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| Just after midnight 30 April 2017.Composite stacked image using the 127mm Refractor with 0.8x focal reducer and field flattener and Canon 600d DSLR. The image is a section of a wider-field and is the composite of stacked 10x 1 minute exposures taken at ISOs 1600, 3200 and 6400. |
" A comet is a minor member of the Solar System which travels around the Sun in an orbit that is generally more eccentric than the orbits of the planets. Comets typically have three parts : The 'nucleus', the 'coma' and the cometary 'tails'.
The nucleus is the solid portion of the comet. The size of cometary nucleii vary. Most are less than a kilometer in diameter but some of the more spectacular comets are larger. Hale Bopp, probably the best comet seen from the UK in the last 200 years was estimated to have a diameter between 40 and 80 kilometers. The cometary nucleus, described by the astronomer Fred L Whipple in 1951 as a 'Dirty Snowball" is thought to be made up from dust and frozen; water ice, carbon monoxide, carbon dioxide, methane and ammonia.
The coma is the most obvious part of the comet when it is close to the Sun. The coma is a diffuse luminous nebulous cloud of gas and dust that surrounds and hides the cometary nucleus from sight. It is formed by the sublimation of ices and the ejection of dust particles from the nucleus. The coma is densest closer to the nucleus where the material being released is at its highest. The sublimation is driven by the warming of the nucleus by sunlight and so the size of the coma varies with distance from the Sun. The luminosity of the coma is mainly produced by fluorescence from a variety of carbon, nitrogen,hydrogen and oxygen ions and the reflection of sunlight by dust particles. The coma is extremely rarefied and consequently even faint stars may be seen shining through it. Comas may be circular or fan -shaped.
The cometary tails. If there are tails of gas and dust, they point away from the nucleus in a direction opposite to the Sun. The tails are only readily apparent when the comet is near to the Sun and usually within two astronomical units (x2 the average distance between the Sun and the Earth). Not all comets have visible tails, but when present, their luminosity is due to both molecular and atomic emission and to the reflection of sunlight. The dust and the gas is forced away from the nucleus by the radiative pressure of the Sun and solar wind interactions. There are two very different types of cometary tail; the plasma or ion tail which is often straight and bluish, consisting of ionized molecules moving at speed and the more strongly curved dust tail. The dust tail shines simply because it is made from billions of tiny solid particles which reflect sunlight.
Comet 2015 V2 Johnson was 152.72 million kilometers from Earth and 259.82 million kilometers from the Sun when it was imaged just after midnight on the 30th April from the Jodrell Plank Observatory. The comet was at Magnitude 7.64 and was located at RA 15 degrees 53 minutes 43 seconds and Dec 43 degrees 56 minutes 14 seconds N."
Kurt Thrust - Director of the Jodrell Plank Observatory
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| Positive and negative images taken from the Observatory showing details of the diffuse fan- shaped coma |
"Most comets move in elliptical orbits (some like Halley's Comet being retrograde). They fall into two distinct classes; 'long period' comets and 'short period' comets. The former are the great majority and have orbital eccentricities very close to unity (1.000 - almost parabolic). The much smaller group of short period comets are regular returnees to our skies following elliptical orbits with the Sun at one focus of the ellipse and at all inclinations to the ecliptic. Most comets reach perehelion (closest point to the Sun) at one or two Astronomical Units (AU) and move within a sphere centred on the Sun. The aphelia (greatest distance from the Sun) of long period comets may extend as far as 50,000 AU with orbital periods as long as a million years or even more. In some cases planetary gravtational effects may make a cometary orbit hyperbolic (ie. where orbital eccentricity is greater than unity), while in others the orbit may be changed into a small ellipse. C2015 V2 (Johnson) will leave our Solar System on a hyperbolic orbit never to be seen again - so when its gone its gone! With every perehelion event, a comet loses material as a result of the intense solar heating and gravitational tidal forces. So eventually short period comets, like the Cheshire Cat, just evaporate away".
Archie Mendes - visiting theoretical astronomer at the Jodrell Plank Observatory - Reydon University - 'School of Computer Modelling and Difficult Sums' - author of "Quantity Surveying and Standard Methods of Measurement in Curved Space Time" or "When the Socks come off!"
