Chandrayaan 2 planned orbit 'compressed' in latest manoeuver, ISRO reveals. What does it mean?

The second Indian mission to explore the moon, Chandrayaan 2, has completed a day in its new orbit around the moon. The Chandrayaan 2 composite – the single, combined unit made up of the orbiter, Vikram lander with the rover tucked away inside it – has successfully completed one of four orbital manoeuvres around the moon to bring the spacecraft within 100 kilometres of the moon's surface.

The Indian Space Research Organisation (ISRO) has said that the planned orbit for the spacecraft has been altered – compressed, to an elliptical orbit of 114 x 4,412 km (nearest x farthest distance in orbit) from the moon. Originally, the planned orbit at this stage was 121 X 4,303 km. Chandrayaan 2 is now at a 118-km altitude, a slight deviation of 3 km from the planned orbit.

Lunar capture and first orbit in the mission's Moon-bound phase. Image: ISRO

But there have been minor deviations from the "planned orbit" in multiple steps along the way:

• The second orbit-raising manoeuvre was meant to put C2 in a 250 x 54,689 km (nearest x farthest distance) elliptical orbit around Earth but instead placed C2 in a 251 X 54,829 km orbit instead. [Off by 1 x 140 km]
• The third earth-bound orbit-raising manoeuvre was meant to put the spacecraft in a 268 x 71,558 km (nearest x farthest distance) elliptical orbit but placed it in a 276 x 71,792 km orbit instead. [Off by 8 x 234 km]
• The fourth earth-bound orbit-raising was meant to put the spacecraft in a 248 x 90,229 km (nearest x farthest distance) elliptical orbit but placed it in a 277 x 89,472 km orbit instead. [Off by 29 x 757 km]
• Final earth-bound orbit-raising was to place C2 in a 221 x 1,43,585 km (nearest x farthest distance) elliptical orbit but was placed in a 276 x 1,42,975 km orbit instead. [Off by 55 x 610 km]
• When the C2 composite first entered into an orbit around the moon on 20 August, it was meant to do so at a 118 x 18,078 km elliptical orbit but was placed in a 114 km x 18,072 km orbit instead. [Off by 4 x 6 km]

If "all spacecraft parameters are normal", as ISRO's most recent update announced, we've got little to worry about. The next update to look forward to is scheduled for 28 August between 5.30 am and 6.30 am IST, is targeting to lower the altitude of the spacecraft to 178 x 1,411 km from the current 114 km x 18,072 km elliptical orbit.

The moon-bound phase of the mission, which was 28 days-long in the earlier launch schedule was shrunk to 13 days in the current one. This is an important part of the orbiter's mission: downtime to surveil its year-long home for the first time, ensure that no damage was caused to its instruments on the journey so far, and a thorough examination of the Vikram lander's landing site in the moon's South Polar region.

In the fourth and final orbital manoeuvre planned for 1 September, the Chandrayaan 2 orbiter-lander composite is expected to enter a circular, 100 km-altitude-orbit around the moon.