At home in the lunar orbit

Published : Dec 05, 2008 00:00 IST

Moon as viewed by Chandrayaan-1s Terrain Mapping Camera on November 4 from a distance of 3,11,200 km.-ISRO

Moon as viewed by Chandrayaan-1s Terrain Mapping Camera on November 4 from a distance of 3,11,200 km.-ISRO

ISRO accomplished with aplomb the complex manoeuvre to insert Chandrayaan-1 into the lunar orbit.

AN unambiguous message that India sent out when Chandrayaan-1 safely entered the moons orbit on November 8 was that the Indian Space Research Organisation (ISRO) could now confidently talk of sending a spacecraft to Mars. Overcoming the challenges posed by the treacherous manoeuvre to insert Chandrayaan-1 into the lunar orbit was no mean task. India succeeded in executing this manoeuvre in its very first attempt.

ISRO accomplished this complex manoeuvre with aplomb at 4.51 p.m. when commands were radioed from the Spacecraft Control Centre (SCC) in Bangalore to Chandrayaan-1s on-board engine to fire. At the end of nearly 14 minutes (817 seconds) of firing of the engine, when the Doppler signals received by the two tracking antennas at Byalalu village near Bangalore showed that everything had been on the dot, there was tempered jubilation among the scientists. And, at 6.33 p.m. on November 12, the engine was fired again, for 58 seconds, at the end of which the spacecraft was in its final lunar orbit of 100 km by 100 km.

The operation, which was done in several missions of the USSR and the U.S., called for precise calculations. When the spacecraft passes by the vicinity of the moon, commands should be given to its engine, called the Liquid Apogee Motor (LAM), to fire at the right moment and exactly for the required length of time. If the engine is under-fired, the spacecraft will skip the moon and fly away into deep space. If the burn duration is in excess, the spacecraft will become sub-orbital and crash on to the moons surface.

The engine should be fired opposite to the direction of the velocity of the spacecraft. In other words, if Chandrayaan-1 were to be travelling from left to right, the firing was to be done from right to left. That is, the thrust should be in the opposite direction so that the spacecraft slows down.

For about 500 ISRO technologists, scientists and radio engineers who were scanning their plot-boards at the SCC, located at ISTRAC, ISROs Telemetry, Tracking and Command Network at Peenya, Bangalore, the final minutes were excruciating.

At 4.51 p.m., commands were radioed to Chandrayaan-1s engine to start firing when the spacecraft passed at a distance of 500 km from the moon. This was to reduce the spacecrafts velocity to enable the moons gravity to capture it.

To Team ISROs delight, as the 817-second firing got under way, it could see on the plot-board the accuracy with which the manoeuvre proceeded, the deceleration of the spacecraft, and its final entry into the lunar orbit.

Chandrayaan-1s aposelene (farthest distance from the moon) was 7,502 km and its periselene (nearest distance from the moon) was 504 km. These were on the dot, as predicted.

G. Madhavan Nair, ISRO Chairman, succinctly captured the mood when he said, The last 20 minutes were so critically important, I can say my heart skipped a beat or two. He explained the dramatic manoeuvre thus: Think of the satellite as a parachute free-falling from an aircraft. We had to ensure that Chandrayaans enormous velocity in deep space was arrested in time and also that it was set on the right track. A host of commands had to be given to ensure that all elements were in good form.

M. Annadurai, project director, Chandrayaan-1, called it an achievement not only for ISRO but for the whole nation. He recalled the hard work put in not only by the Chandrayaan-1 team but the top ISRO management for the past four and a half years to realise this moment.

Annadurai, who led a team of a couple of hundred people in integrating the 11 scientific instruments in the spacecraft, said, Chandrayaan-1 today graduated into a real lunar mission. The first step towards achieving the lunar mission has been taken.

The Doppler signals from the spacecraft received by the two huge antennas with diameters of 32 metres and 18 metres at Byalalu showed that the spacecrafts trajectory was exactly as predicted, he said. There were enough early indications from the behaviour of the accelerometer (on board the spacecraft) that the manoeuvre would be successful, he explained.

For S.K. Shivakumar, Director, ISTRAC, the moment provided a great sense of relief and a wonderful feeling. The SCC, the nerve centre of operations at ISTRAC in communicating with Chandrayaan-1, is his baby. He also led the team that built the 32-metre antenna, which he described as an all-out Indian effort. The 18-metre antenna was built on a turnkey basis by the Germans. He was pleased that the tracking of Chandrayaan-1 by both the antennas was excellent.

Traditionally, it is the launch vehicle that gets the applause. This time, I was pleased that the command network too got accolades, Shivakumar said. He described the signals received from the spacecraft as resounding.

It was a feast for the eyes for B.R. Guruprasad, ISROs Public Relations Officer and an authority on rocketry , who was present at the SCC.

It was a very, very meticulously planned operation. The way Chandrayaan-1s trajectory went in real time was unbelievable. We could see on the monitors the slowing down process the deceleration of the spacecraft before it entered the lunar orbit, said Guruprasad.

