Bangalore (PTI): India's first unmanned spacecraft Chandrayaan-1 entered the lunar orbit on Saturday after ISRO scientists successfully carried out a highly complex and tricky manoeuvre crossing another historic milestone in the country's space programme.

ISRO scientists at the Mission Control Centre near here fired the spacecraft's liquid engine at 1651 hours for a duration of 817 seconds in a hit or miss Lunar Orbit Insertion(LOI) operation in the maiden moon mission, 18 days after it was launched from Sriharikota spaceport.

"For the first time in the history of India, an Indian-made satellite is circling the Moon", a jubilant ISRO Chairman G Madhavan Nair told PTI shortly after a home-grown satellite broke away from the Earth's gravitational field for the first time and reaching the moon. India becomes the sixth country to put a satellite in the moon orbit.

The engine was fired when the spacecraft passed at a distance of about 500 km from the moon to reduce its velocity to enable lunar gravity to capture it into an intended orbit around the moon.

The spacecraft is now orbiting the moon in an elliptical orbit that passes over the polar regions of the moon. The nearest point of this orbit (perilune) lies at a distance of about 504 kms from the moon's surface, while the farthest point (apolune) lies at about 7,502 km.

"Most crucial moment"

A relieved Nair said today's operation was the "most crucial moment" in the mission."We have done it," he declared.

"For the last 20 minutes, almost all our hearts were at a standstill," Nair said immediately after the the chaallenging manoeuvre was carried out.

Nobody else in the world perhaps would have got such a precise lunar orbit as India did in the first attempt, Nair said.

PM congratulates space scientists

Prime Minister Manmohan Singh,who is now in Muscat on an official visit, congratulated the Indian space scientists for the singular achievement.

"This (today's achievement) will be etched in the history of Indian space in golden letters", Nair said.

In the coming days, the height of Chandrayaan-1's orbit around the moon would be carefully reduced in steps to achieve a final polar orbit of about 100 kms height from the moon's surface.

The spacecraft, launched on October 22, had been placed in the Lunar Transfer Trajectory on November four.

The mission, orbiting the earth at a distance of 3,86,000 km, was commandeered from ISRO's Telemetry, Tracking and Command Network (ISTRAC) at Peenya on the outskirts of the city with aid from the Indian Deep Space Network (IDSN) at Byalalu.

Chandrayaan-1 would now be lowered gradually and placed in a circular orbit at a distance of 100 km from the lunar surface.

Important milestone

The successful lunar orbit insertion(LOI) was an important milestone for the Rs 386 crore moon mission whose success depended on today's manoeuvre.

According to space experts, the challenging LOI was not without danger because it meant traversing through an area in which the gravitational forces of the earth and moon nearly cancel each other out.

Consequently, even a small deviation could send the spacecraft into a fresh course towards the moon or earth--or on a path leading into deep space.

Experts recall that about 30 per cent of unamnned moon missions of the US and former Soviet Union failed during LOI phase.

Two dish antennas

The lunar orbit manoeuvre was performed from Chandrayaan-1 Spacecraft Control Centre of ISRO Telemetry, Tracking and Command Network here, while Indian Deep Space Network (IDSN) at Byalalu on the city outskirts supported the crucial task of transmitting commands and continuously monitoring this vital event with two dish antennas, one measuring 18 metres and the other 32 metres.

Expressing happiness that the accuracy that ISRO got in regard to today's lunar orbit is really remarkable, Nair said nobody (space agency) would have secured such a precise orbit in the first attempt.

The achievement goes to the credit of ISRO, which has demonstrated its knowledge base in carrying out long trajectories under the influence of multiple bodies. "We (India) now have a big leadership as far as space is concerned", he said.

ISRO said the performance of all the systems on board Chandrayaan-1 is normal.

Nair said the Moon Impact Probe (MIP) on board Chandrayaan-1 would be released to hit the lunar surface around November 15. "Thereafter, systems (on board Chandrayaan-1) would be switched on one-by-one".

ISRO officials said the primary objective of 29-kg MIP is to demonstrate the technologies required for landing a probe at the desired location on the moon. Through this probe, it is also intended to qualify some of technologies related to future soft-landing missions. This apart, scientific exploration of the moon at close distance is also intended using MIP.

Of the 11 payloads, five are from India, three from European Space Agency, two from the US and one from Bulgaria.

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