Sunday, July 23, 2017

Two visitors from deep space

While I have been spending time familiarizing myself with the 8-inch Celestron EdgeHD Schmidt Cassegrain I recently bought and taking photos with it, I've also been observing with my 15-inch Dobsonian last week at the local club's dark sky site. Normally, it's cloudy and murky here during the summer, but windows do open in the weather of good conditions for observing faint objects from time to time. While I observed the usual Messier, NGC and IC objects that are prominent in the summer skies, I stopped to take a look at two icy visitors to the inner Solar System that are observable right now as soon as the skies are fully dark. They are the long period comet C/2015 V2 Johnson and the periodic comet 71P/Clark.
C/2015 V2 Johnson is a long period comet that has been on the scene for months now, and now it's on it's way back into the frigid, dark outer reaches of the Solar System. Discovered by Jess Johnson of the Catalina Sky Survey, this comet is escaping the Solar System on a hyperbolic trajectory forever after reaching perihelion last June. It's original orbit had an aphelia of about 59,000 astronomical units or nearly a light year away from the Sun, but gravitational perturbations from the giant planets have accelerated it to Solar escape velocity. Through my 15-inch at 227X, it appeared as an elongated object of about eighth magnitude with a bright inner core. Right now C/2015 V2 Johnson in Virgo, soon to be lost in the twilight then daytime skies. It's soon to be a very faint object for large telescopes only, so if you wish to see this comet, take any opportunities available now since it will never return.

The periodic comet 71P/Clark on the other hand has been seen on a number of occasions since it's discovery in 1973 by Michael Clark at the Mt. John University Observatory. This periodic visitor has an orbital period of 5.5 years, an orbital inclination of 9.5 degrees and an eccentricity of .499. That places perihelion at 1.56 A.U., or just outside the orbit of Mars, and aphelion at 4.68 A.U. which brings it close to but not across Jupiter's orbit. It is therefore under Jupiter's gravitational spell. It has been observed at every apparition since it's discovery. Through the 15-inch at 227X, it looked more like a remote and faint galaxy or globular cluster than a comet. It had a round, diffuse appearance with a very weak central brightening, it was a little difficult to see due to the light pollution that pervades the Gulf Coast. This comet is impossible for small telescopes or anyone in badly light polluted areas, therefore plan to look for 71P/Clark in Scorpius from at least a reasonably dark area and an 8-inch or larger telescope. The comet just passed through perihelion three weeks ago, and is now fading quickly. The next appearance of 71P/Clark will be in 2023.

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