Solar Dynamics Observatory: Our Solar Early Warning System [Hackaday]
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
