The image on the television screen is the most startling
I've seen yet. There is a small black spot, similar to what was seen at the
Fragment A impact site. But encircling the spot is a rather sharply-defined
ring. The ring's diameter is about twice that of the spot, and the ring and
spot do not appear to be concentric. A dark area spreading from the spot
occupies roughly the lower right quarter of the ring. Beyond the ring in the
lower right is a very large, smeared swath of dark material, similar to the one
from the Fragment A impact, only this one is much bigger, much wider, and much
darker. A narrow band of light, Jupiter-colored material separates the smudge
from the ring and the spot. The smudge, ring and circle together bear an
uncanny resemblance to a dark bruise under an eye; hence the scientists have
dubbed this image "the black eye."
It soon becomes clear that this is a Hubble Space Telescope
image of the impact site of Fragment G. It was taken with the Planetary Camera,
and Dr. Heidi Hammel is discussing it.
"Let me give you a sense of scale here," offers
Heidi. "There's a bright spot, and there's a little dark ring you can see
around the spot. And then you see a larger sort of smudge to the south. That
little dark ring is 80% of the size of the Earth. That large ring -- that big
smudge -- you could easily fit the Earth inside that diameter. This
is one big impact site!"
A close-up is shown of the Fragment G impact site, taken at with
a green filter. To the left of the "eye" is a smaller black spot. This
spot, we are told, was left behind earlier by Fragment D. "G just barely
missed hitting D," Heidi says.
An image from the methane band looks similar, only the
light and dark regions are reversed. Heidi points out some contrasting
"rays" in the smudge that appear to be spreading outward from the
edge of the inner ring.
"The energy of this event was probably about 25 times
larger than that first impact we showed you," says Heidi, "and that
jibes very well with the sizes of these things. This is a much larger splotch
on Jupiter."
A plume sequence image for the G impact is shown. This plume
peaked out at "about 2200 kilometers above the limb of Jupiter,"
according to Heidi. That's twice the height that was reached by the plume from
Fragment A!
Gene Shoemaker says that the energy of the Fragment G impact
was on the order of 6,000,000 megatons of TNT, whereas the energy of the
Fragment A impact was on the order of 225,000 megatons. Those numbers are
corrections to the off-the-cuff estimates he gave at this morning's press
conference, which were too high. "We were multiplying by the wrong
number," he explains, adding, "That's still a lot of megatons!"
The question and answer session begins, and Bob Cook of Newsday
asks about the interaction between the D and G impact sites, since Heidi had
hinted earlier that she had more to say on that subject. Heidi uses this cue to
introduce an exciting prospect.
"Q used to be the brightest one before G took
over," Heidi begins. "When we were doing our planning sequences early
on, we were planning on Q as our primary target, not G."
What? Q isn't the biggest one? I was counting on having a
couple of days to get my act together with my new telescope! I feel
betrayed. Fragment Q was my primary target, too! Thank goodness I
bought that telescope this morning. I need to get a good look at Jupiter tonight!
Oblivious to my distress, Heidi proceeds with her tale,
using the unfamiliar term "Jovian" to refer to that which is
Jupiter's. "When Q goes in ... a very interesting thing is going to
happen," she states, pausing dramatically. "Q is going to go in, and
exactly one Jovian rotation later -- ten hours -- R is going to hit
right next to the very same longitude as Q!
"And one Jovian rotation after that," continues
Heidi, "S is going to hit the same longitude on Jupiter!
"So you're going to have three -- boom! -- boom! --
boom!" -- she smacks her palm with her fist on each boom for
emphasis -- "right on that very small range of longitude."
"And that is going to make one heck of a mess!"
Heidi concludes. Shoemaker chuckles off-camera. "We're looking forward to
some really interesting chemistry," she says. "You're not just
taking the atmosphere and stirring it up a little bit," Heidi amplifies,
"you're really stirring the atmosphere up a lot!"
Bill Harwood of CBS asks about
the "very clearly-defined ring" around the Fragment G impact spot.
"There are some theorists
who would love to believe that that ring is an atmospheric wave,"
says Heidi. "It's too small to be a seismic wave, but it could be one of
these atmospheric waves that many theorists have been predicting," she
says, naming some names. "If that ring expands, then we'll believe it's a
wave ... if it does not expand with time, then [we'll] have to go back
to the drawing board. We're waiting to get the next sequence of images down, so
that we see what happens to that ring."
