NASA officials say a broken antenna motor is about to silence the $1 billion SOHO spacecraft, one of the world's most complex and successful solar observatories.
The failure will cause a 19-day loss of signal once every three months. It will interrupt some scientific research and crimp the system that warns of approaching geomagnetic storms.
FOR THE RECORD - An article in yesterday's Sun reported that a swivel aboard the SOHO spacecraft allows its high-gain antenna to widen its beam sweep from 14 degrees to 25 degrees. The actual maximum sweep is 25 degrees in each direction, or 50 degrees total. The Sun regrets the error.
Sweeping outward from the sun, these storms can damage satellites, endanger spacewalking astronauts and disrupt communications and power distribution on the ground.
SOHO's outage is "a pretty big deal," said Chris Balch, lead forecaster at the federal Space Environment Center in Boulder, Colo. "As forecasters we had come to rely on SOHO."
Without it, they'll have to rely on less capable instruments on the ground and in orbit. Information on new solar eruptions will be less comprehensive, and confidence in the forecasts will be lowered, Balch said.
"At least it's not a complete blackout. That's good," he said.
The spacecraft itself is still safe, said Paal Brekke, deputy SOHO project scientist at the NASA Goddard Spaceflight Center in Greenbelt. And some solar observations might continue during the periodic blackouts, with data recorded for playback later, officials said.
But the data losses will hinder scientific research measuring the sun's total radiance over time, as well as observations involving helioseismology, the study of waves rippling across the sun's surface.
Engineers will use SOHO's 19-day "vacation" to continue investigating the failure and find a way to minimize the loss of data. If there's no improvement, Brekke said, "we'll lose 20 percent of our observing time every year, which is not too bad. We can live with that."
The Solar and Heliospheric Observatory was built by the European Space Agency and launched by NASA on Dec. 2, 1995.
It was rocketed to an orbit circling the "L1" Lagrangian point - a spot 1 million miles sunward from Earth where the gravitational pull of Earth and the sun balance each other.
From there, SOHO's 12 scientific instruments, X-ray and ultraviolet telescopes have enabled scientists to measure huge eruptions of gas, called coronal mass ejections, that can cause auroras in Earth's upper atmosphere and geomagnetic storms when they're directed toward Earth.
"SOHO is the only spacecraft that can see coronal mass ejections," Brekke said. "It's been the most important solar storm watchdog."
Scientists also study the sun's interior temperature and dynamics; measure temperatures in the sun's corona; identify the atomic particles blowing outward from the sun, and measure their speed and density.
SOHO has even been a hit with the public. Brekke said its Web site averages 12 million hits a month from comet hunters and people looking for screensavers and unusual solar phenomena.
When SOHO's real-time solar images went online, astronomers began to notice a number of small comets, called "sungrazers," drifting past the sun. They had never been visible before SOHO's coronagraph masked the sun's glare.
Since its launch more than seven years ago, SOHO has discovered more than 620 sungrazers, Brekke said, more than any other observatory or astronomer on the ground.
"More than 70 percent have been discovered by amateurs sitting in their living room and downloading images from the Web," he said.
The first of these "La-Z-Boy observers" to report a new sighting to the International Astronomic Union can have the comet named after himself.
SOHO's latest troubles began May 4, when Goddard controllers noticed the high-gain antenna was not pointed properly. Communications were switched to a low-gain antenna that is always in contact with Earth, but it can't handle the large data rates needed to transmit SOHO's scientific data.
Scientists later found that a motor that swivels the high-gain antenna horizontally had failed. The swivel widens the antenna's "view" from 14 degrees to 25 degrees - enough to keep Earth within the beam as SOHO orbits the L1 point. Without it, the beam periodically sweeps beyond the reach of receiving antennas on Earth, cutting off the signal.
The reasons for the breakdown are unknown, Brekke said, but a failed gear and old age are suspects. SOHO was designed for a two-year mission and has survived 7 1/2 years. "This motor has been used basically every day since launch," he said.
SOHO has been in trouble before. It was lost for about three months in 1998 after the failure of gyroscopes needed to keep its antennas pointed toward Earth.
"That was much more serious. We didn't know if we would even be able to locate it," Brekke said. Engineers at Goddard eventually regained contact. They reprogrammed SOHO's computer to use its star-tracking instruments to keep the spacecraft pointed properly. "It was one of the most amazing rescues in space history," he said.
SOHO's Web site is http:// sohowww.nascom.nasa.gov
