The Columbia Accident Investigation Board is preparing to recommend as early as this week that NASA fix foam insulation problems linked to the shuttle tragedy before resuming space flights, an official close to the investigation said.

The recommendation might put a chill on the National Aeronautics and Space Administration's plans. Space agency officials have vowed to fly again by early next year, even though they have not figured out how to fix all the problems related to the foam debris.

Despite the investigation since the Columbia accident Feb. 1, the foam problem remains poorly understood and a huge challenge to the space program. A piece of foam hit Columbia during its launch, striking the orbiter's left wing and likely causing grave damage.

Investigators have determined that the insulating foam applied to the space shuttle tank has contained voids and other defects, but they have not been able to explain precisely why foam falls off during some launches but not others.

Last week, top NASA engineers concluded a preliminary review at a Lockheed Martin plant near New Orleans on proposals to fix at least part of the problem, but have not decided on a course of action, an agency spokesman said. At the same time, senior NASA officials have made optimistic projections about resuming flights as early as January.

Throughout the 20-year history of the program, NASA has made various attempts to reduce foam debris but has never succeeded in eliminating the problem, according to senior NASA officials who have testified before the safety board.

All the foam strikes violate the original safety rules for the shuttle, which said no debris should ever strike the thermal protection system. If Columbia investigators say NASA must abide by that rule, the agency might have a tough time complying.

Neil Otte, deputy manager for the tank program at NASA's Marshall Space Flight Center in Huntsville, Ala., said last month that he doubted that NASA could eliminate all foam debris coming off the external tank.

The Columbia investigation board is preparing to issue a number of interim recommendations this week to give NASA a head start on making fixes to the shuttle rather than waiting for the final report next month.