Neat! Comet Crossing

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Coronal Mass Ejection timed to comet NEAT. Credit: SOHO/SWAN and SOHO/LASCO (ESA & NAS


To solar physicists, the saying that there’s “nothing new under the Sun” couldn’t be further from the truth.

For close observers, the dynamic fireball that makes possible all life on Earth provides a spectacular magnetic show of both heat and light.

NASA and the European Space Agency (ESA) keep a round-the-clock watch on the Sun. To track our star’s health –its weather and light spectra,– the US and Europe share observation time on a satellite called SOHO, or the Solar and Heliospheric Observatory. Its twelve instruments are tuned to even faint changes in the Sun, including the occasional comet that crosses nearby.

One of the spacecraft’s instruments, called the LASCO coronograph, is particularly familiar to solar scientists because it uses a central dark cover to block the brilliant Sun. This blotting-out method allows more sensitive cameras onboard to pick up the fine details of the faint corona and outer atmosphere. The effect is similar to creating an artificial solar eclipse.

Remarkably this solar observatory has such an enormous field of vision that it can take pictures extending outwards more than 13 million miles (or nearly 32 solar radii across). That wide-angle ‘lens’ spans to nearly one-tenth of the distance to the Earth (93 million miles away, or one Astronomical Unit [AU]).

This week, on just such a close approach by a comet called NEAT, solar scientists caught quite a blockbuster of a movie.

As a comet approaches the Sun, it undergoes dramatic changes–sometimes brightening as the pressure of the solar wind pushes its thin dusty wake outwards and elongates its tail. For some comets that approach closely within the LASCO’s field of view (0.1 AU), the more intense ‘wind’ pressures can even split apart the comet’s body.

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Icy-rock core of Halley’s Comet

More and more such comets are first seen by automated telescope patrols, designed to scan the skies looking for objects that could pass close to Earth. These discoveries are given catalog references, as is the case for the NEAT Comet: C/2002 V1. The comet was discovered by NASA’s Near Earth Asteroid Tracking program (thus its tracking name, NEAT). When first caught by such an automated telescope, it was 25,000 times fainter than the human eye can perceive. Because it had no history in the orbital tracking databases, the comet was originally thought to be newly-formed.

But as the comet came closer into view, its highly elliptical orbit became more discernible. Such a sharp oval indicates that its journey inward under the Sun’s huge gravitational field can take a very long time. In the case of the comet NEAT, the last time it would have passed through the inner solar system was 370 centuries ago (37,000 years)!

Initially, the comet became so bright that astronomers wondered whether as it rounded the Sun, they would be able to see it during the day. But during January 2003, the comet failed to brighten as hoped. Not to be disappointed, the science team continued to watch for changes nearer to close-approach.

On February 16, during the comet NEAT’s closest approach, the LASCO pictures and movies became quite out of the ordinary. The comet finally cast out a sizeable tail from its very bright (saturated) comet nucleus.

The movie even caught a Coronal Mass Ejection (CME) off the west limb close to perihelion time. Coronal Mass Ejections (CMEs) are massive (1014 to 1017 grams – ~quadrillion) bursts of plasma and hot gases that are ejected from the sun. The ejection is thought to be caused by instabilities in the Sun’s magnetic field; large ejections are known to interfere with terrestrial power line transmissions and cause geomagnetic storms in Earth’s atmosphere.

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Animation of Sun’s spectacular greeting to passing comet tail. Click image for larger picture. GIF Animation 180 kB. Also see full disk (480 kb) and gallery animation Credit: NASA/ESA/SOHO

In addition to these coronograph images, a full-color sky map tracked the approach of the NEAT comet months in advance.

Although comet NEAT (C/2002 V1) was not visible in the LASCO images until about 16:00 (Universal Time) on February 16, it had in fact been tracked by SOHO since December 31 last year.

At 10,000 foot elevation (above one-third of the Earth’s atmosphere) the 1.2-meter (48-inch) reflector telescope at Haleakala, Hawaii, discovered NEAT as a 17th-magnitude comet. As NEAT passed within one-fourth Mercury’s orbital distance, it brightened 19 orders of magnitude (yet still remained visible only to SOHO, and not naked-eye skywatchers). The rapid increase in brightness is typical of a small comet nucleus. Although the comet has rounded the Sun headed outward, the Southern Hemisphere observers can still glimpse the comet low in the sky [west-southwest] shortly after sunset until March 10.

A full-sky map shows its tracking in widest angle. The SWAN image and movie projects the full sky map in ecliptic coordinates, so anything that moves in this type of movie is moving relative to the stars as seen from SOHO. Comets can be seen by SWAN because their water ice evaporates as they are heated by the Sun. The water in turn is split into oxygen and hydrogen by the radiation from the Sun. The resulting hydrogen cloud is denser than the diffuse gas normally observed by SWAN, so it stands out clearly in their full-sky images, in particular after removing the signal from stars and a smooth background component.

In collaboration with Finnish science teams, researchers snapped the full picture of the sky to complete their diagnostic photo album. The SWAN instrument scans the whole sky using two periscopes (on each side of the spacecraft), looking at how diffuse, slow-moving neutral hydrogen glows in the light from the Sun. In order to protect the instrument from much stronger light than that of the diffuse gas, patches of the sky close to the Sun cannot be observed; the Earth and certain protruding parts of the spacecraft (e.g. solar panels) also has to be avoided – hence the two distinct cut-out sections in its images.

What’s Next

LASCO also takes special observations with extremely short exposure times during the passage, to increase the likelihood of spotting a possible fragmentation of the comet due to its close approach. These images will be available, after processing, due to the non-standard image enhancements that have to be performed in order to remove the image “background”.

NEAT is contributing to the NASA 10-year goal to discover more than 90% of the Near-Earth asteroids larger than 1-km in diameter. Achieving first light on February 8, 2000, it is the largest aperture telescope with a regular Near-Earth object detection program.

The Large Angle and Spectrometric COronagraph (LASCO) instrument is one of 11 instruments included on the joint NASA/ESA SOHO (Solar and Heliospheric Observatory) spacecraft. SOHO was launched on 2 December 1995 at 0808 UT (0308 EST) from the Kennedy Space Center, Cape Canaveral, Florida. The LASCO instrument is a set of three coronagraphs that image the solar corona from 1.1 to 32 solar radii. It is convenient to measure distances in terms of solar radii. One solar radius is about 700,000 km, 420,000 miles or 16 arc minutes. A coronagraph is a telescope that is designed to block light coming from the solar disk, in order to see the extremely faint emission from the region around the sun, called the corona. The SWAN instrument- Solar Wind ANisotropies- is a collaboration between Finnish Meteorological Institute and Service d’Aeronomie. Technical Research Centre of Finland was responsible for the construction, mechanics, thermal design, and central processor of the instrument itself.


 

Related Web Pages:

The Sun as Art
SOHO Observes Comet Kudo-Fujikawa (SOHO Pick of the Week)
Surf the Web to see the Sun-dancing comet (ESA Science Story)
Comet C/2002 V1 (NEAT) (Gary W. Kronk’s cometography)
Comet C/2002 X5 (Kudo-Fujikawa) (Gary W. Kronk’s cometography)
Solar Full Disk (Astrobiology Magazine)

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