Mass and Density Estimates (kg) for Comets

[DaveBowman2001]

Add known parameters for the following comets below:

Density (kg/m³) and mass (kg) estimates:

• 1P/Halley
• 2P/Encke
• 19P/Borrelly
• 46P/Wirtanen
• 55P/Tempel–Tuttle
• C/1995 O1 (Hale–Bopp)
• C/1996 B2 (Hyakutake)
• C/2023 A3 (Tsuchinshan–ATLAS)

Density-only (kg/m³) estimates:

• 17P/Holmes
• 21P/Giacobini–Zinner
• 29P/Schwassmann–Wachmann
• C/1983 H1 (IRAS–Araki–Alcock)
• C/2013 A1 (Siding Spring)

Mass-only (kg) estimates:

• 55P/Tempel–Tuttle
• 109P/Swift–Tuttle

References:

• Jewitt (2004). "From Cradle to Grave: The Rise and Demise of the Comets", Comets II, pp. 659–676
• Liu, et al. (2025). "Great comet C/2023 A3 (Tsuchinshan–ATLAS): Dust loss before perihelion", Astronomy & Astrophysics, Vol. 698, A95
• O'Ceallaigh, et al. (1995). "CCD photometry of comet 109P/Swift-Tuttle", Astronomy & Astrophysics, Vol. 297, pp. 17–20
• Paradowski (2022). "A New Indirect Method of Determining Density of Cometary Nuclei", Acta Astronomica, Vol. 72, 141–159
• Rickman (1989). "The nucleus of comet Halley: Surface structure, mean density, gas and dust production", Advances in Space Research, Vol. 9 No. 3, pp. 59–71
• Sosa, et al. (2009). "Cometary masses derived from non-gravitational forces", MNRAS, Vol. 393 No. 1, pp. 192–214
• Sosa, et al. (2011). "Masses of long-period comets derived from non-gravitational effects", MNRAS, Vol. 416 No. 1, pp. 767–782

[AstroChara]

In the last two commits, a fictional density value has been added to comets with no actual density measurements. This is in contradiction with the current convention for default data files, where fictional data is only added if it is essential for the program to handle the object (such as texture, rotation period), and hence this PR unsuitable for merger. Until these changes are removed, I'll set this PR to a draft.