THE MISSION

The launch of Chandrayaan-1 will be a landmark event for the Indian Space research Organization. This will be the first time that an Indian spacecraft will go beyond the geostationary orbit to explore our nearest neighbor in the solar system, the Moon. Final preparation for launch is going on in full swing at the Satish Dhawan Space Center, Sriharikota. It will be a remote sensing mission with the spacecraft placed in a lunar polar orbit at 100 km with an expected life of two years.

Chandrayaan-1 is a scientific mission aimed at advancing our present understanding of the origin and evolution of the Moon. To achieve this objective a suite of instruments that can provide data on moon at higher resolution than obtained by previous lunar missions have been selected for this mission.

THE PAYLOADS

The Moon Impact Probe (MIP)

Apart from the experiments housed in the main Spacecraft, the Moon Impact Probe (MIP), designed and developed at the Vikram Sarabhai Space Center (VSSC), Trivandrum, sitting in the top deck of the spacecraft, will be released at the very beginning of the mission and will land in an targeted area (near the lunar south pole). It has a camera for taking images during its descent, measure distances from lunar surface using a radar altimeter and will also look for trace amount of gases, that may be present in the lunar atmosphere, using a highly sensitive mass-spectrometer. MIP will communicate all the data to the main spacecraft during its descent to lunar surface that will be complete in less than twenty minutes.

The Chandrayaan-1 mission will also usher a new era in international cooperation in planetary exploration with participation of several countries of Europe and USA that have contributed scientific instruments complementing the Indian instruments. This will enhance the overall science content of the mission. The foreign instruments were selected in mid-2004 based on evaluation of proposals received following an announcement of opportunity by ISRO in early-2004. ISRO has signed memorandum of understanding (MOU) with heads of European Space Agency (ESA), NASA and Bulgarian Academy of Sciences (BAS) that will allow analysis of data from the foreign experiments by Indian scientists along with the foreign scientists.

What are the experiments and observations that are planned in this mission to improve our knowledge about the Moon? The primary aim is to have an accurate estimation of the chemical and mineralogical composition of the whole lunar surface that is essential to infer plausible scenarios for the origin and early evolution of the moon. Such efforts were made earlier by the US missions Clementine (for mineral composition) and Prospector (for chemical composition) in the nineties. The currently operating Japanese Lunar mission, Kaguya (SELENE) is also expected to provide significant information in this regard.

Hyper-Spectral Imager (HySI), Moon Mineral Mapper (MMM) and Infrared Spectrometer (SIR-2)

A suite of Indian and Foreign payloads will try to achieve the above objective. The mineralogical information will be gathered by the Hyper-Spectral-Imager (HySI), design and developed at the Space Application Center, Ahmedabad, the Moon Mineral Mapper (MMM) from Brown University, USA, and an Infrared Spectrometer (SIR-2) from the Max-Planck Institute, Lindau, Germany. All the three payloads (Fig. 3) will collect the reflected sun-light from the lunar surface at various wavelengths in the range of ultra-violet, optical and infrared with extremely high resolution. The three instruments will have overlapping wavelengths for cross-calibration.

How one infers mineralogy from a study of reflected sunlight. In simple terms it is based on the chemical make up of various minerals that lead to strong absorption of light in certain wavelengths and hence intensity of reflected sunlight. Of course a good deal of laboratory calibration and modelling is essential to have quantitative information. If a mineralogical map can be constructed, one can extract chemical information based on concentration of various minerals in a given area of the Moon. In principle these three payloads will allow us to obtain such information for areas as small as 100×100 square meter on the lunar surface.

Chandrayaan-1 X-ray Spectrometer (C1XS)

A very sensitive instrument of the mission is the Chandrayaan-1 X-ray Spectrometer (C1XS; Fig. 4), designed and developed at the Rutherford Appleton Laboratory, UK, with significant Indian contribution from the ISRO Satellite Center (ISAC), Bangalore. This payload will provide direct information on chemical (elemental) composition of the lunar surface over areas of 20 km which is much better than the previous data obtained by Prospector mission using a gamma ray spectrometer that had a spatial resolution of ~150 km.

