While different theories were doing the rounds, a 42-kg, 4.57 billion-year-old meteorite crashed into Piplia Kalan, a village in Pali district on Rajasthan on June 20, 1996. As awe-struck villagers collected splintered meteorite as souvenirs from space, a Geological Survey of India (GSI) arrived and collected about 30 kgs of the meteorite rocks.

Then a PRL team went to Piplia Kalan and brought some fragments of the meteorite that lay stuck in the soil. When a PRL team of planetary scientists Gopalan Srinivasan, Jitendra Nath Goswami and Narendra Bhandari, set out to work, they soon realised that the meteorite probably contained a key to the mystery of meltingasteroids.

After two years of research, the team discovered the first direct evidence that the asteroids melted because they once contained enough of radioactive isotope aluminum-26. The heat generated by the radioactive decay of aluminum-26 into magnesium-26 was the cause of the meltdown. The Piplia Kalan meteorite is believed to be a fragment chipped off a giant asteroid called 4-Vesta, which is believed to be about 500 km in diameter and has a kilogram mass of five with eighteen zeros after it.

Harold Urey, Nobel laureate in chemistry, had proposed in 1955 that the heat to melt asteroid may have come from radioactive aluminum turning into magnesium. But till now no concrete evidence had been found to support that theory.

The results of the team's works were published in the May 21 issue of the prestigious Science magazine of the American Association for the Advancement of Science. The Indian team's work has been hailed as a breakthrough.

There is another important aspect to the team's work. Tillnow space scientists believed that proto-planets or early planets were formed some 20 million years after the formation of the solar system. The team's evidence suggests they were formed within 5 million years of the formation of the solar system. This means that planets were formed much earlier than supposed.

But why did the Piplia Kalan asteroid alone lead to the breakthrough. After all, there are some 15,000 known meteorites in the world. ``The reason'', says Srinivasan, ``is that only about two dozen of so of these meteorites are differentiated ones, meaning ones that have melted and crystallized on a planetary body.''

And how was the calculation made? Aluminum-26 is a radioactive isotope that cannot be detected directly. The quantity of aluminum-26 which was present in meteorites can only be inferred by the amount of magnesium-26 left behind by the decay of the former isotope. Finding more magnesium-26 than is usual on earthly rocks led the scientists to the conclusion that the Piplia Kalan meteoritewas indeed a chip off 4-Vesta, and that the heat that caused the differentiation came from the radioactive decay of aluminum-26.