAL), UK, and Swedish Institute of Space Physics, respectively, will be provided by the European Space Agency (ESA). NASA will provide Mini-SAR, developed by JHU/APL and NAWC, and MMM, developed by Brown University and JPL. RADOM will be provided by the Bulgarian Academy of Sciences. There will be significant technical participation of ISRO in realizing two of the payloads, CIXS and SARA. There will also be active scientific collaboration between ISRO scientists and investigators associated with each of the AO payloads.
The basic characteristics of the payloads are described here. 2.1. The payloads There are 11 instruments including the impact probe. Three payloads (HySI, SIR-2 and MMM), covering different wavelength ranges, will study reflected solar energy from the lunar surface covering the 0.4.3 m. These are: 2.1.1. Hyperspectral imager (HySI) The hyper spectral imager for mineralogical mapping will be operating in the 400–950 nm range employing a wedge filter coupled to an area array detector. It will have 64 continuous channels with a spectral resolution better than 15 nm and a spatial (pixel) resolution of 80 m with a swath of 20 km. 2.1.2. Near infrared spectrometer (SIR-2) SIR-2 is an upgraded, compact, monolithic grating, near infrared point spectrometer based on SIR flown on board ESA’s SMART-1 mission and covers the wavelength region 0.9–2.4 m. The instrument has a spectral resolution of 6 nm. It is a linear CCD array based instrument and will have a resolution of ∼80 m per pixel. 2.1.3. Moon mineral mapper (MMM) The MMM (M 3 ) is a high throughput push broom imaging spectrometer operating in 0.7–3.0 m range with high spatial (70 m per pixel) and spectral (10 nm sampling) resolution. It will have a swath of 40 km. It uses a 2D HgCdTe detector array for measuring reflected solar energy in the above wavelength range. 2.1.4. Terrain mapping camera (TMC) The terrain mapping stereo camera (TMC) in the 500–850 nm band will have three linear array detectors for nadir, fore and aft viewing and will have a swath of 20 km. It will be capable of providing 3D image of the lunar surface with a ground resolution of 5 m with base to height ratio of one. 2.1.5. Chandrayaan imaging X-ray spectrometer (CIXS) Two options were considered for detection of low energy (1–10 keV) fluorescence X-rays from the lunar surface; use of a thermoelectrically cooled X-ray CCD (LEX) or of a swept-charge X-ray detector (SCD) array. The final choice was SCD and CIXS, a modified version of D-CIXS instrument on board SMART-1, proposed by RAL, UK, and supported by ESA was selected in
J.N. Goswami, M. Annadurai / Acta Astronautica 63 (2008) 1215 – 1220
place of LEX. This collimated LEX will have a field of view of ∼30 km and provide detail chemical mapping of the lunar surface for the elements, Mg, Al and Si and also for Ca, Ti and Fe during solar flare times. An X-ray solar monitor (XSM) will be a part of this payload and will continuously monitor the solar X-ray flux essential for analyzing the data on fluorescence Xrays to infer absolute elemental abundance. CIXS will be a collaborative payload between ISRO and RAL with a group of ISRO scientists and engineers involved in various aspects of payload design and fabrication and detector characterization. 2.1.6. High energy X– ray spectrometer (HEX) The high-energy X– ray (30–270 keV) spectrometer (HEX) will employ CdZnTe solid-state detectors and will have a suitable collimator providing an effective spatial resolution of 40 km in the low energy region (< 60 keV). It will employ a CsI anticoincidence system for reducing back-ground and is primarily intended for the study of volatile transport on Moon using the 46.5 keV ray line from 210 Pb decay as tracer. 210 Pb is a decay product of volatile 222 Rn and both belong to the 238 U decay series. The instrument will have a detection threshold of < 30 keV and a resolution of better than 10% at 60 keV. Attempt will be made to infer compositional characteristics of lunar terrain from a study of the continuum background in this energy range as well as low resolution Th and U mapping of terrains enriched in these elements (e.g. KREEP). 