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.