The Comet About to Smash into Jupiter

What observations are astronomers planning?

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 advantage of' 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 kilometers away from the planet, less than a third the distance of Earth from Jupiter at the time. All of the impacts will occur directly in the field of view of its high resolution camera. In addition, instruments that study infrared and ultraviolet light will most likely be used.

Using Galileo to make observations will be challenging however. The amount of data the spacecraft can transmit back to Earth is limited by the capability of its low-gain antenna (the spacecraft's high-gain antenna, which could have transmitted large amounts of data in short periods of time, failed after launch and the time available on the receiving antennas of NASA's Deep Space Network here on Earth. A lot of data frames can be stored in the Galileo tape recorder, but only about 5 percent of them can be transmitted back to Earth. so the trick will be to decide which 5 percent of the data are most likely to include the impacts and 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 impact site of the fragments of Comet Shoemaker-Levy 9 on Jupiter. (Courtesy D. Seal)

The Ulysses spacecraft was designed for solar study and used a "gravity assist'' for 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 kilometers 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 radio receiver that 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 and is about 6.4 billion kilometers 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 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 observe the impact.

A new small spacecraft called Clementine was launched on Jan. 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. Still. it seems probable that attempts will be made to observe "blips'' of light on the edge of Jupiter from the entering fragments or subsequent fireballs.

Stupendous as these collisions will be, they will occur on the far side of a planet half a million miles from Earth. There will be no display visible to the general public. Amateur astronomers may note a few seconds of brightening of the inner satellites of Jupiter during the impacts, and they might observe minor changes in the Jovian cloud structure during the days following the impacts. In the best of cases, these events will be spectacles for the mind to imagine and big telescopes to observe, not a free fireworks display. The real value of this most unusual event will come from scientific studies of the comet's composition. of the impact phenomena themselves, and of the response of a planetary atmosphere to such a series of "insults.''

Atmospheric phenomena on this scale cannot be reproduced, even by nuclear fusion explosions, and have never before been observed. Sixty-five million years ago the Earth was struck by a large asteroid or comet, an event which may have hastened the extinction of the dinosaurs. Better knowledge of the effects of Comet Shoemaker-Levy 9 on Jupiter may allow scientists to predict more accurately just how serious could be the results of future impacts of various-sized bodies on Earth.

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