2.1.7. Lunar laser ranging instrument (LLRI) The LLRI will employ an Nd–Yag laser with energy 10 mJ and employ a 20 cm optics receiver coupled to a Si–APD. It will be operating at 10 Hz (5 ns pulse) and can provide a vertical resolution better than 5 m. The LLRI and TMC will provide complementary data for generating a topographic map of the Moon and the LLRI, in particular, will provide the first such data set for the polar region at higher than 80◦ latitude. 2.1.8. Sub-keV atom reflecting analyzer (SARA) The SARA payload consists of two major subsystems, Chandrayaan-1 low energy neutral atom (CENA) and solar wind monitor (SWIM). CENA detects neutral atom sputtered from the lunar surface by solar wind ions. The CENA sensor has an energy range of 10 eV to 2 keV with an energy resolution of ∼50% and can resolve groups of elements such as H, O, Na/Mg group, K/Ca group and Fe. SWIM is a simple ion mass analyzer consisting of a sensor and an energy analyzer
1217
that provides information on the energy and mass of the incident solar wind ions. Space Physics Laboratory, Thiruvananthapuram, is collaborating in this experiment and is responsible for developing the data processing unit. 2.1.9. Miniature synthetic aperture radar (MINI-SAR) A multifunction miniature radar consisting of SAR, altimeter, scatterometer and radiometer operating at 2.5 GHz will explore the permanently shadowed areas near lunar poles to look for signature of water ice. The mini-SAR system will transmit right circular polarization (RCP) and receive both left circular polarization (LCP) and RCP. The SAR system has a nominal resolution of 150 m per pixel with 8 km swath. 2.1.10. Radiation dose monitor (RADOM) RADOM is a miniature spectrometer–dosimeter that uses a semiconductor detector and measure the deposited energy from primary and secondary particles using a 256 channel pulse analyzer. The deposited energy spectrum can then be converted to deposited dose and incident flux of charged particles on the silicon detector. 2.1.11. Moon impact probe (MIP) In addition to the primary scientific payloads, an impactor carrying a high sensitive mass spectrometer, a video camera and a radar altimeter has also been included in this mission. The impactor will be released at the beginning of the mission after reaching 100 km lunar polar orbit and attempt will be made to impact it in a predetermined location on the lunar surface. Apart from the video imaging of the landing site, the onboard mass spectrometer will try to detect possible presence of trace gases in the lunar exosphere. 3. Mission scenario 3.1. The spacecraft The spacecraft design is adopted from flight proven Indian Remote Sensing Satellite bus coupled with suitable modifications specific to the lunar mission. Apart from the solar array, TTC and data transmission, that are specific to the lunar mission, other aspects of system design have flight heritage. However, some changes specific to lunar mission is also required. These include extending the thrust cylinder and having an upper payload deck to accommodate MIP and few other payloads. Additionally, Chandrayaan-1 will have a canted solar array since the orbit around the Moon is inertially
1218
J.N. Goswami, M. Annadurai / Acta Astronautica 63 (2008) 1215 – 1220
SWIM
XSM
MIP RADOM SIR-2
CENA LLRI
CIXS
MiniSAR
HEX
HySI
M3
TMC
Fig. 1. A view of the Chandrayaan-1 spacecraft showing the placement of the various payloads: terrain mapping camera (TMC); Moon mineralogy mapper (M3); hyper spectral imager (HySI); high energy X– ray spectrometer (HEX); miniature synthetic aperture radar (mini-SAR) antenna; Chandrayaan imaging X-ray spectrometer (CIXS); sub-keV atom reflecting analyzer [SARA: energetic neutral analyzer (SENA); solar wind monitor (SWIM)]; X-ray solar monitor (XSM, CIXS); Moon impact probe (MIP); radiation dose monitor (RADOM); infrared spectrometer (SIR-2) and lunar laser ranging instrument (LLRI). The blue panel is the canted solar array.
