by Gurbir Singh
Date
Event
Delta V (m/s)
Orbit (km)
22/10/08
Launch
254 x 22,932
23/10/08
Earth Bound Orbit 1
344
299 x 37,908
25/10/08
Earth Bound Orbit 2
328
336 x 74,715
26/10/08
Earth Bound Orbit 3
221
347 x 165,015
29/10/08
Earth Bound Orbit 4
77
459 x 266,613
03/11/08
Earth Bound Orbit 5 & Lunar Transfer Trajectory
60
972 x 379,860
05/11/08
TCM while en-route to the Moon
0.8
815 x 378,515
08/11/08
Lunar Orbit Insertion
817
507 x 7,510
09/11/08
Lunar Bound Orbit 1
57
200 x 7,502
10/11/08
Lunar Bound Orbit 2
868
183 x 255
11/11/08
Lunar Bound Orbit 3
31
101 x 255
12/11/08
Lunar Bound Orbit 4
59
101 x 102.8
Table 15‑1 Chandrayaan-1Trajectory from the Earth to the Moon
Key research undertaken by R.V. Ramanan in orbital mechanics defined Chandrayaan-1’s trajectory from the moment it left Indian soil until lunar orbit insertion. The most fuel-efficient trajectory that Ramanan calculated for the PSLV required a series of five Earth orbits, each with an increasing apogee (going further from the Earth) using Chandrayaan-1’s onboard LAM. The initial lunar orbit was highly elliptical, and a series of four LAM burns in lunar orbit were required to achieve the 100-km circular lunar orbit.[831] A circular pole-to-pole 100-km orbit was critical to achieving the spatial resolutions over the lunar poles that the science instruments were designed to capture. Even after the final orbit was achieved, Chandrayaan-1 would not remain precisely in it. Variations in the Moon’s density would change Chandrayaan-1’s orbit, requiring an orbital correction manoeuvre from ISRO engineers every four weeks or so.[832]
Chandrayaan-1 was launched on PSLV-C11 at 10:19 UT from Sriharikota on 22 October 2008 and released twenty minutes later into a transfer orbit around the Earth at 254 km x 22,932 km with an inclination of 17.9o. Following five Earth orbits, Chandrayaan-1 arrived in lunar orbit just over two weeks later. During its first four days in lunar orbit, Chandrayaan-1 circularised its orbit from 507 km x 7,510 km to 101 km x 102 km through a series of four LAM burns.
Figure 15‑3 Chandrayaan-1 Orbit profile. Credit ISRO
Chandrayaan-1’s position throughout this period was regularly and frequently monitored by IDSN, as well as NASA’s Deep Space Network (DSN). These data sets were used by the Precise Orbit Computation using Accelerometer Data (PROCAD) software that was developed especially for Chandrayaan-1.[833] The spacecraft used four high precision accelerometers to autonomously measure its speed during a LAM burn and to terminate the burn dynamically when the required velocity was reached. Usually, the precise position of a spacecraft is calculated by measurements from tracking stations, which can be many hours after an engine burn. Using PROCAD, ISRO was able to determine Chandrayaan-1’s orbit with high precision dynamically at any time, including during an engine burn.[834]
Many nations, one spacecraft
The unmanned lunar landers and rovers of the USSR and the nine Apollo missions of the US had already undertaken extensive scientific work from lunar orbit and on the surface.
