Despite rescue efforts, no one has heard from one of two NASA spacecraft on the far side of the Sun since October 1st.
Solar physicists always worry about the damage to Earth that might occur if the Sun were to unleash a titanic flare and zap our planet with a potent blast wave of energetic particles. So, during the past two decades, NASA has launched a series of spacecraft designed to keep tabs on the Sun and the "space weather" it creates.
Key to this plan is Stereo, the Solar Terrestrial Relations Observatory. Launched in 2006, Stereo A looped around the Moon and swung into a heliocentric orbit "ahead" of Earth, while Stereo B took up a solar orbit "behind" the Earth. They provide views of the Sun and its surroundings from angles we can't see.
Both Stereo craft have drifted to the far side of the Sun, meaning that their dish-shaped radio antennas must point near the Sun to communicate with Earth. To protect these from overheating, mission controllers devised a plan to point each craft away from the Sun (and Earth) and to put it in safe-mode hibernation for about a year.
Tests of these new procedures took place over the past few months. Stereo A checked out fine and began its "time out" on August 20th. A month later engineers were completing Stereo B's final tests, which involved commanding the craft to go into its safe mode and then resume normal operations.Then something went wrong.
There's been no radio contact with the Stereo B spacecraft since October 1st, the day it was supposed to "wake up". Its radio signal came in weakly and then quickly faded away. Up to that point, telemetry showed no indication that anything was amiss.
But now it appears that the spacecraft suffered a double whammy: first the star tracker could not lock onto its correct guide stars, and then a laser gyro in Stereo B's Inertial Measurement Unit, which senses the craft's orientation, failed and started providing bad data to the attitude-control system.
The upshot is that there's no way to know exactly where Stereo B is pointed or the state of its systems. Over the past few weeks, astronomers have been borrowing the giant, 100-meter-wide Green Bank Telescope to try to detect the spacecraft's radio signal, so far without success. The 70-m dishes of NASA's Deep Space Network are also trying to reestablish contact.
All hope is not lost. In 1998, the Solar and Heliospheric Observatory also went AWOL, putting itself in a slow spin with its solar-cell arrays pointed away from the Sun. Eventually, orbital geometry provided enough sunlight on the arrays, and enough electricity, to power the craft, and mission controllers eventually regained control. That was 16 years ago, and SOHO continues to provide daily images of the Sun and its surrounding.
But, at this point, recovery options are few. According to Joseph Gurman, Stereo project scientist at NASA's Goddard Space Flight Center, simulations are under way to deduce the spacecraft's attitude and roll rate based on the few final bits of telemetry received. NASA will then create a review board to brainstorm other recovery schemes — and to make sure this unexpected failure doesn't occur again In particular, they'll doublecheck the safe-mode programming of Stereo A, which will be out of contact with Earth for about four months beginning in March 2015.
Could Earth ever fall victim to a "solar superstorm"? Gets the odds of that happening — and the consequences if it does — in the February 2011 issue of Sky & Telescope.