The NASA commission investigating the space shuttle Columbia disaster will be expanded by three members and given additional assurances of independence amid calls on Capitol Hill for President Bush to appoint a new panel to take over the probe and report directly to him and Congress.

Sheila Widnall, a Massachusetts Institute of Technology aerospace scientist and secretary of the Air Force during the Clinton administration, is one of the three new members who will be formally announced early next week, according to congressional sources. Widnall confirmed yesterday that she was joining the commission.

Retired Adm. Harold W. Gehman Jr., chairman of the shuttle commission, told reporters that he intends to add experts in areas including high-altitude aerodynamics, thermal engineering and civilian government contracting.

"NASA is a great organization and we have no doubts about their competence," Gehman told reporters in Huntsville, Ala., en route with other board members to the Marshall Space Flight Center, which oversees shuttle propulsion systems. "But when something bears directly on our report . . . we will go out and get that technically and independently verified."

Adding expertise in aerodynamics (the study of forces exerted by air or other gases in motion) and in thermal engineering (the study of heat transfer) could help the board figure out why the shuttle came apart. Widnall holds advanced degrees in aeronautics and astronautics from MIT and has done extensive research into fluid dynamics, the behavior of fluids and gases.

Gehman said the original nine-member board appointed by NASA needed to be expanded to broaden the range of expertise and because "we are becoming aware of the size of the job, the amount of work that needs to be done."

But Gehman and NASA officials are also responding to intense pressure from Congress to expand the commission and rewrite its charter to ensure the investigation is totally independent of the space agency that created it.

NASA Administrator Sean O'Keefe has rewritten the commission charter twice within a week to mollify congressional critics and to remove any requirements -- real or perceived -- for the board to coordinate its investigation with NASA officials. O'Keefe will announce additional changes to the charter next week, according to congressional sources. A NASA spokesman said he was uncertain when O'Keefe might take additional action.

Rep. Bart Gordon (Tenn.), ranking Democrat on the House Science Committee, Rep. F. James Sensenbrenner Jr. (R-Wis.), former chairman of the Science Committee, and Sen. Byron L. Dorgan (D-N.D.) this week called for the appointment of a presidential commission similar to the one that investigated the 1986 Challenger accident to supersede the Gehman panel.

Sensenbrenner, who is now the chairman of the Judiciary Committee, has told White House officials "in the strongest terms" that whatever the Gehman commission concludes will lack the credibility of the findings of a truly independent commission, such as the one headed by former secretary of state William P. Rogers that investigated the 1986 Challenger explosion.

The White House yesterday voiced support for the Gehman commission, but a spokesman said the president hasn't ruled out the possibility of appointing a new panel to assume control of the probe.

"From the beginning, we've never ruled anything in or out," said spokesman Scott McClellan. "But we have made it clear that we believe it's important for everybody to support the independent, external [NASA] board in its investigation to uncover all the facts and make sure no stone is left unturned."

"We think this system put in place is working, and we're supportive of it," he said.

Widnall, the only woman to ever head a military service, said that she is confident the board will be given total freedom to pursue all leads.

"I believe all data, all resources, all technical information and expertise will be available to us," Widnall said. "I anticipate we will be completely free to pursue the data and develop alternative scenarios and receive answers to our questions to our satisfaction as we come to our conclusions."

Staff writer Renae Merle at Cape Canaveral contributed to this report.