Ian & Stuart's Australian Mac: Not for Sale/Another.not.for.sale (Australia).iso/Dr. Doyle/Jupiter Comet/Education Notes/sl9-jpl.08

Text File | 1994-04-15 | 9KB | 164 lines

8. How Can These Impacts and Their Consequences be Studied?

Space-Based
        There are at least four spacecraft -- Galileo, Ulysses,
Voyager 2, and Clementine -- with some potential to observe the
Jovian impacts from different vantage points than that of Earth.
There is also the Hubble Space Telescope (HST), in orbit around
Earth, which will view the event with essentially the same
geometry as any Earth-based telescope. HST, however, has the
advantages of perfect "seeing" (no atmospheric turbulence), very
low scattered light, ultraviolet sensitivity, and the ability to
observe much more than two hours each day. HST is scheduled to
devote considerable time to the observation of Shoemaker-Levy 9
before as well as during the impacts.

The Galileo spacecraft has the best vantage point from which to
observe the impacts. It is on its way to Jupiter and will be only
246 million km away from the planet, less than a third the
distance of Earth from Jupiter at that time. All of the impacts
will occur directly in the field of view of its high resolution
camera and 20-25 degrees of Jovian longitude from the limb. Images of
Jupiter will be 60 picture elements (pixels) across, although the
impact site will still be smaller than the resolution of the
camera. Several instruments besides the camera have potential use,
including an ultraviolet spectrometer, a near infrared mapping
spectrometer, and a photopolarimeter radiometer. This last suite
of instruments could acquire light curves (plots of intensity
versus time) of the entry and fireball at many wavelengths from
ultraviolet to thermal infrared (from wavelengths much shorter
than visible light to much longer).

Using Galileo to make these observations will be challenging. The
amount of data the spacecraft can transmit back to Earth is
limited by the capability of its low-gain antenna and the time
available on the receiving antennas of the National Aeronautics
and Space Administration's (NASA's) Deep Space Network here on
Earth. The "commands" that tell the spacecraft what to do must be
sent up several weeks before the fact and before the impact times
are known to better than about 20 minutes with 95% certainty. A
later command that simply triggers the entire command sequence may
be possible. A lot of data frames can be stored in the Galileo
tape recorder, but only about 5% of them can be transmitted back
to Earth, so the trick will be to decide which 5% of the data are
likely to include the impacts and to have the greatest scientific
value, without being able to look at any of them first! After the
fact, the impact times should be known quite accurately. This
knowledge can help to make the decisions about which data to
return to Earth.

The Ulysses spacecraft was designed for solar study and used a
gravity assist from flying close to Jupiter to change its
inclination (the tilt of its path relative to the plane of the
planets) so it can fly over the poles of the Sun. In July 1994 it
will be about 378 million km south of the plane of the planets
(the ecliptic) and able to "look" over the south pole of Jupiter
directly at the impact sites. Unfortunately, Ulysses has no camera
as a part of its instrument complement. It does have an extremely
sensitive receiver of radio frequency signals from 1 to 1000 kHz
(kilohertz, or kilocycles in older terminology) called URAP
(Unified Radio and Plasma wave experiment). URAP may be able to
detect thermal radiation from the impact fireballs once they rise
sufficiently high above interference from the Jovian ionosphere
(upper atmosphere) and to measure a precise time history of their
rapid cooling.

The Voyager 2 spacecraft is now far beyond Neptune (its last
object of study back in 1989 after visiting Jupiter in 1979,
Saturn in 1981, and Uranus in 1986) and is about 6.4 billionJ m
from the Sun. It can look directly back at the dark side of
Jupiter, but the whole of Jupiter is now only two picture elements
in diameter as seen by its high-resolution camera, if that
instrument were to be used. In fact the camera has been shut down
for several years, and the engineers who knew how to control it
have new jobs or are retired. It would be very expensive to take
the camera "out of mothballs" and probably of limited scientific
value. Voyager does have an ultraviolet spectrometer which is
still taking data, and it will probably be used to acquire
ultraviolet light curves (brightness versus time) of the impact
phenomena. The possibility of using one or two other instruments
is being considered, though useful results from them seem less
likely.

A new small spacecraft called Clementine was launched on
January 25 of this year, intended to orbit the Moon and then
proceed on to study the asteroid Geographos. Clementine has good
imaging capabilities, but its viewpoint will not be much different
from Earth's. The impact sites will still be just over the limb,
and Clementine's resolution will be only a few picture elements on
Jupiter. Since the spacecraft will be in cruise mode at the time,
on its way to Geographos and not terribly busy, it seems probable
that attempts will be made to observe "blips" of light on the limb
of Jupiter, from the entering fragments or the fireballs or
perhaps light scattered from cometary material (coma) that has not
yet entered the atmosphere. Useful light curves could result.



Ground-Based
        Many large telescopes will be available on Earth with
which to observe the phenomena associated with the Shoemaker-
Levy 9 impacts on Jupiter in visible, infrared, and radio
wavelengths. Small portable telescopes can fill in gaps in
existing observatory locations for some purposes. Imaging,
photometry, spectroscopy, and radiometry will certainly be carried
out using a multitude of detectors. Many of these attempts will
fail, but some should succeed.

Apart from the obvious difficulty that the impacts will occur on
the back side of Jupiter as seen from Earth, the biggest problem
is that Jupiter in July can only be observed usefully for about
two hours per night from any given site. Earlier the sky is still
too bright and later the planet is too close to the horizon.
Therefore, to keep Jupiter under continuous surveillance would
require a dozen observatories equally spaced in longitude clear
around the globe. A dozen observatories is feasible, but equal
spacing is not. There will be gaps in the coverage, notably in the
Pacific Ocean, where Mauna Kea, Hawaii, is the only astronomical
bastion.

Measuring the light curve of the entering fragments and the post-
explosion fireball can be done only by measuring the light
reflected from something else, one of Jupiter's satellites or
perhaps the dust coma accompanying the fragment. That dust coma
could still be fairly dense out to distances of 10,000 km or more
around each fragment. Moving at 60 km/s, it will be almost
three minutes before all of the dust also impacts Jupiter. Proper
interpretation of such observations will be difficult, however,
because the area of the "reflector", the coma dust particles, will
be changing as the observations are made. Another complication is
the brightness of Jupiter itself, which will have to be masked to
the greatest extent possible. Observations in visible light
reflected from the satellites will be relatively straightforward
and can be done with small telescopes and simple photometers or
imaging devices. This equipment is small enough that it can be
transported to appropriate sites.

Spectroscopy of the entry phenomena via reflected light from one
of the Galilean satellites could be used to determine the
composition of the comet and the physical conditions in the
fireball, if the terminal explosions occur above Jupiter's clouds.
If the explosion occurs below the clouds, there will be too little
light to do useful spectroscopy with even the largest telescopes.

The impact zone on Jupiter will rotate into sight from Earth about
20 minutes after each impact, though quite foreshortened as
initially viewed. Extensive studies of the zone and the area
around it can be made at that time. Such studies surely will
include imaging, infrared temperature measurements, and
spectroscopy using many of the largest telescopes on Earth. These
studies will continue for some weeks, if there is any evidence of
changes in Jupiter's atmosphere and cloud structure as a result of
the impacts.

For example, astronomers will use spectrometers to look for
evidence of chemical changes in Jupiter's atmosphere. Some of the
species observed might be those only present in the deep
atmosphere and carried up by the fireball (if the explosion occurs
deep enough). Others will be the result of changes to the
chemistry of the upper atmosphere, taking place because of the
energy deposited there by the impacts or because of the additional
particulates.