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Bell tone

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Narrator: On December 15, 2011,

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NASA's Solar Dynamics Observatory captured this footage of Come Lovejoy

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approaching the sun. An hour later, it watched as Lovejoy came around the far side

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of the sun and began its long trip back to the outer reaches of the solar system.

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Other NASA spacecraft, such as SOHO and STEREO, also

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saw Lovejoy's close encounter. Lovejoy marked one of the few times that

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orbiting telescopes have been able to watch a so-called "sun grazing" comet survive its

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trip around the sun. Most are not so lucky.

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Besides being interesting to watch, the images and data collected by NASA's

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solar observing fleet can also help scientists learn more about the sun itself.

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One of the biggest features that comets help reveal is the sun's magnetic field.

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Since magnetic fields are invisible, we can only observe them indirectly, like using

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iron filings over a bar magnet. On the sun,

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astronomers can look at where hot plasma in the sun's atmosphere is trapped by fields to see their

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complicated loop structure. But farther away from the sun, where the

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plasma is less dense, this approach doesn't work. Comet

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tails, with their ionized gases, are affected by magnetic fields and so they can act

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as brief tracers. On April 20, 2007,

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Comet Encke had its tail stripped off abruptly by a coronal mass ejection

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that carried a strong parcel of magnetic field through the solar system.

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Even closer to the sun, astronomers were astounded to Comet Lovejoy's tail glowing

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in extreme ultraviolet light as it approached the sun. They now think

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the glow is caused by energetic electrons in the sun's corona interacting with

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oxygen from the comet. The glowing tail followed and illuminated some

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of the sun's magnetic field lines. Careful analysis of the frames

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allows scientists to reconstruct where the field lines were and even, to some

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degree, how strong they were. These comet "tracers" also illuminate

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small structures in the sun's upper atmosphere where they are usually too faint to be

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visible. Continued observation of sun grazing comets will also help

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astronomers understand how hot material in the sun's corona cools, and where

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that energy goes. Finally, long term observations

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of sun grazing comets will help us learn more about the solar wind. Some of

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the particles in the corona are traveling fast enough to escape and travel through the solar

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system. They begin moving at roughly 250,000 miles

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an hour, but start accelerating when they reach around a million miles from the sun's surface.

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By 5 million miles out, they are traveling at up to

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1 million miles per hour. The exact mechanism for this acceleration

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is not known. Comet tails that are blown off by the sun travel with the

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solar wind, and can act like a dye tracer in a river. Because they are

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made of different materials than the usual solar wind, they are distinct and easy to pick out.

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So they can show exactly how the acceleration unfolds.

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Because we are in a period of high sun grazing comet activity, scientists can expect many more

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chances to watch these natural research satellites in the coming years.

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In fact, another large comet is expected to have a close solar pass on

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November 28, 2013. This comet is roughly the size of Hale-Bopp,

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so it should give quite a show. It will also undoubtedly be a treasure

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trove of information for scientists and who knows what new solar secret it