Payload
Origin
Objective
1
Moon Impact Probe (MIP)
India
Vertical analysis of lunar atmosphere
2
Moon Mineralogy Mapper (M3)
US
High-resolution assessment of lunar chemical and mineralogical resources
3
Terrain Mapping Camera (TMC)
India
Topographic mapping
4
Lunar Laser Ranging Instrument (LLRI)
India
Comprehensive 3D mapping and topography
5
Hyper Spectral Imager (HySI)
India
Mineralogical mapping of the lunar surface
6
High Energy X-ray Spectrometer (HEX)
India
Mapping thorium and lead
7
Chandrayaan-1 X-ray Spectrometer (C1XS)
UK but funded by ESA
Mapping relative abundances of magnesium, aluminium, silicon, calcium, iron and titanium
8
Sub keV Atom Reflecting Analyser (SARA)
India, Japan, ESA and Sweden
Analysis of atmospheric content and magnetic field anomalies
9
Near-Infrared Spectrometer (SIR-2)
Germany, ESA, Norway and Poland
Capturing geological, mineralogical and space weathering data of the lunar surface
10
Miniature Synthetic Aperture Radar (Mini-SAR)
US and UK
Measure low energy particles directly from the Sun and those reflected by the lunar surface
11
Radiation Dose Monitor (RADOM)
Bulgaria
Radiation sensor to measure radiation dose at all points along Chandrayaan-1’s journey
Table 15‑2 Chandrayaan-1Payload Overview
The Apollo missions had taken twenty-four men to the Moon, twelve of whom visited the surface in six missions. Both the US and the USSR brought back samples of rocks and soil, 382 kg by the US during the six Apollo missions and 326 grams by the USSR with three unmanned sample return missions. In addition, both nations had conducted extensive experiments from lunar orbit. What science could be done by an Indian mission to the Moon that had not already been done by the numerous missions already completed? Chandrayaan-1 was equipped with an array of eleven instruments. ISRO scientists set three key goals for Chandrayaan-1. Deploying scientific instruments not used during the Moon missions of the 1960s and 1970s, Chandrayaan-1 was to (i) prepare a three-dimensional atlas of the entire surface of the Moon with a resolution of 5–10 m., (ii) identify the location and distribution of lunar minerals, light and heavy elements and (iii) search for and map the distribution of water, if it existed.
Two of ISRO’s instruments, the TMC and LLRI, were designed to capture data to meet the primary objective of the mission, to prepare a high-resolution three-dimensional map.[835] Although 98% of the lunar surface had already been imaged in the past, only specific areas, such as potential landing sites, had been targeted for high-resolution imaging. TMC was designed to cover the entire lunar surface.[836] Data from three separate instruments, HySI, M3 and SIR-2, were compiled to meet the second objective of the mission, to produce a high-resolution map of the Moon’s mineral and chemical composition. HEX and M3 were to detect the presence of water. RADOM, along with TMC, was activated while Chandrayaan-1 was still in Earth orbit. The remaining nine instruments were commissioned gradually between 16 November and 9 December 2008.
The Moon was known to be extremely dry. The Earth’s axis of rotation is inclined at 23° to the plane of the orbit, so both poles get some sunlight over a year. The Moon’s orbit is inclined at 1.5°, so craters at its poles get almost none. If water existed, it would be trapped and frozen in the deep craters at the lunar poles which are never illuminated by sunlight. Chandrayaan-1’s unusual pole-to-pole orbit allowed it to look directly down into these areas during each orbit. If water was present, Chandrayaan-1 had a good chance of detecting it.
Moon Impact Probe
The MIP, at 35 kg, is best considered a separate spacecraft designed to detach from Chandrayaan-1 wh
en it arrived in lunar orbit, descend to, and impact on the surface planting the Indian flag in a predefined location. It was the result of a significant mission redesign introduced at short notice and produced by ISRO at VSSC, Trivandrum. MIP had three key objectives, close-up observation of the Moon, testing technology for navigating to a predefined point on the lunar surface and preparing for future soft-landing missions.
Figure 15‑4 Moon Impact Probe. Credit ISRO
MIP carried three payloads, a Moon imaging system (a camera), a radar altimeter and a neutral mass spectrometer to detect and quantify the chemical constituents in the tenuous lunar atmosphere during the journey from orbit to the surface. The radar altimeter used radio signals 100 times a second to measure altitude during the descent. Trial runs on Earth from aircraft demonstrated an accuracy of between 2 and 4 m. Chandrayaan-1 arrived in lunar orbit on 8 November, and the orbit was circularised by 12 November. MIP separated from Chandrayaan-1 two days later, on 14 November at 20:06 IST; the date was selected to coincide with the birth anniversary of India’s first Prime Minister Jawaharlal Nehru. MIP then started its journey to the surface, impacting at 1.75 km/s near the lunar south pole (latitude 89° south), close to crater Shackleton at 20:31 IST. The impact site was named Jawahar Sthal (Jawahar site).
