Galileo FAQ - General Spacecraft Anatomy
How much does the spacecraft weigh?
At launch, the orbiter had a mass of 2,223 kg (4891 lbs), which includes a 118 kg (260 lbs) science payload and 925 kg (2,035 lbs) of usable propellent. Over 40% of the orbiter's mass at launch is for propellent! The probe's total mass is 339 kg (746 lbs); the probe descent module is 121 kg (266 lbs), including a 30 kg (66 lbs) science payload.
Where is the spacecraft's camera located?
Most people think that the Solid-State Imaging instrument (SSI), which takes photos in visible light, is Galileo's only camera, but there are actually three other cameras on board. A photopolarimeter-radiometer will measure the polarization of light scattered from Jupiter's clouds and the satellites' surfaces, by a process like using polarized sunglasses to cut down on glare. In addition, its infrared channels will sound the atmosphere and measure satellite temperatures. The near-infrared mapping spectrometer will map the satellites, looking for different minerals across their surfaces. It will also study cloud structure and gas composition in the jovian atmosphere. An ultraviolet spectrometer and extreme ultraviolet spectrometer will investigate volatile excape and surface composition of the Galilean satellites, the Io plasma torus, small and large scale properties of the Jupiter clouds, and the composition, structure, and evolution of Jupiter's upper atmosphere.
All four cameras are mounted on a scan platform, located near the bottom of the orbiter. This section of the spacecraft can be "despun," or kept from spinning with the rest of the orbiter--otherwise, all the images would be blurry.
Why is the spacecraft wrapped in black and gold stuff?
Galileo's electronics and science instruments are designed to work in interplanetary space, but, without some sort of insulation, it's too cold for them to operate (just like your camera shutter might freeze if you take it to the North Pole). The black and gold blankets are carefully designed to keep Galileo's innards at a "comfortable" temperature. They also keep micrometeorites from smashing into the spacecraft electronics.
The black blankets, which are made up of 20 different layers, are very efficient insulation. Although only 1/5th of an inch thick, it's three times as good an insulator as the four-inch-thick fiberglass insulation in your attic. The black color is due to carbon in the outer layer, which keeps electrostatic charge from building up in one spot and then shorting out the spacecraft electronics.
Black material in the sun picks up lots of heat, and emits a great deal of infrared light. The "gold" blankets, however, don't absorb a great deal of solar heat, though they do radiate well in the infrared. This material (called "second-surface aluminized kapton") therefore does an even better job of insulation. It's not used on the entire spacecraft because it was developed after Galileo was designed and built. When Galileo's flight path was changed to take advantage of a Venus gravitational assist, there was enough time to use second surface kapton on critical areas of the spacecraft.
Galileo's insulating blankets are 1/5 of an inch thick, but are in fact 60 times more effective than typical household fiberglass insulation at that same thickness. However, that comparison is based on the application for which each insulator was designed.
Fiberglass insulation is designed to work in Earth's atmosphere. Over 90% of the insulation is provided by the air that is trapped between the fiberglass fibers.
Galileo's thermal blankets were designed for use in the vacuum of space and therefore do not have the luxury of using air as an insulator. Alternate layers of Mylar/Dacron netting and aluminized Kapton provide most of the insulation while carbon-polyester coated Kapton (black blankets) and "second-surface aluminized" Kapton (gold blankets) outer layers provide further insulation by absorbing or reflecting the sun's heat.
As such, you can imagine that there is a significant difference between the cost of fiberglass and the cost of the specialized materials that make up Galileo's insulation!
That's the science boom. It's 10.9 meters long, and is designed to minimize the effects of orbiter-generated interference on the magnetometer and plasma wave instruments. The fields and particles instruments are mounted at the end of the boom, and also about 3 meters from the spacecraft.
How did the spacecraft fit inside the shuttle bay?
The spacecraft was folded up, like an elaborate piece of origami. The high-gain antenna and booms were furled up.
Unlike previous planetary spacecraft, Galileo features an innovative "dual spin" design: part of the orbiter rotates constantly at three revolutions per minute, and part of the spacecraft remains fixed in inertial space. This means that the orbiter can easily accomodate magnetospheric experiments (which need to take measurements while rapidly sweeping about) while also providing stability and a fixed orientation for cameras and other sensors. The spin rate can be increased to 10 revolutions per minute for additional stability during major propulsive maneuvers.