In its maiden operational flight, the Geosynchronous Satellite Launch Vehicle – Mark III-M1 (GSLV Mark III-M1) vehicle used for launching Chandrayaan-2 had successfully placed the satellite in the Earth Parking Orbit (EPO) with perigee (closest distance from the earth) of 170 km and an apogee (farthest distance from the earth) of 45,475 km. The apogee of the earth parking orbit is about 6,000 km more than originally envisaged.

As a result, the number of earth-bound steps or manoeuvres needed to take the satellite to the highest orbit of nearly 1,44,000 km apogee has been reduced from seven to six, says K. Sivan, Chairman of the Indian Space Research Organisation (ISRO). The reduced number of manoeuvres will translate into fuel savings.

After the satellite is placed in the Earth Parking orbit, the apogee is increased in steps to take it to the final earth-bound orbit of about 1,44,000 km. The satellite goes around the earth in elliptical orbits and to increase the apogee, the satellite is fired when it comes closest to the earth (perigee).

In the first of the orbit-raising exercise that was carried out on July 24, the perigee was increased from 170 km to about 230 km, while the apogee was reduced from 45,475 km to 45,162 km. Explaining why the orbit raising was done mainly to increase the perigee and not the apogee, Mr. Sivan said to The Hindu: “At 169.7 km altitude at perigee, the satellite is still in the earth’s atmosphere and so is exposed to heat stress while travelling at very high velocity. So we had to raise the perigee.”

The second orbit raising manoeuvre will be undertaken tonight (July 25) when the focus will be to increase the apogee in large measure, while there will be only a small increase in the perigee distance.

“Totally there will be six earth-bound, orbit-raising manoeuvres before the trans-lunar insertion, including the one on July 24. If the satellite had not gained nearly 6,000 km while being placed in the earth parking orbit, then there would have totally been seven orbit-raising manoeuvres,” Mr. Sivan said.

Currently, the ISRO website mentions only five orbit-raising manoeuvres and the perigee distance mentioned is 241.5 km after the first orbit raising exercise. “This will be soon revised,” he said.