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Watching history happen.

Spectacular as Deep Impact was as seen live on NASA TV, it was a washout for most amateur observers watching through telescopes 134 million kilometers away. In the weeks before July 3-4 the comet was fainter than expected, at about magnitude 10.5. Moreover it was quite diffuse, with almost no central condensation.

But for well-placed observers with large telescopes, the event was dramatic indeed--with a tiny point appearing in the comet's dim coma and visibly brightening in the minutes after the collision.

"I had an excellent view of the Deep Impact crash from near Magdalena, New Mexico," wrote Kevin McKeown, who used a 10-inch f/8 reflector at 92[degrees]under a black sky. "Shortly after twilight I recovered Comet Tempel 1 with some difficulty. The comet was a heart- or pear-shaped, filmy smear of light with no central condensation whatever. At the critical time, 11:52 p.m. MDT, there was no flash or anything spectacular.

"However, the show was just beginning! About 2 or 3 minutes after impact, the comet appeared a bit brighter at the center, but perhaps I was imagining things? After 5 minutes I was certain the comet was brightening. It was developing a fairly small, hazy sub-patch within the larger glow. After 8 to 10 minutes it was obvious the comet had brightened considerably--it was not difficult to locate anymore! Its appearance also changed: it was now small-looking, like a condensed galaxy. By 12:05 a.m. or so I could detect a starlike nucleus within the newly formed sub-patch, all set in a large, faint outer haze. I would guess that it brightened by one magnitude over a 25-minute interval."

In Arizona, John Welch and four friends followed the comet in the days before impact using 8- to 18-inch telescopes: "The comet displayed no detail other than a large, dim glow with an ever-so-slight brightening of the coma." On the big night, "We counted down the last seconds to impact as we glued our eyes to the views. After approximately 12 minutes each of us (first with the larger apertures) began to see a brightening of the core! After a couple of minutes to confirm this brightening, we began to cheer and raise our diet sodas in a toast to the event and the people who made it happen. The core continued to brighten by at least two magnitudes compared to some faint field stars until the comet set in the west."

For the next two nights, according to Welch, "The coma continued to display nearly the same brightness. We all agreed this event was at least as dramatic and memorable as Comet Shoemaker-Levy 9 impacting Jupiter."

Similarly, Robert Sheaffer observed with a group of San Diego Astronomy Association members. "Before impact, I could see Tempel 1 in my 11-inch Celestron. It looked like a small fuzzy region, no detail. In a 24-inch Dob, the comet was easy but featureless." Several minutes after crash time, however, "a small, starlike point of light appeared; we all agreed it hadn't been there before. We then looked in a 22-inch scope, and the little 'star' was even clearer."

In the following days the starlike condensation spread out and disappeared. Michael J. R. Begbie, a longtime comet observer in Harare, Zimbabwe, followed Tempel 1 with 15 [degrees]60 binoculars. Two days after impact, he wrote, the coma still displayed "a more condensed central condensation, almost but not quite stellar." But after five days, the comet was "now a difficult object again through this aperture. It has returned fully to the diffuse appearance that it had before the impact, and the coma appears spherical again, with no sign of an extension or any hint of tail."

Some who witnessed Deep Impact's effect realized that they were seeing a unique marker in the long history of humankind. This was the first time that humanity had any effect on the natural universe great enough to be seen from Earth. It surely won't be the last.
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Author:MacRobert, Alan M.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Oct 1, 2005
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