February 21, 2019

Astro-Event: Will Comet McNaught Perform?

During the month of June, keep your eye out for a pre-dawn visitor that may be the naked eye comet of the year. Comet McNaught C/2009 R1 starts the month out June 1st at about +8 magnitude near the star Beta Andromedae, and through the course of the month, skirts the constellations of Andromeda, Perseus, and Auriga. Views will get better starting June 4th, as the Moon passes Last Quarter on its way to New June 12th. The daily apparent motion of the comet will cause it to move semi-parallel to the horizon, never straying above 20° degrees elevation for mid-northern latitude observers about an hour before sunrise.

At closest approach in mid-June, Comet McNaught should be at its most interesting, at a distance of about 1.135 A.U. from Earth and perhaps topping a dark sky naked eye visibility of magnitude +6. If not, comet McNaught should still be a fine binocular object for early risers. Discovered by Robert H. McNaught during NASA’s Siding Spring Survey looking for potentially hazardous Near Earth Objects last fall, Comet McNaught (one of 54 named) will reach perihelion on July 2nd. In addition to Beta Andromedae, some interesting celestial markers it will pass are NGC 752 (June 5th), Gamma Andromedae (June 7th), M34 (June 10th), Mirfak (June 14th), and Capella (June 21st).

Our Astro-term for the week is: Oort Cloud. This is the vast spherical reservoir of comets that exists from a distance of 2,000 A.U. to about 50,000 A.U., a distance equal to about one light year. The concept was coined in 1950 by Dutch Astronomer Jan Oort, who noticed that a large number of isotropic or long period comets had aphelia of about 20,000 A.U. suggesting a large body of cometary material resides at that distance. Comet McNaught belongs to this class, as it is on a hyperbolic orbit and hence making its first trip into the inner solar system. The Oort cloud is estimated to have trillions of objects larger than 1 km in size, but with its enormous radius, these individual nuclei are still dozens of millions of kilometers apart. The Oort cloud is thought to be a remnant of the early solar nebula, and hence, scientists are keen to examine cometary material up close. The entire mass contained in the Oort cloud is estimated to be of the order of 3 X 1025 kilograms, the equivalent of 5 Earths. This shell is periodically perturbed by the gravitational tug of passing stars, causing massive swarms of comets to rain down on the inner solar system throughout its history…thankfully, we’re a small target, and massive planets like Jupiter act as a sort of “cosmic vacuum cleaner” protecting us from a majority of impacts.

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