are building will carry the human senses, and even part of our
reasoning ability, all the way to Saturn, where it will make precise
measurements of many kinds. These observations, in the form of digital
information, will be transmitted back to Earth, where scientists will
analyze the data, draw conclusions, and hopefully advance our
understanding not only of the Saturnian system but also of the solar
system in general and planet Earth in particular.From a design standpoint, then, the spacecraft must incorporate many human-like features, and it must also provide a platform for the science observations. Here are a few highlights about the spacecraft design.
The heart of the mission, of course, will be the science instruments. The Cassini orbiter will be designed to carry equipment for twelve science experiments, and the Huygens probe, which will detach from the orbiter several months after arrival at Saturn, will carry equipment for six more. The orbiter will also contain some innovative new technology that will help keep the mass of the spacecraft down and add to the reliability and longevity of the hardware. The importance of this is that the lighter and sturdier the spacecraft is, the more science equipment it can carry and the longer it can operate.
Thus, the Cassini orbiter is being designed with as much robustness,
autonomy, and fault protection as possible. Robustness is a word
engineers use to describe a system's ability to withstand the rigors of
the environment in which it must operate. For Cassini this means,
among other things, using materials that can withstand temperature
extremes and the impacts of micrometoroids and shielding sensitive
equipment from damage due to solar flares and galactic cosmic rays.
Hardware redundancy is also a critical feature in ensuring a robust
system. Designing for autonomy means creating a machine that can rely
on itself as much as possible to detect and analyze problems and find
solutions. Responsibility for this resides largely with the Cassini
engineering flight computer (EFC).
Fault protection is essential to the success of any robotic space mission. The Cassini spacecraft system fault protection (SFP) is designed to ensure the maintenance of orbiter system integrity and the capability to accomplish major mission objectives in the presence of anomalous conditions such as equipment failure or loss of critical functions. In practical terms, this means that if a fault is detected that might pose a substantial risk to the spacecraft or its subsystems the EFC autonomously initiates appropriate "safing" actions. These may include termination of all pre-programmed activities (except those related to the successful accomplishment of specific mission-critical events) and the establishment of a safe, commandable, relatively quiescent spacecraft state for up to several weeks without ground intervention. This allows time for ground controllers to analyze the problem and send new command sequences to the spacecraft.
Because the orbiter and probe will have a total of 18 scientific
instruments on board, the amount of data to be stored and then
transmitted to Earth will be immense. Data will be stored in the
engineering flight computer, which also provides commands to
operate spacecraft components and instruments. The orbiter will
communicate with Earth via the high-gain antenna (HGA) being developed
by the Italian Space Agency. This antenna will be 4 meters (13.1 feet)
in diameter and will be able to send data at a rate from 5 bits per
second to 249 kilobits per second. Two low-gain antennas (LGAs) will
also be used to transmit data and receive commands.
The complex trajectory and the length of the mission dictate still other requirements. First, because the spacecraft will begin by traveling toward the Sun for the two Venus gravity assists before traveling outward again to Earth, Jupiter, and Saturn, the temperature variations it will encounter are considerable. As described earlier, this means that the spacecraft components must either be made with materials able to withstand these thermal extremes or be shielded or otherwise temperature-controlled.
The long cruise time to Saturn (6.7 years) and the length of the
operational mission (4 years) mean that the orbiter must carry along a
large amount of propellant for in-flight trajectory-correction
maneuvers and attitude control, particularly during the science
observations. The spacecraft will be propelled by redundant 445-newton
(100-pound-foot) main engines that burn nitrogen tetraoxide (N2O4) and
monomethyl-hydrazine (MMH). Sixteen smaller 1-newton (0.2-pound-foot)
engines that burn hydrazine (N2H4) will be used to control attitude and
to correct small deviations from the spacecraft flight path. The
orbiter will be stabilized along all three axes and will not normally
rotate during its flight to Saturn.
Now, let's look at the spacecraft from a "systems" standpoint. Specifically, the spacecraft as a whole is seen as a "system," which is broken down into twelve engineering subsystems, in addition to the eighteen science instruments, which are also called subsystems.
The main body of the orbiter is formed by a stack consisting of a lower
equipment module (LEM), the propulsion module (PM), the upper equipment
module (UEM), and the high-gain antenna (HGA). Attached to this stack
are the remote sensing pallet and the fields and particles pallet, both
with their science instruments, and the Huygens probe system. As
mentioned, the probe system is being built by the European Space Agency
(ESA) and will be deployed into Titan's atmosphere by the orbiter. The
two pallets carry most of the science
instruments on the orbiter.
Other instruments, such as the mapping radar and in-situ sensors (i.e.,
direct-sensing instruments rather than remote-sensing instruments) are
attached to the UEM.
The two equipment modules are used for external mounting of the magnetometer boom and the three RTGs that supply power to the orbiter. The orbiter electronics bus is part of the UEM. The HGA and the two low-gain antennas (LGAs) are used to transmit data and receive commands. One of the two LGAs will be selected when operational constraints prevent pointing the HGA toward Earth.
The engineering subsystems are as follows:
Engineering Subsystems
Scientific Instruments -- Cassini Orbiter
Scientific Instruments -- Huygens Probe
