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 much easier to get to the moon or even mars than to get to saturn or neptune. And so our probes mostly cruised through the relatively cozy confines of the asteroid belt, visiting every world in them and sometimes landing on the surface and making a few holes or even leaving some tracks.
But in this warm and familiar neighborhood, there is still one place that remains mysterious and relatively unvisited: the Sun. This seems strange because 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 can be much worse than just a cold.
While we’ve had a number of satellites in recent decades that specialize in tracking 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 lashings our star can take apart is a tall order. But there’s one satellite that takes everything the sun throws up and turns it into a near-constant stream of high-quality data, and has been doing so for nearly 15 years. The Solar Dynamics Observatory, or SDO, has also provided stunning images of the Sun, such as this CGI-like sequence of a failed solar flare. Similar images captured the imagination during this surprisingly active solar cycle and highlighted the importance of the SDO in our solar early warning system.
Life with a star
In many ways, SDO has its roots in the earlier Solar and Heliospheric Observer, or SOHO, ESA’s highly successful solar mission. SOHO, which was launched in 1995, is placed in a halo orbit at the Lagrangian point L1 and provides near-real-time images and data about the Sun using a suite of twelve scientific instruments. Originally designed for a two-year science program, SOHO continues to operate today, tracking 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 gravity of the two bodies balances out, providing an unobstructed view of our star, has its drawbacks. Chief among them is distance; for 1.5 million kilometers, simply get to L1 is a much more expensive proposition than any geocentric orbit. This distance also greatly complicates radio communication and requires 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 extend some of the capabilities provided by that mission.
SDO grew out of Living with a Star (LWS), a science program launched in 2001 designed to explore the Earth-Sun system in detail. LWS identified the need for a satellite that could continuously monitor the Sun at multiple wavelengths and provide data on its atmosphere and magnetic field at extremely high speeds. These requirements dictated the specifications of the SDO mission in terms of orbital design, spacecraft construction and, curiously, a dedicated communications system.
Geosynchronous, with a Twist
Getting a good look at the Sun from space isn’t necessarily as easy as it might seem. For SDO, designing a suitable orbit was complicated by strict and somewhat conflicting requirements for continuous observation and constant broadband communications. Entrance to SOHO in L1 or setting up shop at one of the other Lagrange points was out of the question given the distances involved, so a geocentric orbit was the only viable alternative. Low Earth Orbit (LEO) would leave the satellite in the Earth’s shadow for half of each revolution, making continuous observations of the Sun difficult.
To avoid these problems, SDO’s orbit was shifted to the geosynchronous earth orbit (GEO) distance (35,789 km) and tilted 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 Moon eclipses the Sun. It also allows a constant line of sight to the ground, greatly simplifying the communication problem.
The Science of the Sun
The main body of the SDO has a pair of solar panels on one end and a pair of high-gain steerable 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 precisely as possible, so only three science instruments were included. All three instruments are mounted on the end of the spaceframe with 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. 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 that spreads the Sun’s spectrum across the CCD detector to cover 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 the entire EUV spectrum every 10 seconds.
To study the Sun’s corona and chromosphere, the Atmospheric Imaging Assembly (AIA) uses four telescopes to produce full-disk images of the Sun at ten different wavelengths from EUV to 450 nm. The 4,096 x 4,096 sensor gives the AIA a resolution of 0.6 arcseconds per pixel, and the optics allow imaging up to nearly 1.3 solar radii to capture fine details in the thin solar atmosphere. The AIA also visualizes the Sun’s magnetic fields as hot plasma gathers along the lines of force and accentuates them. Like all tools on SDO, AIA is built with throughput in mind; it can collect 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 solar 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. The HMI captures disk-wide images of the Sun known as Dopplergrams, which reveal the direction and speed of movement of structures in the photosphere. The HMI is also able to switch a polarizing filter into the optical path to create magnetograms that use the polarization of light as a proxy for magnetic field strength and polarity.
Continuous data and lots of it
Like all SDO tools, HMI is built with data throughput in mind, but with a twist. Helioseismology requires continuous data collection over long observation periods; 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. Thus, the HMI must not only take images of the Sun every four seconds, but must also reliably and completely package them for transmission to Earth.
While most space programs seek to utilize existing communications infrastructure such as the Deep Space Network (DSN), the unique requirements of SDO necessitated a dedicated communications system. The SDO communications system was designed literally from the ground up to meet mission throughput and reliability requirements. A dedicated ground station consisting of a pair of 18-meter parabolic antennas was constructed at White Sands, New Mexico, a location chosen specifically to reduce the potential of rain storms to attenuate the downlink signal in the Ka-band (26.5 to 50 GHz). The two antennas are located about 5 km apart within the download beam width, probably for the same reason; storms in the New Mexico desert tend to be spotty, so it’s more likely that at least one location will always have a solid signal, regardless of the weather.
A comprehensive and highly redundant data distribution system (DDS) has also been developed to ensure that all downstream data is captured and sent to science teams. Each dish has a redundant pair of receivers and servers with RAID5 storage arrays that power a miniature data center of twelve servers and associated storage. The Quality Compare Processing (QCP) system continuously monitors the quality of downlink data from each instrument on board SDO and stores the best available data in a temporary archive before sending it to the science team dedicated to each instrument in near real time.
The numbers involved are impressive. SDO ground stations operate 24/7 and are almost always unmanned. SDO returns approximately 1.3 TB per day, so the ground station has received and sent nearly 7 petabytes of images and data to science teams in its 14 years of operation, almost all of which are available almost immediately as they are generated. .
As impressive as the numbers and the technology behind them may be, it’s the images that grab all the attention, and understandably so. NASA makes all of the SDO data available to the public, and almost every image is stunning. There are also plenty of “greatest hits” compilations, including the reel of X-class flares that led to the spectacular aurora over North America in mid-May.
Like many NASA projects, SDO has far exceeded its planned lifespan. It was designed to capture the middle of solar cycle 24, but managed to stay operational through the solar minimum of that cycle and into the next cycle, and is now closely tracking the peak of solar cycle 25.