Artist's conception of SOHO spacecraft in orbit. Photo: ESA

The joint U.S.-European solar probe SOHO lapsed back into an emergency safe mode Wednesday as controllers were trying to right the $1 billion craft.

The trouble began Sunday when one of the Solar and Heliospheric Observatory's attitude control units reset, causing the satellite to enter Emergency Sun Reacquisition mode. The ESR mode protects the observatory by keeping the solar panels pointed at the sun so onboard batteries can be recharged.

Officials still do not know what caused the control unit to reset, said Bernhard Fleck, European Space Agency's SOHO project scientist. A similar event occurred in November 1997, he said.

By Monday, controllers had commanded SOHO out of ESR and was making progress to return the observatory to normal operations.

But on Wednesday, as the probe was performing an orbital stationkeeping maneuver needed to properly point the observatory, a yet-unknown problem caused the onboard thrusters to shut down eight minutes into a planned 50-minute firing, Fleck said.

The craft was slowly rolling, which should not occur, and communications between the probe and Earth were lost. As SOHO reentered ESR mode, the satellite began transmitting via its low-gain antenna and contact with Earth was reestablished.

"We are analyzing telemetry and trying to determine what happened," Fleck said.

SOHO was launched four years ago Thursday aboard an Atlas rocket from Cape Canaveral Air Station in Florida.