Tiny cracks were found in the propulsion systems of two space shuttles, prompting NASA to delay next month's planned flight of shuttle Columbia.

The launch was previously planned for July 19 and no new date was set. In announcing the delay Monday, NASA said it should be only a few weeks. Columbia was scheduled to carry Ilan Ramon, Israel's first astronaut, and six Americans on a 16-day mission to conduct scientific research.

The cracks, one-10th to three-10ths of an inch long, were found inside the main propulsion system's propellant lines on space shuttles Atlantis and Discovery. Some weren't visible to the naked eye.

The lines feed liquid oxygen and liquid hydrogen to the shuttle's engines.

"These cracks may pose a safety concern and we have teams at work investigating all aspects of the situation," program manager Ron Dittemore said. "This is a very complex issue and it is early in the analysis. Right now there are more questions than answers."

The cracks were first found on Atlantis last week. A weekend check of Discovery found similar cracks. The inspection of Columbia will require its engines to be removed, said James Hartsfield, a spokesman for the Johnson Space Center in Houston.

"The analysis of this situation is really in its infancy," Hartsfield said. "There's a lot we don't know yet."

No decision has been made yet on whether to inspect NASA's fourth shuttle, Endeavour, Hartsfield said. Bad weather in Florida forced Endeavour to land in California last Wednesday after a two-week mission to bring supplies and a new crew to the international space station.

Ramon is a payload specialist, NASA's label for a non-career astronaut. The main scientific experiment on the Columbia flight, from Israel, is to use a camera to study dust and other contaminants in Earth's atmosphere to see how they affect weather.

Because of the Israel connection, extra security was planned at Cape Canaveral for the launch, even though NASA says protection has been high for every space shuttle flight since the Sept. 11 attacks.

Among the six Americans in the crew is Kalpana Chawla, an engineer who was born in India.