Astronomers around the world were waiting for the third "shoe" to drop today as the last of a trio of comet fragments headed for splashdown in the same area of Jupiter.
Reports reaching the Goddard Space Flight Center this morning said that the first -- fragment Q2 of comet P/Shoemaker-Levy 9 -- struck on schedule about 3:45 p.m. yesterday.
Fragment Q2 was accompanied by a fragile companion, Q1, which fell nearby, making a barely discernible mark on the Jovian cloud tops. Scientists today dubbed it a "Q-let."
The second sizable member of the trio fell about 10 hours later, after the planet had rotated through one of its brief days. It was spotted from Flagstaff, Ariz., by Dr. David Schleicher at the Lowell Observatory.
He reported seeing the flash after 1:30 a.m. EDT today as fragment R shot through Jupiter's upper atmosphere as a giant meteor. "That may mean it was a bigger piece than people had anticipated," he said.
The flash was also witnessed by Dr. Imke De Pater at the W.M. Keck Observatory in Hawaii. "The flash from R lasted 20 seconds, and the plume [a fireball of hot gas rising from the impact site] appeared eight minutes later," she said. In another eight minutes, it had faded from sight.
The last of the trio of fragments -- S -- was due to strike the same region of Jupiter one more 10-hour Jovian day later, late this morning, Baltimore time.
Scientists were eager to learn what effects there might be from three hits at almost the same spot.
In all, about 16 fragments of the comet have struck Jupiter since the barrage began on Saturday. Including S, five more were still to fall by about 4 a.m. tomorrow.
Scientists said the Galileo spacecraft, en route to Jupiter and now about 150 million miles away, has had a direct view of the impacts, which have occurred on the back side of the planet, just out of sight as seen from Earth.
The Jet Propulsion Laboratory in Pasadena, Calif., has reported that signals from Galileo indicate that its instruments have recorded the flashes and fireballs.
Astrophysicists hope to use the full series of Galileo's observations to make direct comparisons of the relative power of each impact. No photos are expected from the spacecraft for several months, however, because Galileo's high-speed antenna is not working.
The bombardment of Jupiter by the comet is revealing chemicals that scientists say might explain for the first time the yellows, reds and browns of the planet's visible clouds.
Delighted astronomers also reported yesterday that they have seen the equivalent of northern lights on Jupiter, but at latitudes farther south than they have ever been seen before. It's an effect they think is caused by the falling comet.
But none of their discoveries or computer enhancements, they say, can match the thrill of putting their eye to a small telescope and seeing the dark impact sites for themselves.
The spots -- Earth-sized clouds of gas and dusty debris blown into Jupiter's stratosphere by the comet impacts and explosions -- are the biggest changes and the most prominent features to appear on Jupiter since Galileo started looking in 1610. Astronomers are urging everyone to find a telescope and have a look.
"This is an extraordinary thing," said David Levy, co-discoverer of Shoemaker-Levy 9. "These spots are a major effect from these collisions. They're easy to see [with modest telescopes and good visibility], and this is the time to do it."
University of Maryland astronomer Lucy McFadden, who is coordinating the worldwide observations, stood in line Tuesday night to view Jupiter through an 8-inch amateur telescope at the UM observatory in College Park.
"There's something visceral about putting your eye to an eyepiece," she confessed. "I took my husband, and he said it was sort of a 'Woodstock' experience.
"I could see the [planet's cloud] bands going across and sort of convinced myself I could see some spots."
Later, through the university's 20-inch telescope, Dr. McFadden said, "I actually saw those spots." A video monitor linked to the telescope showed three impact sites. "I could only discern two with my eyeball."
An estimated 500 people trooped through the College Park observatory Tuesday night to have a look.
The UM observatory's open house has been extended through tomorrow night. Located near University Boulevard and and Metzerott Road, the observatory is open from 8 p.m. to 10 p.m.
The Jupiter watch also continues from 8:30 p.m. to 1 a.m. through tomorrow night at the Johns Hopkins University's Bloomberg Physics Center, near the San Martin Drive entrance of the Homewood campus.
Eighty to 100 people have visited the center each night since the comet show began. Call 516-6525 after 5 p.m. for weather updates.
The discovery of sulfur at the spot where fragment "G" crashed down Monday morning was reported yesterday at the Goddard Space Flight Center in Greenbelt by University of Arizona astronomer Roger Yelle.
The Hubble Space Telescope's Faint Object Spectrograph detected the signature of sulfur in the ultraviolet light coming from the center of the impact site's gas plume.
Scientists have long suspected that there must be sulfur at some depth in Jupiter's atmosphere, but it had never been detected before.
Sulfur is one of the best candidates for explaining the yellows, reds and browns seen in Jupiter's uppermost clouds, the ones visible from Earth. But those clouds are known to be made largely of ammonia, and ammonia clouds should be white.
Also, scientists still must figure out whether the sulfur they're seeing really comes from Jupiter's atmosphere, or from the comet.
"I guess we still can't be sure," said astronomer Keith Noll, principal investigator in Hubble's atmospheric spectroscopy experiments.
The "northern lights" spotted on Jupiter were reported by Renee Prange of the French Institute Astrophysique Spatiale, who is also a member of the Hubble upper atmosphere imaging team.
Ultraviolet images shot by Hubble show glowing streaks in Jupiter's Northern Hemisphere that appear as almost mirror images of glows from the impact site of fragment "G" in the Southern Hemisphere.
Dr. Prange said that the northern glows appear further south than ever before. "I think it's a major discovery," she said.
The Jovian auroral displays appear to be triggered by electrically charged particles released during the impacts in the south, and shoot in a looping arc northward along the planet's magnetic field until they fall back into the planet's atmosphere, creating a glow in the north.
It's as if the particles were being repelled from one end of a bar magnet and attracted to the other.
Still unclear, Dr. Prange said, is whether the particles are comet dust that became electrically charged as it fell through the planet's magnetic field, or gas molecules from the planet that became charged in the heat of the fragment's impact and explosion.