Washington: Investigators believe that space shuttle Columbia began shedding pieces a full six minutes before it disintegrated over Texas on February 1, killing Kalpana Chawla and six other astronauts.

Radar images and pictures taken by astronomers and amateur sky-watchers confirm that the spacecraft began losing parts over California well before a puncture in the wing caused its disintegration, investigators said.

These pieces were probably so small that they might have burned up before reaching the ground, James Hallock, member of the Columbia Accident Investigation Board said in Houston.

Astronomers and sky-watchers on the West Coast had photographed the final moments of the shuttle.

"Obviously, it would be very important to understand what those pieces are, particularly the ones that started falling off at the very beginning," Hallock said.

The radar images coincide with video footage showing the partial disintegration of the ill-fated Columbia as it passed over California, Arizona and New Mexico, before bursting apart in flames above Texas, he said.

Meanwhile, NASA (National Aeronautics and Space Administration) amended the charter for probe into the accident and has allowed the Board to hire staff and experts from outside the space agency to validate the investigations carried out by the panel, a NASA spokesman said.

Shiela Widnall, an expert in aircraft turbulence and spiralling airflow will join the probe panel next week.

The new appointment and the charter amendment were done after the Board faced the ire of congressional critics who alleged that only military persons and those linked to NASA or the space agency chief were on the probe panel.

PTI