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Simulation to Predict Comet-Planet Collision
Pittsburgh Supercomputing Center / University of Chicago
Pittsburgh, PA
USA

Year: 1995
Status: Finalist
Category: Science
Nominating Company: Cray Research, Inc.

Advance simulation of the impact of a comet on Jupiter guided scientists in planning their observations and is aiding them in the analysis of data about the event.
Fireworks on Jupiter All telescopes pointed to Jupiter this summer as
Comet Shoemaker-Levy 9 crashed into the giant planet. As people
day-by-day followed the news of Shoemaker-Levy 9's fiery death, a basic
scientific question entered public consciousness: Could a similar large
comet or asteroid someday come hurtling toward Earth? The short answer
is yes. It happened 65 million years ago, with disastrous consequences
for the dinosaurs, and the odds are, say astronomers, it will happen
again within the next 100 million years. What are the chances human life
could survive such a calamity? This summer's event gave scientists an
unprecedented chance to observe doomsday from a safe distance and learn
from it. "It was a unique event," says astrophysicist Mordecai-Mark Mac
Low. "It's the first time we've been able to predict a large planetary
impact and then observe it, and it will probably be the only time in our
lifetimes that an impact this large occurs."

In preparation for the comet crash, Mac Low, a postdoctoral researcher
at the University of Chicago, used the CRAY C90 at Pittsburgh to
forecast what would happen. "Basically, I was trying to predict the
results of the impact so that observers could plan for the event. I
looked at things like how bright the flash of the explosion would be and
how much material from beneath the Jovian clouds would be lifted above
them where it can be observed."

Observers worldwide relied on Mac Low's predictions to help plan for the
event, and the results suggest that Mac Low's simulations provide a
complete description of what actually happened on Jupiter. Using the
C90, Mac Low ran simulations that showed the comet would penetrate less
deeply and explode more violently than other models predicted. Perhaps
the most important result, at least in terms of forecasting the effect
of Earth impacts, is that the simulations agree with a mathematical
theory called "the pancake model."

Forecasting a Big Splash In collaboration with Kevin Zahnle of NASA Ames
Research Center, Mac Low ran three types of simulations: (1) a comet
fragment entering Jupiter's atmosphere until it exploded, (2) the
initial fireball from the explosion, and (3) what happened in the
atmosphere after the initial fireball. The researchers used ZEUS, a
program developed at the Laboratory for Computational Astrophysics of
the National Center for Supercomputing Applications for modeling the gas
dynamics of astrophysical phenomena such as the violent shock waves from
supernovae.

For the last six years, Mac Low has used ZEUS to study interstellar gas
dynamics, and he realized he could apply the same method with relatively
minor changes to simulate a comet crashing into Jupiter's atmosphere --
basically by shifting the scale from light years to kilometers. "The
physics," says Mac Low, "is virtually identical. It's only the details
of the composition of the atmosphere that change, and of course the
length scales, time scales and density changed -- by 20 orders of
magnitude in some cases."

Their simulations predicted that the flash from the explosion would last
about a minute, with about as much brightness as the sunlit side of
Jupiter.

Unfortunately for Earth observers, the comet crashed into the back side
of Jupiter. Mac Low's calculations suggested, nevertheless, that the
fireball would be bright enough to be seen from the NASA spacecraft
Galileo or with Earth telescopes as a reflection off one of Jupiter's
moons.

The strongest prediction from the simulations, the one Mac Low was most
confident of, had to do with how deep the comet would dive into
Jupiter's atmosphere before exploding. Other models showed it going in
hundreds of kilometers, so that its energy is absorbed relatively slowly
-- what one researcher called a "soft catch." Mac Low's results showed,
however, that impact with Jupiter's atmosphere would rip the comet apart
more quickly and violently, with a fierce explosion, after penetrating
only about 110 kilometers below the cloud tops. The resulting plume of
superheated debris, according to Mac Low's study, would shoot hundreds
of kilometers above Jupiter's layered clouds, giving astronomers a good
chance to observe the after-effects and, in the process, learning more
than we know now about the composition of Jupiter's atmosphere.
Inferences from observational data indicate that this prediction was
essentially accurate.

Mac Low's simulations used a computational grid finer than the other
models, suggesting that his results more closely approximated reality.
As a check on this surmise, Mac Low ran his code at much lower
resolution and got a result similar to the other models. "At low
resolution we got one result," says Mac Low, "and at high resolution we
got another, and as we continued increasing resolution the result stayed
the same." The high degree of detail in Mac Low's study -- made possible
by the CRAY C90 -- gave a reasonable basis for astronomers to be
optimistic that they would have a good show to watch in July.