During the 25-minute descent, the onboard instruments relayed data to Earth via Chandrayaan-1. There was an inherent risk of damage to Chandrayaan-1’s instruments from the exhaust of MIP’s engine when it detached and left Chandrayaan-1. To mitigate this risk, a special non-degassing propellant based on Viton rubber was deployed for the tiny (8 g) de-orbit motor and for the two spin motors.[837] During its journey from 100-km orbit to the lunar surface, MIP’s neutral mass spectrometer called Chandra’s Altitudinal Composition Explorer (CHACE) detected evidence of water and carbon dioxide, along with other trace molecules.
Moon Mineralogy Mapper
The M3 was a state-of-the-art high-resolution imaging spectrometer jointly designed and developed by NASA’s Jet Propulsion Laboratory (JPL) and Brown University. A key instrument with a mass of 8.2 kg, it was designed to help map properties of the Moon from polar orbit to improve the understanding of the evolutionary processes responsible for the Moon’s current state.
M3 operated in two modes, target or global. In the target mode, M3 used its onboard telescope to look in detail at narrow strips of the Moon from its 100-km orbit.[838] In the global mode, M3 did not engage the telescope and imaged the whole Moon, albeit at a lower resolution. M3 split moonlight (that is, sunlight reflected by the Moon) with a spectrometer and examined the light at various wavelengths using a solid-state sensor. M3 data measurements of the absorption features of rocks and minerals were designed to help understand lunar geological processes and facilitate a high-resolution assessment of lunar chemical and minerals present.
A payload operations centre was developed by the M3 team to process M3 data. As agreed in the NASA/ISRO MoU for the M3 instrument, NASA delivered a replica of the payload operations centre to the Indian Space Science Data Centre to help process raw science data.[839] M3 was scheduled to make four two-month observational slots over the planned two-year mission. During these active slots, Chandrayaan-1 was to capture and transmit M3 data for two hours each day. However, issues of thermal control and loss of a star tracker resulted in operational challenges that required modifications to the initial schedule. M3’s commissioning phase was extended to 2009. On 31 January 2009, M3 was operated in its global mode for several discrete periods.[840] M3 made observations periodically, rather than the planned 48-minute period, during each of the twelve orbits.[841] Most of the observations were carried out under less-than-favourable viewing conditions in low resolution using global mode. In mid-May 2009, the spacecraft lost the second-star tracker, and ISRO decided to raise the orbit to 200 km from the existing 100 km.[842] Between 18 November 2008 and 16 August 2009, 958 images were recorded by M3 during five observational periods. M3’s discovery of unusual rock types (Mg spinel anorthosite) and a kilometre-sized structure (a crystalline anorthosite exposure) provided enough evidence for the existence of a global magma ocean in the Moon’s early history.[843] M3’s primary discovery was evidence of water (and Hydroxyl) molecules on surface of the Moon. The evidence was stronger at higher latitudes especially at the poles where the temperature was lower compared to that at the lunar equator. This water is not in pools of water on the surface but traces of water molecular and hydroxyl molecules. However, water that is present deep inside a crater could be present as ice frozen since formation. This detection has been confirmed with data from CHACE and other spacecraft.
