13. Colliding worlds

Consumed by the Sun

Fig. .. 

Some comets plunge deep into the Sunís thin, million-degree outer atmosphere, or corona. Instruments aboard the Solar and Heliospheric Observatory satellite, abbreviated SOHO, have recorded their death-defying trip around the Sun. One of its instruments uses an occulting disk to block out the bright light of the visible solar disk, enabling it to detect the comets as they move through the inner corona. A comet often pays a heavy price for this trip, sometimes breaking apart because of the Sunís forces.

Fig. .. 

Other comets are hurtling toward complete meltdown, passing so close to the Sun that the encounter is fatal. Though rarely, if ever, hitting the visible solar disk, or photosphere, these comets can come closer than 50 million meters from it. They are unlikely to survive the Sunís intense heat and gravitational forces at that range. Amateur astronomers from all over the world have examined SOHOís real-time images posted on the Internet, discovering hundreds of previously unknown comets on their death-dive into the Sun.

Most of the comets discovered by SOHO, about 90 percent of them, are small cometary fragments known as the Kreutz sungrazers, which closely approach the Sun from one direction in space. They are named after the German astronomer Heinrich Kreutz (1854-1907) who found that many of the comets, which had come closest to the Sun in the 19th century, seemed to have a common origin with similar orbits. It turned out that they are all fragments of a single large comet that first broke up when passing very close to the Sun thousands of years ago. The original fragmentation may have been witnessed in 321 BC by the Greek philosopher Aristotle (384-322 BC) and by the Greek historian Ephorus (405-330 BC), but it may have occurred much later. The break up gave rise to two main comets, perhaps with orbital period of 350 years and about 700 years, and the two parts were split into more pieces during return visits to the Sun.

Fig. .. 

When a member of the Kreutz sungrazer group moves around its orbit and returns to our vicinity, it can dive into the inner corona and disappear forever. Spectroscopic observations from SOHO indicate that each comet fragment can be very small, just 6 to 12 meters across, despite their spectacular display. Such a tiny object, falling so close to the Sun, would vaporize completely away, like the proverbial snowball in hell.

In its six years of service, SOHO has spotted more than four hundred comets, making it by far the most prolific comet finder in the history of astronomy. Aside from the numerous Kreutz sungrazers, SOHO has found more than forty new comets, which is comparable to the number of comet discoveries during almost any decade throughout the previous two centuries. The other main impetus for recent comet discoveries has been the Lincoln Near Earth Asteroid Research, abbreviated LINEAR, Program, operated by MITís Lincoln Laboratory. It consists of a pair of telescopes dedicated to detecting and cataloging Near-Earth Objects that threaten Earth. LINEAR has completed millions of observations, finding more than seven hundred confirmed near-Earth asteroids or comets. One of them, dubbed Comet LINEAR and also known as C/1999 S4, has been caught breaking up on its way into the Sun, vanishing much further out than the Kreutz sungrazers.

Fig. .. 

When discovered in September 1999, Comet LINEAR was exceptionally bright at a relatively large distance of about 4 AU, which has often happened to other comets during their first trip through the inner solar system. But then something unexpected happened. As it moved closer to the Sun, the comet broke apart into numerous parts. But the comet was as far as 0.8 AU from the Sun when it disintegrated.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University