When mountain-size pieces of the shattered comet P/Shoemaker-Levy 9 begin plunging into Jupiter today, they'll fall into the "back" side of the gaseous planet, just beyond the direct view of Earth's telescopes.
That's why professional astronomers have been mostly gloomy about the chances for back-yard astronomers to see anything of the flashes and fireballs they have predicted during the seven-day barrage.
But Dr. Peter J. T. Leonard, 34, a theoretical astrophysicist at the University of Maryland College Park, believes there is hope for amateurs.
He says that straggling pieces of the broken comet -- rocky iceballs the size of football fields -- may strike the near side of Jupiter after the last of the visible fragments hits on July 22. If so, they would produce brilliant explosions that would be "really obvious, easy to see, possibly even with binoculars," he said.
The first visible chunk of the so-called "string-of-pearls" comet is expected to bore into Jupiter's atmosphere at 3:54 p.m. today, with a second due to follow at 10:49 p.m. The first Hubble Space Telescope photos were to be released on NASA-Select TV some time tonight. The last of the 21 big fragments is predicted to hit at 3:53 a.m. next Friday.
Professional astronomers hope powerful observatories such as Hubble might spot a flash reflected off a jovian moon or a fireball rising over the horizon.
The best hope for sharp-eyed amateurs with quality equipment -- a 14-inch aperture or better, astronomers suggest -- might be to spot changes in Jupiter's cloud patterns after the planet's rotation brings the impact sites into view.
But Dr. Leonard says prospects are better than that because there is more to Shoemaker-Levy 9 than meets the eye.
"We see these 20 fragments and observers . . . think that's all there is," he said. "To me that's ridiculous. There should be lots between the chunks as well that you don't see because it's too small."
In fact, the 20 or 21 large fragments visible in photographs represent only the middle third of a long cloud of comet material, he said. The leading portion of that cloud actually began raining down on Jupiter six days ago, and the trailing portion "will continue to hit six days after the last major fragment hits" on July 22.
That's important because by July 28, he said, the impact points will have moved over the jovian horizon into a direct line of sight from Earth. If so, he believes, even hunks of comet 100 yards long that are invisible from Earth will generate 1-second flashes bright enough to be seen by amateurs.
There are no guarantees, of course, Dr. Leonard said. But then, "I'm a theoretician. I'm allowed to make predictions and it's OK if I'm wrong."
If skies are clear, Jupiter will appear tonight as a bright white "star" high above the southwest horizon after sunset, just to the right of the half moon. It sets in the west about 12:50 a.m.