Mission lost
T.S. SUBRAMANIAN
India’s moon mission Chandrayaan-1 meets a premature death, but the ISRO says it achieved most of its objectives in 10 months.
ISRO
Chandrayaan-1 undergoing tests at the ISRO Satellite Centre in Bangalore on September 15, 2008. It was launched on October 22.
IT was coming, though the Indian Space Research Organisation (ISRO) did not know when. Following the failure of both star-sensors and some power supply systems on board Chandrayaan-1 in mid-July, ISRO engineers and scientists predicted that the spacecraft might meet its end “tomorrow” or “it may last its entire life of two years”. In the event, Chandrayaan-1, India’s first scientific mission to the moon, met a premature death 10 months after its launch.
Around 1-30 a.m. on August 29, ISRO abruptly lost radio contact with Chandrayaan-1. When its radio frequency engineers in the Deep Space Network (DSN) at Byalalu village near Bangalore tried to contact the spacecraft, they could not. The DSN, with its two huge antennae with diameters of 32 metres and 18 m, was the hub of all communication from the ground to the spacecraft. The engineers had received data from the spacecraft until 12-25 a.m. Loss of radio contact meant that no commands could be sent to Chandrayaan-1 to perform various manoeuvres and that no data about the health of its cameras and payloads, including images of the moon’s surface, could be received from it. In effect, Chandrayaan-1 was no more. It was lost. Although it was in orbit, ISRO was unable to locate it.
A press release from ISRO said: “Detailed review of the telemetry data received from the spacecraft is in progress and the health of the spacecraft sub-systems is being analysed.
“… Chandrayaan-1 was launched from Satish Dhawan Space Centre, Sriharikota, on October 22, 2008.
The spacecraft has completed 312 days in orbit, making more than 3,400 orbits around the moon and providing a large volume of data from its sophisticated sensors such as the Terrain Mapping Camera, the Hyper-spectral Imager, the Moon Mineralogy Mapper, etc., meeting most of the scientific objectives of the mission.”
The mission failed because the highly sensitive electronic items on Chandrayaan-1 were baked by solar radiation.
The next day, ISRO Chairman G. Madhavan Nair formally announced the mission’s end. The mission had to be abandoned because ISRO had no possibility of restoring contact with the spacecraft, he said.
High levels of solar radiation on to the moon’s tenuous atmosphere had affected the units supplying power to two computers on board the spacecraft. The lack of power supply resulted in loss of communication with the spacecraft. ISRO had not anticipated such heavy solar radiation. “We have learnt some valuable lessons,” said Madhavan Nair. In future missions, ISRO would look for devices that are less susceptible to radiation, he added.
T.K. Alex, Director, ISRO Satellite Centre, Bangalore, where Chandrayaan-1 was integrated, explained that the
two computers – one primary and the other back-up – controlled the telemetry data flowing from the spacecraft to the ground and the tele-commands given to it from the DSN. The power supply to the primary computer had failed some months ago. And now, the other computer also failed to receive power. This meant that tele-commands could not be radioed to the spacecraft nor could telemetry data be received. “We do not have any telemetry data. So we are not able to make out what is happening,” Alex said.
The spacecraft was hovering 200 kilometres above the moon’s surface. Alex estimated that it would take about 1,000 days for Chandrayaan-1 to crash on the moon. “It will slowly come down. Its periselene will go up and come down. There is dynamics for that,” he explained.
The two star-sensors – primary and redundant – on board Chandrayaan-1 also failed because of “excessive radiation from the sun”, Madhavan Nair said at a press conference in Bangalore on July 17. He acknowledged the failure of the star-sensors only after the media made it public the same day. Solar radiation “can degrade devices in the star-sensors,” he said. Since the star-sensors which helped in orienting Chandrayaan-1 had failed, the spacecraft was being oriented with the help of its gyroscopes, he added.