The Pancake Model The most important result of Mac Low's study is his
finding that the numerical simulations agree well with an analytical
model called the pancake model. This model assumes that once the
aerodynamic force from the comet's impact into the atmosphere overcomes
its material strength the comet flattens like a pancake, which greatly
increases drag -- essentially stopping the comet in its tracks. If these
results prove to be realistic -- and this summer's event could help
determine that -- the pancake model can be used to predict what will
happen from comet and asteroid impacts on Earth and other planets.

"One of the scientific issues we're hoping to get a handle on in terms
of Earth impactors," says Mac Low, "is how big a rock do we need to
worry about? One of the motives for modeling this impact is to see if we
can do reasonably accurate predictions. If we can, we can start talking
about how well the Earth's atmosphere protects us."
These computations provided an immediate scientific benefit by helping
astronomers to plan for the summer 1994 comet crash.

Mac Low's computer simulations were more detailed than others done prior
to the event, and they indicated that, even though the comet was aimed
at the far side of Jupiter, the resulting explosions would be likely to
produce effects observable from the Earth. They also indicated it would
be visible from the Galileo spacecraft. Furthermore, they gave observers
a coherent picture of the expected effects, allowing them to plan
appropriate series of observations for each impact.

Now that the impact has occurred, Mac Low's simulations are proving to
be the most complete description of the explosion. They are able to
explain the major observational results in a coherent story. These
results include the presence of elemental sulfur above Jupiter's visible
clouds after the explosions. The models also appear to yield the size
and mass of the comet fragments, giving unique insight into the
structure and composition of comets. Mac Low's work is playing a central
role in gleaning knowledge of both Jupiter and comets from this
spectacular event.
The results of this research could not have been obtained without using
the techniques of computational gas dynamics running on powerful
supercomputers. Mac Low's simulations gave information that couldn't be
obtained through laboratory techniques. His work exemplifies the value
of computational science, representing a third kind of scientific model
alongside the traditional analytical and experimental approaches.
Mac Low's modeling used a scientific program, ZEUS, developed at the
Laboratory for Computational Astrophysics of the National Center of
Supercomputing Applications, where Mac Low has been a collaborator for
the past seven years. He has contributed to the development of this
program, which has proven itself as a tool in modeling astrophysical gas
dynamics problems such as the violent shock wave produced by a
supernova. ZEUS is freely available to researchers, and is generally
recognized as one of the most versatile programs available in
astrophysical gas dynamics.

Mac Low's insight, once he learned of the predicted comet crash, was to
realize that the problems he had been computing with ZEUS could be
adapted with relatively minor changes to simulate a comet crashing into
Jupiter's atmosphere. Although a supernova is an immensely larger and
more violent astrophysical event than a comet crash, Mac Low understood
that the physics were similar and that only the scales of time, length
and density needed to be changed, along with adding the details of the
Jovian atmosphere. His resourcefulness in seizing this opportunity has
proven to be of great benefit to astronomy in planning for and
interpreting this unique event.
xe 4s computational results appear to give a complete description of
the explosions resulting from the comet fragments entering Jupiter's
atmosphere.

This suggests that the analytical model underlying his approach --
called "the pancake model" -- can be relied on to predict other similar
events. Even more importantly, it can be used in research to predict the
effect of asteroid and comet impacts with Earth's atmosphere. Partly as
a result of this summer's comet crash, planning is now underway -- with
discussion in Congress -- for NASA or another agency to develop a
predictive capability that could provide a measure of protection from
catastrophic events on Earth such as the one that wiped out the
dinosaurs 65 million years ago. Mac Low's work, in collaboration with
Kevin Zahnle, will help to determine the feasibility of such a program.
Mac Low's first obstacle was a technical issue: how to properly model
the entry and explosion of comet fragments. Other attempts to model this
problem suffered badly from underresolution, giving incorrect results.

Because Mac Low had experience working on physically similar problems in
interstellar gas dynamics, using a well developed code (ZEUS) and high-
performance computing (the CRAY C90), he was able to quickly apply
higher levels of resolution to the problem and get answers that agree
with analytical theory and, apparently, with observation.

The second obstacle was time. Results needed to be generated quickly in
order to use them in planning observations of the comet impacts. Major
ground-based and space-based observatories normally develop their
schedules months in advance, which meant that plans for observing the
impact had to be finalized during winter 1993-94, only six to eight
months after Comet Shoemaker-Levy 9 was discovered. This time pressure
meant that normal channels for funding research had to be shortcut. The
availability of supercomputer resources (at Pittsburgh Supercomputing
Center) and funding nominally devoted to other projects proved vital to
producing results in the required time frame.