Comet Ison Updates Thread
Hubble’s scientists debunk theory of Comet ISON being a UFO
It’s not at all certain that Comet ISON will turn out to be the “comet of the century,” as hoped, but a couple of things are certain: It’s not an alien spaceship, and it hasn’t split up into three pieces. Those were apparently questions on the minds of some folks last month, thanks to a flurry of videos and blog postings based on imagery from the Hubble Space Telescope’s archives. The hoohah got hot enough to merit an official response, posted to the Space Telescope Science Institute’sarchive website and its ISON Blog. It all started with a series of images captured by Hubble’s Wide Field Camera 3 UVIS instrument on April 30. Various exposures were combined to produce a widely distributed color picture of Comet ISON against a background field of stars. When Internet sleuths took a close look at the archived image, it looked as if there were three separate objects hiding in the glare of ISON’s coma. Was ISON breaking up?
Was the comet being escorted by two alien spacecraft? No. Just no.
Today we offer a guest guest post from Dr. Richard L. White. Dr. White is an astronomer at the Space Telescope Science Institute and is the Principal Investigator for the Barbara A. Mikulski Archive for Space Telescope (MAST). He is a coauthor on several hundred papers covering topics ranging from cosmology, quasars, stars and galaxies to astronomical data processing and data mining. This article was previously posted at the Barbara A. Mikulski Archive for Space Telescopes news feed. For more information you can also check out our post about making the Hubble color image of Comet ISON.
Some bloggers have noted that the Comet C/2012 S1 (ISON) Hubble images seen here have some surprising features. Here we briefly explain the origin of the image structures.
Quick summary: The image is the result of combining three exposures that produce the three components, and the shapes are produced by the combined motion of the Hubble telescope and the comet. The images look exactly as expected.
The image from April 30, 2013 (released on July 16, 2013) using the F606W filter has attracted the most attention. That image (shown above) is the average of three separate Hubble exposures. The three exposures are available individually for display or download from the web page, as is the combined image
You can see that the three parts of the structure in the combined image result from the three different exposures. The comet itself does not have three pieces. They are an artifact that results from adding up the separate exposures.
The comet does not look the same in each exposure because both the comet and the Hubble telescope are moving during the exposure. The comet is blurred, just as a picture taken out the window of a moving car will be blurred. For this image the Hubble telescope was pointing very steadily and accurately at the background stars and galaxies rather than tracking the comet.
In this case, the largest part of the blurring is due to the motion of Hubble itself rather than the motion of the comet. Hubble is orbiting the Earth every 95 minutes. The third exposure ended 46 minutes after the start of the first exposure. During that time the telescope moved from one side of its orbit to the other, a distance of about 8000 miles (~13,000 km). Because the comet is relatively nearby (compared with the distant stars and galaxies), its apparent position among the stars changes due to the different viewing position. This effect is known as parallax. It is expected when observing solar system targets, and can produce very obvious motion or blurring in images as Hubble whips around the Earth.
There is a nice discussion of the effects of parallax on the Comet ISON images in this YouTube video. It includes detailed modeling of the parallax in these particular images and shows that the observed pattern is exactly what is expected.
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Originally posted by Altaira
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