A summary of Comet 17P observation
This is the second time Comet 17P/Holmes having an outburst (first outburst 1892). According to news, it was first noticed on 2007 Oct 23~24 when its magnitude increased from 17 to 2.5 in five hours. On 2007 Nov 09~10, the comet was observed in Tai Mo Shan with amateurs LT Ng and YC Tang. The sky was fairly good. 17P, accompanied by star Mirfak (Alpha Persei 天船三), resembled a near-Moon sized snowball in 8x42 binoculars. Few single shots were tried by me using Ng’s equipment and Tang’s CF memory card. Two of them were studied as below.
Position of Comet 17P
http://alanchuhk.com/Comet_17P_071109_1708UT_50pct.jpg (Image 8622)
This image was taken northward with a 100-mm focal length camera lens, field of view 12.5 x 8.5 degrees. The bright star near the frame center is Mirfak, magnitude 1.8. The comet is at an altitude of 61 degrees, about 2 degrees east of Mirfak.
Raw Image of Comet 17P
http://alanchuhk.com/Comet_17P_071109_1508UT_Raw_50pct.jpg (Image 8537)
No image enhancement except size reduced by 50%. It looks pink-red due to city light pollusion and characteristics of the camera.
Processed Image of Comet 17P
http://alanchuhk.com/Comet_17P_071109_1508UT_50pct.jpg (Image 8537)
In this processed image, the red of the starry background was suppressed. 17P looks blue, the typical color of a comet. It also has a trace of tail although it is fuzzy and short. Probably this is due to perspective as we view the comet head-on. The coma is asymmetrical with an off-center hot-spot where gas and dust sublimate outward to form the coma. I am not sure the cometary nucleus is inside this hot-spot or not. The coma interior is not quite homogeneous, as suggested by its brightness variation across the envelope. Did the outburst of 17P not uniform ?
Stars occulted by Comet 17P
http://alanchuhk.com/Comet_17P_071109_1508UT_StarOccultation.jpg
In this image which compares the background stars in 17P with The Sky map, at least 10 stars are seen occulted by 17P. The coma is centered at RA = 03hr 36m 38s, Dec = +50 deg 37’ 21” at the given time. Its size is 22 arcmin = 0.367 degrees, measurement uncertainty +- 1 arcmin. It is also known from ephemeris that the distance of 17P from Earth = 1.623 AU = 243 million km (http://cfa-www.harvard.edu/iau/Ephemerides/Comets/0017P.html). Hence, physical diameter of 17P coma = 243 * pi / 180 * 0.367 = 1.56 million km or 1.12 solar diameter (bigger than the Sun).
Average growth rate of coma from Oct 24 to Nov 09
= radius of coma / duration = 780,000 km / 17 days = 1900 km/hr = 0.53 km/s (supersonic).
Now suppose the mass of 17P is 4 * 10^14 kg (a wide guess cited from reference books) and half of the mass goes to the coma, the average density of coma will be mass/volume = 2 * 10^14 kg / (4/3 * pi * the cube of 780,000 km) = 0.000 000 000 000 1 mg/cc, a super-super low level not yet reached by human technology. The coma of a comet is indeed a “visible vacuum”.
Amazing!
Alan Chu
Last update: 2007.11.18, Hong Kong
Observing_Comet_17P_1.doc 1 of 1