Tuesday, July 19, 1994

Has anyone seen the comet impact spots through a small backyard telescope yet? I wonder, as I take my seat in front of the television set for today's 8 a.m. NASA briefing.

This morning's panel is completely different from yesterday afternoon's. Dr. Keith Noll from the Space Telescope Science Institute is back; we haven't seen him since Saturday. Don Savage introduces him as "a member of the Faint Object Spectrograph and High-Resolution Spectrograph teams." Dr. Steve Maran, is "a Hubble research scientist here at the Goddard Space Flight Center" and Dr. Lucy McFadden is "coordinating the worldwide comet observing effort." Lucy will assume Heidi's previous role of primary news reporter, while Steve will act as moderator.

Steve opens the program with news so astonishing that I assume I must have misunderstood. A leading solar system expert, Dr. Clark Chapman of the Planetary Science Institute in Tucson, has called the Fragment G impact site "the most visually prominent discrete spot ever observed on Jupiter."

Visually prominent? I try to process that. Visually. Not infrared. Not ultraviolet. Not in the methane band. But visible light. That would imply that it could be seen with a telescope!

And how can he call it the most prominent spot on Jupiter? He must have meant besides the Great Red Spot...surely he cannot be saying that the Fragment G impact site is now more obvious than the Great Red Spot! I must have heard it wrong. I heard what I wanted to hear, not what was actually said.

It can't possibly be that good!

Jupiter starts spinning before my eyes.

No, I'm not hallucinating -- it's that rotating Jupiter video again. This time it has been cleaned up so that there's hardly any trace of blue at the edges of those orange segments. You really have to look hard to see them.

Jupiter freezes while images showing the various impact sites are superimposed on it in turn, each in its proper place and blending almost seamlessly into it. First the Fragment A impact site is overlaid; then the A and G sites together -- with A definitely looking a lot more smeared; and finally the E, A, and C impact sites together.

The reason why Keith Noll has returned soon becomes clear. His job is to present the first spectrographic results from Hubble. We already know what the impact sites look like from all the images we've seen. Now we are about to get our first concrete data on what kinds of materials are present in those impact sites.

"In astronomy, there are two things we can do with the light that we get from an astronomical object," Keith introduces the subject. "One is to make images, like the ones we have already seen. The other is to break the light out into a spectrum." There are instruments that can do this on the Hubble Space Telescope.

"A spectrum is something that you're familiar with, if you've ever seen a rainbow," Keith enlarges. "But often, we're using light that the human eye can't see. And so, you can think of this as spreading the light into its colors, but these are colors that we don't have names for, because they are not colors that we can see with our eyes." Scientists often refer to these "colors" with the numbers that correspond to their wavelengths in microns (millionths of a meter), nanometers (billionths of a meter), or angstroms (ten billionths of a meter), where a meter is approximately 3.28 feet.

Before Shoemaker-Levy 9 arrived at Jupiter, the Faint Object Spectrograph on the Hubble Space Telescope took a sample of light from a tiny area of Jupiter. "The apertures of spectrometers are generally small," says Keith, "so we have to pick a specific spot." After the G fragment fell, the spectrometer was again pointed at Jupiter, this time right over the dark spot at the center of the Fragment G impact site. As luck would have it, that spot fits almost perfectly into the spectrometer's aperture.

Keith presents the first results in the form of a graph that shows, for various wavelengths, the ratio of the amount of light that was received from the Fragment G impact spot to the amount of light that was received before the G impact.

If there had been no change, the graph would have shown a flat line with a value of one at all wavelengths, because if you divide a number by itself (the "after" value divided by the "before" value), you get one. The dotted red line on the graph represents that outcome.

Instead, all of the values plotted on the graph are less than one. "Jupiter got darker everywhere" within the dark spot, says Keith, emitting less light at all wavelengths. Moreover, "at some wavelengths -- at some colors -- it became darker than at others. And that tells us something about the composition of the material that is absorbing this light."

The graph shows a dip at around the 2000-angstrom (200-nanometer) mark. "That, we know from laboratory experiments, is a place where ammonia gas absorbs light." Keith enlightens us. "So what we're seeing, at least in this spectrum, is an increase in the amount of ammonia. And there are a couple ways you can get that."

"One simple way you can get it is to detonate a large amount of energy in the atmosphere and warm up the clouds, and vaporize what are normally solid ammonia ice crystals," Keith amplifies. "Instead of solid, you have an increased amount of gas -- and that will absorb the light more, for sure."