Harwood also asks about the effect of Jovian winds on the
marks on Jupiter, and Heidi addresses that next.
"Certainly, over the next few days we will be watching
all that material very closely to see how the winds distribute it," Heidi asserts,
"and it will be distributed, there's no question."
"Heidi, do you want to mention that it looks as though
some of the material in the spot from the A impact is being smeared out?"
Gene prompts.
"That's a good point, Gene," Heidi agrees. "In
some of our recent images, the A impact site is not as crisp and sharp as it
once was," she observes. "It looks kind of ... messy."
Gene notes that scientists will be tracking the motion of
those materials, because "it will be a very nice tracer for the velocities
of these upper atmosphere winds, for which there actually is no previous
information." He emphasizes, "Very important information on Jupiter
will come out from that."
"Can you talk about what kind of impact this would have
on Earth, if these things were hitting Earth instead of Jupiter?" queries
John Rutherford of NBC.
"Well," exclaims Heidi, "I told Gene I'd get
him up here to answer that question!" But not until she sets the stage.
"Let me just point out again the scale. Remember that the Earth is about the
size of that ring you saw."
"The physical scale of this effect is terrestrial,"
she continues. "It's that big!"
"You know," she muses, "I wouldn't want to be
on Earth if one of these pieces landed on Earth."
Gene jumps in to define the impact in terms of crater size: "If
we're right about our estimate of the size and the energy ... if it hit a
continent, it would make a 60-kilometer diameter crater," he states.
"Thereabouts."
"How does that compare to the size of a state?" prompts
Heidi. "For example, Rhode Island?"
"The crater would pretty well cover Rhode Island,"
Gene returns. In addition, all that material from the comet that went up so
high "would just go out ballistically all over the Earth." Now it is
Gene's turn to be dramatic.
"It's enveloped the Earth!" Gene exclaims.
"Completely enveloped the Earth!"
interjects Heidi.
"It's gone all over the Earth," Gene
continues. "The material that's blown out of that crater would essentially
blanket the Earth with a layer of fine debris that would just block sunlight. It
would just get dark, all over the Earth."
A number of subsequent questions relate to the relative sizes
of things. Gene states that Jupiter's diameter is about twelve times the size
of the Earth's diameter. Pressed for a direct comparison with the Fragment G
impact site markings, Heidi estimates that "the Earth is a little bit
larger than the sharp black ring, but the Earth is smaller than the whole
smudge area."
David Chandler from The Boston Globe asks why the
smudges on Jupiter are black instead of white.
"The theories are numerous," offers Heidi. Scientists
are theorizing that the dark matter could consist of "cometary material --
comets are very dark" or material of a different color that has been
dredged up from deeper in Jupiter's atmosphere. "I don't have a very good
answer to that question right now," she admits. "I was surprised,
because most of the fresh features that we see on these outer planets ... tend
to be white and bright."
Chandler also has a very good question about the plume
sequence images. "Why is there that dark region between the plume and the
planet?"
Heidi explains that there are two reasons. One is that part
of Jupiter is in shadow. From the Hubble's (and the Earth's) perspective, it is
not being illuminated by the sun. Heidi makes an analogy with the Earth's moon,
which only appears full to us when it is on the opposite side of the Earth from
the sun. The crescent of Jupiter that is in darkness is blocking the light from
the fireball.
"The other part is that these plumes are so high that
the shadow of Jupiter -- just a little sliver of the shadow of Jupiter itself
... is preventing the sunlight from reaching that," Heidi continues.
"But most of that is the limb of the planet."
Heidi asks for the plume image to be displayed again. In
the bottom frame, she narrates, "you see the edge of Jupiter and then you
see that smear, the pancake. That pancake is sitting on the planet. That
pancake's not floating in outer space! The pancake is in the atmosphere,
and that thin little strip there ... that's the unilluminated part of
Jupiter."
Other questions concern Gene's revised estimate of the energy
expended in the Fragment G impact. Jan Smith of Fox Television asks, "How
do you now compare it to the size of the world's bomb arsenal?"
"The number I've always used as the world's arsenal is
ten thousand megatons," Gene replies, "but there are other numbers
floating around -- and I guess the real number is still a state
secret!" He laughs.
