The Sun-observing SOHO spacecraft is experiencing a serious problem that reduces its ability to transmit data to Earth and could threaten the mission's science efforts.

The craft's high-gain antenna does not move properly, engineers said yesterday. In an internal NASA memo obtained by SPACE.com today, an official warned scientists to prepare for up to six weeks without images and data from the probe.

If the problem cannot be fixed, the spacecraft will at best be reduced to a much lower level of performance through the duration of its mission.

Officials expect to put the craft's several cameras and other instruments into a non-working "safe mode" for 4-6 weeks if the glitch is not fixed soon.

A NASA-led team of engineers said that after first noting the problem, they tried to get the antenna to point in a different direction "without any noticeable antenna movement."

SOHO (Solar and Heliospheric Observatory) sits partway between Earth and the Sun. Its ability to transmit data home depends on shifting the antenna's position periodically.

The glitch is likely a "mechanical problem with the antenna drive motor or mechanisms," according to a NASA statement issued yesterday. A NASA spokesperson confirmed by telephone today that the problem indeed appears to involve the motor.

Additional attempted movements suggested by the motor's manufacturer, Moog, were similarly unsuccessful. Engineers warmed the motor overnight hoping to unstick it. If the problem cannot be fixed, "we will probably lose high-rate (i.e., scientific) telemetry this weekend or early next week," the statement said.

Attempts to fix the problem were underway this morning, SOHO Deputy Project Scientist Paal Brekke said in a telephone interview. Results are expected by around 1-2 p.m. ET. If the warming does not work, engineers may decide to warm the motor further, or perhaps cool it, and try again.

SOHO experienced a very serious, unrelated navigation problem in 1998 that put it offline for about two months. The current problem should not doom the mission.

"This is also very serious, but we can still basic do science a large portion of the year," Brekke said.

Shifting the position of the entire spacecraft should allow the antenna to point properly about two months out of three. The craft travels in a large elliptical orbit. Soon it will be in a position that will thwart communication if the antenna can't be moved, Brekke explained. But in another part of the orbit, controllers could flip the craft upside down in order to point the antenna toward Earth.

Charged particles that generate colorful Northern Lights and have been known to knock out satellites might arrive with no warning.

SOHO will continue to maintain a low rate of contact via its low-gain antenna, but that is not useful for sending the high-resolution images of the Sun that SOHO collects continuously.

Meanwhile, engineers and other mission planners are in the process of organizing a teleconference to discuss the fate of the craft, according to the internal document. That meeting could take place mid-day Friday. The document recommends that a press release be issued before the weekend.

SOHO is a project of international cooperation between NASA and the European Space Agency. The probe was launched Dec. 2, 1995 and originally slated for a two-year mission, which was extended to 2003 so scientists could observe an expected peak in solar activity during 2001 and 2002.