How to get information on lunar chemistry by remote sensing in X-rays? The Sun continuously emit low-energy (a few keV) X-rays and during occasional solar flare it can emit X-rays of higher energies up to about ten keV. When X-rays from the Sun hit the lunar surface samples (soils and rocks) they interact with the constituent atoms and excite some of the electrons in these atoms leading to emission of X-rays, the energies of which are characteristics of the atoms. Thus, C1XS will provide direct information on the chemistry of the lunar surface, particularly the concentration of the elements Magnesium, Aluminium and Silicon throughout the mission and also of Calcium, Titanium and Iron during solar flares as excitation of X-rays of the later elements need higher energy solar X-rays. Determination of the Mg to Fe abundance ratio is a key parameter in understanding the evolution of any planetary body, including the Moon.

A major effort will be made to combine the data from the mineralogy payloads (HySI, MMM, SIR-2) and C1XS to make a chemical map of the entire lunar surface. Chemistry of the returned lunar samples from various Apollo and Luna landing sites will provide ground truth to validate the data and cross check results.

Terrain Mapping Camera (TMC)

An understanding of the topography of planetary surfaces and the processes that shaped them is essential to understand the evolution of any solar system object. The Terrain Mapping Camera (TMC) designed and developed at the Space Application Center (SAC), Ahmedabad, will provide unprecedented three dimensional map of the lunar surface at a spatial resolution of five meter and also similar resolution in height. Currently the Japanese lunar mission, SELENE, is aiming at providing such data at ten meter resolution. The information on topography or Digital Elevation Model (DEM) that will be obtained from the TMC will be very useful for many other experiments including the mineralogy instruments (HySI, MMM and SIR-2) as the reflection of sunlight from an inclined or uneven surface will be different from that from a flat surface.

Sub-keV Atom Reflecting Analyzer (SARA)

Moon does not have a global magnetic field. However, presence of local surface magnetic anomalies has been detected from studies conducted in earlier missions. The strength of such anomalies is extremely small. A unique experiment, Sub-keV Atom Reflecting Analyzer (SARA) proposed by a Swedish Group with Indian Collaboration from Space Physics Laboratory, Vikram Sarabhai Space Center, Trivundrum, will try to detect presence of mini-magnetosphere around areas having local magnetic field. The same experiment will also provide low resolution data on chemical composition of the moon.

Moon is continuously bombarded by solar wind, a stream of low energy ions from the Sun, that can have unhindered access to lunar surface in the absence of a lunar atmosphere or magnetic field. The incident solar wind ions sputters very topmost surface of lunar dust and rocks and release neutral atoms that leaves the lunar surface. If there are local surface magnetic fields the solar wind ions will not reach these areas and there will be a drop in neutral atoms released from such areas. Data from SARA will enable identification of such localized areas and modelling of local mini-magnetospheres.

Lunar Laser Range Instrument (LLRI)

Understanding lunar gravity and its variations over the lunar surface can considerably increase our knowledge of the evolution of the Moon. Previous studies suggested presence of lunar gravity anomalies. The global gravity model of the Moon can be best determined if we can perform altimetry of the Moon as accurately as possible. This will be accomplished in the Chandrayaan-1 Mission by the Lunar Laser Range Instrument (LLRI) designed and developed at the Laboratory for Electro-Optics System (LEOS) in Bangalore, an unit of ISRO.

The LLRI consists of a pulsed laser, that shines laser beam on lunar surface several times every second, and a telescope to collect the reflected light. A measurement of the travel time of laser light from the source to moon and back to collector will then allow determination of range of the satellite from the lunar surface and hence lunar altimetry. Several million laser pulses will be executed during the mission. The data on lunar altimetry that can be translated to develop a lunar gravity model better than what we have at present.

There is a global effort to better understand the Moon that can serve as the closest destination for planning for a human-cum-robotic base in future. At present the lunar polar region appears to be the most plausible site for two reasons. First there are mountains near the polar region nearly 5 km in height (e.g. Malapert Mountain near the South Pole), where sunlight is nearly perennial providing possibility of solar energy harnessing. Second, there are also permanently shadowed areas (e.g. base of Shackleton Crater near South Pole), with temperature much below zero degree, acting like a deep freezer, where there is a possibility of having water-ice mixed with lunar soils.

