Around Christmas time, an icy invader from the depths of the solar system may become visible to the unaided. But whether Comet Wirtanen 46P really is visible — and whether it's a dud or astonishing — is uncertain. Comets are notoriously unpredictable, as a review of the last few decades shows.

The following are stories of some comets I've observed.

Comet Halley

The 1910 appearance of Halley, the most famous comet, was breathtaking. When it returned in 1986, we were sad that my wife Cory's grandfather, Henry William Hansen, born in 1893, wasn't here to see it because he had been impressed by it in 1910. He passed away in 1980 and we had hoped he would enjoy its return.

The most recent apparition was a bust though. From my journal entry of April 19, 1986:

"The three of us saw Halley's Comet yesterday, finally. With much grumpiness and confusion, finding boots and cameras, filling a thermos with coffee, we went out to Stansbury Park (Tooele County, Utah). Cory drove. There several hundred people were gathered at the small grassy park-like area where the Salt Lake Astronomical Society was having a 'star party.' As soon as we got out someone told us where to see the comet, and Cory tried to pull the caps off her binoculars to see it. But it wasn't convenient there in the parking lot so we made our way toward the noisy and crowded area around their telescope dome. Finally we saw it. It was invisible to naked eyes, but through binoculars it was a fuzzy ball, bigger and dimmer than the stars. We could not see the tail because of the bright moon. Sky (our son, 6) and I also took a look through a good-sized reflector telescope someone from the astronomical society had set up — the eyepiece lens on it must have been less powerful than … Cory's binoculars because the comet seemed smaller through it. It was also much brighter. 'You're looking at the nucleus,' its owner said. I definitely saw the tail, like a light crease in space pointing downward and a little to the right. I asked what direction the tail was supposed to be pointing, and he said straight up — but quickly added that the telescope reversed everything and turned it upside down too. Anyway, we both saw it. Someone said it was much brighter in March, and the tail could be seen with the naked eye."

Maybe Halley's Comet will make a better impression next time around, in 2061.

Comet Shoemaker-Levy 9

We did not see the actual comet, which was broken into a chain of fragments, but were delighted to see their impacts on the planet Jupiter. Pieces slammed into Jupiter's cloud layers for six days in July 1994. We watched through telescopes the night of July 21, waiting while vast dark smears — the impact sites — rotated into view.

The observing location was Little Mountain near Salt Lake City. Members of the Salt Lake Astronomical Society provided views through telescopes, hot chocolate and cookies.

The air remained steady through the early evening until about 10 p.m., so we saw the large impact sites above Jupiter's south pole. We could see four dark blotches at once in the southern hemisphere, below the planet's most prominent bands. A single hole had been drilled through the clouds on the trailing side of one of the larger smears. The spots, though 475 million miles away, were so clear that we saw distinct edges with lighter material around them.

Comet Hyakutake

My friend (the late) Conrad Bert and I were determined to see the comet at its closest approach, the night of March 25-26, 1996. But cloudy weather ruled out any nearby spot, so I kept driving south and west. By the time we gave up, we were in Nevada. We camped at Valley of Fire State Park. From my journal of March 26, 1996:

"Conrad called me out of my tent, as he had awakened and discovered that not only had the moon set, but the haze and clouds had cleared. There was the comet, stunning in its brightness and size. Before I hit the sack it was so bright that it looked like another huge star burning through the clouds and changing the look of the Big Dipper. After I got up, though, I was dumbfounded by its beauty and scariness. It was filmy but quite distinct, the head bright, a bit of a fan just beyond that, and then the tail stretched straight out toward the east as wide as the nucleus and around 20 degrees long — two fists. Maybe if I studied the tail where it faded out, I could have seen that it was 2½ fists long altogether."

Comet Hale-Bopp

Hale-Bopp, lighting up the sky just a year after Hyakutake, was the most beautiful comet I've seen so far. Its closest approach to Earth was on March 22, 1997, and I was able to see it without astronomical aid from Feb. 18 until May 5. I studied it more than a dozen times.

The night of March 9-10, 1997, Conrad, Cory Maylett and his wife, Rhonda, and I camped at Coral Pink Sand Dunes State Park near Kanab, southern Utah, and had a glorious view. At first, Hale-Bopp was a fuzzy spot on the horizon, behind clouds, at nightfall. In the morning it rose in the northeast but then swung eastward as it climbed.

"Tonight the tail seemed wider and longer than before and through the binoculars the central column of the tail was more pronounced than I'd seen it before," I wrote in my journal of March 26, 1887. "It is wonderfully bright. Last night after we watched it on the porch, I opened the living room drapes and lay down on the carpet with the lights out, and looked at it through the window glass using the binoculars."

Comet McNaught

Comet McNaught (C/2006 P1) was an ordinary streak when it appeared in the skies above the northern hemisphere early in 2007. But it turned into one of the greatest comets once it dipped into southern skies toward the end of January. It unexpectedly sprouted a gigantic, smoky tail of more than a dozen streaks. The finest views available in the United States were from Hawaii, where the peacock tail projected above the horizon at sundown.

Comet Holmes

In October 2007, this usually ordinary comet underwent a dramatic expansion, its coma of material around the head suddenly becoming a million times brighter. In terms of distance across the airy coma, it was the largest object in the solar system.

Comet Holmes was so large that I couldn't get all of it in a single telescopic image. With my gear set up in our front yard, I took 357 exposures of 5 seconds each and combined many of them into snapshots of an exploding comet.

I jotted in my journal: "Some cute ladies dropped by to ask me what I was doing and I was able to point out the comet to several people. It’s visible to the naked eye and easy to find, in Perseus."