Ever since the beginning of the Space Age, the inner planets and the Earth-Moon system have received the lion’s share of attention. That makes sense; it’s a whole lot easier to get to the Moon, or even to Mars, than it is to get to Saturn or Neptune. And so our probes have mostly plied the relatively cozy confines inside the asteroid belt, visiting every world within them and sometimes landing on the surface and making a few holes or even leaving some footprints.
But there’s still one place within this warm and familiar neighborhood that remains mysterious and relatively unvisited: the Sun. That seems strange, since our star is the source of all energy for our world and the system in general, and its constant emissions across the electromagnetic spectrum and its occasional physical outbursts are literally a matter of life and death for us. When the Sun sneezes, we can get sick, and it has the potential to be far worse than just a cold.
While we’ve had a succession of satellites over the last decades that have specialized in watching the Sun, it’s not the easiest celestial body to observe. Most spacecraft go to great lengths to avoid the Sun’s abuse, and building anything to withstand the lashing our star can dish out is a tough task. But there’s one satellite that takes everything that the Sun dishes out and turns it into a near-constant stream of high-quality data, and it’s been doing it for almost 15 years now. The Solar Dynamics Observatory, or SDO, has also provided stunning images of the Sun, like this CGI-like sequence of a failed solar eruption. Images like that have captured imaginations during this surprisingly active solar cycle, and emphasized the importance of SDO in our solar early warning system.
Living With a Star
In a lot of ways, SDO has its roots in the earlier Solar and Heliospheric Observer, or SOHO, the wildly successful ESA solar mission. Launched in 1995, SOHO is stationed in a halo orbit at Lagrange point L1 and provides near real-time images and data on the sun using a suite of twelve science instruments. Originally slated for a two-year science program, SOHO continues operating to this day, watching the sun and acting as an early warning for coronal mass ejections (CME) and other solar phenomena.
Although L1, the point between the Earth and the Sun where the gravitation of the two bodies balances, provides an unobstructed view of our star, it has disadvantages. Chief among these is distance; at 1.5 million kilometers, simply getting to L1 is a much more expensive proposition than any geocentric orbit. The distance also makes radio communications much more complicated, requiring the specialized infrastructure of the Deep Space Network (DSN). SDO was conceived in part to avoid some of these shortcomings, as well as to leverage what was learned on SOHO and to extend some of the capabilities delivered by that mission.
SDO stemmed from Living with a Star (LWS), a science program that kicked off in 2001 and was designed to explore the Earth-Sun system in detail. LWS identified the need for a satellite that could watch the Sun continuously in multiple wavelengths and provide data on its atmosphere and magnetic field at an extremely high rate. These requirements dictated the specifications of the SDO mission in terms of orbital design, spacecraft engineering, and oddly enough, a dedicated communications system.
Geosynchronous, With a Twist
Getting a good look at the Sun for space isn’t necessarily as easy as it would seem. For SDO, designing a suitable orbit was complicated by the stringent and somewhat conflicting requirements for continuous observations and constant high-bandwidth communications. Joining SOHO at L1 or setting up shop at any of the other Lagrange points was out of the question due to the distances involved, leaving a geocentric orbit as the only viable alternative. A low Earth orbit (LEO) would have left the satellite in the Earth’s shadow for half of each revolution, making continuous observation of the Sun difficult.
To avoid these problems, SDO’s orbit was pushed out to geosynchronous Earth orbit (GEO) distance (35,789 km) and inclined to 28.5 degrees relative to the equator. This orbit would give SDO continuous exposure to the Sun, with just a few brief periods during the year where either Earth or the Moon eclipses the Sun. It also allows constant line-of-sight to the ground, which greatly simplifies the communications problem.
Science of the Sun
The main body of SDO has a pair of solar panels on one end and a pair of steerable high-gain dish antennas on the other. The LWS design requirements for the SDO science program were modest and focused on monitoring the Sun’s magnetic field and atmosphere as closely as possible, so only three science instruments were included. All three instruments are mounted to the end of the spaceframe with the solar panels, to enjoy an unobstructed view of the Sun.