Keith relates that other observatories are also reporting results that are "consistent with heating," but that "we still haven't seen anything that we can say for sure is debris from the comet."

Once Keith finishes, it is Lucy's turn to cover the highlights of the reports from other observing sites.

"There are a few recent reports that I personally find exciting," Lucy begins. "I was particularly excited to see an image from the European Southern Observatory taken by Tim Livengood from NASA Goddard Space Flight Center and his colleagues from the European Southern Observatory. It was a 10 micron [wavelength] image of the H impact, and they report that the brightness at 10 microns was fifty times the brightness of Jupiter."

"So, as predicted, the H impact and the following impacts are just as spectacular as the G."

"The radio observatory in Spain has detected CO [carbon monoxide] at radio wavelengths," Lucy reports. "Now we're not sure whether this CO has been produced in Jupiter's atmosphere from chemical reactions, or whether it's remnant CO from the comet itself," she continues, "and it's going to take some time to understand the implications of that."

Both the Hubble Space Telescope and the Kuiper Airborne Observatory have been trying to detect water vapor at the impact sites, but they haven't found any yet, Lucy reports.

From our own Earth's South Pole, we hear an audio report from the South Pole Infrared Explorer observatory that is sponsored by the National Science Foundation.

"We've seen indisputable signature[s] of the impacts of events A, C, E, G, and H so far," reports Dr. Hien Nguyen from the South Pole, with both the G and H impacts being "quite spectacular." By the time Fragment H hit, they had gotten "the hang of it" and could "recognize and anticipate" when the first burst of light would occur and how it would change over time. Therefore, they knew what to expect from Fragment H, and it was "quite a pleasure to see it change as expected."

A red-tinted image of the Fragment G fireball is displayed. An extremely brightly-glowing object appears in the lower left part of a much dimmer round shape that I take to be Jupiter. Judging from this image, it certainly looks as if they did have the timing right, even with Fragment G -- and it does look spectacular!

"The most fun was last night during the Fragment G" impact, Hien enthuses. "We made a big advertisement about Fragment G," and "eight people from the station went out to the observatory." They remained with the three observers working there, "witnessing the collision of Fragment G. It was really fun because, for the first time, the whole station [felt] like we were together."

"Remember, in Antarctica at this time of year, the sun never rises," Lucy points out. "But, more importantly for us this week, Jupiter never sets! It stays above the horizon" -- she traces a wide circle above her head with her finger -- "and basically goes around, at twelve degrees above the horizon.

"So they can observe twenty-four hours a day," she finishes, "and they must be tired!"

On the other hand, the McDonald Observatory in Fort Davis, Texas, has performed infrared imaging in the 2.3 micron methane band in broad daylight! How were they able to do that? Lucy explains that "for infrared measurements, the telescopes are designed so that they can observe during the daytime. The secondary mirror on the telescope actually moves back and forth very quickly, and they can subtract the signal from the daytime sky from the signal of the object that they're looking at."

An orange-toned image taken by the McDonald Observatory appears on the screen, showing Jupiter with a string of four lighter-colored spots near its South Pole. These are, respectively, the impact sites from Fragments A, C, "the site that was hit by both E and F," and Fragment H, according to Dr. Bill Cochran of the University of Texas.

"Once it got dark, we were observing with all the telescopes here," says Bill. "We basically followed Jupiter until ... we were almost looking underneath the cows!"

"From the size of the actual impacts and the way they spread out, we hope to learn about the strength of the [fragment] material and how deep the comet went when it hit Jupiter," he explains. "We also hope, from our spectroscopic studies, to learn about the structure of the deep atmosphere of Jupiter, the parts that are below the clouds that we can't normally see."

Data is coming in faster than these scientists can cope with it. An image of Jupiter showing three impact spots near Jupiter's South Pole and a couple more spots closer to its equator gets displayed, but there is some confusion as to what it represents and which observatory recorded it. After the panelists go around in circles for a bit, Lucy finally calls it off, declaring, "That's all I dare report!"

With astronomers all over the Earth doing whatever it takes to get a look at Jupiter, the panelists are now receiving far more reports than they can present. Steve cites an example: "We even have one report from an astronomer in Korea who said 'We have the monsoon, but we have looked at Jupiter.'"

Following a brief report by Keith on upcoming Hubble Space Telescope activities related to the impacts, Steve summarizes the major findings that were reported this morning. I view that as a noteworthy contribution, since none of the previous briefings has ended with a summary.

