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Astronomical Observatory: Cool Images

Images: Comet LINEAR (C/2001 A2)

Comet LINEAR: Although far from the dazzling display of a great comet, Comet LINEAR (C/2001 A2) has put on the best cometary exhibition of 2001. It was discovered in January by the Lincoln Near Earth Asteroid Research (LINEAR) team sponsored by MIT. It reached its closest point to the sun on May 24, and its closest passage by Earth on June 30. Like the planets, comets orbit the sun, although they typically spend most of their time in the distant regions beyond the planets. Comet LINEAR will reach a solar distance 2400 times that of the earth before finally returning to the inner solar system 41,000 years from now. A three-dimensional interactive graphic of the comet's motion among the planets may be found at the JPL website.

Our movie shows the motion of the central portion (or coma) of the comet past a star over a half hour period on the morning of July 2. At five frames a second, the action in the movie is speeded up 400 times. The field of view of the movie is 2.3 by 3.1 arcminutes. At the distance of the comet (37 million km), 2.3 arcminutes is twice the diameter of the Earth. The comet's speed in this movie is 25 km/s!

The light we see is sunlight scattered off of gas evaporating from the ice in the solid comet nucleus and diffusing away in all directions. The nucleus itself is on the order of a kilometer across, too small to be seen even if it reflected sunlight well. However, the one close up image of a comet nucleus that has been obtained by astronomers, that of Halley's comet, shows the nucleus itself is quite dark. The paradox of dark ice may be understood by considering the appearance of a mound of snow at the edge of a mall parking lot in late winter. Sunlight melts the shiny snow on the surface of the mound, leaving a dark crust of dirt and gravel that was scooped up with the snow. The dark crust covers an interior that is still mostly pure snow.

The false color image has the same angular scale as but a wider field of view (8.6x5.6 arcminutes) than the movie. Brightness here is rendered as a series of colors alternating with dark bands. The levels of the bands are equally spaced in brightness. This presentation makes it easier to see that the coma extends over the entire image, although with ever less intensity as distance increases from the nucleus. Notice also that the coma is particularly extended towards the lower right. At the time of this image, the direction towards the sun is towards the upper left. Wind from the sun's outer atmosphere pushes the tenuous outer coma away from the sun, forming the famous comet tail on very large size scales.

Comets are thought to be remnants left over from the epoch of planet formation. Astronomers study their size, composition, and the distribution of their orbits as tests of models of the dynamic evolution of the solar system and of planet formation.

Processing: These images were made by Philip Ammar, a freshman, and Larry Molnar. The 22 images in the movie are 20-second exposures made between 2:41 and 3:09 am EDT on the morning of July 2, 2001 with no filter. Each image was dark subtracted and flat field corrected. The images were aligned to keep the star (GSC 4668:282) at the same position in each one. An inverse exponential transfer function was applied to bring out the low light levels without saturating the comet core.

The false color image is the average of four 20-second exposures made between 3:16 and 3:22 am EDT the same morning. Each image was dark subtracted and flat field corrected. They were coadded keeping the comet core at the same location in each.

Orientation and scale: North is up and East is to the left. The fields of view are 2.3x3.1 and 8.6x5.6 arcminutes for the movie and the false color, respectively. The star in the movie, GSC 4668:282, is 14th magnitude, and has J2000 coordinates 0h25m41s -2deg39'30".

As an extended object, the comet cannot be easily described with a single magnitude. The light inside a 12 arcsecond radius equals that of an 11th magnitude star; that within an 80 arcsecond radius equals an 8th magnitude star. There is also additional light beyond the edge of our image which, though it is low intensity, adds to a significant total because of the large area covered.

Content updated 7/6/01


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