fixed resulting in large variation in solar incidence angle. There is a need to have a gimbaled high gain antenna system for downloading the payload data to the Indian Deep Space Network (IDSN). Thus the spacecraft will be cuboids in shape of approximately 1.50 m side, with a liftoff mass of ∼1.3 ton with bus element accounting for ∼0.4 ton, payload ∼0.1 ton and propellant ∼0.8 ton. At lunar orbit it will be ∼0.6 ton. This would be a three-axis stabilized spacecraft generating about 750 W of peak power using canted single sided solar array and will be supported by a Li-Ion battery for eclipse operations. The spacecraft would adopt bipropellant system to carry it from EPO through lunar orbit, including orbit and attitude maintenance in lunar orbit. The propulsion system would carry required propellant for a mission life of two years, with adequate margin. The TTC communication would be in the S-band. The scientific payload data that will be stored in a solid-state recorder will be later played back and down linked in X-band through 20 MHz bandwidth by a steer able antenna pointing at DSN. Fig. 1 provides the spacecraft configuration with payloads.
3.2. Launch vehicle and mission sequence Chandrayaan-1 will be launched by PSLV-XL, a variant of flight proven indigenous Polar Satellite Launch Vehicle (PSLV) from Satish Dawan Space Centre, Shriharikota. The spacecraft will be injected into 260 km × 24, 000 km orbit. After separation from the launcher, the solar panel is deployed and the spacecraft is raised to Moon rendezvous orbit by three consecutive in-plane perigee maneuvers to achieve the required 3,86,000 km apogee. After the third perigee burn, a quick estimate of the achieved lunar transfer trajectory (LTT) is made and a mid-course correction is imparted at the earliest opportunity. The spacecraft coasts for about five days in this trajectory prior to the lunar encounter. During the coasting phase the spacecraft would stay mostly in the sun-pointed mode and at the same time ensuring good communication link to ground. The major maneuver of the mission, called lunar orbit insertion (LOI) leading to lunar capture, is carried out at the peri-selene part of the trajectory. The maneuver ensures successful lunar capture in a polar, near circular 1000 km-altitude
J.N. Goswami, M. Annadurai / Acta Astronautica 63 (2008) 1215 – 1220 LUNAR ORBIT INSERTION (LOI) FINAL ORBIT ~100 km
LUNAR CAPTURE AT ~1000 km
LUNAR TRANSFER TRAJECTORY ETO
EPO
MID-COURSE CORRECTION
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these instruments. SSR-2 is sized to record the data for about seven orbits covering the entire ground trace of the Moon before being played back for ground transmission. The other SSR namely SSR-1 will record data from optical imaging instruments or MiniSAR for subsequent transmission to ground. Two ground terminals one with 18 m antenna and another with 32 m antenna have been established near Bangalore as a part of IDSN. Even though the already tested 18 m terminal (during SMART-1 EOL mission and some of the ongoing planetary missions) will suffice for Chandrayaan-1, the newly installed 32 m antenna would cater to the futuristic planetary exploration programme as well. 3.4. Chandrayaan-1 project management
Fig. 2. Chandrayaan-1 mission sequence.
orbit around the Moon. After successful capture and checks of various spacecraft sub-systems, the altitude is lowered through a series of in-plane corrections to 200 km near circular orbit for studying the nature of orbit perturbations under the influence of the lunar gravity field. Following this, the target altitude of 100 km circular, polar orbit is achieved. Fig. 2 depicts the planned Chandrayaan-1 mission sequence.
Chandrayaan-1 is carrying 11 scientific instruments from an equal number of institutions. Accommodation of these instruments and to meet their stringent technical requirements in a small satellite bus turned out to be a challenging task. The difficulty and complexity of the task has further increased because of the geographical locations of various laboratories involved in design and development of these instruments at various Indian laboratories and foreign institutions with varying approach and work cultures. Chandrayaan-1 has provided an opportunity to demonstrate true multinational cooperation in the field of planetary exploration missions in general and for lunar science missions in particular.
