SENDING a spacecraft on a lunar mission involves navigating it, controlling it and communicating with it as it travels deep into space towards the moon. The moons gravity model is not uniformly spread as is that of the earth and is far more complex. Precise orbit-manoeuvring capabilities should be engineered into Chandrayaan-1 to prevent it from falling. That is why the Indian Space Research Organisations (ISRO) top brass was circumspect on October 22 when the PSLV-C11 put Chandrayaan-1 into its initial orbit with an apogee of 22,866 kilometres and a perigee of 256 km. This is only the first step. We have a long way to go in time and space before we reach the moon, they cautiously told reporters.
There are two or three ways of reaching the moon. The Russians and the Americans have used the tested approach of sending the spacecraft into a low-parking earth orbit and then boosting it to the lunar orbit.
ISRO has repeatedly changed its plans for the manoeuvres to send Chandrayaan-1 to its final orbit of 100 km 100 km above the moon. The initial plan was to put Chandrayaan-1 in a transfer orbit with an apogee of 36,000 km and a perigee of 285 km. Then commands to the propulsion system of the spacecraft were to send it into an extended transfer orbit. Subsequent manoeuvres were to send the spacecraft around the moon and then its orbit was to be reduced in stages until the spacecraft was finally put into a 100-km lunar orbit. In this plan, Chandrayaan-1 would have reached the moon in five and a half days.
However, this plan was given up and a new plan was drawn up to put Chandrayaan-1into an initial earth orbit of 22,866 km 256 km, raise it to a geostationary orbit of 37,000 km 300 km, and again raise it to 73,000 km 300 km. Following this, Chandrayaan-1 was to be propelled beyond the moon with an apogee of 3,87,000 km, brought back to circle the earth and then taken back to the moon, where its velocity was to be reduced to insert it into its lunar orbit and finally slot it into a 100-km lunar orbit. One of these manoeuvres, of propelling Chandrayaan-1 to the moon, bringing it back to the earth and manoeuvring it again to the moon, would have taken 11 days. That is why the ISRO top brass said on October 22 that Chandrayaan-1 would ultimately be placed into the lunar orbit at an altitude of 100 km on November 15, that is, three weeks from October 22.
ISRO gave up this strategy, too, and as on October 30, Chandrayaan-1s orbit had been raised four times. First, it was raised on October 23 from its initial orbit of 22,866 km 256 km to 37,000 km 305 km, in which it took 11 hours to go round the earth once. Then, on October 25, its orbit was raised again to 74,715 km 336 km, in which it took about 25 hours to go round the earth once.
In the third orbit-raising manoeuvre on October 26, the spacecraft went deep into space and reached almost half the distance to the moon, with an apogee of 1,64,600 km and a perigee of 348 km. In this orbit, it took 73 hours to go round the earth once.
In the fourth manoeuvre on October 29, commands were radioed to fire its engine and take the spacecraft to a highly elliptical orbit with the farthest distance of 2,67,000 km (two-thirds of the distance to the moon) and the nearest distance of 465 km. In this orbit, the spacecraft would take six days to go round the earth once.
The fifth manoeuvre to raise the orbit was scheduled for November 4, to take it to a distance of 3,84,000 km. It will partially circle the moon and then go into a highly elliptical orbit around it.
On November 8, after Chandrayaan-1 reaches the vicinity of the moon, its velocity will be reduced by rotating it in the opposite direction. The velocity is reduced so that the moons gravity captures the spacecraft and puts it into a lunar orbit. This has to be done with precision, in stages. Otherwise, the spacecraft will fly past the moon.
If things go as planned, on November 15 the spacecraft will be slotted into the final orbit at an altitude of 100 km above the moon.
T.S. Subramanian
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