Impacts on Jupiter
T
here have been two recent impacts on Jupiter – one
natural, one man-made. Each has provided us with a
great deal of new information.
In March 1993 three American comet-hunters, Eugene
and Carolyn Shoemaker and David Levy, discovered what
they described as a ‘squashed comet’; it was their ninth
discovery, so the comet became known as Shoemaker–
Levy 9 (SL9). It was unlike anything previously seen. It
was orbiting Jupiter, and had been doing so for at least
20 years; calculation showed that on 7 July 1992 it had
skimmed over the Jovian cloud tops at a mere 21,000 km
(13,000 miles) and the nucleus had been torn apart, so that
it had been transformed into a sort of string-of-pearls
arrangement. Over 20 fragments were identified, and were
conveniently lettered from A to W.
It was soon found that the comet was on a collision
course, and that the fragments would hit Jupiter in July- The first fragment, A, impacted on 16 July, just on
the side of Jupiter turned away from Earth, but the planet’s
quick spin soon brought the impact site into view. The
other fragments followed during the next several days, and
produced dramatic effects; there were huge scars, visible
with a very small telescope. The cometary fragments were
only a few kilometres in diameter, but were travelling at
tremendous speed. Spectacular pictures were obtained
from ground observatories and from the Hubble Space
Telescope; the effects of the impacts were detectable for
months.
The Galileo probe to Jupiter was launched from the
Shuttle in October 1989, and after a somewhat roundabout
journey reached Jupiter in December 1994. It was made
up of two parts: an entry vehicle, and an orbiter, which
separated from each other well before arrival. The entry
probe was scheduled to plunge into the Jovian clouds,
and transmit data until it was destroyed; the orbiter would
orbit Jupiter for several years, sending back images of the
planet and its satellites.
The high-gain antenna, a particularly important part of
the communications link, failed to unfurl; some data were
lost, but much of the planned programme could be carried
out. The entry probe plunged into the clouds on schedule, and
continued to transmit data for 75 minutes, by which time it
had penetrated to a depth of 160 kilometres (100 miles).
Some of the results were unexpected. For instance, it
had been thought that the strong Jovian winds would be
confined to the outer clouds, and would slacken with
increasing depth, but this did not happen; by the time
contact was lost, the winds were just as strong as they had
been at the surface, indicating that the driving force was
not the Sun, but heat radiating from the interior. There
was much less lightning activity than had been expected,
and, most surprising of all, the Jovian atmosphere was
dry; the amount of water was very low, so that presumably
the water shown after the Shoemaker–Levy impact came
not from Jupiter, but from the dying comet.
Later analysis showed that the Galileo entry probe had
plunged into the clouds in an unusually ‘dry’ area of the
planet – an equivalent of a Jovian desert, so that the lack
of water was not typical. One mystery at least was solved,
but it cannot be claimed that we have as yet anything
like a complete knowledge of the interior of Jupiter. We
can hardly hope for another cometary impact, so that
presumably we must wait for another deliberate entry
into Jupiter’s cloud layer.ATLAS OF THE UNIVERSE
▼ Comet collision – a Hubble
Space Telescope image
showing numerous Comet
Shoemaker–Levy 9 impact
sites on Jupiter shortly
after the collision of the last
cometary fragment (north
is to the top). The impact
sites appear as dark
‘smudges’ lined up across
the middle of Jupiter’s
southern hemisphere and
are easily mistaken for
‘holes’ in the giant planet’s
atmosphere. In reality, they
are the chemical debris
‘cooked’ in the tremendous
fireballs that exploded in
Jupiter’s atmosphere as
each fragment impacted the
planet. This material was
then ejected high above the
bright multicoloured cloud
tops where it was caught
in the winds of the upper
atmosphere and eventually
dispersed around the planet.▲ Impact of the largest
fragment of Comet
Shoemaker–Levy 9,as
viewed from the Hubble
Space Telescope.C Atl of Univ Phil'03stp 2/4/03 3:09 pm Page 100