Friday, Jun 26, 2009
- The search for deposits of water is high on the agenda of the Lunar Reconnaissance Orbiter and the Lunar Crater Observation and Sensing Satellite. Indias Chandrayaan-1 probe, too, may join the quest.
Forty years after man first set foot on the Moon, the United States has despatched two unmanned lunar spacecraft to Earths natural satellite to pave the way for humans to return there. The search for deposits of water is high on the agenda of the Lunar Reconnaissance Orbiter (LRO) and the Lunar Crater Observation and Sensing Satellite (LCROSS). Indias Chandrayaan-1 probe, too, may well join the quest.
Discovering water on the Moon would be like finding a gold mine, said U.S. space agency NASA in a recent press document. It estimates that getting a bottle of water to the Moon would run to about $50,000 (around Rs. 24 lakh) at current launch costs. So the ability to extract water locally would be immensely useful if humans want to establish bases on the celestial body.
It is believed that water could have been brought to the Moon by comets and meteorites that have crashed on its surface over billions of years. Likewise, hydrogen ions streaming out from the Sun might have combined with oxygen from chemical compounds in the lunar soil and turned into water. The question is whether all this water has boiled off in the face of the Moons scorching day time temperatures and its low gravitational hold.
In a paper published in 1961, three scientists at the California Institute of Technology put forward the idea that water may well be present in appreciable quantities in shaded areas in the form of ice. The paper appeared in the Journal of Geophysical Research around the same time President John F. Kennedy committed the U.S. to landing a man on the Moon.
Some 30 years later, two U.S. space probes that went to the Moon, Clementine and Lunar Prospector, provided evidence that water might persist as patches of ice mixed with soil at the bottom of craters at the poles. Sunlight never reaches the bottom of some craters at the lunar poles, which therefore remain at temperatures far below the freezing point of water. So these would be ideal locations for trapping water ice on the Moon.
But the evidence has been disputed and scientists continue to argue vigorously about whether or not Earths nearest neighbour holds any water.
Last Friday, the LRO and the LCROSS were launched from Cape Canaveral in Florida. The two spacecraft, along with Chandrayaan-1, will undoubtedly throw a great deal of new light on the issue. The LRO entered the lunar orbit on Tuesday. Once the spacecraft is commissioned, a whole slew of instruments on it will look for signs of water ice and hydrogen in different ways.
Meanwhile, the LCROSS and the spent upper stage of the Atlas rocket that launched the two spacecraft have swung past the Moon for the first time. NASA plans to send the empty upper stage, weighing over 2,000 kg, hurtling into a crater near the lunar south pole at a speed of about 9,000 km per hour. The impact, scheduled for October 9, is expected to hurl vast quantities of soil from the bottom of the crater to a height of several km. The plume of debris will then catch the sunlight, making it possible for instruments on various spacecraft and telescopes on the ground to analyse material that has lain hidden in the crater for billions of years.
The LCROSS will separate from the upper stage some hours before the latters final plunge to the Moon. Following four minutes behind the upper stage, the spacecraft will fly through the debris plume created by the crash and relay the data collected by its instruments to Earth. The spacecraft too will then slam into the Moon, creating a second debris plume.
Chandrayaan-1 is busy surveying the Moon. Looking for water is one of its tasks too. In addition, the Indian lunar probe and the LRO could carry out a complicated radar-based duet.
One body of evidence that favours the presence of water ice on the Moon comes from bistatic radar observations made with the Clementine probe in 1994. In bistatic radar, the radio signal is emitted from one location and the return echo picked up at another place. In the case of Clementine, the radar on the spacecraft aimed its signal at the Moons south pole and the signal that bounced back was received by an antenna on the Earth.
Now with both Chandrayaan-1 and the LRO equipped with radars, for the first time, there is a chance to carry out bistatic radar observations using two satellites in the lunar orbit.
The Mini-RF radar on the LRO is a more advanced version of the U.S.-supplied Mini-SAR on the Indian spacecraft. Moreover, the radars on Chandrayaan-1 and LRO were designed to operate cooperatively in a bistatic mode, with Chandrayaan-1 transmitting and LRO receiving, observed Paul Spudis and others of the Mini-SAR team in a paper published in Current Science earlier this year.
Such bistatic radar observations could provide the best evidence for water ice on the Moon, Dr. Spudis told this correspondent when he was in India for the Chandrayaan-1 launch last year.
Both the U.S. and Indian scientists are known to be enthusiastic about using Chandrayaan-1 and LRO for bistatic studies. But the logistics of how to go about it must first be agreed upon by the space agencies of the two countries.
If both spacecraft are in the same orbit, their radars will be able to work together for extended periods of time, allowing larger areas to be mapped in this fashion. On the other hand, if the satellites are in different orbits, bistatic observations are only possible when the two spacecraft are so aligned that radar emissions from one can bounce off the Moons surface and be received by the other.
Chandrayaan-1 is currently orbiting the Moon at a height of about 200 km. The LRO, on the other hand, is intended to work at a height of just 50 km. Now that the LRO is in the Moons gravitational clutches, NASA plans to initially hold the spacecraft in an elliptical commissioning orbit of 30 km by 216 km for about 60 days. During this period, the spacecraft will be checked out and its instruments tested. One possibility is for the bistatic observations to be carried out some time during this commissioning phase or shortly afterwards.
Although a news report that appeared recently in Nature suggested that bistatic observations using LRO and Chandrayaan-1 would take place this summer, there has so far been no official word on the matter from the two space agencies.
In 1998, NASAs Lunar Prospector spacecraft measured the energy of neutrons coming off the Moons surface and found indications of hydrogen at the poles. The data were consistent with deposits of hydrogen in the form of buried water ice, said William Feldman of the Los Alamos National Laboratory and his colleagues in a paper in the journal, Science.
Last year, Vincent Eke of Durham University in the U.K. and others published research that reanalysed the Lunar Prospector data. When the Lunar Prospector made a map of the hydrogen abundance, it essentially took a blurred image, said Dr. Eke in an email. He and his colleagues had used a sophisticated image reconstruction algorithm to undo the blurring.
The new result that we have found is that the hydrogen, which Lunar Prospector discovered about 10 years ago, is not merely near to the lunar poles but it is actually concentrated in the permanently shadowed polar craters, he told this correspondent in the email.
If hydrogen exists in the form of water, Dr. Eke and his colleagues estimate, the top one metre of soil in the lunar craters could be holding many billions of litres of water. The question now is: what will Chandrayaan-1, the LRO and the LCROSS find?
The Hindu : Opinion / Leader Page Articles : Searching for water on the Moon