"So I've used what's been commonly used in some of the
literature, ten thousand megatons," Gene continues. Applying that to
Fragment A, he states, "nucleus A's energy is about twenty times
that." Gene had also heard an estimate of eighty thousand megatons for the
world's arsenal, which would make Fragment A's energy only three times as great.
For Fragment G, Gene goes back to using the ten thousand
megatons standard. "When you get to six million megatons, that's a whole
lot higher, and that's getting in the ballpark of four hundred or five hundred
times what I think is the more conventional, published estimate of the world's
nuclear arsenal."
"What was the actual size of the fragment?" asks a
reporter from NHK, the Japanese public broadcasting station. "And also,
what was the time that the fragment actually impacted into Jupiter?"
"Those are two questions for which we would very much
like to have the answers!" Heidi exclaims.
They both address the timing question, then Heidi declares, "We
still don't know the sizes of these fragments. I think if Hal were here, he'd
probably say that this was a three-kilometer body."
"I would say the height of the plume ... based on the
modeling that we've done for a one-kilometer impactor, is consistent with that,"
Gene agrees. "In other words, that estimate by Hal Weaver for three
kilometers looks pretty golden to me."
Jim Reston from Esquire asks, "If you could see
the full plume without the shadow, would it mimic the fireball of
Hiroshima?"
"We don't think so ... no," decides Gene. "What
happens in this case is that the plume is erupting from substantial depth. So
there is a column that goes down below the cloud tops that you don't see. But
what we see at the surface is just this big bump that comes up
initially. Now, as it flattens out, it will ... get kind of mushroom-y
out at the edges, so that there is a gap between the bottom of that and the
ammonia cloud deck."
"We noticed that the plume was somewhat asymmetric,"
Heidi adds, "and I think that's because this is a directional thing. The
comet pieces are not coming straight down onto the atmosphere; they're coming in
at some angle."
"The plume doesn't really necessarily rise straight up,"
she concludes.
"Some of it blows out sideways," Gene chimes in.
Questions for which no answers can be provided remind us
that we are watching scientific discoveries as they unfold. Jim's follow-up
question is whether the triple punch of Fragments Q, R, and S in the same
location will increase the likelihood of a permanent cyclone forming on
Jupiter.
"I'll tell you the answer to that in about two or three
days -- " Heidi laughs, " -- after it happens!"
"I really can't even speculate at this point,"
Heidi demurs when pressed further. "Certainly, it's going to disturb the
atmosphere a lot more than a single impact on one site would. I won't go much
further than that, though."
In answer to a question from an L.A. Times reporter, some
mind-boggling numbers are thrown out, with a warning from Heidi that "these
are very, very preliminary. We haven't had much time to do detailed
analysis." Heidi then dictates, "The diameter of that sharp, thin
ring is about 7500 kilometers, and the diameter of the white ring surrounding
that, the inside of the black eye, as it were, that's about 15,450 kilometers.
A follow-up question for Gene asks him to compare the size
of Fragment G with the object believed to have killed off the dinosaurs when it
crashed into Earth 66 million years ago, forming a huge crater in what is now
Mexico. Gene explains that, although the exact size is difficult to determine, the
minimum diameter of that crater is estimated to be around 180 kilometers. However,
the Fragment G impact site, at 60 kilometers, is "three times
smaller," so its energy would have been "about 27 times less."
"This is a big event," Gene concludes,
"but it still isn't the dinosaur killer."
A reporter asks how much Internet email traffic the
panelists are receiving: "How crowded is the electronic huddle?" (Ed.
note: This was before the Internet exploded into the popular and pervasive
means of communication it is now. In 1994, few people had heard of the Internet,
and even fewer still had Internet access or email. It was only just beginning
to trickle into people's homes and was used primarily for research and
education. It was also a lot smaller -- and a lot slower!)
"We have an email distribution network set up for the
professional astronomy community," Heidi explains, "and that is
actually working extraordinarily well. People are being very concise about
reporting their observations, and therefore ... we have, in our parlance, very
high signal-to-noise. A very good amount of information is coming through."
However, public areas of the Internet are experiencing
slowdowns. "When I saw...this morning that one of the images from the IRTF
[Infrared Telescope Facility] showing the G impact plume was posted on one of
the Mosaic bulletin boards, I tried to connect, and I could not
get through," laments Heidi. "There's a big traffic jam on the
information superhighway right now!"
"But this is an extraordinary event," she
observes. "I think that's why. Normally we don't have that kind of
difficulty."