Terrain Mapping Camera
TMC was developed by ISRO at the SAC in Ahmedabad. At almost 7 kg, the stereo colour camera was designed to capture high-resolution colour images at 5 m resolution. TMC was equipped with three viewing windows, looking straight down from orbit, forward in the direction of travel (by an angle of 25o) or behind (by an angle of 25o). It used a digital linear Active Pixel Sensor (APS), which an enhanced form of CCD producing data directly in a digital output. Images from the TMC were used to compile a 3-D atlas of the Moon’s surface, both its near and far sides. The first images taken by the TMC were not of the Moon but the Earth. A week after launch, while still in Earth orbit, the TMC was commanded to image the Earth as part of the testing and commissioning activity.[844]
TMC had imaged 50% of the Moon, mostly around the key Polar Regions, when the mission came to a premature end. The high-resolution images from the TMC recorded detailed views of the central peak of the crater Tycho and combined with HYSI data confirmed the existence of crystalline feldspars in the lunar highlands. TMC also recorded images of a well-preserved lava tube near the landing site of Apollo 15. Data from the TMC supported the theory that the surface of the Moon was covered by an ocean of magma for the first billion years of its 4.5 billion-year-long history.
Lunar Laser Ranging Instrument
LLRI was built by ISRO’s Laboratory for Electro-Optics Systems in Bangalore. At about 10 kg, LLRI was one of the heavier instruments in the science payload. It was used to measure the lunar terrain (heights of mountains and depths of valleys) to a resolution of 5 m. Activated for the first time on 16 November; it was designed to systematically cover the day and night side of the Moon. LLRI directed pulses of infrared laser light down to the lunar surface ten times a second. The faint reflections were collected by its 7-cm optical telescope and directed onto a sensitive photodetector. LLRI data was combined with TMC data to produce a topographical map of the lunar polar regions. LLRI allowed scientists to study the morphology of the lunar surface features and to help provide a density distribution of the crust.
Hyper Spectral Imager
HySI, an optical camera using a solid-state image sensor, was designed to acquire stereoscopic data to produce a mineralogical map of the lunar surface. It had a mass of 4 kg and was developed by ISRO’s SAC, Ahmedabad. The Active Pixel Sensor used in HySI was of the same type used in TMC. With a spatial resolution of 80 m, it had the ability to image the Moon in 20 km wide strips from an orbit of 100 km.
HySI targeted deep craters and their central peaks where the material from the lower crust or upper mantle could be observed and analysed. It produced images in a narrow range of wavelengths in 32 contiguous bands that helped produce a mineralogical map of the Moon by combining its data with that from other instruments, especially C1XS, M3 and SIR-2. HySI data was also used to confirm the discovery of a new spinel-rich rock type on the lunar far side composed of magnesium and aluminium minerals.
High Energy X-Ray Spectrometer (HEX)
HEX was developed by ISRO’s SAC in Ahmedabad, with contributions from ESA. Although referred to as an X-ray Spectrometer, the higher energies of the detected radiation were more consistent with gamma-rays. HEX was the first instrument designed to capture data on high-energy X-rays in the lunar environment, and at 16kg, it was also the heaviest instrument on-board.
Unlike the
Earth, the Moon does not have a significant magnetic field or an atmosphere. Consequently, high-energy radiation and cosmic rays from the Sun and the cosmos can reach the lunar surface and penetrate up to a meter below.[845] This radiation interacts with some constituents that make up the rocks and soil, raising their energy level (of the constituent atoms and molecules) to an excited state. Such excited states are not stable and return to their ground state by emitting X-rays or gamma-rays. It is this radiation that HEX was designed to detect. The precise energies of these X-rays and gamma-rays indicate the chemical composition of the lunar surface. On Earth, when the Sun heats up the equator, volatile material, such as water, moves towards higher latitudes. HEX was designed to detect and measure the transport of such volatile material from the high temperature lunar equatorial region (more than 100°C) towards the lunar poles (less than 0°C).
Radioactive elements, such as uranium, thorium, radium and lead, as well as potassium, which have a very long half-life, provide the Moon with an internal heat source. By detecting the gamma-rays from the decay of these heavy elements and combining it with data from TMC and HySI, HEX was to map the distribution of these elements across the lunar landscape. However, immediately after arrival in lunar orbit, Chandrayaan-1 experienced thermal control problems that limited the operations of HEX and other instruments. HEX was not able to operate between noon and midnight (lunar time) because its sensitive detector could not be adequately cooled.[846] However, it did collect data on the high-energy X-ray background from the Moon, averaging over a significant portion of the lunar surface.[847]