Of the three science packages, the Extreme UV Variability Experiment, or EVE, is the only instrument that doesn’t image the full disk of the Sun. Rather, EVE uses a pair of multiple EUV grating spectrographs, known as MEGS-A, and MEGS-B, to measure the extreme UV spectrum from 5 nm to 105 nm with 0.1 nm resolution. MEGS-A uses a series of slits and filters to shine light onto a single diffraction grating, which spreads out the Sun’s spectrum across a CCD detector to cover from 5 nm to 37 nm. The MEGS-A CCD also acts as a sensor for a simple pinhole camera known as the Solar Aspect Monitor (SAM), which directly measures individual X-ray photons in the 0.1 nm to 7 nm range. MEGS-B, on the other hand, uses a pair of diffraction gratings and a CCD to measure EUV from 35 nm to 105 nm. Both of these instruments capture a full EUV spectrum every 10 seconds.
To study the corona and chromosphere of the Sun, the Atmospheric Imaging Assembly (AIA) uses four telescopes to create full-disk images of the sun in ten different wavelengths from EUV to 450 nm. The 4,096 by 4,096 sensor gives the AIA a resolution of 0.6 arcseconds per pixel, and the optics allow imaging out to almost 1.3 solar radii, to capture fine detail in the thin solar atmosphere. AIA also visualizes the Sun’s magnetic fields as the hot plasma gathers along lines of force and highlights them. Like all the instruments on SDO, the AIA is built with throughput in mind; it can gather a full data set every 10 seconds.
For a deeper look into the Sun’s interior, the Helioseismic and Magnetic Imager (HMI) measures the motion of the Sun’s photosphere and magnetic field strength and polarity. The HMI uses a refracting telescope, an image stabilizer, a series of tunable filters that include a pair of Michelson interferometers, and a pair of 4,096 by 4,096-pixel CCD image detectors. The HMI captures full-disk images of the Sun known as Dopplergrams, which reveal the direction and velocity of movement of structures in the photosphere. The HMI is also capable of switching a polarization filter into the optical path to produce magnetograms, which use the polarization of light as a proxy for magnetic field strength and polarity.
Continuous Data, and Lots of It
Like all the SDO instruments, HMI is built with data throughput in mind, but with a twist. Helioseismology requires accumulating data continuously over long observation periods; the original 5-year mission plan included 22 separate HMI runs lasting for 72 consecutive days, during which 95% of the data had to be captured. So not only must HMI take images of the Sun every four seconds, it has to reliably and completely package them up for transmission to Earth.
While most space programs try to leverage existing communications infrastructure, such as the Deep Space Network (DSN), the unique demands of SDO made a dedicated communications system necessary. The SDO communication system was designed to meet the throughput and reliability needs of the mission, literally from the ground up. A dedicated ground station consisting of a pair of 18-meter dish antennas was constructed in White Sands, New Mexico, a site chosen specifically to reduce the potential for rainstorms to attenuate the Ka-band downlink signal (26.5 to 50 GHz). The two antennas are located about 5 km apart within the downlink beamwidth, presumably for the same reason; storms in the New Mexico desert tend to be spotty, making it more likely that at least one site always has a solid signal, regardless of the weather.
To ensure that all the downlinked data gets captured and sent to the science teams, a complex and highly redundant Data Distribution System (DDS) was also developed. Each dish has a redundant pair of receivers and servers with RAID5 storage arrays, which feed a miniature data center of twelve servers and associated storage. A Quality Compare Processing (QCP) system continually monitors downlinked data quality from each instrument aboard SDO and stores the best available data in a temporary archive before shipping it off to the science team dedicated to each instrument in near real-time.
The numbers involved are impressive. The SDO ground stations operate 24/7 and are almost always unattended. SDO returns about 1.3 TB per day, so the ground station has received almost 7 petabytes of images and data and sent it on to the science teams over the 14 years it’s been in service, with almost all of it being available nearly the instant it’s generated.
As impressive as the numbers and the engineering behind them may be, it’s the imagery that gets all the attention, and understandably so. NASA makes all the SDO data available to the public, and almost every image is jaw-dropping. There are also plenty of “greatest hits” compilations out there, including a reel of the X-class flares that resulted in the spectacular aurorae over North America back in mid-May.
Like many NASA projects, SDO has far exceeded its planned lifespan. It was designed to catch the midpoint of Solar Cycle 24, but has managed to stay in service through the solar minimum of that cycle and into the next, and is now keeping a close watch on the peak of Solar Cycle 25.