3.3. Lunar observational plans 3.5. Indian Space Science Data Centre (ISSDC) All the imaging instruments needs appropriate solar illumination. Since the solar angle and hence illumination changes with time as the Moon moves in its orbit, useful imaging can be done only for selected period of time. In a duration of one year there will be two prime imaging seasons of 60 days each separated by a gap of 120 days. During the prime imaging season ± 60◦ latitude is covered. During the four months interim period between the two prime imaging seasons, 60–90◦ of north/south polar regions will be covered. Due to poor solar illumination in this region, the effective spatial resolution will be degraded in achieving a reasonable signal to noise ratio. During the two-year period of mission life, the entire Moon will be imaged with a possibility of repetitive coverage of selected regions. MiniSAR polar imaging is planned during non-imaging seasons. The X-ray payloads, LLRI, SARA and RADOM are kept ‘ON’ continuously and a separate solid-state recorder (SSR-2) is provided for recording data from
The IDSN in Bangalore will receive the payload data from Chandrayaan-1. The data in its raw form along with auxiliary data will be sent to the Indian Space Science Data Centre (ISSDC) that is being set up in Bangalore. ISSDC would process the raw data and convert those into user-friendly form. The data centre will also archive all the payload data in PDF format and will be the repository and focal point for Chandrayaan-1 science team. 4. Summary The Chandrayaan-1 mission, the first Indian planetary exploration mission, is in advanced stage of preparation and will be launched in 2008. It will have 11 scientific payloads including an impact probe. The mission is truly international in character with two IndoEuropean collaborative experiments and additional experiments from USA, Germany and Bulgaria, that were
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selected based on proposals received in response to Announcement of Opportunity, to meet the basic scientific objectives of the mission. With a life time of two years and plans for whole Moon mapping at high spatial and
spectral resolution, a successful Chandrayaan-1 mission will substantially improve our understanding of several key questions regarding the origin and evolution of the Moon.
Chandrayaan-1 mission to the Moon Jitendra Nath Goswamia,∗ , Mylswamy Annaduraib a Planetary and Geosciences Division, Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India b ISRO Satellite Center, Bangalore 560017, India
Received 5 December 2007; received in revised form 20 March 2008; accepted 6 May 2008 Available online 1 July 2008
Abstract Chandrayaan-1 is the first Indian planetary exploration mission that will perform remote sensing observation of the Moon to further our understanding about its origin and evolution. Hyper-spectral studies in the 0.4–3 m region using three different imaging spectrometers, coupled with a low energy X-ray spectrometer, a sub-keV atom analyzer, a 3D terrain mapping camera and a laser ranging instrument will provide data on mineralogical and chemical composition and topography of the lunar surface at high spatial resolution. A low energy gamma ray spectrometer and a miniature imaging radar will investigate volatile transport on lunar surface and possible presence of water ice in the polar region. A radiation dose monitor will provide an estimation of energetic particle flux en route to the Moon as well as in lunar orbit. An impact probe carrying a mass spectrometer will also be a part of the spacecraft. The 1 ton class spacecraft will be launched by using a variant of flight proven indigenous Polar Satellite Launch Vehicle (PSLV-XL). The spacecraft will be finally placed in a 100 km circular polar orbit around the Moon with a planned mission life of two years. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction The current decade has seen a revival in the field of planetary exploration with several new initiatives to explore the various solar system objects. In particular, a renewed effort to explore the Moon has started in 2003 with the Smart-1 mission of ESA that will be followed by Japanese, Chinese, Indian and American missions to Moon during the next two years. A proposal for a mission to the Moon was put forward by Indian planetary scientists in the late nineties and the Indian Space Research Organization (ISRO) initiated spadework for such a mission in 2001. As a part of this new
∗ Corresponding author. Fax: +91 79 6301502.
