Comet C/2012 S1 ISON
Comet C/2012 S1 ISON was discovered by Vitali Nevski and Artyom Novichonok by 0.4-m telescope of the International Scientific Optical Network (ISON) in Russia on September 22, 2012. Following astrometric observations and precovery observations by Mount Lemmon Survey and Pan-STARRS revealed a long period orbit that will bring comet to its perihelion on November 28, 2013. According to SWIFT, the gamma-ray satellite, the comet nucleus has approximately 5 km in diameter. Coming from the Oort cloud on hyperbolic orbit, it is believed that this is the first entry of the comet into the inner Solar system. The brightness and appearance of the comet early in 2013 suggested that the comet might reach visual magnitude of -17 at perihelion which could make it the brightest comet of the century, visible even during the daylight. It will fly around the Earth at the distance of 0.43 AU on December 26. However, the brightness started to slow down on March and April and even on October 2013 and the brightness is few magnitudes below the original estimate. This could be related to physical processes on its surface and interior or depletion of reservoir of fresh ices on the surface. Therefore, the maximum expected brightness of the comet has been revised to -6. The physical length of the tail as measured on October 5, 2013 at Lowell Observatory (Meech K., Kaluna H. - personal communication) is about 500,000 km. Still, the comet brought attention and a intense observing campaign was established - so far, the comet has been observed by Hubble, in infrared, optical, gamma and radio, from the orbit of Mars (Mars Reconnaissance Orbiter) and vivid media coverage is ongoing.
The comet will pass in extremely small distance above the Sun's surface - only 1.65 diameter of the Sun, it is not clear whether the comet survives the fly-by (Figure 1). The black-body temperature at the perihelion distance would reach 2700°C and the nucleus will be inside of the Roche's limit within the disintegration due to tidal forces is very likely.
Figure 1 (by Nicolas Biver): The figure shows activity of long periodic comets as a function of heliocentric distance. Because the surface of the new incomers is fresh, the activity is high and that could cause total disintegration and depletion of the comet near the Sun (see C/1999 S4, C/2010 X1, C/2002 04). It is also not clear whether the comet will survive. The Hubble and Spitzer observations set the upper limit on nucleus diameter to 4 km. If the nucleus is larger than 1 km, dense enough and does not have fast rotation and is not destroyed by tidal forces, it might survive (Knight M. and Walsh K.)
Comet ISON was observed at the AGO Modra observatory by Stefan Gajdos and Jaroslav Simon on October 5, 2013. The stacks of 5 and 12 frames are shown on figures 2 and 3.
Figure 2: Stack of 5 images, Gajdos, Simon, AGO Modra.
Figure 3: Stack of 12 images, Gajdos, Simon, AGO Modra.
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