his is the first time ever that an Indian spacecraft has been sent lakhs of kilometres away from earth. This has been done by incrementally changing the apogee (farthest point from earth). I realised that several people did not understand or know how this was done. And that made me write a lead piece in today’s Science and Technology pages of The Hindu explaining the basics.
The piece first addressed why all the firings happened at the perigee (nearest point to earth) — the need for 3-4 ground stations to track the spacecraft before it is fired to the next higher orbit, using less energy to achieve this, and finally to increase the apogee.
Next, I looked at why the perigee increases very little while the apogee increases by several hundreds of kilometres. That is to do with the duration of firing. The firing at the perigee does not happen at one instant but over a period of several seconds. This is responsible for the perigee distance to also increase.
And while we are discussing about the duration of firing, it is interesting to note that the duration of firing is the most when it comes to raising the spacecraft from the initial orbit to the first orbit (1060 seconds), and the least to take it from the fourth to the fifth orbit (150 seconds).
Finally, I addressed how the spacecraft is able to reach the 100 km circular orbit around the moon. This is by slowing down the velocity of the spacecraft. Reducing the velocity is done by firing Chandrayaan-1 at both perigee and apogee. But the firing itself takes place only after Chandrayaan-1 is flipped by 180 degrees.
Next, I looked at why the perigee increases very little while the apogee increases by several hundreds of kilometres. That is to do with the duration of firing. The firing at the perigee does not happen at one instant but over a period of several seconds. This is responsible for the perigee distance to also increase.
And while we are discussing about the duration of firing, it is interesting to note that the duration of firing is the most when it comes to raising the spacecraft from the initial orbit to the first orbit (1060 seconds), and the least to take it from the fourth to the fifth orbit (150 seconds).
Finally, I addressed how the spacecraft is able to reach the 100 km circular orbit around the moon. This is by slowing down the velocity of the spacecraft. Reducing the velocity is done by firing Chandrayaan-1 at both perigee and apogee. But the firing itself takes place only after Chandrayaan-1 is flipped by 180 degrees.
