From: Naval Research Laboratory
Posted: Friday, September 27, 2002
The Naval Research Laboratory's (NRL's) Large Angle Spectrometric Coronagraph (LASCO), which is flying aboard the Solar and Heliospheric Observatory (SOHO), has been credited with discovering its 500th comet. This achievement makes LASCO the most productive comet finder ever. Most of the comets that LASCO has discovered are in the family of Kreutz sun-grazers, which are thought to derive from a single massive comet that had fragmented into many pieces, because they all have very similar orbits. The original comet was probably last seen about 12,000 years ago.
LASCO is a wide-field white light and spectrometric coronagraph developed for flight aboard ESA-NASA's SOHO mission. The launch took place on December 2, 1995. The mission began routine observations in May 1996 upon completion of commissioning activities after arriving in an orbit about the L1 Lagrangian point. In this orbit one million miles from Earth toward the sun, the spacecraft is always in sunlight and able to continuously view the sun. LASCO detected its 500th comet on August 12, 2002. Before the development of space-borne coronagraphs in the 1960s, only about ten sun-grazers had ever been detected. The NRL white light coronagraph, SOLWIND aboard the USAF Space Test Program P78-1 satellite, recorded images of another member of the Kreutz group in August, 1979. This was the first comet ever discovered using satellite instrumentation. SOLWIND and the coronagraph on the Solar Maximum Mission discovered a total of 16 comets from 1979-1989. The increased sensitivity of LASCO detects about one comet a week, compared to the earlier rate of about one comet every 8 months, explains Dr. Russell Howard, head of NRL's Solar Physics Branch.
LASCO views the space around the sun, looking for outbursts of solar activity by blocking out the solar disk to create an artificial eclipse. This artificial eclipse allows the Sun's very faint outer atmosphere -- the corona -- to be studied. LASCO is able to detect comets that would normally be lost in the glare of the sun. Most of these sun-grazers are eventually vaporized in the solar atmosphere. Other non-sungrazing comets have been observed such as in the spring of 1996, when LASCO recorded unprecedented images of the bright Comet Hyakutake making its close approach to the sun.
LASCO was not built for the purpose of detecting comets. Its main job is to detect and even anticipate the origin of coronal mass ejections (CMEs), the solar sources of space weather. CMEs, which were first discovered by NRL in 1971, can reach velocities ten times higher than solar wind, and when directed toward Earth, can have a serious impact on the earth's magnetic field and upper atmosphere. CMEs can cause aurorae and geomagnetic storms, disrupt communications and military systems, and even damage electrical power grids.
LASCO is a set of three coronal telescopes, using nested, concentric fields of view at the center of which a dark occulting disk blocks out direct light from the sun's brilliant photosphere; thus simulating a continuous total eclipse of the sun. The instrument observes the inner, the intermediate, and the outermost regions of the sun's corona over an unprecedented range of distances. LASCO was the first instrument to record coronal features from just above the solar limb, where the coronal glow is about one million times fainter than the solar disk, all the way out to nearly 30 solar radii (nearly 13 million miles) above the surface, where the ever-fainter corona blends into and becomes the solar wind.
High-resolution charged-coupled-device cameras in each telescope have provided detailed images in near-real time to the world-wide-web, with exceptional dynamic range, while large digital memories and high-speed microprocessors have supported extensive onboard image processing and image data compression that allow transmission of up to 10 full coronal images per hour.
Amateur astronomers in Europe have discovered the great majority of the LASCO comets. They download the LASCO real- time images that are put onto the web and analyze them for transient features. Many false detections have occurred because the CCD detectors are also sensitive to energetic particles such as cosmic rays. The tracks caused by energetic particles look like comet tracks, but occur randomly from image to image. Thus they don't appear to be in an orbit toward the sun.
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