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The archetypal scientist tests theories by designing and carrying out
experiments. Astronomers, however, are barred from this path. They can observe,
but they lack the luxury of being able to do an experiment themselves –
they must wait for nature to do it for them. Many of these natural experiments,
some of them key to further understanding, simply don’t happen in our lifetimes.
And when something does happen – say, when a supernova in a nearby galaxy
explodes – astronomers are usually unprepared because they cannot monitor
every star.

So, an event predicted for July of this year – the crash of a broken
comet into Jupiter – is doubly extraordinary. There will be a succession
of nearly two dozen explosions, any one of which, if it happened on our
own planet, would threaten the future of civilisation. Not only is it an
exceedingly rare and important experiment, but we have known that it is
going to happen since last May, two months after the discovery of the comet.
Instead of tracking the fading aftermath of an unexpected event, astronomers
will be able to aim most of the world’s mountain-top telescopes toward
Jupiter every evening from 16 to 22 July. The newly refurbished Hubble Space
Telescope will be looking too, along with a host of other Earth-orbiting
and interplanetary spacecraft. Even amateur astronomers, using back-yard
telescopes, may witness some signs of the crash with their own eyes.

In mid-January, I joined 200 fellow astronomers at the University of
Maryland to compare notes and coordinate preparations for this grand cosmic
experiment. In late January, new observations from the Mauna Kea observatory
in Hawaii showed changes in the comet’s appearance, including a fading of
its ‘wings’ – the extensions of the comet beyond the ends of the line of
visible fragments – and a brightening of the tails that stream away from
the main fragments. Only a few weeks ago, a sharp picture from the Hubble
Space Telescope was released, showing several pieces drifting to the side
of the main train of fragments. More pictures from Hubble are anticipated
at monthly intervals as the date of the crash looms. Unquestionably, the
crash of Comet Shoemaker-Levy 9 (S-L 9) into Jupiter is the most widely
anticipated event in the history of modern astronomy.

The comet whose fiery demise is now commanding such attention was already
a broken shadow of its former self when it was discovered, in late March
1993, from the observatory at Palomar Mountain in southern California. Originally
recognised by comet-hunter Carolyn Shoemaker of Lowell Observatory, Flagstaff,
Arizona, as an elongated, ‘squashed’ comet, S-L 9 was soon revealed to be
at least 21 separate comets, strung out like beads on a string. We now
know that the comet had been circling Jupiter for over a century, and then
passed less than 1.3 times Jupiter’s radius from the planet’s centre on
7 July 1992, which means that it sailed less than 1/6 of Jupiter’s diameter
above the planet’s cloud tops. The planet’s huge ‘tidal’ forces ripped it
into the pieces we see today.

We normally think of tides as the bulges in sea level caused by the
Moon’s gravity tugging more at the front part of Earth than the back. But
when an object passes as close to a large planet as S-L 9 did, the differential
pull is more than sufficient to rip the object apart. S-L 9’s fragments
have continued on their separate, slightly diverging paths ever since.

However, it was not inevitable that they would all carry on and run
into Jupiter. The disintegrated comet might have continued orbiting Jupiter
for decades more. Or it might have escaped back into a separate interplanetary
trajectory around the Sun, like the path it followed before it became a
temporarily captured satellite of the planet. There was also a tiny chance
that, immediately after break-up, S-L 9 might have crashed into one of Jupiter’s
four big moons, the Galilean satellites, which the Italian astronomer Galileo
Galilei discovered in the 17th century using his innovative telescope.

Crater chain

Mysterious chains of craters on Ganymede and Callisto, two of the Galilean
satellites, appear in photographs taken by the Voyager1 spacecraft in 1979.
These were recently reanalysed by Paul Schenk of the Lunar and Planetary
Institute in Houston, Texas, and his findings testify that about 15 comets
did previously crash into the two moons after break-up. But the four Galilean
satellites block only a very small fraction of the heavens as seen from
Jupiter, so a comet’s chances of running into a moon are extremely small.
The chains we see imply that tens of millions of comet break-ups must have
happened in the last several billion years, or about one every century.
S-L 9, of course, sailed on by, and last summer, 30 million miles out, it
slowly turned back and began falling straight towards Jupiter.