E-mail addresses: [email protected] (J.N. Goswami), [email protected] (M. Annadurai). 0094-5765/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2008.05.013
initiative to have dedicated planetary science missions, the Chandrayaan program was conceived. Chandrayaan-1, the first such mission is now in an advanced stage of preparation and will be launched in 2008. This paper describes the scientific objectives, the various payloads, the mission details, observational plans and some project management aspects of the Chandrayaan-1 mission. 2. Scientific objectives and payloads The primary scientific objective of the Chandrayaan1 mission is to further our understanding of the origin and evolution of the Moon based on high resolution selenological and chemical mapping of the Moon. A suite of baseline payloads, identified to meet this scientific objective, include a terrain mapping camera (TMC), a hyper-spectral imager (HySI), a low energy
1216
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X-ray spectrometer (LEX), a high energy X– ray spectrometer (HEX) and a lunar laser ranging instrument (LLRI). These payloads, developed at the Space Application Center, Ahmedabad, the ISRO Satellite Center, Bangalore, Physical Research Laboratory, Ahmedabad, and the Laboratory for Electro-Optics System, Bangalore, will provide simultaneous mineralogical, chemical and photo-geological mapping of the lunar surface at resolutions better than previous and currently planned lunar missions. They will allow (i) direct estimation of lunar surface concentration of the elements Mg, Al, Si, Ca, Ti and Fe with high spatial resolution ( 30 km), (ii) high resolution (∼100 m) UV–VIS–NIR mapping of the lunar surface to identify abundances of various lunar minerals, (iii) high resolution 3D mapping of the lunar surface and (iv) nature of volatile transport on Moon, particularly to colder lunar polar regions. An impact probe carrying mass spectrometer, video camera and a radar altimeter, developed at the Vikram Sarabhai Space Center and the Space Physics Laboratory, Thiruvananthapuram, will complement these baseline payloads and will be a technological fore runner for future proposed lunar landing missions. ISRO also offered opportunity to the international scientific community to participate in Chandrayaan-1 mission and several payloads that complement and supplement the basic objectives of the Chandrayaan-1 mission have been selected for inclusion in this mission. These are: a miniature imaging radar instrument (Mini-SAR) to explore the polar regions of the Moon to look for possible presence of water ice, two infrared spectrometers (SIR-2 and Moon mineralogy mapper: MMM) for extending the range beyond that of the HySI, a low-energy X-ray spectrometer (CIXS) for high resolution chemical mapping, a sub-keV atom reflecting analyzer (SARA) for detection of solar wind sputtered low energy neutral atoms as well as studies of lunar surface magnetic anomalies and a radiation dose monitor (RADOM) for monitoring energetic particle flux in the lunar environment. Three of the payloads, SIR-2, CIXS and SARA, developed at the Max-Planck Institute, Lindau, Rutherford Appleton Laboratory (RAL), UK, and Swedish Institute of Space Physics, respectively, will be provided by the European Space Agency (ESA). NASA will provide Mini-SAR, developed by JHU/APL and NAWC, and MMM, developed by Brown University and JPL. RADOM will be provided by the Bulgarian Academy of Sciences. There will be significant technical participation of ISRO in realizing two of the payloads, CIXS and SARA. There will also be active scientific collaboration between ISRO scientists and investigators associated with each of the AO payloads.
