Indian Space Research Organisation (ISRO) spokesman S Satish said the sky is overcast and there are heavy rains, but it is not a cause for worry.

A delay will happen only in case of a cyclone or lightning. Rains will not hamper the spacecraft's lift off, ISRO sources said.

"Only cyclone-related incidents and lightning could force a delay," the sources said.

The lift-off is slated around 0620 hours on board indigenously-built rocket, PSLV-C11, from the spaceport of Sriharikota on the east coast in Andhra Pradesh, some 100 kms north of Chennai.

The event would mark India's entry into select band of lunar explorers -- the European Space Agency (ESA), Japan, China, the US and Russia which have undertaken moon missions.

"Basically, this (Chandrayaan-1) is meant for a comprehensive mapping of the lunar surface," ISRO Chairman G Madhavan Nair said.

"Earlier missions (by others) focused on specific regions or looked at one aspect or other only. It's for the first time (in the world) that we will have the entire lunar surface mapped up."

Chandrayaan-1 is seen by some analysis as a move by India to catch up with Asian rivals China and Japan, and not lag behind in the race for moon. Coupled with the "pride factor", Chandrayaan-1 would signal India's rising international stature, seen as a reflection of its space prowess as well as build on its technological capability to undertake inter-planetary missions in the coming years.

The venture, India's first spacecraft mission beyond earth orbit, is truly complex. It's for the first time that an Indian satellite would travel close to four lakh kms, more than ten times the distance that an indigenously-built spacecraft have covered till date. Work on this most ambitious space mission of India began in April 2004.

Once the 1380-kg Chandrayaan-1 spacecraft is lofted into space by the 316-tonne PSLV-11, it would first reach a highly elliptical Initial orbit and thereafter the satellite's Liquid Apogee Motor (LAM) is fired at appropriate moments that would finally take it to an orbit at a height of 100 kms around the moon around November eight.

Chandrayaan-1 carries 11 payloads (scientific instruments) - five from India, three from ESA, two from the US and one from Bulgaria. It aims to undertake remote-sensing of the moon in the visible, near infrared, microwave and X-ray regions of the electromagnetic spectrum. With this, preparation of a three-dimensional atlas of the lunar surface is envisaged.