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RE: Comet Hale-Bopp
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Title: Evidence for fresh frost layer on the bare nucleus of comet Hale--Bopp at 32 AU distance
Authors: Gy. M. Szabó, L. L. Kiss, A. Pál, Cs. Kiss, K. Sárneczky, A. Juhász, M. R. Hogerheijde

Here we report that the activity of comet Hale--Bopp ceased between late 2007 and March, 2009, at about 28 AU distance from the Sun. At that time the comet resided at a distance from the Sun that exceeded the freeze-out distance of regular comets by an order of magnitude. A Herschel Space Observatory PACS scan was taken in mid-2010, in the already inactive state of the nucleus. The albedo has been found to be surprisingly large (8.1ħ0.9%), which exceeds the value known for any other comets. With re-reduction of archive HST images from 1995 and 1996, we confirm that the pre-perihelion albedo resembled that of an ordinary comet, and was smaller by a factor of two than the post-activity albedo. Our further observations with the Very Large Telescope (VLT) also confirmed that the albedo increased significantly by the end of the activity. We explain these observations by proposing gravitational redeposition of icy grains towards the end of the activity. This is plausible for such a massive body in a cold environment, where gas velocity is lowered to the range of the escape velocity. These observations also show that giant comets are not just the upscaled versions of the comets we know but can be affected by processes that are yet to be fully identified.

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Comet Hale-Bopp discovered in 1995



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Comet C/1995 O1 (Hale-Bopp)
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On March 9, 1997, a solar eclipse in China, Mongolia and eastern Siberia allowed observers there to see the comet Hale-Bopp (formally designated C/1995 O1) in the daytime. Hale-Bopp had its closest approach to Earth on March 22, 1997 at a distance of 1.315 AU.
Hale-Bopp had its closest approach to Earth on March 22, 1997 at a distance of 1.315 AU 

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The comet may have been observed by ancient Egyptians during the reign of pharaoh Pepi I (2332-2283 BC). In Pepi's pyramid in Saqqara is a text referring to an "nhh-star" as a companion of the pharaoh in the heavens, where "nhh" is the hieroglyph for long hair.



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Comet Hale-Bopp was discovered on July 23, 1995



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Comet Hale-Bopp has been spotted beyond the orbit of Neptune, far enough from the sun to be without its dirty tail
When a comet is far from the sun, it freezes and stops shedding material. In the case of Hale-Bopp, the Great Comet of 1997, that freeze-out has taken a while - astronomers have spotted Hale-Bopp out past the orbit of Neptune, where it's finally chilling out.

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Title: Frozen to death? -- Detection of comet Hale-Bopp at 30.7 AU
Authors: Gy. M. Szabó, K. Sárneczky, L. L. Kiss

Comet Hale--Bopp (C/1995 O1) has been the single most significant comet encountered by modern astronomy, still having displayed significant activity at 25.7 AU solar distance in late 2007. It is a puzzling question when and where this activity will finally cease. Here we present new observations with the ESO 2.2m telescope at La Silla to check the activity of Hale--Bopp at 30.7 AU solar distance. On 2010-12-04, 26 CCD images were taken with 180 s exposure times for photometry and morphology. The comet was detected in R and had a total brightness of 23.3ħ0.2 mag, referring to an absolute brightness of R(1,1,0)=8.3. The profile of the coma was star-like at a seeing of 1.9", without any evidence of a coma or tail extending farther than 2.5" (=55,000 km in projection) and exceeding 26.5 mag/arcs² surface brightness. The measured total brightness corresponds to a relative total reflecting surface, a_RC, of 485 km², nine times less than three years before. The calculated a_RC value would imply a nucleus with 60--65 km radius assuming 4% albedo. This size estimate is in significant contradiction with the previous results scattering around 35 km. Therefore we suggest that the comet may still be in a low-level activity, despite the lack of a prominent coma. Alternatively, if the nucleus is already dormant, the albedo should be as high as 13%, assuming a radius of 35 km. With this observation, Hale--Bopp has been the most distant comet ever observed, far beyond the orbit of Neptune.

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Hale-Bopp had its closest approach to Earth on March 22, 1997 at a distance of 1.315 AU.
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Comet Hale-Bopp (formally designated C/1995 O1) was arguably the most widely observed comet of the twentieth century, and one of the brightest seen for many decades. It was visible to the naked eye for a record 18 months, twice as long as the previous record holder, the Great Comet of 1811.
Hale-Bopp was discovered on July 23, 1995 at a great distance from the Sun, raising expectations that the comet would brighten considerably by the time it passed close to Earth.

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