November 1982:JPL space report

GALILEO ON TARGET

The writer Mark Twain once cabled a correction from Europe to the Associated Press, “The report of my death was an exaggeration.” In a similar vein, the press has reported the end of the Galileo mission to Jupiter several times in the last year. Now, however, continued funding seems assured, and Galileo, along with the Voyager 2 encounters with Uranus (1986) and Neptune (1989), should form the backbone of JPL’s interplanetary programme for this decade.

The basic plan of the mission is to insert a 1600 kg spacecraft into orbit about Jupiter after first deploying a 335 kg probe into the Jovian atmosphere. The orbiter will conduct measurements in the Jovian system concerning the planet, rings, and Galilean satellites, with at least 11 satellite encounters within 24 months after insertion into orbit about the planet. Some of these encounters, using satellite gravity-assist, will be as close as a few hundred km and will provide unmatched opportunities for determining the chemical composition and fine-scale structure of these bodies. Supplementing the optical imaging system will be an ultraviolet spectrometer, an infrared spectrometer and a radiometer for measuring temperature profiles, as well as several field-and-particle instruments. The Galileo Probe will carry instruments designed to measure the temperature, pressure and chemical composition of the Jovian atmosphere. The orbiter will provide a relay link communicating the probe measurements back to Earth.

The May 1985 launch will involve an Air Force Inertial Upper Stage (IUS) rather than the wide-body Centaur that was originally planned (the Shuttle is to be used for the initial launch phase). In order to compensate for the loss in capability resulting from this change, two actions are being taken: (1) a kick stage will be added on top of the IUS, and (2) the trajectory will incorporate a gravity assist from Earth. This type of trajectory, termed a AVEGA (AV-Earth-Gravity-Assist), features a loop in the inner Solar System of approximately two years’ duration (shaped by a velocity change or AV) which is followed by a very close, 200 km flyby of the Earth in order to be boosted into the Earth-Jupiter leg of the trajectory. Arrival at Jupiter is scheduled for late 1989 or early 1990, depending upon the exact trajectory which is selected. JPL’s Mariner 10 mission to Venus and Mercury was the first mission to employ gravity-assist techniques.

PLANETARY IMAGE FACILITY

The Regional Planetary Image Facility located at JPL is a joint effort between the Laboratory and NASA in order to provide a reference library of planetary and satellite images. The emphasis is, of course, upon images returned by space probes, but Earth-based photography and relevant topographic and geologic materials are also available.

An extensive list of missions comprises the source for the collection including Rangers 7-9; Surveyors 1, 3, 5-7; Lunar Orbiters 1-5; Apollos 8, 10-17; Mariners 4,6, 7, 9, 10; Vikings 1, 2 and Voyagers 1, 2.

The Facility, which opened in 1979, contains approximately 225,000 images accessible in photographic files and also on a versatile, computerised filing system using a video disc. Approximately 20 parameters, including planetary latitude and longitude, spacecraft range-to-planet, and lighting angle can be specified by the user of the video disc system. The programme responds to this input with a listing of all images which satisfy these criteria. The images themselves can then be displayed on a TV screen in black and white, with plans to extend the capability to a colour display of those images which were originally produced in colour.

The video disc itself is a silvery plate that resembles an ordinary gramophone record in shape. It was produced by first photographing the original prints one-by-one onto 35 mm film, transferring these to 1 in. video tape and then, finally, loading the contents of the tape onto the video disc.

Head Librarian Leslie Pieri demonstrated the remarkable properties of the disc by stepping through a sequence of Viking Orbiter images at the rate of five frames per second. As I watched the TV screen I partially enjoyed the perspective of the Orbiter since the images are stored sequentially, by consecutive orbit.

VIKING MISSION CONTINUES

In the summer of 1976 the planet Mars was visited by a small fleet of four spacecraft: two orbiters and two landers. Today the orbiters are silent but the Viking 1 lander continues to return data from its location in Chryse Planitia, the Plains of Gold. It is scheduled to return imaging, meteorology, radio science and engineering data up to the late 1980s.

More than 4,500 pictures have been returned from the landers and over 50,000 were received from the two orbiters. The latter pair mapped about 97% of the Martian surface at a resolution of 300 m, while 2% of the surface was resolved at the 25 m level. Although much valuable material was obtained by the imaging sequences taken from the landers, most of it was of a static nature; very few changes occur on the Martian surface. Only two small surface slides were noted in four years of the extended mission (the follow-on to the initial mission segment), in addition to seasonal coloration changes due to the transport of material, and the phenomenon of winter frost.