April-May 1981:Voyager 1 at Jupiter

Approaching Jupiter

As Voyager 1 approached Jupiter at the beginning of February 1979 the disc of the planet had been growing steadily in size and soon filled the full field of view of the narrow angle camera. It then became necessary to image the planet by taking photo mosaics, initially of a 2 x 2 format and later, by February 21st, 3x3. Each of the 2 x 2 mosaics were made by shuttering once through each of three filters (violet, orange and green) with the camera centred on four different points. Each set of twelve images was then reconstructed to make a single colour picture of the entire planet's disc. This process would be continued with ever larger mosaics as the spacecraft closed in on Jupiter.

Although the photographs which were being returned were becoming increasingly spectacular they were more than just pretty pictures to the scientists of the imaging team. Amongst the specific objectives of the imaging experiment were:

•The study of global atmospheric circulation.

•The horizontal and vertical structure of the clouds.

•Cloud colouration.

•The vertical structure of the upper atmospheric levels.

•Specific features such as the Great Red Spot.

•The geology of the Jovian satellites.

•The search for lightning and auroras on the night side of the planet.

In addition to the imaging, other experiments continued to probe the planet and its satellite system. Infrared surveys were made every day and compared with infrared images taken from Earth. At shorter wave-lengths the ultra violet spectrometer began to zero in on the larger moons. The instrument's narrow field of view was set up near to the satellite and then scanned across and just beyond it, allowing measurements of both the satellite and any nearby gases associated with it.

Towards the end of February members of the scientific community began to arrive at Pasadena to take up residence for the period of closest approach and excitement began to build up at mission control. The engineers of the flight team however continued to guide the spacecraft in towards the planet and made a short firing of the hydrazine thrusters to fine tune the trajectory. By now Voyager 1 was already within the Jovian system having passed the orbit of tiny Sinope some 23 million km (15 million miles) from Jupiter on February 10th.

Two weeks from closest approach the UV spectrometer was scanning across the width of the orbit of Callisto seeking evidence of gaseous clouds associated with some of the inner satellites. The photopolarimeter was also active pointing at the heart of the Jovian system, searching for sodium atoms and mapping the distribution of this element relative to both Io and the planet’s magnetic field. Although the most intensive effort would be made around the time of closest approach a few pictures of the major moons were returned starting with Callisto on February 18th. By now Voyager 1 was travelling towards its target at over 13 kilometers per second (nearly 30,000 mph) and still slowly accelerating; maximum velocity at encounter was predicted to be 36 kilometers per second. At this time the twin probe Voyager 2 was 100 million km from Jupiter and quietly cruising through interplanetary space.

Ten days from Jupiter mission controllers stopped thinking that their spacecraft was going to the planet and realised that it was in effect already there. All of the various instruments were now reacting to the Jovian environment in their own way and the magnetometers and plasma instruments indicated that the crossing of the bow shock wave was imminent. Auroral type activity had been detected around both Io and Jupiter and the planetary radio astronomy experiment was busy investigating the intense radio emissions associated with Jupiter and its electromagnetic environment.

It is not our purpose to describe the Jovian system or the scientific results from the Voyager mission in detail as these are given excellent coverage in other, more specialist, publications and within Spaceflight, but brief mention will be made of some of the significant results. Voyager 1 encountered Jupiter's bow shock, the area where the solar wind responds to the magnetic field of the planet, no less than three times. The first crossing came at about 7 a.m. PST on February 28th,nearly 6 million km from the planet, and later that day the solar wind increased squashing the magnetosphere back towards Jupiter and a second crossing was made six hours later. The next day the solar wind had overtaken the spacecraft again pushing the bow shock to within 5.1 million km of the planet.

On March 4th Voyager’s narrow angle camera recorded an image on which were subsequently discovered evidence of a previously unknown ring around Jupiter. A time exposure of 11 minutes detected a thin, flat ring of particles around the Jovian equator. On the original photograph the ring is split into six and the background star trails appear to oscillate due to a slight nodding motion of the spacecraft. This residual motion is due to the long instrument booms projecting from the vehicle.

As Voyager 1 closed in towards its closest approach, scheduled for the early hours of March 5th PST, the cameras started to zoom in on specific features in the planet's turbulent atmosphere, examining some of them several times to search for short term changes. The Great Red Spot, for example was imaged several times, the final mosaic on the inbound leg being made about 8 hours from periapsis and consisting of no fewer than eighty-one frames. A few hours before periapsis Amalthea, a tiny ellipsoidal moon with a major axis only 270 km long and just over 100,000 km from Jupiter, was photographed and shown to reflect 50% more red than violet light. Soon after, the cameras were turned on the innermost of the large moons and an imaging mosaic covering about one third of the visible side of Io was made. Intensive studies of Io continued as Voyager made its closest approach to Jupiter, and then the imaging data was directed to the onboard tape recorder since Earth occultation was rapidly approaching.

