By the start of 1996, the subsystems for the Cassini spacecraft were being assembled in the 'clean room' of JPL's Spacecraft Assembly Facility.
The Attitude and Articulation Control Subsystem (AACS) that will enable the spacecraft to maintain its bearings in space was connected to the Command and Data Subsystem (CDS) that will serve as its 'brain', and to the Power and Pyro Subsystem that will regulate its electrical power supply. Meanwhile, at White Sands in New Mexico, one of the main engines built by Lockheed-Martin in Denver, Colorado, was run for 200 minutes in order to certify its design.
In February, the first scientific instrument - the University of Iowa's Radio and Plasma Wave Spectrometer - was fitted, and the others followed over the next few months. Although each instrument had been thoroughly tested by its builder, it was necessary to re-run the tests in order to verify that they all worked in conjunction with the spacecraft's systems. As each instrument was added, tests had to be run to make sure that it did not upset the existing suite. To facilitate testing, a new Science Operations and Planning Computer in the Distributed Operations Interface Element enabled the science team to remotely operate their instruments and receive its data at their home institutions. In addition to providing each team with unprecedented access to the engineering and science data from its hardware, this 'distributed' strategy significantly reduced costs and saved time. Meanwhile, experts on planetary rings met to discuss the estimates of ring particle distribution to be used by Cassini trajectory designers in identifying a safe flight path for the Saturnian orbital tour. Recent studies of Saturn's rings helped to ensure that the flight path would avoid areas that might pose a hazard. It was also decided that the minimum altitude for a Titan fly-by should be 950 kilometres. As the detached layers of haze in the moon's upper atmosphere extend to an altitude of 700 kilometres, the spacecraft will actually pass through the tenuous thermosphere. If it were to penetrate any more deeply, the 'drag' would impair its ability to control its orientation.
In March, in Germany, the integration of the Huygens probe started, and in Italy the Radio Frequency Electronic Subsystem was assembled. At the Kennedy Space Center, a full-scale mock up of the Cassini spacecraft (dubbed the 'Trailblazer') was run through the procedures for testing and mating with the launch vehicle adapter in order to identify likely handling problems and to develop alternatives. In April, JPL took receipt of a fully functional engineering model of the Huygens probe. This was mated with the spacecraft and subjected to a full range of tests in order to verify that Cassini could communicate with a probe. Meanwhile, in Europe, the real probe was being subjected to thermal, vibrational and shock testing. Once the Trailblazer was returned to JPL, it served as a model for tailoring the many individual sections of the thermal blanket that would protect the vehicle from the space environment. Once the propulsion module had been certified leak-free by Lockheed-Martin, it was shipped to JPL in July.
By this point, science planning by more than 250 scientists from 17 countries had reached an 'intermediate' level of detail, having established the general objectives for the study of Saturn, its magnetosphere, its rings, its icy companions and Titan. As a large number of individual observations would have to be made, the task was to plan the orbital tour so that the spacecraft would be in position to make the observations required by each science investigation. At a series of meetings in the USA and Europe, the scientists, engineers and mission planners evaluated different orbital tour options to determine which offered the best opportunities for most of the instruments. Enceladus has only a few impact craters disfiguring its remarkably smooth surface, and Cassini is to try to ascertain whether Enceladus has an internal heat source that occasionally melts the ice sufficiently to erase its craters. A search will be conducted for geysers venting water ice crystals into space where they form the tenuous 'E' ring. If this can be established, then Enceladus will have to be considered in the same class as the ice-enshrouded Jovian moon, Europa. Another high-priority target is Iapetus, which has a very large dark patch on its leading hemisphere. Cassini is to determine the composition of this material, and reveal whether it derives from the interior or was deposited by some external process. Another high priority is the ring system and, hopefully, Cassini will identify the reason for its tremendous complexity. The physical structure of the magnetosphere will be charted to establish how it interacts with the moons, rings and solar wind. Observations of Saturn will characterise its atmosphere at different altitudes, identify the forces driving the zonal winds, and monitor the behaviour of the weather system over an extended period. At the science group's meeting at Caltech in the final week of July, it was decided to develop an orbital tour providing about 40 Titan fly-bys and a total of up to half a dozen of the most interesting icy moons. The integration of Cassini's main components was completed on 25 September, and by 11 October the engineering model of the Huygens probe had also been integrated. At that point, all manner of people were drawn to the visitors' gallery to admire the three-storey-tall robotic explorer. Engineers and scientists who had previously seen only their individual subsystems, and mission planners who had a broader perspective but no 'hands-on' involvement with the hardware, came to see the results of their endeavours. At long last, Cassini-Huygens was a spacecraft. As R.J. Spehalski, the Program Manager, observed, ''People seeing it for the first time are saying 'Holy smokes!'...''