Comets are fragile conglomerations of dust and ice, and several have
been seen to disintegrate in the past. Some fall apart for no known reason,
others when they are near the Sun’s heat and gravitational pull. There is
one known previous case of a comet breaking up because of passing close
to Jupiter: in 1886, astronomers extrapolating backwards from the path
of the split comet Brooks 2 concluded that it must have passed near the
planet. But unlike S-L 9, Brooks 2 was in an interplanetary trajectory,
so there was almost no chance of its running into Jupiter after it had split.

The comets that are temporarily trapped into paths around Jupiter usually
are not broken up, however. Eugene Shoemaker of Lowell Observatory in Flagstaff,
one of the co-discoverers of S-L 9, estimates that Jupiter may have one
or two satellite comets at any given time. They typically remain for a couple
of decades, before leaking out into the Solar System again.

Nevertheless, as a target for such careening comet-satellites, Jupiter
is much bigger than any Galilean moon, and its strong gravity pulls them
in more efficiently; so occasionally they do collide. Shoemaker estimates
that a comet – either a temporary orbiter like S-L 9 or one like Brooks
2, which is just passing by – crashes into Jupiter about three times every
century. The spectacle we are about to witness, of a comet breaking up and
then crashing into Jupiter, is extraordinarily rare, occurring perhaps only
once every few thousand years.

Lucky break

In fact, if S-L 9 had not broken up, astronomers would never have known
that it existed. Until it was torn apart in 1992, Comet Shoemaker-Levy 9
was simply too small, dark, and faint to be seen. An astronomy student from
Uruguay studying in Sweden, G. Tancredi, was photographing the sky near
Jupiter in 1992, hunting for comets. His plates show no sign of S-L 9 before
the break-up. But after the break-up, S-L 9’s fragments and dust presented
a vastly larger surface area to reflect sunlight, making an easy target
for the Shoemakers and their colleague David Levy of the University of Arizona
in Tucson to spot.

The comet is focusing unaccustomed attention on Jupiter. With the heavens
full of stars and galaxies to monitor, the planet has been studied only
sporadically until now. Jupiter is the largest planet in the Solar System,
a gas giant made mostly of hydrogen and helium, with no solid surface.
Through telescopes we see its multihued bands of ammonia clouds. The Great
Red Spot, an anticyclonic ‘storm’, is roughly the size of the Earth. In
the 1960s, an international photographic Jupiter patrol kept watch on Jupiter’s
changing cloud patterns on a nearly hourly basis, 24 hours a day, using
small telescopes spaced around the world. Then NASA had funds to underwrite
it; but proposals to mount a new Jupiter watch, with modern instruments
to complement the Galileo spacecraft’s forthcoming (1995-1997) orbital tour
of the planet, were turned down for lack of funds. So an unexpected comet
crash would likely have been missed, were it not for the lucky chance that
S-L 9 broke up first. For professional astronomers, this has meant bringing
together all the sophisticated instruments of modern astrophysics.

In choosing their tools, the astronomers have had to devote much time
– nearly a year, in fact – to thinking about what they might see in July.
New techniques are being pioneered, specialised instruments are under construction,
and the astronomers are trying to find ways of coordinating the worldwide
effort. There has been time to book telescopes which are normally reserved
months in advance. Programmers are writing the software that will be radioed
up to Earth-orbiting satellites and spacecraft so that they can observe
the event, an effort that ordinarily takes months or years of development
and testing.

What will be the pay-off from all this advance planning? Whatever happens
when S-L 9 smashes into Jupiter’s mid-southern latitudes in July, it will
be duly recorded from sites around the world. The instruments and methods
that will be used in observing the event are being designed to measure what
theoreticians predict will happen. Nearly all the researchers believe something
will be visible; but few of them are brash enough to make definite predictions.
So astronomers will also be monitoring Jupiter during July and August, at
all wavelengths from extreme ultraviolet to long-wave radio, so they can
be sure of capturing every detail of the show.

