The next great sun-watching spacecraft
By Irene Yachbes
(SPACE.com) -- Beneath the buffer zone of the atmosphere and magnetic field, Earth is an oasis nourished by the light and heat of the sun. However, the solar fireball can adversely impact our increasingly technology-driven society as well as our climate.
The main line of defense against solar storms is early detection by the aging Solar and Heliospheric Observatory (SOHO). But SOHO is not expected to last too many more years.
The Solar Dynamics Observatory (SDO), which is being designed and built at the Goddard Space Flight Center, is expected to replace SOHO and provide much improved sun-watching capabilities. SDO will monitor the sun's behavior and help scientists better predict its effects on Earth and near-Earth space.
SDO is the first of NASA's Living With a Star (LWS) missions and is scheduled to begin its five-year mission in April, 2008. The goal of the LWS missions is to understand how the sun affects our climate, communication systems, as well as space and atmospheric craft.
"LWS is kind of science with an edge," said SDO project scientist Barbara Thompson.
SDO will provide nearly continuous observations of changes in the sun's magnetic field, its interior, and ever-changing energy outputs, with an ambition of improving the timeliness and accuracy of space weather forecasts.
Better forecasts are needed during displays of solar moodiness, like those of last autumn when the sun's activity is said to have taken out power to a Swedish city and fried two Japanese satellites.
Over a two-week period in late October and early November of last year, the sun let loose with a smorgasbord of space weather spectacles: eleven X-class flares (explosions of stored energy powerful enough to prompt global blackouts and long radiation storms), including the strongest ever recorded; two near-record full-halo coronal mass ejections (releases along magnetic lines of solar material into space); this solar cycle's largest sunspot group; and two substantial proton storms which together lasted five days.
The tools of the trade
Three instruments comprise SDO's gear for exploring the sun's magnetic personality -- the source of solar variability -- and resultant physical and energetic properties, in what Thompson calls "a very coherent suite."
"We have 3 extremely experienced instrument teams and we're really lucky to have them," said mission systems engineer John Ruffa.
Lockheed Martin's Atmospheric Imaging Assembly (AIA) will "look sort of like a pan flute," featuring four huge telescopes. It will produce awe-inspiring images and link solar interior and surface changes, mission officials say.
University of Colorado's Extreme Ultraviolet Variability Experiment (EVE) will measure spectral energy flux deposits into Earth's atmospheric layers. The observations will clue scientists in to how our atmosphere shelters us and how these wavelengths affect airborne systems. "When you see the results of this," said Thompson, "it will be extraordinary... to see a flare go off on the sun and see the Earth's atmosphere go PHMMPHH!"
Stanford University's Helioseismic & Magnetic Imager (HMI) will measure magnetic field changes. HMI will also generate 3-D pictures of the solar interior to predict imminent magnetic field changes by monitoring sound waves ricocheting beneath the surface.
"It's cool. It's reverse modeling really," Thompson explained. "Instead of taking a structure and finding out what vibrational modes it would exhibit, we're taking the vibrational modes and asking what is the structure that exhibits these modes... It's a tough science. You've got to measure for a long time and you've got to be extremely accurate, but if you do that you can ... infer a lot about the interior changes in the sun."
Knowing your roots
Many of the necessary algorithms and models for this stellar mission can be derived from predecessor programs.
"Thanks to previous missions we think we know which measurements are vital for understanding the changes happening at Earth," Thompson said. "They've done a lot of the hard work."
Thompson continued. "We're fairly certain we've got a lot of the right measurements so we're going to have to just go for it and pump up the volume quite a bit." Quite a bit is an understatement:Whereas the Transition Region and Coronal Explorer (TRACE) spacecraft has provided partial solar views, AIA will "hose down" 16 mega-pixel multiple wavelength images to one huge picture of the entire sun. EVE derives inspiration from the Thermosphere Ionosphere Mesosphere Energetics and Dynamics spacecraft and Solar Radiance and Climate Experiment. EVE brings to the table greater sensitivity and extremely rapid cadence, or frequency of images, better for capturing instantaneous phenomena like flares. HMI, too, will feature the 16 mega-pixel technology. HMI's "precursor MDI flew on SOHO but we have significantly greater resolution that the Solar and Heliospheric Observatory (SOHO) had," explained Ruffa.
A fallible hero
According to Thompson, SOHO is "the real inspiration for SDO." SOHO, whose trailblazing science has spawned more than 1,500 scientific papers during its eight-plus years of service and whose images of the sun have captivated audiences worldwide, could survive until SDO's debut.
But SOHO's mission has been troubled at times. Three times the spacecraft faced near-death experiences. "The long ones were just devastating," Thompson said of SOHO's blackouts last summer. "Science stopped."
This latest problem came as a result of a degraded high-gain antenna motor aboard the spacecraft. The SOHO crew conceived an innovative solution to resurrect the observatory with planned partial data losses every three months.
The team also pulled SOHO from the wreckage in 1998, when a software glitch put the craft into a flat spin such that it lost power, and later when its gyroscopes failed. Despite these hardships, SOHO has continued to be a beacon for solar scientists and for folks who like really cool pictures.
Even on its best behavior, though, SOHO trickles down just about one image every fifteen minutes.
A solar mission on steroids
SDO will make its way to geotransfer orbit via an Atlas V or Delta IV launch vehicle, where a bi-propellant propulsion system (weighing about half of SDO's 3,200 kilograms) will then boost the craft into inclined geosynchronous orbit. There it will provide the crucial, nearly continuous science data at a whopping 150 megabits per second at Ka-band.
"Just a couple times a year we'll have eclipses that are not much more than an hour," said Thompson. "We can stand it."
Tremendous data rate is the piéce de resistance that sets SDO apart from its predecessors.
"It will be like taking high definition TV once every ten seconds," said outgoing project manager Ken Schwer in a video at SDO's Preliminary Design Review, which took place at Goddard from March 9--12. "Everybody is used to gigabytes and their hard drives... We'll actually bring down about 1.5 terabytes of data a day. Some people refer to this mission as a solar mission on steroids, because it's bringing down so much data."
"We have a belt-and-suspenders approach to make sure we don't lose any data... Data comes down 24 hours a day, 7 days a week. It never stops," said Ruffa.
From its position in geosynchronous orbit, SDO will trace a small figure-8 over a ground station at White Sands, a new facility that will be expressly built for the observatory.
"The ground station basically operates as a bent pipe," said Ruffa. "It takes the data in, autonomously compares it, stores the good data, and sends it to the Science Operations Centers (SOCs). If the SOCs need a re-transmission they make a request and the data is autonomously sent back. It is very, very autonomous so it reduces cost and manpower requirements... There will never be a downtime for this ground station."
It's easy for SDO engineers to maintain their sunny disposition -- they see SDO's practical daily applications.
"And with the President's new initiative on going back to the Moon and to Mars," Ruffa said, "a mission like ours is very important because every time we have electronics, satellites, and personnel that go out into space they're affected by [solar] radiation."
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