The basic characteristics of the payloads are described here. 2.1. The payloads There are 11 instruments including the impact probe. Three payloads (HySI, SIR-2 and MMM), covering different wavelength ranges, will study reflected solar energy from the lunar surface covering the 0.4.3 m. These are: 2.1.1. Hyperspectral imager (HySI) The hyper spectral imager for mineralogical mapping will be operating in the 400–950 nm range employing a wedge filter coupled to an area array detector. It will have 64 continuous channels with a spectral resolution better than 15 nm and a spatial (pixel) resolution of 80 m with a swath of 20 km. 2.1.2. Near infrared spectrometer (SIR-2) SIR-2 is an upgraded, compact, monolithic grating, near infrared point spectrometer based on SIR flown on board ESA’s SMART-1 mission and covers the wavelength region 0.9–2.4 m. The instrument has a spectral resolution of 6 nm. It is a linear CCD array based instrument and will have a resolution of ∼80 m per pixel. 2.1.3. Moon mineral mapper (MMM) The MMM (M 3 ) is a high throughput push broom imaging spectrometer operating in 0.7–3.0 m range with high spatial (70 m per pixel) and spectral (10 nm sampling) resolution. It will have a swath of 40 km. It uses a 2D HgCdTe detector array for measuring reflected solar energy in the above wavelength range. 2.1.4. Terrain mapping camera (TMC) The terrain mapping stereo camera (TMC) in the 500–850 nm band will have three linear array detectors for nadir, fore and aft viewing and will have a swath of 20 km. It will be capable of providing 3D image of the lunar surface with a ground resolution of 5 m with base to height ratio of one. 2.1.5. Chandrayaan imaging X-ray spectrometer (CIXS) Two options were considered for detection of low energy (1–10 keV) fluorescence X-rays from the lunar surface; use of a thermoelectrically cooled X-ray CCD (LEX) or of a swept-charge X-ray detector (SCD) array. The final choice was SCD and CIXS, a modified version of D-CIXS instrument on board SMART-1, proposed by RAL, UK, and supported by ESA was selected in
J.N. Goswami, M. Annadurai / Acta Astronautica 63 (2008) 1215 – 1220
place of LEX. This collimated LEX will have a field of view of ∼30 km and provide detail chemical mapping of the lunar surface for the elements, Mg, Al and Si and also for Ca, Ti and Fe during solar flare times. An X-ray solar monitor (XSM) will be a part of this payload and will continuously monitor the solar X-ray flux essential for analyzing the data on fluorescence Xrays to infer absolute elemental abundance. CIXS will be a collaborative payload between ISRO and RAL with a group of ISRO scientists and engineers involved in various aspects of payload design and fabrication and detector characterization. 2.1.6. High energy X– ray spectrometer (HEX) The high-energy X– ray (30–270 keV) spectrometer (HEX) will employ CdZnTe solid-state detectors and will have a suitable collimator providing an effective spatial resolution of 40 km in the low energy region (< 60 keV). It will employ a CsI anticoincidence system for reducing back-ground and is primarily intended for the study of volatile transport on Moon using the 46.5 keV ray line from 210 Pb decay as tracer. 210 Pb is a decay product of volatile 222 Rn and both belong to the 238 U decay series. The instrument will have a detection threshold of < 30 keV and a resolution of better than 10% at 60 keV. Attempt will be made to infer compositional characteristics of lunar terrain from a study of the continuum background in this energy range as well as low resolution Th and U mapping of terrains enriched in these elements (e.g. KREEP). 2.1.7. Lunar laser ranging instrument (LLRI) The LLRI will employ an Nd–Yag laser with energy 10 mJ and employ a 20 cm optics receiver coupled to a Si–APD. It will be operating at 10 Hz (5 ns pulse) and can provide a vertical resolution better than 5 m. The LLRI and TMC will provide complementary data for generating a topographic map of the Moon and the LLRI, in particular, will provide the first such data set for the polar region at higher than 80◦ latitude. 2.1.8. Sub-keV atom reflecting analyzer (SARA) The SARA payload consists of two major subsystems, Chandrayaan-1 low energy neutral atom (CENA) and solar wind monitor (SWIM). CENA detects neutral atom sputtered from the lunar surface by solar wind ions. The CENA sensor has an energy range of 10 eV to 2 keV with an energy resolution of ∼50% and can resolve groups of elements such as H, O, Na/Mg group, K/Ca group and Fe. SWIM is a simple ion mass analyzer consisting of a sensor and an energy analyzer
1217
