Illustration of Odyssey with description of its components. Photo: NASA/JPL

The shape of 2001 Mars Odyssey is anything but uniform, but its size can most easily be visualized by mentally placing the spacecraft inside of a box. Pictured this way, the box would measure 2.2 meters (7.2 feet) long, 1.7 meters (5.6 feet) tall and 2.6 meters (8.5 feet) wide. At launch Odyssey weighs 725.0 kilograms (1598.4 pounds), including the 331.8-kilogram (731.5-pound) dry spacecraft with all of its subsystems, 348.7 kilograms (768.8 pounds) of fuel and 44.5 kilograms (98.1 pounds) of instruments.

The framework of the spacecraft is composed mostly of aluminum and some titanium. The use of titanium, a lighter and more expensive metal, is an efficient way of conserving mass while retaining strength. Odyssey's metal structure is similar to that used in the construction of high-performance and fighter aircraft.

Most systems on the spacecraft are fully redundant. This means that, in the event of a device failure, there is a backup system to compensate. The main exception is a memory card that collects imaging data from the thermal emission imaging system.

Command and Data Handling
All of Odyssey's computing functions are performed by the command and data handling subsystem. The heart of this subsystem is a RAD6000 computer, a radiation-hardened version of the PowerPC chip used on most models of Macintosh computers. With 128 megabytes of random access memory (RAM) and three megabytes of non-volatile memory, which allows the system to maintain data even without power, the subsystem runs Odyssey's flight software and controls the spacecraft through interface electronics.

Interface electronics make use of computer cards to communicate with external peripherals. These cards slip into slots in the computer's main board, giving the system specific functions it would not have otherwise. For redundancy purposes, there are two identical strings of these computer and interface electronics, so that if one fails the spacecraft can switch to the other.

Communication with Odyssey's sensors that measure the spacecraft' orientation in space, or "attitude," and its science instruments is done via another interface card. A master input/output card collects signals from around the spacecraft and also sends commands to the electrical power subsystem. The interface to Odyssey's telecommunications subsystems exists through another card called the uplink/downlink card.

There are two other boards in the command and data handling subsystem, both internally redundant. The module interface card controls when the spacecraft switches to backup hardware and serves as the spacecraft's time clock. A converter card takes electricity produced by the power subsystem and converts it into the proper voltages for the rest of the command and data handling subsystem components. The last interface card is a single, non-redundant, one-gigabyte mass memory card that is used to store imaging data. The entire command and data handling subsystem weighs 11.1 kilograms (24.5 pounds).

Telecommunications
Odyssey's telecommunications subsystem is composed of both a radio system operating in the X-band microwave frequency range and a system that operates in the ultra high frequency (UHF) range. It provides communication capability throughout all phases of the mission. The X-band system is used for communications between Earth and the orbiter, while the UHF system will be used for communications between Odyssey and future Mars landers. The telecommunication subsystem weighs 23.9 kilograms (52.7 pounds).

Electrical Power
All of the spacecraft's power is generated, stored and distributed by the electrical power subsystem. The system obtains its power from an array of gallium arsenide solar cells on a panel measuring seven square meters (75 square feet). A power distribution and drive unit contains switches that send power to various electrical loads around the spacecraft. Power is also stored in a 16-amp-hour nickel-hydrogen battery.

The electrical power subsystem operates the gimbal drives on the high-gain antenna and the solar array. It contains also a pyro initiator unit, which fires pyrotechnically actuated valves, activates burn wires, and opens and closes thruster valves. The electrical power subsystem weighs 86.0 kilograms (189.6 pounds).

Guidance, Navigation and Control
Using three redundant pairs of sensors, the guidance, navigation and control subsystem determines the spacecraft's orientation, or "attitude." A Sun sensor is used to detect the position of the Sun as a backup to the star camera. A star camera is used to look at star fields. Between star camera updates, a device called the inertial measurement unit collects information on spacecraft orientation.

This system also includes the reaction wheels, gyro-like devices used along with thrusters to control the spacecraft's orientation. Like most spacecraft, Odyssey's orientation is held fixed in relation to space ("three-axis stabilized") as opposed to being sta-bilized via spinning. There are a total of four reaction wheels, with three used for primary control and one as a backup. The guidance, navigation and control subsystem weighs 23.4 kilograms (51.6 pounds).

Propulsion
The propulsion subsystem features sets of small thrusters and a main engine. The thrusters are used to perform Odyssey's attitude control and trajectory correction maneuvers, while the main engine is used to place the spacecraft in orbit around Mars.

The main engine, which uses hydrazine propellant with nitrogen tetroxide as an oxidizer, produces a minimum thrust of 65.3 kilograms of force (144 pounds of force). Each of the four thrusters used for attitude control produce a thrust of 0.1 kilogram of force (0.2 pound of force). Four 2.3-kilogram-force (5.0-pound-force) thrusters are used for turning the spacecraft.

In addition to miscellaneous tubing, pyro valves and filters, the propulsion subsystem also includes a single gaseous helium tank used to pressurize the fuel and oxidizer tanks. The propulsion subsystem weighs 49.7 kilograms (109.6 pounds).

Structures
The spacecraft's structure is divided into two modules. The first is a propulsion module, containing tanks, thrusters and associated plumbing. The other, the equipment module, is composed of an equipment deck, which supports engineering components and the radiation experiment, and a science deck connected by struts. The top side of the science deck supports the thermal emission imaging system, gamma ray spectrometer, the high-energy neutron detector, the neutron spectrometer and the star cameras, while the underside supports engineering components and the gamma ray spectrometer's central electronics box. The structures subsystem weighs 81.7 kilograms (180.1 pounds).

Thermal control
The thermal control subsystem is responsible for maintaining the temperatures of each component on the spacecraft to within their allowable limits. It does this using a combination of heaters, radiators, louvers, blankets and thermal paint. The thermal control subsystem weighs 20.3 kilograms (44.8 pounds).

Mechanisms
There are a number of mechanisms used on Odyssey, several of which are associated with its high-gain antenna. Three retention and release devices are used to lock the antenna down during launch, cruise and aerobraking. Once the science orbit is attained at Mars, the antenna is released and deployed with a motor-driven hinge. The antenna's position is controlled with a two-axis gimbal assembly.

There are also four retention and release devices used for the solar array. The three panels of the array are folded together and locked down for launch. After deployment, the solar array is also controlled using a two-axis gimbal assembly.

The last mechanism is a retention and release device for the deployable 6-meter (19.7-feet) boom for the gamma ray spectrometer. All of the mechanisms combined weigh 24.2 kilograms (53.4 pounds).

Flight Software
Odyssey receives its commands via radio from Earth and translates them into spacecraft actions. The flight software is capable of running multiple concurrent sequences, as well as executing immediate commands as they are received.

The software responsible for the data collection is extremely flexible. It collects data from the science and engineering devices and puts them in a variety of holding bins. The choice of which channel is routed to which holding bin, and how often it is sampled, is easily modified via ground commands.

The flight software is also responsible for a number of autonomous functions, such as attitude control and fault protection, which involves frequent internal checks to determine if a problem has occurred. If the software senses a problem, it will automatically perform a number of preset actions to resolve the problem and put the spacecraft in a safe standby awaiting further direction from ground controllers.