Monday 28 April 2014

Observing Comet 17P/Holmes with the WISE Space Telescope.

Comet 17P/Holmes is a Jupiter Family Comet with a 6.89 year orbit that takes it from 2.06 AU from the Sun (2.06 times the average distance at which the Earth orbits the Sun, considerably outside the orbit of Mars) to 5.18 AU from the Sun (slightly inside the orbit of Jupiter). Since its discovery in 1892 it has been observed to undergo three massive outbursts when it increased dramatically in brightness; the last of these was in October 2007, when its brightness increased by a factor of almost a million, and it briefly became visible to the naked eye. Each of these outbursts has come 6-9 months after the comet reached its perihelion (the closest point in its orbit to the Sun).

Image of 17P/Holmes taken on 2 November 2007. Syuichi Nakano/IQC/Havard Cometary Science Archive.

In a paper published in on the arXiv database at Cornell University Library on 22 April 2014, a team of scientists led by Rachel Stevenson of the Jet Propulsion Laboratory at the California Institute of Technology, discuss the results of a study of a series of images of 17P/Holmes taken by the Wide-Field Survey Explorer (WISE) Space Telescope on 14-15 May 2010, when 17P/Holmes was 5.1 AU from the Sun (i.e. 5.1 times the average distance between Earth and the Sun), and roughly five months short of reaching its aphelion (the point in its orbit when it is furthest from the Sun).

Based upon the data gathered by the WISE telescope, Stevenson et al. were able to determine that 17P/Holmes has a core with an equivalent diameter of 4.135 km (i.e. a spherical body with the same volume as the comet would have a diameter of 4.135 km). It is surrounded by a cloud of material (halo) with a temperature of 134 K (-139.15˚C) which, while very cold, is slightly warmer than would be predicted for particles at this distance from the Sun, suggesting that the particles are either too small to radiate heat effectively, have a rough surface which does not radiate heat evenly, or are large enough to maintain thermal gradients internally (the cloud is probably made up by a mixture of particles showing all three properties). Dynamical modelling of the material in the observed tail of 17P/Holmes suggests that it was mostly made up of material produced by the October 2007 eruptive episode.

Comet 17P/Holmes as observed by the WISE mission on 14-15 May 2010 at a wavelength of 22 µm. The nucleus is located in the south-east corner of the image. Celestial north (N) and east (E) are marked, as are the solar ( Sun symbol.svg) and velocity (v) vectors. (Note celestial east is reversed with respect to normal maps, since the observer is looking up). Stevenson et al. (2014).

This still leaves the question as to why 17P/Holmes undergoes periodic spectacular outbursts, unlike those seen in other Jupiter Family Comets, which typically only undergo moderate outgassing at perihelion. None of the properties uncovered by WISE are atypical for such a comet, and previous studies have shown that its chemical composition is also typical. Stevenson et al. suggest that the outbursts may be related to eccentricities in the orbit of 17P/Holmes, which at perihelion in 2007 was 0.12 AU closer to the Sun than it had been at its previous perihelion in 2000. This might potentially cause solar heat to reach deeper into the comet, potentially reaching pockets of highly volatile gasses such as carbon monoxide or carbon dioxide, which would result in a more spectacular outburst than at the previous perihelion. However Stevenson et al. also note that this explanation would require 17P/Holmes to have a remarkably high internal tensile strength to survive such a process. Estimates of internal tensile strengths for the Jupiter Family Comets 16P/Brooks 2 and Shoemaker-Levy 9, which were both broken apart by tidal forces produced by the planet Jupiter, suggest that they had internal tensile strengths of 0.1-0.38 kPa (kilopascals), while the Stevenson et al. explanation for the outbursts on 17P/Holmes would require it to have an internal tensile strength in the 10-100 kPa range.

The orbit of 17P/Holmes. JPL Small Body Database Browser.

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