Dan Blackburn from KCOP-TV in Los Angeles asks if they have
any information about the depths to which the comet fragments are reaching into
Jupiter's atmosphere.
"I don't think we have a good answer for that question
yet," Heidi concedes, noting that scientists around the world will be
using telescopes to take spectroscopic measurements of Jupiter and look for the
signature of water. If they find it, that will tell them how far down into
Jupiter's atmosphere a particular fragment has penetrated, based on scientists'
predictions of how deep in the atmosphere the water clouds are located. (They
can't see the water clouds, because they are hidden beneath other clouds.)
Another Los Angeles television reporter has a two-part
question. First, "Will anything be left of Jupiter once this is all
over?" And second, "What would the impact be on Los Angeles if it
were hit?"
Both panelists burst out laughing at the first question. "I
think Jupiter's going to hang in there," Heidi reassures him. "It's a
pretty big planet."
Gene fields the second question, noting that Los Angeles is
his hometown. "It'll take out all of L.A. County," Gene declares. "In
fact, I just wouldn't want to be in Southern California, period!"
Mark Courot of the Houston Chronicle asks whether the
Hubble Space Telescope might have missed the impacts had their timing been
different. How much luck was involved in capturing the Fragment G image?
"The impacts are always occurring on the far
side," Heidi reminds us, so for any given impact, "it wouldn't matter
if it came in early or late; we would never be able to see it."
The timing of the Hubble Space Telescope's images is limited
by its orbit around the Earth. "It orbits every 96 minutes," explains
Heidi, "and obviously we can only look at Jupiter when the Space Telescope
is on the side of the Earth [from which] Jupiter is visible; it can't look through
the Earth. So, in some sense we have been extremely lucky to have
captured three of these impact events in the middle of one of the
Hubble's visibility windows."
"We were fortunate to have the Hubble in the right
place at the right time to be able to capture this one," Heidi exults,
referring to the Fragment G impact. "We couldn't have done anything if the
Hubble had been on the other side of the Earth."
"I think part of the answer to the question, Heidi, is
that we already had very good predictions of the impact times," Gene puts
in. "They were certainly good to within fifteen minutes. So that was extraordinarily
important in the timing, in the planning, for these images. That was a very
essential part of the planning."
"Well, not really, Gene," Heidi counters. "Because
we had to do the planning and the sequences many weeks ago -- months
ago, when the impact times were not well-known. And the Hubble's orbit was not
particularly well-known, either, because that has to be adjusted fairly
frequently." As the predictions kept changing, Heidi recounts, "I'd
be getting very nervous, and then I'd be very happy, and then I'd
be nervous again." But in the end, "I ended up being very
happy!"
Traci Watson of U.S. News & World Report asks
whether the fragment appears to be from a comet or an asteroid. Its name
notwithstanding, it would seem that the jury is still out on the origin of "Comet"
Shoemaker-Levy 9. Heidi says that the best answer to Traci's question will come
from spectroscopy of the impact sites. Again, it's too early to tell.
The most important question -- to my mind, at least -- is
also the last one, by Bill Harwood of CBS. "This is something that should
be clearly visible to amateurs, is it not?"
"Well, these black spots are starting to get pretty
darned big," Heidi acknowledges, "close to the size of the Great Red
Spot. And I believe there have been some reports from visual observers who are
starting to see them.
"Now, remember, you have a very high-resolution image
in front of you with the Hubble Space Telescope," Heidi cautions, "and
with a ground-based telescope, much of this is smeared out."
"But the short answer is that, yes, this is
starting to get large enough that amateurs may be able to spot those, if
they're very careful observers and know what to look for."
This is what I have been waiting to hear!
"And that three-impact sequence may stir up a [really]
big portion of atmosphere; it may get very dark," Heidi continues. She
thinks that, if enough material is present, "it's possible that a slightly
darker band will develop in this latitudinal region."
Gene picks up where Heidi left off with the amateur
astronomers. "I think there's a good chance that amateurs with eight-inch
diameter telescopes -- or bigger -- in good sites, where they have very
good seeing, will be able to see these," he predicts. "Already there
are reports from quite reliable observers."
My hopes are deflated somewhat. My telescope is less than
four inches in diameter. But at least now they're seriously entertaining the possibility
that some amateur telescopes will be able to see the comet impact spots.
Tonight I will get to find out if mine is one of them!
