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astronautix.com Mars Odyssey


Nation: USA.

Mars Odyssey was part of NASA's Mars Exploration Program as reformulated after a series of probe failures. Mars Odyssey was scheduled to arrive at Mars in October 24, 2001, with the primary science mission spanning January 2002 through July 2004. The mission was to map the amount and distribution of chemical elements and minerals that make up the Martian surface. The spacecraft was to especially look for hydrogen, most likely in the form of water ice, in the shallow subsurface of Mars. It would also record the radiation environment in low Mars orbit to determine the radiation-related risk to any future human explorers who may one day go to Mars.

The three primary instruments carried by 2001 Mars Odyssey were:

During and after its science mission, the Odyssey orbiter would also support other missions in the Mars Exploration program. It would act as a communications relay for U.S. and international landers, including the Mars Exploration Rovers to be launched in 2003.

The framework of the spacecraft was composed mostly of aluminium and some titanium. The use of titanium, a lighter and more expensive metal, was an efficient way of conserving mass while retaining strength.

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

Command and Data Handling

All of Odyssey's computing functions were performed by the command and data handling subsystem. The heart of this subsystem was 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 allowed the system to maintain data even without power, the subsystem ran Odyssey's flight software and controlled the spacecraft through interface electronics. The entire command and data handling subsystem weighed 11.1 kilograms.

Telecommunications

Odyssey's telecommunications subsystem was composed of both a radio system operating in the X-band microwave frequency range and a system that operated in the ultra high frequency (UHF) range. It provided communication capability throughout all phases of the mission. The X-band system was used for communications between Earth and the orbiter, while the UHF system was to be used for communications between Odyssey and future Mars landers. The telecommunication subsystem weighed 23.9 kilograms.

Electrical Power

All of the spacecraft's power was generated, stored and distributed by the electrical power subsystem. The system obtained its power from an array of gallium arsenide solar cells on a panel measuring seven square meters. A power distribution and drive unit contained switches that send power to various electrical loads around the spacecraft. Power was also stored in a 16-amp-hour nickel-hydrogen battery. The electrical power subsystem operated the gimbal drives on the high-gain antenna and the solar array. It contained also a pyro initiator unit, which fired pyrotechnically actuated valves, activated burn wires, and opened and closed thruster valves. The electrical power subsystem weighed 86.0 kilograms.

Guidance, Navigation and Control

Using three redundant pairs of sensors, the guidance, navigation and control subsystem determined the spacecraft's orientation. A Sun sensor was used to detect the position of the Sun as a backup to the star camera. A star camera was used to look at star fields. Between star camera updates, a device called the inertial measurement unit collected information on spacecraft orientation.

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

Propulsion

The propulsion subsystem featured sets of small thrusters and a main engine. The thrusters were used to perform Odyssey's attitude control and trajectory correction manoeuvres, while the main engine was used to place the spacecraft in orbit around Mars. The main engine, which used hydrazine propellant with nitrogen tetroxide as an oxidiser, produced a minimum thrust of 65.3 kilograms of force. Each of the four thrusters used for attitude control produce a thrust of 0.1 kilogram of force. Four 2.3-kilogram-force thrusters were used for turning the spacecraft. In addition to miscellaneous tubing, pyro valves and filters, the propulsion subsystem also included a single gaseous helium tank used to pressurise the fuel and oxidiser tanks. The propulsion subsystem weighed 49.7 kilograms.

Structures

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

Thermal control

The thermal control subsystem was responsible for maintaining the temperatures of each component on the spacecraft to within their allowable limits. It did this using a combination of heaters, radiators, louvers, blankets and thermal paint. The thermal control subsystem weighed 20.3 kilograms.

Mechanisms

There were a number of mechanisms used on Odyssey, several of which were associated with its high-gain antenna. Three retention and release devices were used to lock the antenna down during launch, cruise and aerobraking. Once the science orbit was attained at Mars, the antenna was released and deployed with a motor-driven hinge. The antenna's position was controlled with a two-axis gimbal assembly. There were also four retention and release devices used for the solar array. The three panels of the array were folded together and locked down for launch. After deployment, the solar array was also controlled using a two-axis gimbal assembly. The last mechanism was a retention and release device for the deployable 6-meter boom for the gamma ray spectrometer. All of the mechanisms combined weigh 24.2 kilograms.

