The most critical phase of Chandrayaan-3 will begin when the spacecraft starts moving closer to the moon from the 100 km circular orbit, Isro chief S Somanath said on Monday.The orbit determination process of Chandrayaan-3 will be planned between August 9 and 17 to place the spacecraft in a 100 km circular orbit.Somanath said that overall, India's third lunar mission is in good health.