On Guard: NASA Plans Trio of Sun-Watching Missions

With some researchers predicting the onset of severe solar weather in a few years, NASA is planning to triple team the Sun by keeping one ancient craft alive to provide critical support for two fancy new missions.

The SOHO spacecraft has been the workhorse of space weather forecasting since it began observing the Sun in 1995. It has weathered storms of its own, suffering electrical problems and various malfunctions that rendered it all but dead on more than one occasion.

But SOHO (Solar and Heliospheric Observatory) has endured, longer than any other solar-dedicated observatory. Designed as a two-year mission, funding to operate the craft has been extended twice in the past so it could cover a complete 11-year solar cycle.

And now, even though it is being upstaged by the more glamorous 3-D Solar TErrestrial RElations Observatory (STEREO) images, SOHO has received funding through 2009. Why? Turns out, there are crucial observations only SOHO can do in the effort to monitor the raging Sun. Even next year, when the Solar Dynamics Observatory (SDO) is slated to launch, SOHO will remain a vital part of the three-mission observation team.

"SOHO has been a godsend in terms of advancing our science in the years it's been up," said Joseph Kunches, lead forecaster at the NOAA Space Environment Center. "SOHO has already taken us and lifted the whole profession up to the first floor of the building from the basement, and now we're trying to go higher."

Home runs

Currently, forecasters rely on statistics to predict when a storm will hit, for instance using information from an average magnetic storm. In contrast, the triple fleet will provide real-time information. Plus, rather than a window of, say, six hours, Kunches said the forecasters might be able in the future to say that stormy conditions will clear up in the next 45 minutes with some confidence.

"Right now, we have somebody standing at home plate and the Sun is the pitcher and it's throwing us these fast balls," Kunches said in a phone interview. STEREO will provide "first-base and third-base coaches," watching what's coming from the "pitcher" to the "batter," he said. "They'll be able to tell us better than ever when [a pitch or a solar storm] is going to arrive at home plate, at the Earth."

Adding SDO to the team will be like a sharp-eyed umpire, upping the chances of "hitting one out of the park," he said, extending the analogy.

"We want to hit home runs. We want to know what's coming and be able to prepare for it as best we can and get the word out," Kunches said.

Tough to replace

The jury is still out as to whether the coming solar cycle will be characterized by lots of intense solar storms or a calm period. There is strong evidence on both sides, Kunches said.

Considered SOHO's replacement, SDO is the first mission to be launched for NASA's Living With a Star (LWS) program, which aims to determine the causes of solar variability and how these peaks and troughs in solar storms impact Earth.

But SOHO's Large Angle and Spectrometric COronagraph Experiment (LASCO) coronagraphs, which routinely capture massive solar eruptions called coronal mass ejections (CMEs), can't be matched by either STEREO or SDO. These major storms sometimes slam into Earth, threatening satellites, communications and even terrestrial power grids.

"LASCO is the only show in town," said Bernhard Fleck, SOHO project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.

The LASCO instrument uses a Sun-shade to block out the main body of the Sun, simulating a solar eclipse. That way, astronomers can watch activity in the Sun's outer atmosphere, which is up to 10 million times fainter than the light from the solar disk. Without the bright-light blocker, forecasters would not be able to observe a solar ejection in the corona and beyond, a step that is crucial for forecasting how such space weather might impact Earth.

"This instrument is capable of observing what is happening in the high atmosphere of the Sun," said Guillermo Stenborg, SOHO-LASCO operations scientist at Goddard.

One set of instruments aboard SOHO monitors solar activity in the disk itself, and once the charged ejections reach the outer atmosphere LASCO's coronagraphs take over, tracking the event in interplanetary space out to a distance of 432,000 miles (695,500 kilometers) from the Sun.

Supporting roles

If it survives long enough, SOHO will also be used for the crucial task of calibrating SDO to make more accurate readings of the data it collects.

Meanwhile, Fleck told SPACE.com, he and other SOHO engineers are working to automate SOHO mission procedures and observations to "fly very cheaply." Instead of scientists monitoring the status of instruments, "this will be done by computers which will alert the spacecraft engineers and instrument teams in case of an anomaly," Fleck said. "The plan is to move to unmanned night passes in the fall."

If SOHO can hang in there, it will also make up for a shortcoming of the STEREO mission plan. STEREO's two spacecraft are drifting apart at about 45 degrees each year. Within four years, the probes will be so widely spaced they won't be watching the portion of the Sun that faces us, and they won't be able to properly monitor how solar storms will affect Earth.

SOHO can fill in the gap.

"SOHO is actually the third eye of STEREO," Fleck said, and even now many of STEREO's observations are enhanced by adding SOHO data. In order to whip up a 3-D image, astronomers need three points of reference, so STEREO's twins provide two points while SOHO generates the third.

Currently, SOHO and its instruments appear in good shape, and Stenborg said battery power (a typical reason for instrument conk-outs on an old mission) is not an issue.

Stenborg said most of the instruments will be decommissioned a year or two after SDO is launched, which is set for August 2008. "They will be taken out of operation except for LASCO and a couple of other instruments, especially because SDO has no LASCO kind of instrument," he said.

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