What Chandrayaan-1 aims to accomplish

Although India has earlier built multi-purpose INSATs which combined communication and meteorology payloads, Chandrayaan-1 is a novel clubbing together of remote-sensing and communication payloads. Chandrayaan-1’s orbit may be similar to the geo-synchronous transfer orbit of a communication satellite.

The complexities of the remote-sensing payloads in Chandrayaan-1 are also similar to those of the regular remote-sensing satellites. The comparison stops there.

For “all [the Indian] instruments on board Chandrayaan-1 have been made for the first time [in the country]. We had to develop prototypes and test them for high levels of endurance in the environment,” said T.K. Alex, Director, ISRO Satellite Centre, Bangalore, which built the spacecraft.

For this spacecraft, every system and sub-system is critical. “We have made sure that their reliability is good. For every item, we had a redundant system. If an item or a sub-system did not work, we had a standby. We had two star-trackers. We had two gyroscopes, two transmitters, two receivers and so on,” he added.

Chandrayaan-1 carries on board 11 instruments — five from India and six from abroad.

These scientific payloads will help in preparing a three-dimensional atlas of the entire topographic surface of the moon, spot minerals such as thorium, magnesium, aluminium, silicon, iron and titanium, and in confirming the reported presence of water ice in the moon’s polar regions.

The spacecraft’s communication sub-system transmits this precious information gathered by these 11 instruments to the earth in ‘X-band’ through its dual gimballed antenna, which has been made in India. M. Annadurai, Project Director, Chandrayaan-1, has been the dynamic driving force behind the integration of the 11 instruments in the spacecraft bus.

Mission wise, the journey of Chandrayaan-1 towards the moon is extremely complicated. “For the first time, we are sending a spacecraft beyond the earth’s orbit deep into space,” Dr. Alex said.

The moon is nearly four lakh km away from earth. The manoeuvres for propelling Chandrayaan-1 into the lunar orbit will be done in stages. The spacecraft will be initially put in low, elliptical orbit.

Its altitude will be increased precisely in stages. “Finding the direction in which the spacecraft is pointing is important. Finding the direction and position of the spacecraft in its orbit are the important challenges in accomplishing this mission,” he explained.

The direction of Chandrayaan-1 is found by using star-trackers and gyroscopes, both of which have been developed by the Indian Space Research Organisation (ISRO) laboratories.

The star-tracker images the sky and gets the direction in which the spacecraft is travelling from ten stars. The positions of the bright stars in the sky are kept in the memory of Chandrayaan-1’s computer by a technique called pattern-imaging.

The computer automatically identifies the star-cluster and establishes the direction in which the spacecraft is travelling. Chandrayaan-1’s position in orbit is found by a technique called “satellite tracking,” which is done by a chain of tracking stations spread all over the globe.

“The most important part is that Chandrayaan-1 should reach the moon at the precise time and required velocity when the moon is exactly at the desired place,” explained Dr. Alex.

Once the spacecraft reaches the moon’s vicinity, the former’s velocity is reduced by giving commands to it and it is put in an orbit of 100 km by 5,000 km around the moon. The altitude is reduced to a circular orbit of 100 km around the moon.

Later, the scientific instruments are switched on. The Moon Impact Probe, one of the 11 instruments, is ejected from Chandrayaan-1 and it hits the lunar surface.

The five Indian payloads are Terrain Mapping Camera (TMC) , Hyperspectral Imager (HySI), Lunar Laser Ranging Instrument (LLRI),

High Energy X-ray Spectrometer (HEX) and MIP. The TMC is a charged coupled device (CCD) camera which will take images of the near and far side of the moon which will enable preparation of the 3-D atlas of the entire lunar surface. This will help in understanding the origin and evolution of the moon.

The HySI, which is also a CCD camera, will provide mineralogical mapping of uranium and thorium deposits.

These images will help in identifying the mineralogical compositions in the moon’s deep crater region. The LLRI will provide the accurate height of moon’s hills and mountains, and depths of craters. This information will be useful in getting an improved model of lunar gravity.

HEX will enable exploration of the moon’s polar regions. The MIP, which will crash-land on the moon, is a forerunner to India landing rovers on the moon.

It has a video-camera, which will take pictures of the lunar surface every second of its 20-minute descent to the moon. Its altimeter will measure the MIP’s altitude from the moon every second of its descent.

Its mass spectrometer will analyse moon’s thin atmosphere. The high-resolution and low resolution optics in the cameras of the Indian instruments have been fabricated by the Laboratory for Electro-Optic Systems (LEOS), Bangalore.

Dr. Alex, who was the founder-director of LEOS, said: “We get raw, special glass, grind it and polish it into mirrors and lenses of very large sizes. We can fabricate at LEOS lenses and mirrors of half-a-metre to one metre diameter.”

Of the six instruments from abroad, three are from the European Space Agency (ESA), two from the U.S. and one from Bulgaria.

The three ESA payloads are Chandrayaan-1 Imaging X-ray Spectrometer (CIXS) which will measure the presence of magnesium, aluminium, silicon, iron and titanium on the moon’s surface; Sub keV Atom Reflecting Analyser (SARA) will study the moon’s surface composition, the way in which its surface reacts with solar wind and so on; Smart Near Infrared Spectrometer (SIR-2) will study the lunar surface to explore its mineral resources.

Bulgaria’s Radiation Dose Monitor (RADOM) will characterise the moon’s radiation environment. NASA’s Mini Synthetic Aperture Radar (MiniSAR) will detect water ice in the moon’s permanently shadowed polar regions. The Moon Mineralogy Mapper (M3) also of NASA will map lunar minerals.

http://www.thehindu.com/thehindu/seta/2008/10/23/stories/2008102350011500.htm