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Wednesday, Apr 07, 2004

Opinion

Opinion - Leader Page Articles

The Moon mission

By N. Gopal Raj

The Chandrayaan-1 will attempt high-resolution mapping of the lunar surface's chemical composition.

IN THREE TO four years' time, a Polar Satellite Launch Vehicle (PSLV) will lift-off from the Satish Dhawan Space Centre, Sriharikota, carrying India's first scientific probe to another celestial body. The Chandrayaan-1 is expected to spend two years circling the Moon, during which time the probe's onboard instruments will map the lunar surface and analyse its chemical constituents in great detail.

The Cold War period saw the heyday of lunar exploration, when the United States and the Soviet Union competed fiercely in space. Even before the first humans set foot on the Moon in 1969, the Soviet Union and America had despatched 40 spacecraft (some of which failed) to the Earth's natural satellite. The numerous Luna, Ranger, Zond, Surveyor, and Lunar Orbiter missions mapped the Moon from orbit and also landed on its surface for more detailed analyses. Apart from the lunar samples brought back by the Apollo astronauts who landed on the Moon, the Soviet Union used robotic landers and later rovers to collect and bring back moon rocks. But once the race to send men to the Moon was over, both countries appeared to lose interest. Apollo 17 in 1972 was the last time humans landed on the Moon; four years later, the Soviet Union's Luna-24 returned with about 170 grams of rock and soil collected from the Mare Crisium (Sea of Crisis).

Almost a decade-and-a-half would pass before another spacecraft, the Hiten mission from Japan, visited the Moon. Then the U.S. sent two scientific satellites to the Moon, the Clementine in 1994 and the Lunar Prospector in 1998. Now there is a resurgence of interest. Last year, the European Space Agency launched its first Small Mission for Advanced Research in Technology (SMART) satellite. Japan is expected to send two satellites within the next year or so. The Lunar-A will carry two penetrators, which will be dropped so that they burrow into ground and relay back quake and thermal data that can be used to understand the internal structure of the Moon. The SELENE will carry a wide range of instruments to map and analyse the Moon's surface and sub-surface structures from orbit.

Both China and India have announced that they will be sending scientific probes to the Moon. The Chinese one is to be launched by 2006 as a prelude to unmanned and later manned landings. With President George W. Bush having announced his vision for returning to human exploration of the Moon, the U.S. expects to send a series of robotic missions to the lunar surface, starting from 2008, to prepare for the manned missions.

Despite all the data gathered by satellites, the lunar samples that were brought back and analysed, along with lunar meteorites collected from Antartica, the "Moon is still a very mysterious object," says Narendra Bhandari of the Physical Research Laboratory in Ahmedabad who has been closely involved in formulating the scientific investigations that the Chandrayaan-1 will carry out.

The Moon's origin is still a major mystery. How did the Earth acquire such a large satellite that in many ways shares the composition of its outer layer, the mantle? Currently, the most widely accepted hypothesis is that a gigantic body the size of Mars slammed into the Earth about four-and-a-half billion years ago when it was not fully formed. The force of the impact flung vast quantities of molten rock and debris into space, and these later solidified to form the Moon. Although this hypothesis explained many things about the Moon, there was till much that it left unexplained, pointed out Dr. Bhandari during a talk at the recent National Space Science Symposium in Kottayam.

The Chandrayaan-1 will attempt high-resolution mapping of the chemical composition of the lunar surface materials. A low energy x-ray fluorescence spectrometer that will detect x-rays emanating from the lunar surface and is expected to reveal the abundance of elements like magnesium, aluminium, silicon, calcium, titanium and iron. A hyperspectral imager, capable of differentiating light of various colours, will be used to identify many minerals. This data is expected to be useful not only in understanding the Moon's chemical makeup, but also give clues to its formation and evolution.

Although water can be expected on the Moon, not even trace amounts of it could be found in the lunar rocks and soils that have been analysed. The sunlit side of the Moon is excessively hot (about 130 degrees C) while the dark hemisphere is bitterly cold (minus 170 degrees C). So, it is thought that water and other volatile materials could have evaporated and ultimately got concentrated at the poles, which never see sunlight. This seemed to have been borne out when the Lunar Prospector found signals indicative of hydrogen-bearing compounds around the poles, which could be water in the form of ice. If it is indeed water, there could be two billion tonnes of it on the Moon, according to one estimate. The presence of such large quantities of water can be a decisive factor for a permanent manned base on the Moon. The Chandrayaan-1's high energy x-ray/gamma-ray spectrometer will detect certain radioactive elements that can be used to study the movement of volatile materials.

Images from the Chandrayaan-1's terrain mapping stereo camera will allow a high-resolution atlas of the Moon to be prepared. There will also be a laser ranging equipment onboard so that the satellite's precise height above the lunar surface can be determined, which is a key input required for using the data from the camera and other instruments.

The simultaneous high-resolution mapping of the moon's minerals, chemicals and geology that the Chandrayaan-1 would carry out had not been done before, observed Dr. Bhandari in his talk at Kottayam. Moreover, the Chandrayaan-1, along with the satellites sent by other nations, would provide prolonged and continuous scientific observations of the Moon, which could help resolve many outstanding questions, he added.

"The various sensors proposed for the Indian lunar mission all have merit," agrees Carle Pieters of the Planetary Geosciences Group at Brown University in the U.S., an experienced researcher who has been involved with remote sensing of the moon for decades. The combined X-ray experiments make a solid package. A hyperspectral imager has never been flown to the Moon and has exceptional value. In an email, she also pointed out that a high resolution camera for terrain mapping was an essential asset. The low-resolution global topography of the moon provided by the Clementine satellite was enormously interesting as the first investigation, but completely inadequate for serious exploration.

The proposed Indian mission would be a major improvement in the current data available for the moon and was likely to provide significant breakthroughs, says Paul Lucey of the Hawai'i Institute of Geophysics and Planetology. On the negative side, the European SMART-1 may obtain a portion of the data proposed by the Indian mission and the Japanese SELENE mission would collect a very similar, but not identical, data set. However, these missions had not begun collecting data and there was no way to know if they would be successful.

In planning the Chandrayaan-1 mission, the Indian Space Research Organisation has opted to use its proven capabilities as far as possible. The Chandrayaan-1 satellite will be based on the same basic satellite structure as the Kalpana-1, India's first dedicated meteorological satellite. As with Kalpana-1, the PSLV will first place the Chandrayaan-1 in an oval orbit. The Chandrayaan-1's onboard rocket motor will be used to move the satellite into more elliptical orbits that will take it farther away from the earth till it is captured by the Moon's gravity.

The Chandrayaan-1 would have to be designed for considerable onboard autonomy, according to M. Annadurai, the mission's project director. Crucial manoeuvres, including entry into lunar orbit, could occur when the satellite was not being monitored and controlled by the ground station, he pointed out at the Kottayam meeting.

At the Kottayam meeting, both Dr. Bhandari and Mr. Annadurai said that more missions must follow Chandrayaan-1. In his presentation, Mr. Annadurai pointed out that a lunar satellite launched by the PSLV would weigh only 500 kg when it entered into orbit around the Moon. The current generation Geosynchronous Satellite Launch Vehicle (GSLV) could increase that to 780 kg. The next generation GSLV Mark-III, which ISRO is currently developing, would be able to send 2.5 tonnes to the moon and that would suffice for a lunar lander mission. The Indian launch vehicles could be used to send scientific satellites to Mars and Venus as well.

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