When NASA launched the Solar Dynamics Observatory (SDO) in February 2010, engineers placed it in what’s called a geosynchronous orbit over Earth.
The idea is that the craft circles our planet at the same speed as Earth’s rotation about its axis. To an observer on the planet’s surface, the satellite seems to return to the same place in the sky at exactly the same time every day.
In SDO’s case, the orbiter traces a figure-eight over the Pacific that seems to hang from northern Mexico:
—Orbital map courtesy NASA/SDO
The benefit of geosynchronous orbit is that it allows SDO to almost constantly face the sun, collecting scads of data and beaming the results continuously back to mission control.
[5/5 UPDATE: A couple commentors have pointed out that a geosynched orbit alone doesn't mean SDO is almost always in sunlight. This is the downfall of writing in a rush! I neglected to say that the orbit is also *inclined* with respect to Earth's Equator, and the craft is flying pretty high—22,000 miles, or 36,000 kilometers, above the planet. Those factors contribute to the Earth seldom being in SDO's line of sight.]
Occasionally, though, the craft dips into Earth’s night side for two- to three-week periods, interrupting solar observations.
In addition, three times a year SDO will pass through the moon’s shadow, when our only natural satellite transits—or passes in front of—the sun, from the spacecraft’s perspective. (For a nice visual reference, see Bad Astronomer Phil Plait’s drawing of SDO’s orbital geometry.)
This morning astronomers using NASA’s Solar Dynamics Observatory captured one of these lunar transits against an image of the sun seen in extreme ultraviolet light. Click the image to zoomify.
—Picture courtesy NASA/SDO
As huge as the sun is, SDO is taking its pictures in such high resolution that you can even zoom in on the dark blot of the moon and see the bumps of mountains along its limb.
Such amazing views offer more than just a “wow” factor, too.
The SDO science team can use the hard features along the moon’s edge to calibrate their telescope, fine tuning the optics to make future images of our increasingly active sun even sharper.