Some CometsLike it Hot
Â July 7, 2000 -- In October
1965 comet Ikeya-Seki swooped past the Sun barely 450 thousand
kilometers above our star's bubbling, fiery surface. Gas and
dust exploded away from the comet's core as fierce solar radiation
vaporized the icy nucleus. Most comets wouldn't survive passing
as close to the Sun as the Moon is to the Earth, but Ikeya-Seki
literally came through with flying colors. When the comet emerged
from perihelion (closest approach to the Sun) it was so bright
that observers on the street with very clear skies could see
it during broad daylight if the Sun was hidden behind a house
or even an outstretched hand.
Right: This 4-minute exposure of comet Ikeya-Seki was captured by Roger Lynds at Kitt Peak, Arizona, on the morning of 1965 October 29. Copyright© Roger Lynds, all rights reserved. [more information from American Meteor Society]
"In Japan (where observers spied the comet 1/2 degree from the Sun) it was described as 10 times brighter than the Full Moon," recounted Brian Marsden of the Harvard Center for
Ikeya-Seki, a.k.a. "The Great Comet of 1965", is a member of the family of comets called Kreutz sungrazers (after the nineteenth-century German astronomer who studied them in some detail). These ill-fated visitors to the inner solar system have been seen to pass less than 50,000 km above the Sun's photosphere. Most never make it past perihelion -- they are completely obliterated. But the few that do, like Ikeya-Seki, can be very bright.
The nucleus of Ikeya-Seki was probably some kilometers across. Tinier pieces of Ephorus's comet streak past the Sun every day. Measuring perhaps only ten meters in diameter, they brighten briefly as they approach the Sun and disappear forever when they vaporize above the photosphere. Most of the faint fragments must have escaped detection entirely..
Now, thanks to coronagraphs on board the orbiting ESA/NASA Solar and Heliospheric Observatory (SOHO), amateur and professional astronomers can easily monitor the sky around the Sun for the telltale streaks of faint sungrazers. All that's needed is a computer and a connection to the internet.
"In late1998 we put SOHO's realtime coronagraph movies online so that anyone with an internet connection could access the data" says Doug Biesecker, a solar physicist at the Goddard Space Flight Center and SOHO's champion comet hunter with 47 finds. "Over a three year period before that time we had found 58 comets near the Sun in SOHO images. Now the total is up to nearly 170. Amateur astronomers watching coronagraph movies on the web are responsible for nearly all of the new finds this year. They're keeping me very busy!"
Editor's note: The pace of SOHO comet discoveries has accelerated to such an extent that during the author's 45 minute interview with Biesecker, two new comets were found!
A coronagraph is a device that blocks the glare of the Sun so that the faint corona, as well as surrounding stars and planets, are visible. It's a sophisticated version of the black disk Stephen Maran used to see Ikeya-Seki in 1965. The SOHO spacecraft carries two coronagraphs, one with a 3-degree field of view (the "C2" coronagraph) and another with a 16-degree field of view (the "C3" coronagraph). SOHO is located at the L1 point 1.5 million kilometers from the Earth in the direction of the Sun. It enjoys an uninterrupted view of our star.
Right: The SOHO C2 coronagraph captured this image of a sungrazing comet 0.75 degrees from the Sun on April 29, 2000. The solid brick-colored disk in the middle is the coronagraph's occulting disk; the white circle shows the true size of the Sun. The comet was noticed by four different amateur astronomers who were monitoring images from SOHO's realtime data page. All four (M. Boschat, T. Lovejoy, M. Oates, R. Gorelli) are credited with the discovery. The same comet was visible a day earlier in wider-angle C3 images, but it was much fainter. This 4-frame animation of the comet illustrates why it is easier to find sungrazers when they are very close to the Sun.
One of the most successful amateur comet hunters is Michael Boschat. He's credited (or shares credit) with a dozen discoveries since March 2000.
"I use the C3 512 x 512 pixel images," explains Boschat. "They appear on the SOHO site every 30 minutes and I download them as soon as they do. After I have four images I begin to loop them using GIF animation software that can be found on the Internet. I usually loop them at four frames per second looking for an object that is moving towards the Sun in a steady manner. I also use a magnifying glass to watch the possible comet move. After I feel it is a comet I put my mouse arrow as near as possible to the object to get the X and Y coordinates then send all that information off via email to Douglas Biesecker at Goddard."
All of the comets identified in images from SOHO are called
"comet SOHO" followed by a number denoting the order
of discovery. This differs from the traditional convention of
naming a comet after the person who finds it. The most recent
confirmed sungrazer, as of July 4, 2000, was comet SOHO-143.
The International Astronomical Union's official designation for
SOHO-143 is C/1998 K15, because the actual images were obtained
in 1998, with the K15 indicating that this was the fifteenth
comet (of any description) found during the second half of May.
"It started in the early 1980s with the SOLWIND mission, which also carried a coronagraph," explains Biesecker. "SOLWIND detected 6 sungrazers and they were all named after the satellite. The tradition continued for the Solar Maximum Mission (10 comets) and now for SOHO (143 confirmed comets and counting). It's reasonable because all of the comet finds have to be confirmed by mission scientists who are familiar with the hardware. Cosmic rays, noise in the detectors and other factors can mimic comets and we have to carefully examine each one. It's really a team effort."
"In the early 1980s there were also the 'IRAS' comets, found by the Infrared Astronomical Satellite," added Marsden. "Most of the comets found nowadays from the ground--and far from the sun--are named 'LINEAR', acronym for the Lincoln Near Earth Asteroid Research team, which scans the sky intensely with a U.S. Air Force telescope." (One of these is about to become a naked eye object in late July, 2000.)
Above: Comet Shoemaker-Levy 9 was
broken into many pieces during a close encounter with the planet
Jupiter in 1992. Two years later it came so close to the planet
that the fragments actually plunged into Jupiter's atmosphere.
Like the pieces of SL-9, most sungrazing comets are debris from
a single comet that broke apart when it passed by the Sun perhaps
2000 years ago. [more information]
Biesecker says he hopes the recent spate of amateur discoveries will continue unabated.
|Note: An asterisk denotes a professional member of the SOHO team. Others are amateur astronomers.|
If you're interested in joining the hunt for sungrazing comets,
a good place to start is the Solar and Heliospheric Observatory's
images web page where coronagraph data are posted every 30
minutes, and sometimes even more frequently. Data from the satellite
are available to the general public at the same time as to the
scientific community. If you think you've found something, first
review the basic
criteria for a discovery before forwarding the details to
scientists at the Goddard Space Flight Center. Confirmed finds
are posted daily on the "What's
New" area of http://sungrazer.nascom.nasa.gov.
SOHO (the Solar and Heliospheric Observatory) is a mission of international cooperation between NASA and the European Space Agency. It is managed by the Goddard Space Flight Center for the NASA HQ office of Space Science.