In a historic event, the Indian space program achieved a unique feat on November 14, 2008 with the placing of Indian tricolor on the Moon's surface. The Indian flag was painted on the sides of Moon Impact Probe (MIP), one of the 11 payloads of Chandrayaan I spacecraft that successfully hit the lunar surface at 20:31 hrs on November 14, 2008. This is the first Indian built object to reach the surface of the Moon. The point of MIP's impact was near the Moon's South Polar Region. Weighing 34 kg at the time of its launch onboard Chandrayaan I, MIP carried a video imaging system, a radar altimeter, and a mass spectrometer. The video imaging system took the pictures of the Moon's surface as MIP approached the lunar surface. The radar altimeter measured the rate of descent of the probe and the mass spectrometer made a detailed study of the extremely thin lunar atmosphere.

India has joined a select band of countries who have undertaken lunar missions by launching the first unmanned mission to the Moon, Chandrayaan I. In a historic flight conducted from Satish Dhawan Space Centre (SDSC), Sriharikota on October 22, 2008, the Polar Satellite Launch Vehicle, PSLV-C11, successfully launched the 1380 kg Chandrayaan I spacecraft into a transfer orbit with a perigee of 255 km and an apogee of 22,860 km, inclined at an angle of 17.9 degree to the equator.

PSLV-C11 used for launching Chandrayaan I is the updated version in its standard configuration. Weighing 316 tons at lift-off, the vehicle used larger strap-on motors (PSOM-XL) to achieve required higher payload capability. PSOM-XL uses 12 tons of solid propellants instead of 9 tons used in the earlier configuration of PSLV. PSLV is a four stage launch vehicle employing both solid and liquid propulsion stages. PSLV is the trusted workhorse launch vehicle of ISRO with 13 consecutively successful flights.

Chandrayaan I further expands our knowledge about Earth's only natural satellite - the Moon. With well-defined objectives, Chandrayaan I mission intends to put an unmanned spacecraft into an orbit around the Moon and to perform remote sensing of our nearest celestial neighbor for about two years using eleven scientific instruments built in India and five other countries.

The primary objectives of Chandrayaan I are:

• To upgrade the technological base in the country.
• To place an unmanned spacecraft in an orbit around the Moon.
• To conduct mineralogical and chemical mapping of the lunar surface.

Chandrayaan I aims to achieve these objectives through high-resolution remote sensing of Moon in the visible, near infrared, microwave, and X-ray regions of the electromagnetic spectrum. A 3-dimensional atlas of the lunar surface and chemical and mineralogical mapping of entire lunar surface will be prepared.

The Spacecraft. Chandrayaan I spacecraft weighing 1380 kg at the time of its launch is a 1.5 m cuboid. The spacecraft is powered by a single solar panel generating electrical power of 700 W. A Lithium ion battery supplies power when the solar panel is not illuminated by the Sun. To make Chandrayaan I spacecraft travel toward the Moon, its Liquid Engine is used. Liquid propellants needed for the engine as well as thrusters are stored onboard the spacecraft. Chandrayaan I spacecraft's dual gimballed antenna transmits the scientific data.

The Payloads. There are 11 payloads (scientific instruments) through which Chandrayaan I intends to achieve its scientific objectives. They include five instruments designed and developed in India, three instruments from European Space Agency (one of which is developed jointly with India and the other with Indian contribution), two from the United States, and one from Bulgaria.

The Indian payloads of Chandrayaan I are:

Terrain Mapping Camera (TMC), a CCD camera that maps the topography of the Moon, which helps in better understanding of the lunar evolution process.

Hyperspectral Imager (HySI), another CCD camera, is designed for mapping of the minerals on the lunar surface as well as for understanding the mineralogical composition of Moon's interior.

Lunar laser ranging instrument (LLRI) provides necessary data for accurately determining the height of lunar surface features.

High Energy X-ray Spectrometers (HEX) are designed to help explore the possibility of identifying Polar regions covered by thick water-ice deposits as well as help in identifying regions of high uranium and thorium concentrations.

Moon Impact Probe (MIP) demonstrates the technologies required for landing a probe at the desired location on the Moon. It is also intended to qualify some of the technologies related to future soft landing missions.

The Journey

After circling the Earth in its transfer orbit, Chandrayaan I spacecraft was put into elliptical "extended transfer orbits" by repeatedly firing its liquid engine in a pre-determined sequence. Subsequently, the liquid engine was again fired to make the spacecraft travel to the vicinity of the Moon by following a path called "Lunar Transfer Trajectory (LTT)."

When the spacecraft reached the vicinity of the Moon and passed at a few hundred kilometers from it, the spacecraft's liquid engine was fired again so that the spacecraft slowed down sufficiently to enable the gravity of the Moon to capture it into an elliptical orbit. Following this, the height of the spacecraft's orbit around the Moon was reduced in steps. After careful and detailed observation of the orbit perturbations, the orbital height of Chandrayaan I was finally lowered to its intended 100 km height from the lunar surface. Moon Impact Probe was ejected from Chandrayaan I spacecraft at the earliest opportunity to hit the lunar surface in a chosen area. Later, cameras and other scientific instruments were turned "on" and tested. This leads to the operational phase of the mission which lasts for about two years during which Chandrayaan I spacecraft explores the lunar surface with its array of instruments that includes cameras, spectrometers, and SAR.

The Ground Segment. ISRO Telemetry, Tracking and Command Network (ISTRAC) established the ground segment of Chandrayaan I which consists of the following:

The Indian Deep Space Network receives the data sent by Chandrayaan I spacecraft. Besides, it sends commands to the spacecraft at a power level of up to 20 kW. IDSN consists of two large parabolic antennae - one with 18 m diameter and the other 32 m diameter - at Byalalu, situated at a distance of about 35 km from Bangalore. Of these, the 32 m antenna with its ‘seven mirror beam waveguide system', was indigenously designed, developed, built, installed, tested, and qualified. The 18 m antenna can support Chandrayaan I mission, but the 32 m antenna can support spacecraft missions well beyond the Moon.

The Spacecraft Control Centre located near the ISTRAC campus at Peenya, North of Bangalore, is the focal point of all the operational activities of Chandrayaan I during all the phases of the mission.

The Indian Space Science Data Centre forms the third element of Chandrayaan I ground segment. Also located at Byalalu, ISSDC receives data from IDSN as well as other external stations that support Chandrayaan I, stores, processes, archives, retrieves, and distributes scientific data sent by Chandrayaan I payloads to the user agencies.

Conclusions

The cameras on board Chandrayaan I, namely, the terrain mapping camera (TMC) and hyper-spectral imager (HySI) have been switched on and excellent quality pictures of the lunar surface have been obtained. All the payloads are operating satisfactorily. Chandrayaan I has successfully demonstrated the country's capability and expertise in accomplishing highly complex space missions. The success of Chandrayaan I has paved the way for undertaking missions to the Moon and beyond.