WASHINGTON – Whatever caused a Soyuz capsule to fall to earth like a rock on Saturday, scaring its crew of three and space enthusiasts around the world, has got NASA and its Russian engineers stumped.

There was smoke inside the Russian-built space capsule as it plummeted to earth. The crew of three – including U.S. astronaut Peggy Whitson – were tossed around in their seats, squeezed by a force of up to 10-times gravity. And they hit the ground hard, at least 260 miles off course.

They were, according to the Russian news agency Interfax, in mortal danger. The agency quoted an unidentified space official as saying part of the spacecraft had failed to separate from the crew capsule, meaning it entered the atmosphere with a hatch rather than its heat shield leading the way.

But as engineers on both sides of the world look for answers, one thing is clear: the ship that NASA is relying on to ferry astronauts to space after the shuttle retires has a serious problem – bad enough, critics say, that NASA should reconsider its plans for human spaceflight after 2010.

“If a problem occurs with the Soyuz, we are in big, big trouble,” said Rep. Dave Weldon, R-Fla., a leading critic of NASA’s plans to rely on the Russians and the sponsor of a bill to keep the shuttle fleet flying beyond 2010.

NASA plans to pay Russia to ferry American astronauts to the international space station until at least 2015, when the agency hopes to launch the shuttle’s successor.

The foundation of the plan is the Soyuz, a time-tested capsule with almost 90 consecutive safe landings over a 34-year period. Apollo 15 was on the launch pad at Kennedy Space Center the last time there was a Soyuz fatality.

What is perplexing engineers now is the cause of a second straight steep descent through the Earth’s atmosphere at an angle of more than 30 degrees, an approach known as a “ballistic” re-entry.

A Soyuz automatically makes a ballistic re-entry if its guidance systems fail.

“It’s clearly something that should not have occurred,” NASA associate administrator Bill Gerstenmaier told a press conference in Washington Tuesday. He had been in Russia for the landing.

The last Soyuz return, which carried two cosmonauts and a Malaysian space tourist, also made a ballistic re-entry in October 2007. The Russians blamed that on a broken cable.

A ballistic re-entry is tough on a crew but not necessarily dangerous. Gerstenmaier said he couldn’t confirm the Interfax report that the crew was in peril on Saturday.

He said the Russians suspect a faulty cable could again be the cause. Or possibly the explosive bolts that separate the crew capsule from an instrument compartment malfunctioned. It will take at least a month for investigators to analyze data from the capsule’s flight recorder, inspect the capsule and come up with possible causes, he said.

NASA critics are worried that the agency may be downplaying the incident, for fear that it could interfere with its request to buy more Soyuzes.

“I don’t know how to reliably interpret everything they (NASA) are telling me about things like this,” said Weldon.

In February, NASA chief Mike Griffin asked Congress to permit the agency to order additional Progress unmanned supply ships and Soyuz crew capsules form Russia.

NASA already has agreed to buy $700 million worth of services from Russia through 2011, when a congressional waiver of the Iran-North Korea-Syria Non-Proliferation Act expires. The law prevents NASA from buying space station-related goods and services from Russia as long as it continues to help Iran acquire missiles and other advanced weapons.

Now, space advocates say, pressure will increase on NASA to look for alternatives.

Officials in Brevard County and members of Florida’s congressional delegation have been pushing the Bush administration to give money meant for the Soyuz to U.S. commercial operators to develop an alternative. Last month Brevard County passed a resolution saying “resources to be spent on procuring Russian Soyuz…would best be devoted to the development and procurement of domestic crew and cargo logistic capabilities.”

As yet, however, no American-built replacement has been tested.