How Chandrayaan-1 will be put in the moon’s sphere of influence

he enhanced capabilities of the Polar Satellite Launch Vehicle (PSLV) and accurate modelling of the forces that act on the Chandrayaan-1 satellite in orbit make India’s mission to Moon possible next week. The PSLV will put the satellite into an elliptical orbit under the influence of earth’s gravity. The inbuilt rockets of the satellite will then push it to the moon’s sphere of influence. Final destination The final destination is a circular Lunar orbit 100 kilometres above the surface of the Moon. The first challenge for the engineers of ISRO will be to put the satellite into the transfer orbit around the earth. The PSLV has been modified to lift the 1,304 tonne satellite and attain a highly elliptical orbit. The nearest point (perigee) of this orbit will be about 250 kilometres and the farthest point (apogee) will be about 22,860 km away from earth. The launch vehicle will have to achieve a velocity of about 26,000 km an hour to place the satellite into the transfer orbit. This, it will do in just over 18 minutes, or 1,096 seconds, to be exact. The capacity of the strap-on-booster motors of PSLV has been increased from nine to 12 tonnes of solid propellant to achieve that. (Because of the increased length of the strap-ons, they are referred to with the suffix XL.) The first stage of the vehicle together with its six strap-on boosters carries 320 tonnes of propellants. The third stage also uses solid propellant while the second and fourth stages use liquid propellants. Once the launch Vehicle puts the satellite into orbit, the inbuilt thrusters are used to move it into an extended transfer orbit. Then a trajectory to transfer the satellite into the moon’s gravitational sphere is achieved through multiple manoeuvres to extend the apogee beyond 3.8 lakh kilometres. The calculation of the gravitational and other forces acting on the satellite at this and earlier stages is crucial in guiding the satellite into the right orbits. The Indian Space Research has prepared models for this, and the calculations have been validated in reference to models used by other space agencies. The manoeuvre The manoeuvre to insert the satellite into Lunar orbit will be done when the moon is at its nearest position to earth. The Indian Space Research Organisation is hoping to use a window available early in November. For this, the launching is to be done between October 22 and 28. Before the moon is in position, a trial will be done by extending the apogee beyond the position where the moon would be at the time of insertion. When the satellite falls into the Lunar orbit, it will be about 500 km (peri-seline) from surface of the Moon on an elliptical orbit that will extend to 5000 km (apo-seline). The orbit will then be reduced to 100 km in steps by slowing down the satellite. Corrections and Clarifications (A sentence in the third paragraph of a report "How Chandrayaan-1 will be put in the moon's sphere of influence" ("Science & Technology" page, October 16, 2008) was "The PSLV has been modified to lift the 1,304 tonne [Chandrayaan-1] satellite and attain a highly elliptical orbit". It was an error. The Indian Space Research Organisation describes Chandrayaan-1 as weighing 1,304 kg at launch and 590 kg at lunar orbit.)

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