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.
(Due to problems with the Clementine spacecraft, no observations of the impacts are likely. -- jf)
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.
The faint rings of Jupiter, mentioned in Section 5 , can be usefully observed from the ground at infrared wavelengths. Shoemaker-Levy 9 debris might bring in new ring material by hitting the two small satellites embedded in the rings (Metis and Adrastea). The rings surely will be monitored for some time using infrared imaging array detectors, which are sensitive to wavelengths more than eight times as long as red visible light.
In Section 5, note was made of the Jovian magnetosphere, which makes its presence known at radio wavelengths, and the Io torus of various ions and atoms, which can be mapped spectroscopically. Either or both of these could be affected sufficiently by the intruding dust from Shoemaker-Levy 9 to be detectably changed. Radio telescopes will surely monitor the former and optical telescopes the latter for weeks or months looking for changes. Jupiter's intense electromagnetic environment is responsible for massive auroral emission near the planet's poles and less intense phenomena across the face of the planet. These may also be disrupted by the collisions and/or the dust "invasion," making auroral monitoring a useful observing technique.
In summary, the phenomena directly associated with each impact from entry trail to rising fireball will last perhaps three minutes. The fallback of ejecta over a radius of a few thousand kilometers will last for about three hours. Seismic waves from each impact might be detectable for a day, and atmospheric waves for several days. Vortices and atmospheric hazes could conceivably persist for weeks. New material injected into the Jovian ring system might be detectable for years. Changes in the magnetosphere and/or the Io torus might also persist for some weeks or months. There is the potential to keep planetary observers busy for a long time!