Io, innermost of the four major satellites, was generally expected to be similar to Earth’s Moon with a cratered surface but photographs taken at long range showed Io to be unusually smooth. As the distance fo the satellite decreased dark spots with faint outer rings were observed and these were assumed to be impact craters. As resolution improved however it became obvious that this was not the case since no impacts could be detected amongst a host of unusual landforms. Just before the closest approach to Io a circular feature 50 km in diameter and surrounded by a radiating pattern of flow was observed. It began to look as if Io had a recent history of extensive vulcanism which had overlaid the ancient terrain. Such activity had been predicted in a paper published only days before by Peale, Cassen and Reynolds. Seldom in the history of science can such spectacular confirmation of a prediction have occurred so soon after publication. Eventually more than one hundred volcanoes would be identified.

Some three and a half hours after periapsis the spacecraft, as seen from Earth, slipped behind the planet allowing a series of radio science and UV investigations. Before occultation the S and X band radio transmitters were adjusted to equalise the signals through the atmosphere. The transmission characteristics of the atmosphere and the relative distortion of the two different frequencies as the signals passed through the increasing depth of gases allowed studies to be made of the ionosphere and the nature of materials in the clouds.

Fifty-three minutes later, and for the first time since separation from the launch vehicle some eighteen months before,Voyager 1 lost sight of the Sun. As the sunlight became increasingly dimmed by the clouds the ultraviolet spectrometer was used to probe the composition and temperature of the atmosphere and was able to detect a variety of chemicals which had eluded the less sensitive instruments on the earlier Pioneer spacecraft. Whilst occulted by the planet. Voyager made its closest approach to Europa although even then the flyby distance was in excess of three quarters of a million km. Photos revealed a body which was lightly coloured and globally of low contrast. In addition the moon was covered by a number of intersecting streaks some tens of kilometers wide and extending over a thousand kilometers or more. Since at this range the resolution was equivalent to a view of Earth's Moon occupying only one third of a domestic television screen little else could be discerned initially. Voyager 2 was planned to make a much closer approach to Europa on its inbound passage through the Jovian system later in the year.

Both Sun and Earth occultation lasted about two hours, and then the spacecraft began to pull away from the planet and attention turned towards the satellites Ganymede and Callisto. Pictures of Ganymede revealed a surface remarkably similar to the Earth's moon but, as the resolution improved, peculiar grooved terrain, consisting of parallel ridges and troughs up to 15 km wide and hundreds of km long were observed. Extensive coverage of the Jupiter-facing side of Ganymede was achieved,while the opposite hemisphere would be studied by Voyager 2. The closest approach of Voyager 1 to Ganymede was 112,030 km allowing a best surface resolution of 2 km.

The final satellite to be examined in detail by Voyager 1 was Callisto, outermost of the Galilean moons, orbiting at a mean distance of 1.8 million km from Jupiter. The Jupiter-facing hemisphere of Callisto was also imaged at high resolution, between two and seven km per line pair, and showed itself to be different again from its companions. The spacecraft flew within 124,000 km of Callisto and showed that the surface was very heavily cratered. In addition to the craters a massive system of concentric rings was discovered in the crust. Centred on a bright circular region 10 degrees north of the equator, these rings stretched out to a distance of 1,500 km. As Voyager 1 receeded from Callisto on the afternoon of March 6th the intensive activities of the near encounter phase began to draw to a close. The spacecraft would continue to look back upon Jupiter for several weeks yet and analysis of the data returned during closest approach had hardly begun. Work began immediately to analyse as much of the data as possible in the short time remaining before Voyager 2 arrived, thus allowing maximum benefit to be obtained from the second spacecraft.

It was during this post-encounter period that examination ot a long range photograph of Io, taken for navigation purposes,revealed a curious asymmetry. The photograph had been overexposed to bring out details of the background stars and JPL engineer Linda Morabito noticed an unusual umbrella shaped blob on the limb. After attempts to explain it away as an artifact had failed, it was realised that one of Io's volcanoes had been caught in the act of erupting. What else could have thrown an enormous cloud 270 km above Io? Once the reality of the feature was established a major effort was put under way to locate plumes and no less than a further seven active volcanoes were discovered on Io.