that provides information on the energy and mass of the incident solar wind ions. Space Physics Laboratory, Thiruvananthapuram, is collaborating in this experiment and is responsible for developing the data processing unit. 2.1.9. Miniature synthetic aperture radar (MINI-SAR) A multifunction miniature radar consisting of SAR, altimeter, scatterometer and radiometer operating at 2.5 GHz will explore the permanently shadowed areas near lunar poles to look for signature of water ice. The mini-SAR system will transmit right circular polarization (RCP) and receive both left circular polarization (LCP) and RCP. The SAR system has a nominal resolution of 150 m per pixel with 8 km swath. 2.1.10. Radiation dose monitor (RADOM) RADOM is a miniature spectrometer–dosimeter that uses a semiconductor detector and measure the deposited energy from primary and secondary particles using a 256 channel pulse analyzer. The deposited energy spectrum can then be converted to deposited dose and incident flux of charged particles on the silicon detector. 2.1.11. Moon impact probe (MIP) In addition to the primary scientific payloads, an impactor carrying a high sensitive mass spectrometer, a video camera and a radar altimeter has also been included in this mission. The impactor will be released at the beginning of the mission after reaching 100 km lunar polar orbit and attempt will be made to impact it in a predetermined location on the lunar surface. Apart from the video imaging of the landing site, the onboard mass spectrometer will try to detect possible presence of trace gases in the lunar exosphere. 3. Mission scenario 3.1. The spacecraft The spacecraft design is adopted from flight proven Indian Remote Sensing Satellite bus coupled with suitable modifications specific to the lunar mission. Apart from the solar array, TTC and data transmission, that are specific to the lunar mission, other aspects of system design have flight heritage. However, some changes specific to lunar mission is also required. These include extending the thrust cylinder and having an upper payload deck to accommodate MIP and few other payloads. Additionally, Chandrayaan-1 will have a canted solar array since the orbit around the Moon is inertially
1218
J.N. Goswami, M. Annadurai / Acta Astronautica 63 (2008) 1215 – 1220
SWIM
XSM
MIP RADOM SIR-2
CENA LLRI
CIXS
MiniSAR
HEX
HySI
M3
TMC
Fig. 1. A view of the Chandrayaan-1 spacecraft showing the placement of the various payloads: terrain mapping camera (TMC); Moon mineralogy mapper (M3); hyper spectral imager (HySI); high energy X– ray spectrometer (HEX); miniature synthetic aperture radar (mini-SAR) antenna; Chandrayaan imaging X-ray spectrometer (CIXS); sub-keV atom reflecting analyzer [SARA: energetic neutral analyzer (SENA); solar wind monitor (SWIM)]; X-ray solar monitor (XSM, CIXS); Moon impact probe (MIP); radiation dose monitor (RADOM); infrared spectrometer (SIR-2) and lunar laser ranging instrument (LLRI). The blue panel is the canted solar array.
fixed resulting in large variation in solar incidence angle. There is a need to have a gimbaled high gain antenna system for downloading the payload data to the Indian Deep Space Network (IDSN). Thus the spacecraft will be cuboids in shape of approximately 1.50 m side, with a liftoff mass of ∼1.3 ton with bus element accounting for ∼0.4 ton, payload ∼0.1 ton and propellant ∼0.8 ton. At lunar orbit it will be ∼0.6 ton. This would be a three-axis stabilized spacecraft generating about 750 W of peak power using canted single sided solar array and will be supported by a Li-Ion battery for eclipse operations. The spacecraft would adopt bipropellant system to carry it from EPO through lunar orbit, including orbit and attitude maintenance in lunar orbit. The propulsion system would carry required propellant for a mission life of two years, with adequate margin. The TTC communication would be in the S-band. The scientific payload data that will be stored in a solid-state recorder will be later played back and down linked in X-band through 20 MHz bandwidth by a steer able antenna pointing at DSN. Fig. 1 provides the spacecraft configuration with payloads.