A few days later the package was moved to the Solar-Thermal Vacuum Chamber for the 'shake and bake' tests. Over the next few months, enormous speakers blasted the vehicle with acoustic vibrations in order to subject it to the random shocks that it would experience during launch. The customised thermal blankets were then fitted, the chamber was reduced to vacuum, and the spacecraft was subjected to the extreme temperatures that it would have to endure in space. The thermal blankets comprised a multitude of segments. Although the fabric looked like gold foil, the shiny golden hue was due to the placement of a transparent layer of amber-coloured material on top of the extremely reflective aluminised fabric. The segments were finely sewn together as a strong but extremely lightweight protective conformal shield. ''Our blankets are built unlike any others,'' said Mark Duran, supervisor of the 'shield shop' that had made the protection for JPL's previous vehicles. On such a long mission, the durability of the shield is a major factor. ''Our goal in blanketing Cassini is to keep temperatures on board the spacecraft at room temperature,'' noted thermal-requirements engineer Pamela Hoffman. In space, temperatures on exposed parts of the spacecraft will range from about -220°C to +250°C. If the spacecraft maintains a fixed orientation relative to the Sun, the illuminated exposed structure will bake and the shaded parts will freeze, therefore the blanket fabrics must withstand this extreme radiation environment. For Cassini, the blankets employ as many as 24 layers of different fabrics, including aluminised kapton, mylar, dacron and other special materials. Some of the blankets were sewn with layers of beta cloth, which is a canvas-like carbon-coated fabric that is especially effective in protecting against micrometeoroids. The blankets must also satisfy electrical standards. The spacecraft will be flying through environments full of charged particles that could cause an electrical arc to form across the blankets, so thin accordion-like strips of aluminium were sewn into every layer of each blanket to ensure that an arc would be electrically grounded.
To ensure that Cassini would be ready for transfer to the Kennedy Space Center in April 1997, testing continued right through the end-of-year festivities. In the past, JPL had put spacecraft in environmentally stable containers and driven them across the country. However, 'anti-nuke' activists had started registering their protests against the use of RTG power units by taking pot shots at the containers -a practice hardly likely to ensure safe passage of what they regarded as a hazardous material! With Cassini being so large, it was decided that the Air Force should transport it in a C-17 Globemaster-II on 21 April. Once the high-gain antenna and the propulsion module had been integrated, and Huygens mounted on its side, the entire vehicle was thoroughly checked-out to verify its functionality. Once the spacecraft was ready, one final item was added. "The 'signature disk' idea began as a very small in-house effort to let team members place their signatures on the spacecraft that they had helped to build, but then we realised we could accommodate many signatures, and we opened it up to everyone,'' mused Spehalski. As soon as the announcement was made in November 1995, cards had begun arriving from around the globe with as many as 35,000 signatures being received during peak weeks. In excess of 600,000 signatures were eventually submitted by citizens of 81 countries. ''It has blossomed into a 'message in a bottle' to Saturn,'' Spehalski noted. The project-within-a-project was managed by Charles Kohlhase, the science and mission design manager. ''We received signatures from individuals young and old, whole families, hundreds of classes of students and indeed whole schools,'' he said. Some of the names were in solemn memory of deceased family members. For completeness, the signatures of the two astronomers after whom the mission had been named were retrieved from archives. At least one person claimed to be a descendant of Giovanni Domenico Cassini. Every submitted signature was electronically scanned by Planetary Society volunteers and the entire set was then stored utilising the recently developed Digital Versatile Disk (DVD) technology. The artwork on the disk was designed by Kohlhase to celebrate various elements of the Cassini mission, its scientific targets, and the flags of the countries from which the largest numbers of signatures were received. The disk was inserted into a shallow cavity on the side of the spacecraft, beneath the Remote Sensing Pallet, in between two pieces of aluminium where it will be safe from micrometeoroid impacts. The completed spacecraft was then mounted on its launch vehicle adapter and driven to Launch Complex 40 for mating with the Centaur rocket stage. The overall package was so heavy that a Titan IV-B, the most powerful rocket in the Air Force's fleet, would be required to dispatch it.
On 3 October, after persistent attempts by anti-nuke activists to have the launch prohibited, the White House's Office of Science and Technology Policy granted the space agency approval to proceed. In calling for ''an end to nuclear proliferation in space'' the protesters were rather naive, as interplanetary space is flooded with solar and cosmic radiation. Their immediate concern, however, was their assessment of the environmental threat from the plutonium dioxide in the RTGs and the radioisotopic heaters in the event of a catastrophic failure. The White House deemed the risk to be acceptable. ''NASA and its interagency partners have done an extremely thorough job of evaluating and documenting the safety of the Cassini mission. I have carefully reviewed these assessments and have concluded that the important benefits of this scientific mission outweigh the potential risks,'' said J.H. Gibbons, the director of the Office of Science and Technology Policy, having signed the approval document. ''The launch of spacecraft cannot be made completely risk-free,'' pointed out Louis Friedman, the executive director of the Planetary Society, ''but the public can take satisfaction in knowing that NASA is being careful, prudent, and smart.''
The Cassini-Huygens spacecraft hanging from a crane during its integration at the Kennedy Space Center.
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