The mission began at 6.22 a.m. on October 22 when the Polar Satellite Launch Vehicle (PSLV-C11), an advanced version of the standard PSLV, rose from the second launch pad at the spaceport at Sriharikota in Andhra Pradesh. It was a dramatic lift-off because the Sriharikota island had been buffeted by rain and thunderstorm for the previous five days. By then ISRO lost 10 hours of the 49-hour countdown to the lift-off.

But by T minus four hours, ISRO was back on track, doing in parallel many pre-launch operations that should be done in sequence. Eighteen minutes and 20 seconds after the lift-off, the fourth stage of the PSLV put Chandrayaan-1 into a perfect elliptical earth orbit with an apogee of 22,866 km and a perigee of 256 km (Frontline, November 21, 2008).

From then on, ISRO executed five manoeuvres to increase the ellipticity of the spacecrafts orbit to propel it towards the moon. The manoeuvres were executed by radioing commands at the opportune moments from the SCC to its LAM to fire and increase the apogee height in order to take the spacecraft towards the moons vicinity. The five manoeuvres took the apogee height to 37,900 km, 74,715 km, 164,600 km, 2,67,000 km and finally 3,80,000 km. The fifth manoeuvre took place on November 4 when the LAM was fired to take Chandrayaan-1 to its apogee of 3,80,000 km. The moon is 3,84,000 km from the earth.

During this journey of Chandrayaan-1, its Terrain Mapping Camera (TMC), a scientific instrument built by India, was successfully operated twice to take pictures, first of the earth and then of the moon. Then, on November 8, came the most crucial manoeuvre of inserting Chandrayaan-1 into the lunar orbit.

The USSR and the U.S. had floundered in their initial attempts to insert their spacecraft into lunar orbits. The details of the USSRs failures are not available.

The U.S. attempted its test flights of Ranger 1 and Ranger 2 to the moon in 1961. Its Ranger 3 missed the moon in January 1962 by 36,793 km.

Ranger 4 crashed on the lunar far side in April 1962. In October 1962, Ranger 5 of the U.S. missed the moon by 724 km. In January 1964, Ranger 6 did a hard-landing on the moon and its television camera failed. In July 1964, Ranger 7 did a hard-landing and its television camera took pictures of the moon.

Japan and China succeeded in their first attempts, in 1990 and 2007 respectively, with their Hiten and Change-1 spacecraft. Both entered the lunar orbit. On November 8 afternoon, there was palpable tension in the SCC at ISTRAC as ISRO technologists and radio frequency engineers sat in front of their consoles and radioed commands for retro-firing to Chandrayaan-1.

B.N. Suresh, former Director, Vikram Sarabhai Space Centre (VSSC), and currently Director, Indian Institute of Space Science and Technology, both located at Thiruvananthapuram, explained that this complex manoeuvre called for reorienting Chandrayaan-1 to 180{+0} and giving commands to the LAM to fire in order to push the spacecraft in the opposite direction. That is, orders for firing the LAM should go when the spacecraft is in its apogee of about 3,80,000 km and exactly for the required length of time.

Suresh said this would push the spacecraft in the opposite direction of its velocity. That would reduce Chandrayaan-1s velocity, the moons gravity would become a dominant force, and Chandrayaan-1 would be captured by the lunar gravity. So, instead of being a satellite of the earth, Chandrayaan-1 would become the moons satellite, Suresh said.

S. Ramakrishnan, Director (Projects), VSSC, explained that the precision of firing the LAM at the right time and the motors burn duration are important and it should give the precise delta velocity to the spacecraft.

He described the accuracy with which Chandrayaan-1 was inserted into the lunar orbit as a fantastic achievement.

Ramakrishnan explained the significance of this achievement thus: We now have the capability to send an Indian spacecraft to another planetary body. We can now confidently talk of sending an Indian spacecraft to Mars.

After the lunar orbit insertion manoeuvre was done and Chandrayaan-1 achieved an aposelene of 7,502 km and a periselene of 504 km, ISRO executed a series of manoeuvres to reduce the spacecrafts orbit and bring it closer to the moon. These manoeuvres were the opposite of what was done for the spacecraft to approach the moon.

On November 9, ISRO gave commands to the LAM to fire to reduce Chandrayaan-1s periselene from 504 km to 200 km. The aposelene remained the same at 7,502 km. On November 10, the SCC performed a difficult manoeuvre of steeply reducing the aposelene from 7,502 km to 255 km and the periselene from 200 km to 187 km. The periselene was reduced from 187 km to 101 km on November 11.

Uncertainties besieged the manoeuvre to reduce Chandrayaan-1s orbit as well, for the moons gravity field is not well characterised. It is not symmetrical like that of the earth and it has not been fully understood yet. As Annadurai said, these orbit reduction manoeuvres also went off very well.

After studying the final orbit for a couple of days, ISRO will switch on Chandrayaan-1s 11 scientific instruments one by one.

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