Chandrayaan-2, launched from the Satish Dhawan Space Centre in Sriharikota on July 22, is India’s first lander mission to the moon.
“Today, the Chandrayaan-2 mission crossed a major milestone. The lunar orbit insertion manoeuvre carried out at about 9 am, for about 30 minutes, precisely injected Chandrayaan-2 in a pre-defined orbit (around the moon), in a perfect way… All the systems onboard are functioning normally. The spacecraft is in perfect health,” Indian Space Research Organisation (ISRO) Chairman K Sivan told reporters in Bengaluru.
The spacecraft has been injected into an elliptical orbit that is 114 km away from the moon’s surface at its nearest point and 18,072 km at its farthest. Sivan said this orbit would be changed through another series of manoeuvres, to let the spacecraft eventually attain a near-circular orbit of 100 km around the moon.
At this point, the Vikram lander, along with the small Pragyaan rover, is slated to separate from the main composite module and start its incremental descent towards the moon’s surface. The separation is scheduled for September 4, following which the lander and rover would position themselves in a lower orbit, while the landing is planned to take place at 0140 hours IST on September 7. The main spacecraft module would continue to go around the moon in its orbit for at least one full year.
Sivan gave an account of the precision involved in inserting Chandrayaan-2 into the lunar orbit. He said because Vikram is meant to land in the region around the moon’s South Pole, Chandrayaan-2 needed to attain an orbit that had an inclination of 90 degrees with respect to the lunar equator. In other words, the lunar orbit selected for Chandrayaan-2 had to pass directly over the polar regions.
“This is a unique requirement that only Chandrayaan-2 has… other countries that have landed (their spacecraft, all in the equatorial regions of the moon) did not have this constraint. With today’s manoeuvre, Chandrayaan-2 is now going around the moon in an orbit of 114 km x 18,072 km with an inclination of 88 degrees. In due course, this orbit would be brought down to 100 km x 100 km, and further to 100 km x 30 km. At that time, the inclination of the orbit would also be 90 degrees,” he said.
After its launch on July 22, the spacecraft had been put in an earth-bound orbit. It went around the earth till August 14, raising its orbit incrementally five times, before beginning its six-day journey towards the moon with higher energy. Sivan said when the spacecraft left its earth-bound orbit for the final time on August 14, it started with a velocity of 10.9 km per second (39,240 km per hour).
“To give you an idea of the precision required for today’s manoeuvre, if the velocity of the spacecraft was different (from 10.9 km per second or 39,240 km per hour at the initial point) by even 10 cm/ sec (0.36 km per hour), the spacecraft would not have been able to attain the required inclination in lunar orbit. It would have been off by at least seven degrees,” he said.
The ISRO chairman said Chandrayaan-2 had come under the influence of moon’s gravity on Monday and had started gaining velocity, after having slowed down considerably, to well below 2 km per second (7,200 km per hour), during its journey. He said the spacecraft, at one point, had accelerated to 2.4 km per second (8,640 km per hour), which if left unchecked, would have made it fly past the moon. In order to keep it in the lunar orbit, it had to be slowed down to 2.1 km per second (7,560 km per hour), just like a moving vehicle needs to slow down while travelling from a straight road into a roundabout.
Over the next few days, Chandrayaan-2 will do a series of four ‘burns’ to slow down its speed further and get into a lower orbit, eventually reaching an orbit of 100 km x 100 km. The first of these manoeuvres is planned for Wednesday early afternoon. The other manoeuvres will take place on August 28, August 30 and September 1, Sivan said.
On September 3, before the planned separation, a three-second operation will be carried out to check the functioning of all onboard systems on the Vikram lander. The next day, the lander and the rover will separate from the main spacecraft through a manoeuvre that will last 6.5 seconds, Sivan said.
After separating, Vikram will not immediately land on the moon’s surface. Instead, it will go around the moon for three days, during which all its parameters will be checked. Its powered descent will start in the early morning of September 7, around 1.40 am, and it is expected to land within 15 minutes, the ISRO chairman said.