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A Coronagraph is an instrument which studies the Sun's outer atmosphere, the corona. From Earth the corona is most easily seen during a total solar eclipse. It is also possible to cover the bright disk of the Sun with a disk to create a sort of mini-eclipse and allows us to see the Sun's fainter outer atmosphere. If we do this from space we do not have to worry about the bright blue sky which makes this harder to do from Eath's surface. Each STEREO spacecraft has two coronagraphs which study the Sun from space. These two coronagraphs, which are part of STEREO's SECCHI suite of imagers, have different fields of view, using different sized occulting disks to allow us to see the faint corona at different distances from the Sun's surface.
The image is from STEREO SECCHI's Cor2 coronagraph on the Ahead spacecraft. The white circle shows the size and location of the Sun's surface. The color isn't "real". We color code the images so we can tell them apart quickly. The main feature to notice in this image is a faint coronal mass ejection (CME). CMEs are huge magnetic eruptions which blast off the Sun and can affect us here on Earth.
Originally posted by Paroxysm
Keep in mind that the object in question looks to be roughly 1/20 the size of the white circle drawn in the center (the sun). Mercury is not 1/20 the size of the sun.
[edit on 2-7-2009 by Paroxysm]
Originally posted by Phage
(re: LASCO image of planets).
About STEREO STEREO (Solar TErrestrial RElations Observatory) is the third mission in NASA's Solar Terrestrial Probes program (STP). This two-year mission, launched October 2006, will provide a unique and revolutionary view of the Sun-Earth System. The two nearly identical observatories - one ahead of Earth in its orbit, the other trailing behind - will trace the flow of energy and matter from the Sun to Earth. They will reveal the 3D structure of coronal mass ejections; violent eruptions of matter from the sun that can disrupt satellites and power grids, and help us understand why they happen. STEREO will become a key addition to the fleet of space weather detection satellites by providing more accurate alerts for the arrival time of Earth-directed solar ejections with its unique side-viewing perspective.
The spacecraft is being designed to fly for five years. However, since satellites go through a lot of testing and retesting, they often keep working long past their initial mission life. SOHO for example, which was built to fly for five years, in 2005 celebrated its 10 year anniversary in 2005!