Flight Software

Odyssey received its commands via radio from Earth and translated them into spacecraft actions. The flight software was capable of running multiple concurrent sequences, as well as executing immediate commands as they were received. The software responsible for the data collection was extremely flexible. It collected data from the science and engineering devices and put them in a variety of holding bins. The choice of which channel was routed to which holding bin, and how often it was sampled, was easily modified via ground commands. The flight software was also responsible for a number of autonomous functions, such as attitude control and fault protection, which involved frequent internal checks to determine if a problem had occurred. If the software sensed a problem, it automatically performed a number of preset actions to resolve the problem and put the spacecraft in a safe standby awaiting further direction from ground controllers.

Mars Orbit Insertion

Odyssey will arrive at Mars on October 24, 2001. As it nears its closest point to the planet over the northern hemisphere, the spacecraft will fire its 640-Newton main engine for approximately 22 minutes to allow itself to be captured into an elliptical, or egg-shaped, orbit. If the launch occurs early in the period, Odyssey will loop around the planet every 17 hours. About three orbits after insertion, the spacecraft will fire its thrusters in what is called a period reduction manoeuvre so that it orbits the planet approximately once every 11 hours.

Aerobraking will then be used to transition from the initial elliptical orbit to the circular science orbit. During each of its long, elliptical loops around Mars, the orbiter will pass through the upper layers of the atmosphere each time it makes its closest approach to the planet. Friction from the atmosphere on the spacecraft and its wing-like solar array will cause the spacecraft to lose some of its momentum during each close approach, known as an "a drag pass." As the spacecraft slows during each close approach, the orbit will gradually lower and circularise.

Aerobraking will occur in three primary phases that engineers call walk-in, the main phase and walk-out.

The walk-in phase occurs during the first four to eight orbits following Mars arrival.

The main aerobraking phase begins once the point of the space-craft's closest approach to the planet, know as the orbit's "periapsis," has been lowered to within about 100 kilometres above the Martian surface. As the spacecraft's orbit is reduced and circularised during approximately 273 drag passes in 76 days, the periapsis will moved northward, almost directly over Mars' north pole. Small thruster firings when the spacecraft is at its most distant point from the planet will keep the drag pass altitude at the desired level to limit heating and dynamic pressure on the orbiter.

The walk-out phase occurs during the last few days of aerobraking when the period of the spacecraft's orbit is the shortest. The aerobraking drag pass events will be executed by stored onboard command sequences. The drag pass sequence begins with the heaters for the thrusters being warmed up for about 20 minutes. The transmitter is turned off to conserve power during the drag pass. The spacecraft then turns to the aerobraking attitude under reaction wheel control.

Following aerobraking walk-out, the orbiter will be in an elliptical orbit with a periapsis near an altitude of 120 kilometres and an "apoapsis" -- the farthest point from Mars -- near a desired 400-kilometer altitude. Periapsis will be near the equator. A manoeuvre to raise the periapsis will be performed to achieve the final 400-kilometer circular science orbit. The transition from aerobraking to the beginning of the science orbit will take about one week.

The high-gain antenna will be deployed during this time and the spacecraft and science instruments will be checked out. NASA’s Langley Research Center in Hampton, Va., will provide aerobraking support to JPL’s navigation team during mission operations. Langley’s role includes performing independent verification and validation, developing simulation tools and assisting the navigation team with trade studies and performance analysis.

Mapping Orbit

The science mission begins about 45 days after the spacecraft is captured into orbit about Mars. The primary science phase will last for 917 Earth days. The science orbit inclination is 93.1 degrees, which results in a nearly Sun-synchronous orbit. The orbit period will be just under two hours. Successive ground tracks are separated in longitude by approximately 29.5 degrees and the entire ground track nearly repeats every two sols, or Martian days.

During the science phase, the thermal emission imaging system will take multispectral thermal-infrared images to make a global map of the minerals on the Martian surface, and will also acquire visible images with a resolution of about 18 meters. The gamma ray spectrometer will take global measurements during all Martian seasons. The Martian radiation environment experiment will be operated throughout the science phase to collect data on the planet's radiation environment. Opportunities for science collection will be assigned on a time-phased basis depending on when conditions are most favourable for specific instruments.

Relay Phase

The relay phase begins at the end of the first Martian year in orbit (about two Earth years). During this phase the orbiter will provide communication support for U.S. and international landers and rovers.


Specification

Total Mass: 725 kg. Total Payload: 45 kg. Total Propellants: 349 kg.



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Last update 3 May 2001.
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