

Planetary Spacecraft as Observers


Planetary Spacecraft as Observers

Three NASA spacecraft missions launched some years ago to explore various planets and the sun will observe the events surrounding the Shoemaker-Levy 9 impacts with Jupiter from unique perspectives far removed from Earth. They are Voyager 2, Galileo and the joint NASA/European Space Agency Ulysses mission. Voyager, Galileo and Ulysses will be the only spacecraft actually with a direct view of the nightside impact points for the many comet fragments.

Voyager 2, launched in August 1977 and now leaving the solar system after flying by and observing Jupiter, Saturn, Uranus and Neptune, will be some 6 billion kilometers (3.7 billion miles) from Jupiter in mid-July when the events begin. It will use its ultraviolet spectrometer and its planetary radio astronomy instrument to detect, time and measure emissions stimulated by the comet impacts.

The Galileo spacecraft is now on the final leg of its flight path to Jupiter after a complex interplanetary trajectory involving gravity-assist flybys of Venus and Earth and two encounters with asteroids. It is scheduled to send a probe into Jupiter's atmosphere and begin a two-year Jupiter orbital tour in December 1995.

Galileo will be about 240 million kilometers (150 million miles) from Jupiter between July 16 and July 22 when the comet fragments are colliding with the planet, and will view the nightside impact regions directly. At that range, its camera can resolve Jupiter as a disc 60 pixels across, comparable to the capability of many large Earth-based telescopes. (A pixel, or picture element, is one point in the digitized picture.)

Galileo's near-infrared mapping spectrometer, the photopolarimeter and the ultraviolet spectrometer will measure all of the light from Jupiter, attempting to detect increases at the time of the impacts. Plasma-wave sensors and the dust detector will be operating to detect radio emissions from the impacts and to search for changes in Jupiter's dust environment.

The Galileo imaging team plans to try several different strategies to capture the Shoemaker-Levy phenomena. One approach is to collect many time-lapse exposures in mosaic patterns, because the times of impact will be uncertain to within minutes. The timing uncertainties and data storage limitations mean that this procedure can be used to observe only a few of the fragment collisions. Multi-color imaging - in which the camera takes several exposures in a row through various colored filters - will be attempted for other impacts.

The commands to execute this complex, multiple-instrument observation program must be transmitted to the spacecraft many days in advance, with limited opportunity for later updates. The spacecraft will then carry out the program when the time comes, while the flight team monitors its actions and receives some science data sent back at a low rate over Galileo's low-gain antenna. After the Shoemaker-Levy events have concluded, the flight team will search through data recorded on Galileo's onboard tape recorder and will command the spacecraft to selectively transmit observations of interest. This selective procedure has been proven in the two asteroid encounters. It may be as late as October or November 1994 before the majority of Galileo's data from these events are transmitted to Earth from the spacecraft.

Ulysses, the NASA/ESA mission to study the poles of the Sun, will also search for possible radio emissions caused by Shoemaker-Levy. During July the spacecraft will be in the midst of its southern passage around the Sun - measuring the solar wind as well as fields and particles from positions less than 20 degrees from the Sun's south pole.

At that time Ulysses will be about 400 million kilometers (250 million miles) from the sun, and about twice that distance from Jupiter. Its combined radio and plasma wave instrument will also search for radio emissions from Jupiter's magnetosphere that show effects of the Shoemaker-Levy impacts.

JPL Scientist Contacts on Shoemaker-Levy 9 Observations

SL9 Ephemeris (where the pieces are, when they'll hit, etc):
Dr. Donald Yeomans, 818-354-2127.
Dr. Paul Chodas, 818-354-7795.

Galileo Mission:
Project Manager: William J. O'Neil, 818-354-4195.
Project Scientist: Dr. Torrence Johnson, 818-393-7957.

Voyager Mission:
Project Manager: George P. Textor, 818-306-6001.

Ulysses Mission:
Project Scientist: Dr. Edward J. Smith, 818-354-2248.

Goldstone Radio Astronomy Observations (NASA/JPL Jupiter Patrol)
Dr. Michael J. Klein, 818-354-7132

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