Alan Hale

For the Daily News

The inner solar system is awash with short-period comets. These are objects that, at some point in the not-too-distant past, were captured into short-period orbits – typically, five to seven years – by the giant planet Jupiter and its enormous gravitational field.

A few hundred of these comets have been discovered – with more being found all the time – and as many as two dozen or more pass through the inner solar system every year. Most of these are far too dim to be of any interest to the backyard sky-watcher, however there are a few that can, on occasion, become bright enough to be of interest. Such comets can become even more interesting if they come close to Earth, and as it happens one of these comets will be passing quite close to our home planet a little later this month.

The comet in question was initially discovered back in December 1948 by a Japanese amateur astronomer, Minoru Honda, who discovered eleven additional comets between the early 1940s and the late 1960s. A few days after Honda’s discovery, two members of a comet-hunting team based at the Skalnate Pleso Observatory in then-Czechoslovakia, present-day Slovakia, Antonin Mrkos and Ludmilla Pajdusakova, independently discovered the same comet. The comet thus goes by the rather cumbersome name of Honda-Mrkos-Pajdusakova, although for obvious reasons this is usually informally shortened to H-M-P, or, more formally, the comet is referred to by its official designation, 45P.

Comet 45P has an orbital period of close to 51Ž4 years, and has been seen at almost every return since its original discovery. It comes closer to the sun – approximately 50 million miles – than do most short-period comets, and on several returns has become bright enough to be visible in relatively small backyard telescopes. On the previous return, in 2011, it passed only 5.6 million miles from Earth, however the comet was located deep in southern skies at the time, and the approach time also coincided with the full moon and with widespread cloudy weather throughout the southern hemisphere, so this cometary encounter was not as widely observed as it otherwise might have been.

On its present return Comet 45P was closest to the sun on this past New Year’s Eve. For a couple of weeks prior to that, and for a week or so afterwards, it could be detected low in the southwestern sky after dusk, and actually became bright enough that it could be detected with binoculars. After that it sank into the twilight and disappeared from view as it has raced on ahead of Earth in their respective orbits, and earlier this week it passed almost directly between the Earth and the sun.

Having pushed on ahead of Earth, Comet 45P is now emerging into the morning sky, and by the end of this week will already be rising before the beginning of dawn. Over the course of this coming week it travels northwestward through the constellation of Aquila, the eagle, the northern part of the constellation Ophiuchus, somewhat to the north of the bright star Rasalhague, and on into the constellation of Hercules. It should at least be bright enough to detect with binoculars during this time, and it is conceivable that it might even be bright enough to see with the unaided eye from dark rural sites.

Comet 45P comes closest to Earth, 7.7 million miles, on Feb. 11 although that unfortunately coincides with the full moon. Afterwards it continues traveling rapidly westward, across the constellation of Bootes and then the constellation of Canes Venatici, southeast of the Big Dipper, and will be visible in the northeastern sky during the later evening hours. By that time it will be fading rapidly, however, and late this month, when it will be located somewhat north of the hindquarters of the constellation Leo, lion, it will likely have faded to the point where only moderate-size backyard telescopes will be able to detect it. Even these instruments will be unable to follow it for much longer.

While both Earth and Comet 45P will continue on in their respective orbits for the indefinite future, they do not come anywhere as close to each other again for quite some time; they’ll pass 51 million miles from each other in 2027 and 32 million miles from each other in 2032, but there are no significantly closer approaches until late in the 21st Century – and even these are nowhere as close as the present one. Curiously, however, Comet 45P will make a couple of very close approaches to Venus during the late 21st Century.

But while Comet 45P won’t be making any more close approaches to Earth for quite some time, there will be other close-approaching comets from time to time to take up the slack. Indeed, another fairly bright short-period comet, which goes by the equally cumbersome name of Tuttle-Giacobini-Kresak but which is normally referred to by its official designation of 41P, will be coming quite close to Earth in early April. That particular comet has a most interesting observational history, and will be discussed in this column at the appropriate time.

Alan Hale is a professional astronomer who resides in Cloudcroft. Hale is involved in various space-related research and educational activities throughout New Mexico and elsewhere. His web site is http://www.earthriseinstitute.org.