An antenna glitch has forced NASA engineers to put the sun-watching SOHO spacecraft to sleep for at least two weeks, causing a data outage that will temporarily suspend its use as a space weather forecasting tool.
The move comes about a week after SOHO engineers realized the spacecraft's high gain antenna was not working properly. It also comes the day after the fifth anniversary of SOHO's first brush with death, when software changes almost resulted in loss of the spacecraft in 1998.
"Not only did we get [SOHO] back then, but we got it back with 98 percent of the instruments undamaged," said Joe Gurman, U.S. project scientist for the Solar and Heliospheric Observatory (SOHO).
Engineers tracked the current problem to SOHO's high-gain antenna pointing motor responsible for keeping the transmitter pointed at Earth and the data flowing. The antenna is used to beam sun images and observations to Earth from the craft's 12 onboard instruments. Normally, it moves both horizontally and vertically, but recently the horizontal movement stopped working properly.
Telemetry from SOHO was lost Thursday at 7:50 p.m. EDT (Friday, June 27, 1:50 a.m. GMT), during a contact through the DSS-46 26-meter antenna of the Deep Space Network (DSN). The spacecraft entered a telemetry "keyhole" -- a window of time where, due SOHO's orbit, communication from the antenna is not possible -- expected to last until July 14.
Gurman told SPACE.com that SOHO engineers have been in continuous contact with the antenna motor's manufacturer to better understand the device's failure and hopefully find some way to repair it. Engineers did try to fix the problem by first heating, then cooling the motor.
"There have been a series of attempts to use temperature to repair the motor, but these have all been unsuccessful so far," Gurman said.
SOHO should be in "sleep mode" from anywhere between 18 to 20 days as the craft's orbit causes the high-gain antenna to turn away from Earth. Although researchers will experience a data outage during that time, ground controllers will use one of two onboard low gain antennas on SOHO to monitor the spacecraft's health and safety.
Gurman said ground controllers will command SOHO to flip 180 degrees during the outage to reposition the high-gain antenna so it points toward Earth later in the craft's orbit and can resume sending data home.
Launched in 1995, SOHO is a fundamental tool in space weather forecasting because of its constant eye on the Sun. Space weather scientists use the craft to detect coronal mass ejections, solar phenomena that can knock out satellites and interfere with communications. The mission is the result of collaboration between NASA and the European Space Agency (ESA).
An unintended anniversary
The recent difficulties with SOHO aren't the first headaches for mission scientists. This month, June 25 to be exact, marks the five-year anniversary of a time when scientists almost lost the spacecraft.
In 1998, changes in the way software functioned onboard SOHO inadvertently caused flight controllers to send the craft farther away from Earth and put it in a flat spin. The spin turned SOHO's solar panels edge-on to the Sun, and the spacecraft lost power, researchers said.
Project scientists had to use the Arecibo Radio Telescope in Puerto Rico and California's Goldstone telescope to track SOHO and send commands. Eventually, the craft's solar panels began to receive sunlight and NASA engineers regained control of the spacecraft.
The close call aside, SOHO has already surpassed its original mission lifetime. NASA and the ESA officials originally sent the craft on a two-year mission, and many researchers didn't expect SOHO to function more than five years let alone almost eight.
"It's really exceeded expectations," Gurman said of SOHO. "Of course that doesn't mean it's not important, because people depend on it."
In addition to monitoring space weather for forecasters, SOHO has also helped astronomers discover more than 600 comets over the course of its lifetime.
advertisement
