Chandrayaan-1: Why ISRO changed the orbit-transfer strategy

Published in The Hindu on October 23, 2008

Chandrayaan-1
In the revised strategy, there will be five earth-bound orbits. The first orbit will have an apogee (farthest point from the earth) of 23,000 km and perigee of 255 km. — Photo: ISRO

The Indian Space Research Organisation (ISRO) had earlier planned two earth-bound orbits for the Chandrayaan-1 spacecraft before it moved towards the moon. Much like in the case of the earth orbits, two lunar orbits were planned for before the spacecraft reached the final lunar orbit destination — circular orbit of about 100 km from the moon’s surface.

But ISRO had revised the orbit-transfer strategy. In the revised strategy, there will be five earth-bound orbits. The first orbit will have an apogee (farthest point from the earth) of 23,000 km and perigee of 255 km.

Increasing apogees

The second orbit will have an apogee of about 1,60,000 km and the third orbit will have an apogee of about 2,60,000 km. The fourth earth-bound orbit will have an apogee of about 3,87,000 km. It will take about 11 days to complete the fourth earth-bound orbit. During the fifth earth-bound orbit, which will have an apogee of about 3,84,000 km, the spacecraft will approach the moon’s North pole at a safe distance of a few hundred kilometres.

To calibrate the systems

“Basically, we wanted to calibrate our systems, such as the ground tracking system. When we get out of earth’s influence [gravity], there will be an influence of other planets, sun and moon’s gravity. We have theoretical knowledge of this influence. But getting actual data will be more useful to calibrate our systems,” said Dr. Madhavan Nair, Chairman of ISRO.

“The fourth and fifth orbits go up to the final point before it [the spacecraft] comes back. So this will help us to calibrate the systems better,” he said.

“[The earth-bound orbits] will tell us how far our assumptions and models are correct,” Dr. Nair said.

To avoid errors

The earlier orbit-transfer strategy involving just two earth-bound orbits would have also provided the essential data. “When we do it [in fewer and smaller orbits] in a short period, we have to evaluate all the parameters in a shorter time. So some errors can arise,” said Dr. Nair, explaining why the orbit-transfer strategy was changed.

In the revised strategy, there will be two lunar orbits before the spacecraft reaches the final circular orbit of about 100 km from the moon’s surface.

But will the revised strategy, which will involve longer period in space before it reaches its final destination, lead to more fuel consumption and hence affect the total mission duration? “The fuel consumption will be the same and the mission life will also be the same,” he stressed.

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