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Comets and Asteroids |
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CCD Astrophotography |
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Comets and asteroids, as visitors from the outer regions of our
Solar System, have similarities. Asteroids range from 500 to a few miles in diameter and generally orbit between Mars and Jupiter. Some have eccentric orbits and pass close to the Earth and Sun on a periodic cycle. Near the Sun they can even exhibit some cometary behaviour. Their make up can be carbonaceous and mineral, consisting of agglomerated remnants from the origin of our Solar System, and hence their interest. An asteroid (e.g. 85 Io) in a starfield can be found by it's movement over a period of time. The larger bodies such as Ceres and Pallas
are reasonably bright and can be readily found with
binoculars, and are called Minor Planets. Asteroids and larger bodies are
discovered regularly. 2004 DW is a recently found large planetoid some 900 miles in
diameter.
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85 Io, mag 10.6, imaged in Aquila on Sept 4 2003 by 200mm Schmidt-Newtonian at prime focus. Note the movement of 85 Io against the starfield in superimposed images taken over 35 minutes. Exposure: 2 minutes.
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Most comets approach the Sun from a distant comet reservoir region beyond Neptune called the Oort Cloud. The orbits of comets are more eccentric i.e. less circular than asteroids, so they may only visit our Solar System once. But some can be trapped in so doing and become periodic visitors such as Comet Halley, which visits every 76 years. Comet
2P Encke, a short-period comet, visits every 3.3 years. Their ice and dust content makes them exhibit a tail or coma as they approach the Sun. It's radiation causes the ice and dust to volatilise and released gases to ionise. The ionised gas tail, which can be coloured, thus points away from the Sun. The comets and indeed asteroids have been proposed as bringing both water and life itself through organic molecules to Earth during it's early bombardment after formation. Alternative
life producing mechanisms include (electro)chemical reactions and also "black smokers" on ocean floors.
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Schmidt-Newtonian images of C/1999 S4 LINEAR on 18 July 2000 at 01:00 hrs. I min. exposures. The lower shows two superimposed images 16 minutes apart to demonstrate that the ion tail does not follow the comets path. The tail is in the direction of the Sun's radiation.
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Dependent upon the comet's make up and closest approach to the Sun (perihelion) it may appear as a nebulous glow or present a significant tail. Imaging of comets allows path parameters to be measured to determine if the comet is periodic.
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My Comet C1/2002 Ikeya-Zhang images were taken in March 2002 but this same comet may have visited us in 1652 and earlier.
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Comet C1/2002 Ikeya Zhang S/N image on 29 March 2002 with red, green and blue filters. 1 to 2.5 min. exposures.
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The unpredictable nature of comets means that they are often imaged even under poor seeing conditions. Such images need to be enhanced, e.g. for the removal of background illumination, as for my Comet C/2000 WM1 LINEAR images in December 2001.
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Some comets (Borrelly) and asteroids (Eros) have been photographed close up by space probes and also landed on. The Contour probe launched July 2002 was to rendezvous with Comet Encke in Nov. 2003 but was lost. NASA Stardust probe sampled the dust of comet Wild 2 in Jan. 2004 for return to Earth in 2006. Rosetta's lander is due to descend on comet Churyumov Gerasimenko in Nov. 2014. Comets may unpredictably break up as with Comet C/1999 S4 LINEAR. Debris from comets and asteroids can cross Earth's orbit and burn in the atmosphere to display as meteorite showers
and fall to the earth as meteorites. The Leonid showers are associated with the visit of Comet Tempel-Tuttle.
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Comet C/1998 M5 LINEAR of predicted mag. 10.1 imaged in Lyra on 16 Dec. 1998 at 18:03 hrs. with Schmidt-Newtonian at prime focus. The larger star at lower right is GSC-3118-1245 of mag. 11.96. The field of view is 8.5 x 8.5 arcmin. 3 by 3 min. exposure.
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Comet C/1999 H1 Lee of mag. 10.0 imaged in Pegasus on 19 October 1999 at 23:20 hrs with Schmidt Newtonian at prime focus. The RHS star (double) is Tyc 2242-1981-1 of mag. 8.27. Field of view is 8.5 x 8.5 arcmin. Moonlight background illumination removed. 4 min. exposure.
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PJM's Astronomy Gallery
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