As the moon mission began progressing from the realms of imagination to
reality, scientists put their heads together to design the pe
rfect spacecraft to carry
India's soaring aspirations. After slogging it out for months, straddling numerous calculations and combinations, what emerged from the drawing boards was a cuboid spacecraft. It had a single solar array to generate power.
As the spacecraft took shape, the scientists monitored its progress on computers. The equipment was housed in a closely-guarded room at the Isro satellite centre in Bangalore. During launch, the spacecraft will weigh 1,304 kg. Once it reaches the lunar orbit covering a distance of nearly 3,86,000km from the Earth, its weight will go down to 590kg as most of its propellants will be depleted during the five-and-a-half-day journey.
Of its total weight, the 11 scientific payloads it'll carry will weigh 80kg. Initially, when the design of the spacecraft was conceived, the weight of the payloads was kept at 35kg. One of the reasons why it shot up was the inclusion of the 29kg Moon Impact Probe, an Indian payload which was initiated at the instance of former President A P J Abdul Kalam. The probe will crashland on the lunar surface to identify future landing sites and explore the moon from a close range.
After freezing the design, scientists got down to preparing it and, by September, it was in flight mode. But takeoff was still a far cry as the spacecraft had to undergo several crucial tests to assess if it could weather the hostile space environment while encircling the moon for two years. Chandrayaan-I would take 117 minutes for every revolution at an altitude of 100km above the lunar surface. In the thermovac test, the spacecraft was lowered into a chamber with temperature extremes.
"This test imulates the actual difficult space environment," said an Isro official. Next came the vibration test in which the spacecraft was placed on a table and vibrated just like an actual launch. Third in line was the acoustic test where the roar of four jet planes was simulated simultaneously to check if the spacecraft could survive the literal blast of the launch. Isro officials recalled a case a few years ago when a few cows, which had accidentally strayed into the launch zone, had gone deaf because of the sound and fury of the takeoff.
The lunar vehicle scored full marks in all these tests and is now ready for launch in the early hours of October 22. After takeoff, once it detaches from the rocket and enters the geo-stationary transfer orbit, the spacecraft will operate on its own for a few days.
The spacecraft will generate power through what is known as a canted single-sided solar array. Which means Chandrayaan's energy will be gathered from the sun. The solar array with light-sensitive cells will generate 700 watts during peak period. During eclipse, it will be powered by lithium batteries.
Chandrayaan is equipped with things like thrusters, which will execute fuel burns to alter the trajectory whenever needed. It has an inertial measurement unit consisting of accelometers and gyroscopes to measure the attitude.
The design of the spacecraft is so advanced that flight controllers on Earth can instruct it to bring a back-up on line if there is a technical hitch. It has three solid state recorders on board to record data from various payloads.
While SSR-I will store science payload data, SSR-II will carry spacecraft altitude information, satellite housekeeping and other auxilliary data besides science payload data.
Isro fixes Oct 22 as launch date
India's maiden moon mission, Chandrayaan-1, is scheduled to take off from Sriharikota at 6.20 am on October 22, Isro officials said on Monday.
"We are definitely targeting October 22. But if the weather becomes unfavourable, we will postpone the launch to October 23. The window of opportunity will remain open till October 26.We are keeping our fingers crossed and praying that the weather gods will be on our side on October 22," an official said.
Isro experts had recently told TOI that while rain wouldn't be an impediment to the launch, the mission cannot take off amid thunder, lightning and cyclonic conditions as these could damage the electronic circuit of the rocket and the spacecraft.
The moon-bound spacecraft with the 11 scientific instruments-six from abroad and five of India-was moved from the Isro Satellite Centre in Bangalore to the Sriharikota launch centre on October 3 where it is undergoing some more pre-launch tests at present. Once these are completed, the process of integrating the spacecraft with the fourth stage of the Polar Satellite Launch Vehicle (pslv) will be initiated.
Officials said all the four stages of the rocket have been completed. A regular PSLV has six strap-on motors attached to the first stage of the rocket that use nine tonnes of solid propellants. But, in the case of the rocket which will carry Chandrayaan to the moon, the length of the strap-ons has been extended from 10 metres to 13.5 metres and each will use 12 tonnes of propellants. It is for this reason that this particular PSLV for the moon mission has been designated as PSLV-XL.
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