Pick of The Week

SOHO Comet 900 (January 25, 2005)

Hi-res TIF (2.9M)

On January 15, 2005, SOHO discovered its 900th comet. Though it was small and very faint, it marks another milestone in comet discoveries. To celebrate this event, the SOHO project is pleased to announce a new contest for its web visitors: the 1,000th Comet Contest. SOHO (which has already become the most prolific comet hunter in history) is on track to find its 1,000th comet sometime this summer or early fall. Your challenge, should you choose to accept it, is to guess which day and at what time SOHO 1,000th comet will reach its closest approach to the Sun (called perihelion). For contest rules, links to our best comet images and movies, and the chance to vote, go to: http://soho.nascom.nasa.gov/comet1000/.

The number of comet discoveries made by SOHO have been a surprise to just about everyone, including those who designed and produced the SOHO instruments that have revealed them. They also highlight the power of the internet, as about three-fourths of the discoveries were made not by professional astronomers, but by amateur stargazers monitoring the SOHO website. The pages feature new images of the sun and its corona about every 30 minutes. Amateur enthusiasts scour the images for evidence of bodies in motion that just might be comets.

"This has been a big surprise," said Douglas Biesecker, a NOAA scientist formerly with NASA Goddard Space Flight Center in Greenbelt, Maryland. "We certainly expected to discover comets...however, we only expected a few per year at most."

What are comets? They are small bodies made of ice and dust that speed around the solar system along elliptical orbits. Some of these are pulled toward the Sun as a result of the Suns gravitational strength. As they near the Sun, they begin to evaporate and develop elongated tails. These tails tend to point away from the Sun due to pressure exerted on them by the constant outflow of the solar wind. Most of the comets exist out beyond Pluto in what is called the Oort Comet Cloud many millions of them can be found. Comets are old. They are believed to have been formed in the beginnings of our solar system. (More information about and images of comets can be found at http://www.solarviews.com/eng/comet.htm)
In current news, NASA has launched its Deep Impact mission to probe into comet Tempel 1 on July 4, 2005. You can learn more here: http://deepimpact.jpl.nasa.gov/.

How come SOHO can see so many comets? SOHOs two LASCO instruments create an artificial eclipse every day. Because of their specialized ability to block out much of the light coming from the Sun with their occulting disks, these LASCO instruments are quite good at detecting comets. The scientific value derived from observing the comets comes from watching how the comets react to the Sun, especially to solar storms.

Images from LASCO usually appear on the SOHO site within 20 minutes after they are taken. Almost immediately, amateur astronomers around the world carefully pore over the images, trying to determine if they can spot the movement of a comet. It is not so easy as it may sound. There are always some specks appearing here and there on almost any given image and many of the comets discovered are very faint. The comet hunters need to find four to six frames of motion to establish a possible comet discovery before describing and reporting their findings.The question is when will the number reach that magic number 1000! Good luck with the contest!

Previous Picks of the Week

SOHO began its Weekly Pick some time after sending a weekly image or video clip to the American Museum of Natural History (Rose Center) in New York City. There, the SOHO Weekly Pick is displayed with some annotations on a large plasma display.

If your institution would also like to receive the same Weekly Pick from us for display (usually in Photoshop or QuickTime format), please send your inquiry to steele.hill@gsfc.nasa.gov.


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