Comet 55P/Tempel-Tuttle:
Research at Lowell Observatory

Several different projects involving comet Tempel-Tuttle are underway at Lowell observatory.

Photometry of the comet is being done using narrowband comet filters to isolate the light emitted by different gases. These measurements are used to determine the amount of each gas species that is being released from the nucleus, which gives us clues to the composition of the comet. The amount of light being reflected by the dust is also measured, which tells us how much dust is carried off with the gases. These observations are made at different heliocentric distances so we can determine how the activity levels change with time as the comet approaches and recedes from the sun. These results will then be compared to results from other objects to see how T-T fits into our general understanding of comets.

Imaging of Tempel-Tuttle is being done to observe changes in the comet's appearance and morphology as it approaches the sun. This monitoring process provides a long-term picture of how the dust tail forms and changes with time. In addition, several nights of observing are scheduled for the time around closest approach, when the comet's appearance should change rapidly as it sweeps by the Earth. A sequence of images at the time of closest approach, combined with the long-term monitoring will provide enough information to produce detailed models of the dust tail. These models will give us a measure of the number and sizes of the particles that make up the dust tail, which can then be related to the upcoming meteor shower.

The images will also be used for other studies. Initially, they will be used to look for clues to the rotation of the nucleus and to determine the positions of any active regions that may be located on the surface. Other studies, which depend on what the images show, will also be performed.

The images shown here (displayed in false color) were obtained at the 42" Hall telescope at Lowell Observatory, using a 2048x2048 CCD (smaller sections of the overall frame are shown for clarity). A wideband R filter was used to isolate reflected light from any dust particles in the coma.
T-T Nov 16 Image Tempel-Tuttle on November 16, 1997. The comet was at a heliocentric distance of 1.85 AU and a geocentric distance of 2.22 AU. This image consists of 10 300-second exposures added together to increase the signal of the comet. T-T, at the center of the image, is stellar in appearance.
T-T Dec 30 Image Tempel-Tuttle on December 30, 1997, when it was 1.36 AU from the Sun and 0.77 AU from the Earth. This image consists of 8 separate exposures added together. The coma extends to a distance of about 20,000 miles from the comet. No distinct tail is visible at this time, however, the inner coma is elongated in the SE-NW direction, which suggests that the effects of radiation pressure are starting to be seen.
T-T Jan 18 Image Tempel-Tuttle as it appeared during its closest approach to the Earth. This image was obtained on January 18, 1998, when T-T was 1.18 AU from the Sun and 0.36 AU from the Earth. The coma now extends to a distance of at least 30,000 miles from the nucleus, but there is still no tail visible in the raw data. The long star trails are due to the rapid motion of the comet as it passes the Earth.
T-T Jan 18 Processed Image Preliminary data processing of the January 18 image. This section of the above image, with the nucleus at center, has had a 1/rho profile subtracted off to enhance features in the inner coma. The result shows that the dust emission is not uniform around the nucleus. In addition, further processing shows a possible short, faint dust tail extending to the NW.

Comparison of images from several nights, processed as described above. Images are from Jan 17-19 (Top Row) and Jan 22-25 (Bottom Row). The most obvious changes, e.g., the direction of peak emission and the apparent motion of the stars, are due to the changing viewpoint as the comet passes the Earth. Other, more subtle changes in the morphology of the dust envelope are due either to the changing viewpoint or to inherent changes in the comet itself.
T-T Jan 17 Processed Image T-T Jan 18 Processed Image T-T Jan 19 Processed Image
T-T Jan 22 Processed Image T-T Jan 23 Processed Image T-T Jan 24 Processed Image T-T Jan 25 Processed Image

Comparison of images obtained with the R and V filters (Left and right, respectively, displayed in false colors). Both are 300 second exposures obtained on Jan 25, with the V image obtained shortly after the R image. Both have been processed in the same manner and are displayed at the same scale. The R image primarily shows the dust emitted from the comet, while the V image is primarily gas and ions. Note that the R frame shows a narrow tail extending to the left and slightly upward, that does not appear in the more diffuse morphology of the V image. This is apparently a dust tail composed of very small particles that are being pushed away from the sun.
T-T Jan 25 Processed R Image T-T Jan 25 Processed V Image

False color image of Tempel-Tuttle obtained with the R filter on February 19, 1998, when the comet was 0.98 AU from the sun and 1.22 AU from the Earth. A faint tail can be seen extending to the upper left (the anti-solar direction), even in unprocessed images. The tail is also visible in images taken with the V filter, although it is not as well defined.
TT Feb 19 image

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