Tense 30 minutes for Chandrayaan as it settles into lunar orbit

New Delhi: Around 1530 hrs on Saturday a critical manoeuvre will be performed that will aim to inject India's lunar probe, Chandrayaan-1, into a circumpolar orbit around the moon. If successful, the  manoeuvre, referred to as the Lunar Orbit of Injection, will either unshackle Chandrayaan from the earth's gravitational pull forever and make it a lunar satellite - as it is meant to - or send it crashing towards the moon or the earth, or on a trajectory leading into deep space.

In this context it may be stated that about 30 per cent of American and erstwhile Soviet unmanned moon missions would fail executing this critical manoeuvre.

Chandrayaan-1 is currently in a highly elliptical orbit around the earth.

The Indian Space research Organisation carried out the last of the earth-bound manoeuvres on the Chandrayaan on 4 November, which deployed the satellite into an orbit with an apogee (the farthest point in the orbit from the earth) of 384,000 km and a perigee (the nearest point) at 1,019 km. At its farthest point the satellite actually encircles the moon as well.

In its present trajectory, the moon's gravity will begin to dominate the probe at a distance of around 60,000 km from the moon. This distance would have been reached around midnight Friday. The moon's gravitational pull will also add increasing velocity to the satellite. In order to exit the earth's gravitational pull and be enslaved by lunar gravity the satellite would need to be slowed down. It is this function that would need to be performed this evening when its current trajectory brings the Chandrayaan within 500 km of the moon, just above the lunar north pole.

This is the most critical time for the lunar probe as a number of factors come into play simultaneously. At this point, the satellite's orientation is earth-facing and it is no longer in a closed elliptic orbit, but in an open hyperbolic one. This is a critical moment, for if velocity is not reduced at the correct time, the lunar probe will escape moon's gravity and be lost in space irretrievably.

The first part of the critical manoeuvre is to change the Chandrayaan's earth facing orientation by 180 degrees with the help of on-board reaction wheels. Once this is carried out retro-rockets will be fired for about 800 seconds with the purpose of slowing down the velocity of the hurtling satellite, from about 2 km/sec to about 1.5 km/sec.

If successful, the manoeuvre would bring the satellite under the moon's influence and the lunar gravitational pull will slowly tilt the probes' orbit by nearly 90 degrees southwards to make it a circumpolar one. In this Lunar Orbit of Injection, the satellite's closest point from the moon (perilune) would be 500 km and the farthest point (apolune) about 7,500 km.

It will revolve around the moon roughly once every 10 hours.

By today afternoon, health checks on the probe would have begun and a little before the probe reaches the lunar north pole, its orientation will be turned around to ensure that its new orientation is exactly opposite to its velocity vector.

The firing of the retro-rockets is expected between 1730 hrs and 1800 hrs and the results of the manoeuvres are expected to be known within an hour.

Four more velocity reduction operations will be carried out, twice at perilune and twice at apolune, to bring it into final pole-to-pole circular orbit of 100 km radius, which the Indian lunar orbiter expects to attain on 15 November.