Astronomers think that each of the larger S-L 9 fragments is about a
kilometre across, maybe as much as 3 or 4 kilometres. Each of the fragments
that will plunge into Jupiter’s atmosphere at 60 kilometres per second will
unleash explosive energies equivalent to millions (perhaps tens of millions)
of megatons of TNT. That is hundreds of times the combined nuclear weapons
arsenal on Earth, and would make a crater 40 kilometres across, about the
size of a large crater on the Moon. The 21 detonations might add up to a
bang comparable to the largest impacts on Earth during the last few billion
years, including the one that hit Mexico 65 million years ago and wiped
out most species, probably including the dinosaurs.

Surface shaper

S-L 9’s crash will give astronomers a chance to witness this process,
which has been most important in shaping the surfaces of planets in our
Solar System. Until the space age, people believed that it was a rare and
exotic event for a comet or an asteroid to crash into a planet. They were
forced to think again, however, when the early Mariner and Voyager spacecraft
found craters on Mars and Mercury, and then on the moons of Jupiter, Saturn,
Uranus, and Neptune – often to the point of crater-on-crater saturation.

Despite extensive plate tectonics and erosion, which reshape terrains
and erase pre-existing craters, nearly 200 surviving terrestrial craters
have been found. Even the largest of these giant impacts do little damage
to Earth as a planet, but can be devastating to that thin shell – the ecosphere
– which sustains life. Every million years, an asteroid or comet strikes
that can drastically change our global climate. Every hundred million years
or so, a comet or fragment striking with the energy of S-L 9 before its
break-up changes the course of evolution of life on our planet. It has happened
before and it will happen again.

Until 1994, such impacts could not be studied directly. By scanning
the skies for objects, and by counting the scars such objects leave on planetary
surfaces, we could calculate how often they happen. But to begin to appreciate
what might happen when a comet strikes, we had to resort to uncertain theory,
and complex computer codes running on elaborate Cray computers. Now, we
can watch it live.

One might wonder why the tiny comet should have such an influence on
such a big planet. Comparing the mass of the S-L 9 fragments with Jupiter
is a bit like comparing a gnat with an elephant. But just as the atmospheric
distribution of dust ejecta arising from an impact on Earth changes global
climate, so the impacts on Jupiter should profoundly affect that planet’s
stratosphere (the only bit we can see through our telescopes). Each S-L
9 detonation deep in Jupiter’s atmosphere should create a fireball of hot
gases a thousand kilometres across, which will erupt through the clouds
with a brilliance approaching that of Jupiter itself, and splash back down
in arcs spanning thousands of kilometres of the planet’s stratosphere.

If there were any life forms floating in Jupiter’s stratosphere, they
would certainly be poisoned as a result of the impact. A cubic kilometre
of cometary material, or an equivalent amount of Jupiter’s lower atmosphere
dredged up into the stratosphere by the fireball, could increase the concentration
of some chemical compounds (carbon monoxide, for example) by as much as
a factor of 100, according to Gordon Bjoraker of NASA’s Goddard Space Flight
Center in Maryland. Astronomers will search for the characteristic spectra
of such materials, and will watch them spreading through Jupiter’s stratosphere
and evolving through chemical reactions and photolysis by sunlight. Stratospheric
hazes analogous to wispy cirrus clouds or smogs may form. These would change
Jupiter’s appearance as seen through amateur telescopes, enough to be noticeable
by observers familiar with Jupiter’s pre-crash appearance.

Astronomers will also study the explosions themselves. The main impacts
will occur on Jupiter’s night side, just half an hour before sunrise on
Jupiter. Unfortunately, they will occur just beyond the point of visibility,
on the far side of the planet as viewed from Earth. The Galileo spacecraft,
which is off to the side (see figure opposite) as it streaks toward its
1995 encounter with Jupiter, is lucky enough to have a direct view of the
impact sites. It can take excellent pictures of the events, but only about
5 per cent of them will ever be radioed back to Earth because of its crippled
main antenna. Members of the Galileo imaging team, myself included, are
counting on Earth-based astronomers to observe the explosions indirectly
and tell us when they happened, so we can then pick from Galileo’s digital
tape recorder just those images that contain the explosions.