During the question and answer session, Bill Harwood asks about the lack of water observations, spawning a lengthy discussion.

"We had generally assumed that there'd be large amounts of water vapor and ammonia vapor condensing around these impact sites," Keith reiterates, "and that would create bright, white clouds, even in visible wavelengths."

"Our models of the atmosphere of Jupiter predicted water clouds beneath the high ammonia clouds," Lucy concurs.

Yet, no signs of water vapor have been seen thus far. Both panelists express surprise and spend some time discussing and debating the reasons why.

"This is why we do science," Steve observes. "If everything we predicted happened, the people who predicted it would pat themselves on the back, and everybody else would say, '...we already knew that.' "It's the unknown that you're discovering here...and that's what's so great about it!"

Bob Cook from Newsday asks the question that I've been wanting to ask, about that "most visually prominent" spot quote. "Does it outdo the Red Spot itself?"

"Yes," Steve replies, to my great surprise and delight. "Maybe the G does not appear as big as the Red Spot, but the Red Spot is not as dark," he explains. "Contrast can be almost as important as size when you're trying to visually pick out something."

A reporter from the San Jose Mercury News asks if we know yet whether Fragment K was bigger than Fragment G.

"K impacted at 6:30 this morning," Lucy responds. "I got up at five, and I didn't swing past my computer [nor log onto one here] ... so we don't have any reports yet."

It is barely 8:30. "Information is traveling very fast in this whole thing, but not quite that fast," Keith agrees. "We still have to drive!" They laugh.

"We're relying on the laws of celestial mechanics -- and the success with the first seven fragments -- [to] say that it did strike Jupiter this morning," Steve quips.

The Galileo spacecraft, which is nearing Jupiter, has been making observations of the comet impacts and storing them. However, due to the great distance involved and some technical difficulties, transmitting that data from Jupiter to Earth will take a couple of months.

How long do the scientists think the spots will last?

"Jupiter has a whole series of jet streams, much like on the Earth," notes Keith, so "the material -- or the disturbances in the clouds -- whichever it is" will get "stretched out and diffused by the winds on Jupiter." They might last a week or more, and then slowly fade out. "I don't think the expectations are that these spots will be permanent."

Lucy agrees that they will be temporary, lasting "on the order of months," but she qualifies it with, "Your guess is as good as mine!"

"It's also worth pointing out that nobody expected the Great Red Spot to last for 300 years, but it's still going strong," Steve contributes.

A reporter from Science magazine asks Lucy, "Has anything been seen of G2? The fragment that split off of G?"

Lucy seems intrigued, yet momentarily taken aback. "I'd forgotten that G had split up," she murmurs, digging through her stack of papers, "and I do have a report in here of someone sighting G1 and G2."

While she shuffles, Steve cautions, "Remember, there was one report yesterday, but it was the only one."

Lucy finds what she is looking for, and announces that five observers at the Anglo-Australian Telescope "saw bright flashes associated with the impacts of fragments G2 and G1" that were "detected seven minutes apart." These flashes were reportedly "tens to hundreds of times brighter" than those they had seen previously.

Steve seems somewhat skeptical. "It's hard to imagine something like that being mistaken when they're so bright," he muses, "but there are lots of other observers who only saw one."

"That was with a near-infrared imaging spectrometer," Lucy offers in their defense. "They have both spatial and spectral information."

Mark Courot of the Houston Chronicle wonders what questions the scientists have about the comet fragments' composition.

"The debate has been asteroid or comet," Keith summarizes. Unfortunately, "that's still not clear, because we're not seeing any evidence of the debris one way or the other."

Just as Don Savage is concluding his closing teaser with highlights from tomorrow's press conference at noon, someone off-camera attracts his attention with a late-breaking news report. With Don's permission, Malcolm Niedner of the Goddard Space Flight Center walks over to Don's microphone to announce a new finding from the International Ultraviolet Explorer (IUE) spectrograph. They've obtained some ultraviolet images that show the same type of darkening observed by the Hubble Space Telescope's spectrograph, confirming the presence of ammonia at the G impact site. Niedner urges Keith to speak with Principal Investigator Renee Prange directly so that they can compare results.

I am so overwhelmed with new information that I can't even begin to process that report, but its sudden inclusion at the last minute convinces me that it must be important. What's more important to me, however, is getting on with my telescope upgrade!