3.2. Launch vehicle and mission sequence Chandrayaan-1 will be launched by PSLV-XL, a variant of flight proven indigenous Polar Satellite Launch Vehicle (PSLV) from Satish Dawan Space Centre, Shriharikota. The spacecraft will be injected into 260 km × 24, 000 km orbit. After separation from the launcher, the solar panel is deployed and the spacecraft is raised to Moon rendezvous orbit by three consecutive in-plane perigee maneuvers to achieve the required 3,86,000 km apogee. After the third perigee burn, a quick estimate of the achieved lunar transfer trajectory (LTT) is made and a mid-course correction is imparted at the earliest opportunity. The spacecraft coasts for about five days in this trajectory prior to the lunar encounter. During the coasting phase the spacecraft would stay mostly in the sun-pointed mode and at the same time ensuring good communication link to ground. The major maneuver of the mission, called lunar orbit insertion (LOI) leading to lunar capture, is carried out at the peri-selene part of the trajectory. The maneuver ensures successful lunar capture in a polar, near circular 1000 km-altitude
J.N. Goswami, M. Annadurai / Acta Astronautica 63 (2008) 1215 – 1220 LUNAR ORBIT INSERTION (LOI) FINAL ORBIT ~100 km
LUNAR CAPTURE AT ~1000 km
LUNAR TRANSFER TRAJECTORY ETO
EPO
MID-COURSE CORRECTION
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these instruments. SSR-2 is sized to record the data for about seven orbits covering the entire ground trace of the Moon before being played back for ground transmission. The other SSR namely SSR-1 will record data from optical imaging instruments or MiniSAR for subsequent transmission to ground. Two ground terminals one with 18 m antenna and another with 32 m antenna have been established near Bangalore as a part of IDSN. Even though the already tested 18 m terminal (during SMART-1 EOL mission and some of the ongoing planetary missions) will suffice for Chandrayaan-1, the newly installed 32 m antenna would cater to the futuristic planetary exploration programme as well. 3.4. Chandrayaan-1 project management
Fig. 2. Chandrayaan-1 mission sequence.
orbit around the Moon. After successful capture and checks of various spacecraft sub-systems, the altitude is lowered through a series of in-plane corrections to 200 km near circular orbit for studying the nature of orbit perturbations under the influence of the lunar gravity field. Following this, the target altitude of 100 km circular, polar orbit is achieved. Fig. 2 depicts the planned Chandrayaan-1 mission sequence.
Chandrayaan-1 is carrying 11 scientific instruments from an equal number of institutions. Accommodation of these instruments and to meet their stringent technical requirements in a small satellite bus turned out to be a challenging task. The difficulty and complexity of the task has further increased because of the geographical locations of various laboratories involved in design and development of these instruments at various Indian laboratories and foreign institutions with varying approach and work cultures. Chandrayaan-1 has provided an opportunity to demonstrate true multinational cooperation in the field of planetary exploration missions in general and for lunar science missions in particular.
3.3. Lunar observational plans 3.5. Indian Space Science Data Centre (ISSDC) All the imaging instruments needs appropriate solar illumination. Since the solar angle and hence illumination changes with time as the Moon moves in its orbit, useful imaging can be done only for selected period of time. In a duration of one year there will be two prime imaging seasons of 60 days each separated by a gap of 120 days. During the prime imaging season ± 60◦ latitude is covered. During the four months interim period between the two prime imaging seasons, 60–90◦ of north/south polar regions will be covered. Due to poor solar illumination in this region, the effective spatial resolution will be degraded in achieving a reasonable signal to noise ratio. During the two-year period of mission life, the entire Moon will be imaged with a possibility of repetitive coverage of selected regions. MiniSAR polar imaging is planned during non-imaging seasons. The X-ray payloads, LLRI, SARA and RADOM are kept ‘ON’ continuously and a separate solid-state recorder (SSR-2) is provided for recording data from
The IDSN in Bangalore will receive the payload data from Chandrayaan-1. The data in its raw form along with auxiliary data will be sent to the Indian Space Science Data Centre (ISSDC) that is being set up in Bangalore. ISSDC would process the raw data and convert those into user-friendly form. The data centre will also archive all the payload data in PDF format and will be the repository and focal point for Chandrayaan-1 science team. 4. Summary The Chandrayaan-1 mission, the first Indian planetary exploration mission, is in advanced stage of preparation and will be launched in 2008. It will have 11 scientific payloads including an impact probe. The mission is truly international in character with two IndoEuropean collaborative experiments and additional experiments from USA, Germany and Bulgaria, that were
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selected based on proposals received in response to Announcement of Opportunity, to meet the basic scientific objectives of the mission. With a life time of two years and plans for whole Moon mapping at high spatial and
spectral resolution, a successful Chandrayaan-1 mission will substantially improve our understanding of several key questions regarding the origin and evolution of the Moon.