Two payloads in the Chandrayaan-1 Mission will have emphasis on study of the polar region. The High Energy X-ray (HEX) Spectrometer designed and developed at the Physical Research Laboratory (PRL) in association with ISRO Satellite Center (ISAC), Bangalore, will try to validate the concept based on which one expects water to be present in the permanently shadowed regions of the Moon. The other is the Miniature Imaging Radar Instrument (Mini-SAR), an instrument from the Applied Physics Laboratory, John Hopkins University, USA, that will try to detect possible presence of water ice within the first few meters of the permanently shadowed surface of the Moon.

The sunlit areas of the Moon reach very high temperature in excess of 100 degree centigrade and conversely the shadowed region are like deep-freezers with temperature below minus hundred degrees. Thus any volatile atom or molecule (e.g. water molecule) present on the sunlit side of the moon will have sufficient energy to hop around and if it reaches a permanently shadowed cold region it will get permanently trapped. There are such regions near the lunar poles as the Sun-Moon angle does not allow sunlight to reach the base of the craters in the polar region. One therefore expects volatiles like water, brought in to the Moon by comet impacting on it or production of water molecule by solar wind interaction with lunar soils, to be concentrated in the permanently shaded polar region of the moon.

High Energy X-Ray Spectrometer (HEX)

The HEX experiment, that will for the first time detect emission of energetic photons in the energy range 30 to 270 keV from a planetary surface, using a new generation solid state X-ray detector, will try to quantify the volatile transport process on the moon. It will look for movement of the radionuclide Radon-222, which is a volatile and produced from decay of Uranium. Even though the overall concentration of Uranium in moon is low, its decay produces Radon that should also hop around and move to the permanently shadowed polar region leading to its elevated concentration in these areas before it eventual decay. The HEX experiment will look for the decay of Radon through the detection of photon of ~45 keV emitted during such decay. An excess of this signal at the polar region, compared to other areas of Moon, will validate the hypothesis of volatile transport on Moon to permanently shaded polar region. This experiment will also map the U and Th concentration on the lunar surface regions.

Miniature Imaging Radar Instrument (Mini-SAR)

The Mini-SAR experiment will have an antenna that will beam high frequency polarized beam and collect and analyze the reflected beam to look for signature of water ice within the top layers of the permanently shadowed polar region and particularly within the Shackleton Crater near South Pole. One expects differences in the property of the reflected beam for pure lunar soil layers and layers with embedded water ice. A claim was made during the Clementine Mission to Moon in 1994, for signature of possible presence of water in this crater.

Radiation Dose Monitor (RADOM)

Any long duration space mission has to worry about radiation environment in space and possible damage that intense radiation may cause to any of the payloads or the spacecraft subsystems. Such damage can happen by impact of high energy particles from Sun emitted during solar flares and also such particles coming from outside the solar system. The Radation Dose Monitor (RADOM) instrument provided by the Bulgarian Academy of Sciences will detect energetic particle environment around the spacecraft both during its journey to the Moon as well as during its orbit around the moon throughout the mission duration. It will be possible to take appropriate actions to safeguard the instruments and the spacecraft sub-system in case of enhanced radiation dose experienced by the spacecraft during the mission duration.

RADOM is an ultra-compact detector that weighs only a few hundred grams and has been flown in many earlier space missions including in International Space Station for monitoring radiation dose.

With a wide range of instruments, the Chadrayaan-1 should provide much better high resolution data on Lunar mineral composition and chemistry, on lunar gravity, and its feeble local magnetic field. These will definitely improve our current understanding of the origin and evolution of the Moon. The data obtained from the mission for the lunar polar region is expected to provide vital information on the possibility of available resources and sites that may be conducive for planning future human and robotics base on the moon. This mission with its international character will hopefully usher a new age of international cooperation in the field of planetary exploration. All the scientists, both Indian and Foreign, participating in the Chandrayaan-1 mission are eagerly waiting for its launch and successful journey to the lunar orbit.

While Chandrayaan-1 will improve our current understanding of the Moon it will also raise new questions that will require further investigations. It is essential that we complement the remote sensing effort by another lunar mission that will land in an area considered vital from our understanding based on Chandrayaan-1 data as well as those obtained by the Japanese Mission (Selene) and the US mission (Lunar Reconnaissance Orbiter) to be launched next year. The Government of India is very supportive of such an idea and it has recently given approval to ISRO for the Chandrayaan-2 mission. Discussions are also on for mission to Mars and Asteroids. India will soon emerge as an important contributor to our quest to explore and understand our neighbours in the solar system.
A film on Chandrayan(India’s Moon Mission):