Several ISRO scientists and engineers conceded that the mission had exposed their inadequate knowledge of the radiation environment above the moon’s surface. An ISRO expert admitted: “Obviously, our understanding of the radiation in the space above the moon was not up to the mark. Nobody tells you what it is and why it is so. We did our job to the best of our wisdom, but it was not enough.”
In deep space, there are several factors that cause radiation and one of them is the sun. Power generated in the battery on board Chandrayaan-1 was wheeled by a module to a distribution system which, in turn, fed the power supply to the computers.
The computers were called the attitude and orbit control systems (AOCS) because they controlled the spacecraft’s attitude and orbit. It was in the power distribution system that the temperature had shot up, resulting in the snapping of power supply, the expert said. With the power supply to the AOCS cut off, there was no way of conversing with the spacecraft.
ISRO
A picture of the moon’s surface taken by Chandrayaan-1’s Terrain Mapping Camera on November 15, 2008. Taken over the polar region of the moon, the picture shows many large and numerous small craters. The bright terrain on the lower left is the rim of the 117-km-wide Moretus crater.
Another top ISRO engineer said it was “definitely plausible” that solar radiation was the reason for the failure of power supply to the two computers. The moon is about 3,84,000 km away from the earth. This was the first time that ISRO was sending a spacecraft into deep space. Before this, it had only orbited communication satellites 36,000 km above the earth.
The engineer said: “What the radiation in space beyond 36,000 km is, we did not know. Everybody does not report everything [that is, other countries that have sent spacecraft to the moon do not reveal all their data about radiation in outer space]. There could have been uncertainties in our modelling of the radiation above the moon. The solar radiation in the space environment that affected Chandrayaan-1 was obviously more than what we had assessed or anticipated.”
Radiation in deep space is severe. “Raw radiation will come in. Chandrayaan-1 was full of electronic devices. These electronic devices worked on the movement of electrons. They were affected by the bombardment of extreme radiation. So these electronic devices must be ruggedised to operate in the harsh environment of space,” he explained.
This was the first time India had stepped into deep space. “Our inadequate understanding of solar radiation/solar winds seems to be the reason for the failure of Chandrayaan-1,” he said.
The journey
Chandrayaan-1 was put in its initial orbit by a Polar Satellite Launch Vehicle (PSLV) of ISRO from the spaceport at Sriharikota on October 22, 2008. After a series of manoeuvres, the spacecraft reached its final circular orbit 100 km above the moon on November 12. It did so after commands were radioed from the Spacecraft Control Centre situated at the ISRO Telemetry, Tracking and Command Network (ISTRAC), Bangalore. Its orbit was to pass over the poles of the moon.
M. Annadurai, Project Director, Chandrayaan-1, had then noted: “The entire team is very happy that in three weeks from the launch, we could safely send Chandrayaan-1 to the moon without any hiccups.”
Chandrayaan-1 had 11 scientific instruments – five made in India and six procured from abroad. The spacecraft was a novel combination of communication and remote-sensing applications. Its remote-sensing payloads were to help in prospecting for minerals and chemicals in lunar soil, including helium-3, and to look for the possible presence of water-ice in the permanently shadowed polar regions. The purpose of Chandrayaan-1’s Terrain Mapping Camera (TMC), built by India, was to help in preparing a three-dimensional atlas of the entire surface of the moon.
Another special payload, called the Moon Impact Probe (MIP), was also built by India. It was to separate from Chandrayaan-1 and crash on the moon’s surface. The MIP had three devices: a video camera to take pictures of the moon’s surface as the MIP approached the lunar surface before crashing on it; an altimeter that calculated how far the MIP was from the lunar surface every second of the MIP’s descent; and a mass spectrometer to “sniff” the very thin atmosphere over the moon. The MIP, with India’s flag painted on its sides, crashed on the moon’s surface on November 14, the birthday of Jawaharlal Nehru, celebrated as Children’s Day. “This is ISRO’s gift to the children of India on the occasion of Children’s Day,” Annadurai said that day.
However, Chandrayaan’s troubles began in November. The power sub-system of the AOCS (the primary computer which controls the satellite and sends telemetry data to the ground) failed. Then other power sub-systems began to fail one after the other.
