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“Everything went exactly as planned, and on dot”

R. Ramachandran

Chandrayaan-1 frees itself from the Earth’s gravity and is captured by the Moon’s

NEW DELHI; The Indian Space Research Organisation (ISRO) achieved a milestone on Saturday when Chandrayaan-1 was successfully inserted into the lunar orbit following a complex manoeuvre (see The Hindu, November 8) that enabled it to free itself from Earth’s gravity and be captured by the Moon.

In this operation, once the spacecraft was at a distance of about 500 km from the Moon, the satellite orientation was turned around so that it faced in the direction opposite to the original orbit direction. The on-board retro rockets were then fired to slow down the spacecraft for it to be captured by the Moon’s gravity. The firing was carried out at 16:51 hrs IST, which lasted for 817 seconds, according to the ISRO press release. This, according to M. Annadurai, the Chandrayaan-1 project director, succeeded in applying a braking velocity of 366.8 m/s.

“Everything went exactly as planned, and on dot,” remarked Mr. Annadurai, who sounded happy and fully satisfied with the operation. While the exact orbit determination will take a couple of hours, the ISRO release gave the nominal orbit parameters as 504 km perilune (the nearest point from the Moon’s surface) and 7502 km apolune (the farthest point). According to Mr. Annadurai, the spacecraft’s period of revolution in this orbit is about 10.5 hrs.

Asked why the operation happened about half an hour earlier than the originally expected time of 17:30 hrs, Mr. Annadurai said the actual solar radiation pressure on the satellite — something which is only an estimate based on modelling — required that the torque to be applied on the satellite using the on-board momentum wheels to achieve the correct orientation was less than anticipated. Since the correct orientation could be achieved a little before 17:00 hrs itself and the position of the spacecraft was also correctly above the north-pole, the firing was carried out earlier, he said.

Mr. Annadurai added that the Doppler signals from the spacecraft also showed that the orbit achieved was exactly as planned with the orbit inclination being almost exactly 90 degrees (to the equator). This pole-to-pole orbit was being continuously monitored both from Byalalu, where ISRO had set up its 32 m deep space antenna, and from Canberra. “We had built in redundancy for monitoring the orbit. Both have confirmed that the orbit achieved is as desired,” Mr. Annadurai said. When asked about if there was any concern at any point of time, he said “No concern, but only anxiety because we were doing it for the first time.”

He further said that the next deboost manoeuvre that will take the spacecraft from the present Lunar Orbit of Insertion (LOI) to the first lower orbit LO-1, which will have a perilune of 125 km, will be carried out after two and a half revolutions in the present orbit.

“Most critical”

“This was the most critical manoeuvre which was successfully achieved to make it one of the best missions of ISRO,” said Dr. V. Adimurthi, the associate director of the Vikram Sarabhai Space Centre, who is closely involved in the project.

“The inclination achieved is almost exact and the trajectory correction manoeuvre required after insertion was minimal,” he said. “This was due to the fact the original lunar transfer trajectory itself was near perfect,” he added. “Now keeping the spacecraft in the lunar orbit is routine stuff.”

“We are extremely happy,” said S. K. Shiva Kumar, director, ISRO Telemetry, Tracking and Command (ISTRAC) center in Peenya, Bangalore. “It was a flawless operation. We have been able to achieve everything we wanted to do,” he added.

Rumours scotched

The achievement also quells the various rumours that were floating since Friday in internet blogs and websites like www.N2YO.com, which claimed that Chandrayaan was losing altitude and the orbit had gone astray. The point is that all these unofficial websites that claim to track satellites make use of the ‘Two Line Element (TLE)’ data set put out by the NASA/North American Aerospace Defence Command (NORAD) for each satellite. Now this data set is derived using a model that is strictly valid only for earth-bound satellites and not for deep space probes such as Chandrayaan. Once the spacecraft escapes the Earth’s gravity, the Keplerian orbit parameters derived from an Earth-bound model are no longer valid. Using the NORAD TLE data set in such situations invariably leads to wrong conclusions, a space expert points out.

Full text of ISRO press release

Chandrayaan takes pictures of moon

Chandrayaan 1: In depth

Chandrayaan-1 gets closer to moon

How Chandrayaan-1 is raised to higher orbits

Chandrayaan-1 bound for Moon

Chandrayaan’s orbit raised

“ISRO can put an Indian into space before 2015”

Off to the Moon: Editorial

‘Once Chandrayaan goes near the moon, we will be there to track it’

Going boldly where others have gone before

Corrections and Clarifications

The third paragraph in a report "'Everything went exactly as planned, and on dot'" (November 9, 2008) said ". the ISRO release gave the nominal orbit parameters as 504 km perilune (the nearest point from the Moon's surface) and 7,502 km apolune (the farthest point)." The accompanying graphic in a report "India's Moon mission a big success" (November 9, 2008, page 1) gave it as periselene and aposelene. The Science Correspondent clarifies that both mean the same thing. The terminology is derived from Latin and Greek respectively. Luna, Latin for moon, is the moon goddess in Roman mythology, whereas selene is the moon deity in Greek mythology.

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