In November and December 2011, professional and amateur astronomers reveled in observing a sun-grazing comet that dove close to the Sun and survived for a return flight back to the outer solar system. Astronauts on the International Space Station (ISS) enjoyed their own surreal view of the comet as it appeared on Earth’s horizon on the day of the solstice.
ISS Commander Dan Burbank captured a series of digital photographs of Comet Lovejoy on December 21, 2011, as it rose above Earth’s limb. The ISS was passing from eastern Australia southeast toward New Zealand, between 17:35:50 to 17:43:02 Universal Time (6:35 to 6:43 a.m. local time on December 22). In an interview with WDIV-TV, Burbank described the moment as “the most amazing thing I have ever seen in space.”
Note how the tail of the comet points away from the Sun even as the comet itself is moving in the same direction, away from our star. Every comet has two tails, one of ice and dust, the other of ions, or charged particles. The heat and pressure of sunlight sloughs off the ice and dust, pushing it away from the Sun. Likewise, the solar wind strips ions off of the comet surface, though not necessarily in the same direction as the tail of debris and ice. The ion tail is not visible in this image.
The comet, officially designated C/2011 W3 (Lovejoy), was discovered by Australian amateur astronomer Terry Lovejoy on November 27, 2011. It belongs to a group of comets known as the Kreutz sungrazers, which are thought to be pieces of a much larger comet that broke up centuries ago. The comets are termed sungrazers because their orbits take them quite near—and often into—the Sun.
Comet Lovejoy is remarkable for diving through the superheated solar corona (atmosphere) to within 120,000 kilometers of the Sun’s surface and surviving the encounter. The event was recorded by five NASA and European spacecraft.
In the ISS image you can also see green and yellow airglow paralleling the Earth’s horizon line (or limb) before it is overwhelmed by the light of the rising Sun. Airglow is the emission of light by atoms and molecules in the upper atmosphere after they are excited by ultraviolet radiation.
Image Credit: NASA
Explanation from: http://visibleearth.nasa.gov/view.php?id=76853