The mission went into a crisis when the power sub-system of the spacecraft’s primary star-sensor failed on April 26. The star-sensor could not withstand the radiation from the sun. The back-up star-sensor failed next. The star-sensors are vital instruments used for determining the orientation of the spacecraft. They provided the reference for orienting Chandrayaan-1 to the required area of the moon.
The star-sensors are also called star-trackers. The star-sensors and gyroscopes helped find the direction in which Chandrayaan-1 was travelling. The star-trackers image the sky and receive information about the direction in which the spacecraft is travelling from the position of 10 stars. The positions of the bright stars in the sky were kept in the memory of Chandrayaan-1’s computer by a technique called pattern imaging. The two star-sensors were made by ISRO’s Laboratory for Electro-Optics Systems (LEOS) in Bangalore. Both of them failed in April/May 2009. So Chandrayaan-1 was put in “gyro-mode” to orient it towards the moon.
An ISRO press release on May 20 made no mention of the failure of the star-sensors. It merely spoke of an orbit-raising manoeuvre done on May 19. The press release said: “After the successful completion of all the major mission objectives, the orbit of Chandrayaan-1 spacecraft, which was at a height of 100 km from the lunar surface since November 2008, has now been raised to 200 km.” The press release claimed that the spacecraft’s orbit was raised “to enable further studies on orbit perturbations, gravitational field variation of the moon and also enable imaging of the lunar surface with a wider swath.”
However, informed ISRO officials revealed that the orbit was raised to preclude the possibility of the spacecraft spiralling down to the moon’s surface. “In the 100-km orbit, you will have to perform manoeuvres every day and you will strain the gyroscope,” an official said. He added that “it was basically a component failure” in the star-sensors that had led to their malfunctioning.
But ISRO admitted the failure of the star-sensors only on July 17. Madhavan Nair said the failure was a “handicap” but he argued that “90 to 95 per cent” of the objectives of India’s moon mission had been achieved. “We could collect a large volume of data including 70,000 images of the moon.” The images provided breathtaking views of lunar mountains and craters, especially craters in the permanently shadowed regions of the moon’s polar region. It also collected data on the chemical and mineral content of the moon’s soil.
S. Satish, Director, Publications and Public Relations, ISRO, also asserted that Chandrayaan-1 had met more than 95 per cent of its objectives.
The mission had two objectives: engineering and scientific. Among the engineering objectives were propelling the spacecraft over a distance of nearly 400,000 km to the moon and inserting it in a lunar orbit; deliberately crashing the MIP on the moon’s surface; and establishing the necessary ground structure including the two antennae at Byalalu for conducting the mission operations. “The engineering objectives have been met 100 per cent,” Satish said.
The scientific objectives included mapping the lunar surface, preparing a three-dimensional map of the lunar surface, mapping the chemicals and minerals in the lunar soil, and studying the radiation environment above the moon. Satish said: “Using the TMC and the Hyper-spectral Imager, the mapping of the lunar surface in different resolutions has been successfully completed. Preparation of the three-dimensional map is in progress. The Moon Mineralogy Mapper and Chandrayaan-1 X-ray Spectrometer have performed exceedingly well, providing quality data on the chemical and mineralogical content of the lunar soil. All this data are being analysed by ISRO and the other space agencies that sent their payloads on the Chandrayaan-1 mission.”
Radiation Dose Monitor, another payload, had worked non-stop, producing data on the radiation environment of the moon.
“The mission has provided a wealth of valuable data, which need to be analysed for clues on the evolution of the moon and the earth. The maximum life envisaged for Chandrayaan-1 was two years. However, within 10 months, most of the mission objectives have been met,” he said.
About ISRO not making public the setbacks in the mission, he argued it was not necessary to go public about every failure as failures were common in complex space missions. However, when the back-up systems also failed, thus jeopardising the mission, it was necessary to make the failures public. “ISRO has done this,” Satish said.
