The Hindu : National : SRE returns today

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Monday, Jan 22, 2007
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National

National

SRE returns today

N. Gopal Raj

Experiments to create new materials using the effects of microgravity were carried out

THIRUVANANTHAPURAM: India's maiden effort at a capsule for microgravity work, the Space capsule Recovery Experiment (SRE) that was launched by the Polar Satellite Launch Vehicle, is scheduled to return to Earth on Monday (January 22). As the SRE-I circled overhead for 12 days, two science experiments were carried out onboard the unmanned capsule.

Even at a height of a few hundred kilometres above the Earth, the planet's gravitational field is still about 90 per cent of its strength on the ground. A capsule in orbit is somewhat like a lift in a building that hurtles down out of control. The lift is then in free fall and its unfortunate occupants feel weightless for a brief period. In a space capsule, the conditions of free fall and weightlessness last as long as it remains in orbit. The capsule's inhabitants and contents experience a force that is only a tiny fraction of the normal tug of gravity on the surface of the earth.

Microgravity changes the way things happen in space. A candle's flame, normally teardrop shaped on the ground, becomes spherical. Liquids do not automatically take the shape of their container and, instead, turn into a blob. Particles suspended in a liquid do not settle down at the bottom.

Microgravity experiments can help improve our understanding of basic processes, according to Brij Kumar Dhindaw, professor in metallurgical and materials engineering at IIT Kharagpur. Professor Dhindaw was a co-investigator for two microgravity experiments carried out during Space Shuttle flights in 1996 and subsequently in 1997. (Kalpana Chawla, the Indian-born astronaut who died when the Space Shuttle Columbia disintegrated, had performed the second set of experiments.)

On earth, as molten metal or glass cool down and solidify, the hotter and lighter parts of the liquid rise to the top while the cooler and heavier parts sink. Such convection currents play a big role in determining the quality of the solidified material, pointed out Professor Dhindaw. The convection currents could not be eliminated when experiments were carried out on the ground and were difficult to model mathematically.

Under microgravity, on the other hand, solidification could take place without convection, he remarked. That gave a much clearer picture of the part played by convection currents during solidification. "A better understanding of how solidification occurs can help improve processing methods," Professor Dhindaw told this correspondent.

One experiment being performed onboard the SRE-I is for growing what is known as a quasicrystal in space. In an ordinary crystal, atoms repeat after a certain length, and the crystal therefore has a perfectly ordered arrangement of atoms, explains K. Chattopadhyay, chairman of the Department of Materials Engineering at the Indian Institute of Science in Bangalore, who is the principal investigator for the experiment. But it was possible to make crystals with an ordered arrangement of atoms that did not repeat. Such crystals were known as quasicrystals.

Quasicrystals are often very strong and wear-resistant, according to Dr. Chattopadhyay. These days, top-of-the-line electric shavers had a quasicrystal coating on their blades for increased life. For reasons that were not yet understood, quasicrystals were also non-wettable. So these materials were also being used as a long-lasting non-stick coating for frying pans.

"On Earth, it is very difficult to grow a perfect quasicrystal and we want to see if we can grow one in space," he told this correspondent.

Many quasicrystals are produced through a reaction between a liquid and chemical compounds in the form of fine particles. But as the reaction proceeds, the particles start to settle and their distribution in the liquid changes. That change affects the quality of the quasicrystal produced.

In microgravity, sedimentation does not occur. So, in space, "we should be able to grow a perfect, defect-free quasicrystal," explained Dr. Chattopadhyay. Besides, the reaction that produced quasicrystals was still only imperfectly understood. The experiment could also lead to insights on factors that influenced the reaction and how the reaction could be better controlled on earth.

Bone substitute

The other experiment on the SRE-I by the National Metallurgical Laboratory at Jamshedpur to develop a better bone substitute. It is hoped that in microgravity perfect nanocrystals of hydroxyapatite, a major constituent of bone, would form and self-assemble on a semi-solid framework. Uniformity in shape and size of the crystals, each about one-thousandth the thickness of a human hair, was difficult to achieve under normal gravity, said a scientist with the project.

After the SRE-I splashes down in the Bay of Bengal off Sriharikota, the materials produced by the in-flight experiments must be retrieved and analysed. Only then will the scientists know how well they have succeeded.

Although a second SRE has been sanctioned, it is not expected to fly before the end of the decade.

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