Since the beginning of the space age, the inner planets and the Earth-Moon system have received the lion’s share of attention. Which makes sense; it is much easier to get to the Moon, or even Mars, than to get to Saturn or Neptune. And so our probes have largely crossed the relatively cozy confines within the asteroid belt, visiting every world within them and sometimes landing on the surface and punching a few holes or even leaving a few footprints.
But there is still one place within this warm and familiar neighborhood that remains mysterious and relatively unvisited: The Sun. This seems strange, since our star is the source of all energy for our world and the system at large, and its continuous emissions across the electromagnetic spectrum and its occasional physical explosions 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 much worse than just a cold.
While we’ve had a number of satellites over the past few 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 abuse of the Sun, and building anything to withstand the blows our star can throw at it is a difficult task. But there’s a satellite that takes everything the Sun gives out and turns it into a near-constant stream of high-quality data, and has been doing so for nearly 15 years now. The Solar Dynamic Observatory, or SDO, has also provided stunning images of the Sun, like this CGI-like sequence of a failed solar flare. Such images have captured the imagination during this surprisingly active solar cycle and highlighted the importance of the SDO in our solar early warning system.
Living with a star
In many ways, SDO has its roots in the previous Solar and Heliospheric Observatory, or SOHO, ESA’s hugely successful solar mission. Launched in 1995, SOHO is placed in a halo orbit at the Lagrangian L point1 and provides near-real-time images and data on the sun using an array of twelve scientific instruments. Originally planned for a two-year science program, SOHO continues to operate to this day, watching the sun and acting as an early warning for coronal mass ejections (CMEs) and other solar phenomena.
Although L1, the point between the Earth and the Sun where the two bodies’ gravitation balances, offers an unobstructed view of our star, has its drawbacks. Chief among them is distance; in 1.5 million kilometers, just going to L1 it is a much more expensive proposition than any geocentric orbit. Distance also makes radio communications much more complicated, requiring specialized deep space network (DSN) infrastructure. SDO was conceived in part to avoid some of these shortcomings, as well as to take advantage of what was learned on SOHO and to extend some of the capabilities provided by that mission.
SDO grew out of Living with a Star (LWS), a science program that began in 2001 and was designed to explore the Earth-Sun system in detail. LWS identified the need for a satellite that could view the Sun continuously at multiple wavelengths and provide data on its atmosphere and magnetic field at an extremely high speed. These requirements dictated the SDO mission specifications in terms of orbital design, spacecraft engineering and surprisingly, a dedicated communications system.
Geosynchronous, with a twist
Getting a good look at the Sun for space isn’t necessarily as easy as it might seem. For SDO, designing a suitable orbit was complicated by the stringent and somewhat conflicting requirements for continuous observations and continuous high-bandwidth communications. Joining SOHO in L1 or setting up shop at any of the other Lagrangian points was out of the question due to the distances involved, leaving a geocentric orbit as the only possible alternative. A low Earth orbit (LEO) would have left the satellite in Earth’s shadow for half of each rotation, making continuous observation of the Sun difficult.
To avoid these problems, SDO’s orbit was pushed to the 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 only a few brief periods during the year when the Earth or the Moon eclipse the Sun. It also allows a continuous line of sight to the ground, which greatly simplifies the communication problem.
The Science of the Sun
The main body of the SDO has a pair of solar panels on one side and a pair of high-gain directional 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 on the bottom 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 does not image the entire disk of the Sun. Instead, EVE uses a pair of EUV multiple 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 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 Sun’s corona and chromosphere, the Atmospheric Imaging Assembly (AIA) uses four telescopes to create full-disk images of the Sun at 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 up to almost 1.3 solar radii, to capture fine details in the thin solar atmosphere. AIA also visualizes the Sun’s magnetic fields as hot plasma gathers along the lines of force and highlights them. Like all instruments in SDO, AIA is built with throughput in mind; it can collect a full set of data 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 the strength and polarity of the magnetic field. 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 x 4,096 pixel CCD image detectors. HMI captures complete images of the Sun known as Dopplergrams, which reveal the direction and speed of movement of structures in the photosphere. The HMI is also capable of passing a polarization filter in the optical path to produce magnetograms, which use the polarization of light as a proxy for the strength and polarity of the magnetic field.
Continuous data and lots of it
Like all SDO instruments, the HMI is built with data flow in mind, but with a twist. Helioseismology requires the accumulation of data continuously over long periods of observation; the original 5-year mission plan included 22 separate HMI runs lasting 72 consecutive days, during which 95% of the data was to be captured. So not only does the HMI have to take images of the Sun every four seconds, it has to package them reliably and completely for transmission to Earth.
While most space programs try to make use of existing communications infrastructure, such as Deep Space Network (DSN), the unique requirements of SDO necessitated a dedicated communications system. The SDO communications system was designed to meet mission throughput and reliability needs, literally from the ground up. A dedicated ground station consisting of a pair of 18-meter dish antennas was constructed at White Sands, New Mexico, a location specifically chosen to reduce the chance of 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 beam, presumably for the same reason; storms in the New Mexico desert tend to be patchy, making it more likely that at least one spot will always have a strong signal, regardless of the weather.
To ensure that all downlink data is captured and sent to the science teams, a complex and highly redundant Data Distribution System (DDS) was also developed. Each platter has a pair of redundant receivers and servers with RAID5 storage arrays, which feed a small data center of twelve servers and attached storage. A Quality Comparison Processing (QCP) system continuously monitors the quality of downlink data from every instrument on board the SDO and stores the best available data in a temporary archive before sending it to the team science dedicated to each instrument in near real time.
The figures involved are impressive. SDO ground stations operate 24/7 and are almost always unattended. SDO returns about 1.3 TB per day, so the ground station has taken almost 7 petabytes of images and data and sent them to science teams over the 14 years it’s been in service, with almost all of it available almost as soon as it was created. . .
As impressive as the numbers and the engineering behind them are, it’s the images that grab all the attention, and understandably so. NASA makes all SDO data available to the public, and almost every image is stunning. There are also plenty of “greatest hits” compilations out there, including a reel of X-class flares that resulted in spectacular auroras over North America in mid-May.
Like many NASA projects, SDO has far exceeded its planned lifespan. It was designed to capture 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 closely monitoring the peak of Solar Cycle 25.