The GSLV Mark III-M1 vehicle launched Chandrayaan-2 satellite in the Earth Parking Orbit with an apogee (farthest distance from the earth) of 45,475 km, which is about 6,000 km more than originally envisaged. As a result, the number of orbit-raising manoeuvres needed to take the satellite to the highest orbit has been reduced from six to five, leading to savings in fuel.

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 six to five. The sixth will be a trans-lunar injection, 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 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 says: “At 170 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 July 26 early morning when the focus will be to increase the apogee to 54,689 km, while there will be only 20 km increase in the perigee distance (from 230 km to 250 km).

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

The other orbit-raising manoeuvres will be on 1) July 29 when the apogee will be increased to 71,558 km, 2) August 2 when the apogee will be increased to 90,229 km, and 3) August 6 when the apogee will be increased to 1,43,585 km. “It may be noted that the orbit achieved after every manoeuvre and the orbit planned for the next manoeuvre depends on the system performance during each manoeuvre,” the website says.

Published in The Hindu on July 25, 2019