What will they look like? Mordecai-Mark Mac Low of the University of
Chicago has evaluated his colleagues’ computer models of the impacts. Here
is his best guess: first, each projectile will streak through Jupiter’s
atmosphere like a brilliant meteor, generating a 1 to 2-second flash so
bright that Earth-based astronomers hope to detect it by reflection from
Jupiter’s moons. The disintegrating comet fragment will plunge through Jupiter’s
ammonia clouds, on down through its water cloud deck, and finally explode,
perhaps 60 kilometres further down. A huge fireball, analogous to that preceding
the ‘mushroom cloud’ in a nuclear weapons test, will erupt into Galileo’s
view. Glowing with a temperature of perhaps 2000 K to 5000 K, the fireball
will cool and fade during the ensuing minute. Even brighter than the meteor
flash, the fireball’s glare will reflect from Jupiter’s satellites, its
rings, and any comet material lingering near the planet.

Seismic waves from the explosions will penetrate deep into Jupiter.
Our knowledge of Jupiter’s interior is practically nil, but there may be
the opportunity to do some ‘Jovian seismology’. The impact of the comet
will offer a golden opportunity to probe beneath the planet’s thick atmosphere.
Several ingenious astronomers, including a team led by Donald Hunten who
plan to use NASA’s Infrared Telescope Facility in Hawaii, hope to record
the perturbations of Jupiter’s atmospheric temperatures, a matter of only
a hundredth of a degree, that are caused by seismic waves in the planet’s
atmosphere. (A wave changes the pressure of the gas it moves through, and
so its temperature.) More likely to be visible, especially to observers
with the best telescopes, such as Hubble, are atmospheric gravity waves
that will be generated by the splashback of the fireball’s remnants.

Little attention has been given to anything other than the predicted
impacts, and the ensuing changes that may occur in Jupiter’s southern stratosphere.
But last year’s photos of S-L 9 show that ‘wings’ of debris both precede
and follow the main cometary fragments. Furthermore, faint haloes surround
each fragment, and tails extend away from them. The wings, haloes, and tails
are thought to be composed of dust that was dispersed when S-L 9 was ripped
apart in 1992. When some of the dust is caught up in Jupiter’s enormous
magnetosphere, unexpected changes may ensue – and radio astronomers will
be listening in. Some dust will strike Jupiter’s night side, too, producing
a meteor storm or even auroral displays spectacular enough to be seen from
Galileo, and perhaps even from the Earth.

Unseen fragments

I am interested in the possibility that S-L 9’s wings, haloes, and tails
may contain many unseen comet fragments – much bigger than the hypothesized
grains of dust and ice, but much smaller than the 21 largest fragments.
Sharp new images by the newly repaired Hubble Space Telescope have shown
some changes, including a couple of previously unforeseen fragments, although
another has disappeared; but Hubble has not yet focused on the now-fading
wings. In any case, particles smaller than a hundred metres across will
remain invisible, even to Hubble, until their couple of seconds of glory
as they streak into Jupiter’s atmosphere. Their brilliant, fiery death could
be recorded by Galileo, and might be observable direct from Earth.

Pieces that became dislodged from the big pieces in 1992 with velocities
of only a few centimetres per second could hit the narrow dark-side crescent
of Jupiter that is visible from Earth, just beyond the morning horizon.
By early August, even debris in the middle of the southwestern wing of S-L
9 will begin striking that same Earth-facing part of Jupiter. So astronomers,
both amateurs and professional, should not give up looking for flashes when
the last big one strikes on 22 July.

During the course of our Solar System’s history, tidal disruptions of
comets may have been one of the major causes of cratering on planets. But
nobody knows what range of fragment sizes are produced by such tidal disruptions.
We also don’t know much about comets, those icy entities stored in a kind
of outer deep-freeze ever since the Solar System was formed. What size of
building blocks accreted to form comets, four-and-a-half-billion years ago?
My colleague at the Planetary Science Institute, Stuart Weidenschilling,
believes ‘cometesimal’ building blocks may be a few tens of metres across;
if so the large fragments of S-L 9 could be surrounded by swarms of such
smaller pieces.

Looking for Jovian meteors may provide unique insights to these fundamental
questions of planetary science. And observations of the mighty explosions
of the main fragments are sure to provide new insights about comets and
the planet Jupiter, and about fundamental planetary processes in general.

Clark Chapman is a senior scientist at the Planetary Science Institute,
a division of Science Applications International Corporation, in Tucson,
Arizona.