2001 Mars Odyssey is an orbiter carrying three packages of science experiments designed to make global observations of Mars to improve our understanding of the planet's climate and geologic history, including the search for liquid water and evidence of past life. The mission will extend across a full Martian year, or 29 Earth months.

Launch Vehicle

Odyssey will be launched on a variant of Boeing's Delta II rocket called the 7925 that includes nine strap-on solid-fuel motors. Each of the nine solid-fuel boosters is meter (3.28 feet) in diameter and 13 meters (42.6 feet) long; each contains 11,765 kilo-grams (25,937 pounds) of a propellant called hydroxyl-terminated polybutadiene (HTPB) and provides an average thrust of 485,458 newtons (109,135 pounds) at liftoff. The casings on the solid rocket motors are made of lightweight graphite epoxy.

The main body of the first stage houses the Rocketdyne RS-27A main engine and two Rocketdyne LR101-NA-11 vernier engines. The vernier engines provide roll control during main engine burn and attitude control after main engine cutoff before the second stage separation. The RS-27A main engine burns 96,000 kilograms (211,000 pounds) of RP-1 (rocket propellant 1, a highly refined form of kerosene) as its liquid fuel and liquid oxygen as an oxidizer.

The second stage is 2.4 meters (8 feet) in diameter and 6 meters (19.7 feet) long, and is powered by an Aerojet AJ10-118K engine. The propellant is 3,929 kilograms (8,655 pounds) of a liquid fuel called Aerozine 50, a 50/50 mixture of hydrazine and unsymmetric dimethly hydrazine. The oxidizer is 2,101 kilograms (4,628 pounds) of nitrogen tetroxide. The engine is restartable and will perform two separate burns during the launch.

The third and final stage of the Delta II provides the final velocity required to place Odyssey on a trajectory to Mars. This upper stage is 1.25 meters (4.1 feet) in diameter and consists of a Star-48B solid-fuel rocket motor with 2,012 kilograms (4,431 pounds) of propellant and a system called active nutation control that provides stability after the motor ignites. A spin table attached to the top of the Delta's second stage supports, rotates and stabilizes the Odyssey spacecraft and Star-48B upper stage before they spin up and separate from the second stage. The Odyssey spacecraft is mounted to the Star-48B by a payload attachment fitting. A yo-yo despin system decreases the spin rate of the spacecraft and upper stage before they separate from each other.

During launch and ascent through Earth's atmosphere, the Odyssey spacecraft and Star-48B upper stage are protected from aerodynamic forces by a 2.9-meter--diameter (9.5-foot) payload fairing that is jettisoned from the Delta II during second stage powered flight at an average altitude of 136 kilometers (73.6 nautical miles).

Launch Period

The orbiter launch period extends for 21 days, opening on April 7 and closing on April 27. The first 12 days of the launch period from April 7 through 18 make up what is considered the primary launch period; a secondary launch period runs from April 19 through 27. If Odyssey is launched during the secondary period, science data return at Mars may need to be reduced slightly because of higher arrival speeds and a longer aerobraking periods. Arrival dates at Mars vary with launch dates, and range from October 17 to 28, 2001.

Daily Windows

Two nearly instantaneous launch opportunities occur each day during the launch period each is separated by 30 to 90 minutes depending on the day. On April 7 the first is at 11:02 a.m. EDT and the second is at 11:32 a.m. EDT. The opportunities become earlier each day through the launch period.

Liftoff

Odyssey will lift off from Space Launch Complex 17 at Cape Canaveral Air Station, Florida. Sixty-six seconds after launch, the first three solid rocket boosters will be discarded followed by the next three boosters one second later. The final three boosters are jettisoned two minutes, 11 seconds after launch. About four minutes, 24 seconds after liftoff, the first stage will stop firing and be discarded eight seconds later. About five seconds later, the second stage engine ignites. The fairing or nose cone will be discarded four minutes, 41 seconds after launch. The first burn of the second stage engine occurs at 10 minutes, three seconds after launch.

At this point the vehicle is in low Earth orbit at an altitude of 189 kilometers (117 miles). Depending on the actual launch day and time the vehicle will then coast for several minutes, once it is in the correct point in its orbit, the second stage will be restarted at 24 minutes, 32 seconds after launch.

Small rockets will then be fired to spin up the third stage on a turntable attached to the second stage. The third stage will separate and ignite its motor, sending the spacecraft out of Earth orbit. A nutation control system (a thruster on an arm mounted on the side of the third stage) will be used to maintain stability during this the third stage burn. After that, the spinning upper stage and the attached 2001 Mars Odyssey spacecraft must be despun so that the spacecraft can be separated and acquire its proper cruise orientation. This is accomplished by a set of weights that are reeled out from the side of the spinning vehicle on flexible lines, much as spinning ice skaters slow themselves by extending their arms. Odyssey will separate from the Delta third stage about 33 minutes after launch. Any remaining spin will be removed using the orbiter's onboard thrusters.

About 36 minutes after launch the solar array is unfolded and about eight minutes later it is locked in place. Then the spacecraft turns to its initial communication attitude and the transmitter is turned on. About one hour after launch the 34-meter-diameter (112 foot) antenna at the Deep Space Network complex near Canberra, Australia will acquire Odyssey's signal.

Interplanetary Cruise

The interplanetary cruise phase is the period of travel from the Earth to Mars and lasts about 200 days. It begins with the first contact by the DSN after launch and extends until seven days before Mars arrival. Primary activities during the cruise include check out of the spacecraft in its cruise configuration, checkout and monitoring of the spacecraft and the science instruments and navigation activities necessary to determine and correct Odyssey's flight path to Mars.

There are science activities planned for the cruise phase including payload health and status checks, instrument calibrations, as well as data taking by some of the science instruments as spacecraft limitations allow.

Odyssey's flight path to Mars is called a Type 1 trajectory that takes it less than 180 degrees around the Sun. During the first two months of cruise, only the Deep Space Network station in Canberra will be capable of viewing the spacecraft. Late in May California's Goldstone station will come into view, and by early June the Madrid station will also be able to track the spacecraft. The project has also added the use of a tracking station in Santiago, Chile, to fill in tracking coverage during the first seven days following launch.

The orbiter will transmit to Earth using its medium-gain antenna and it will receive commands on its low-gain antenna during the early portion on its flight. At some point during the first 30 days after launch, the orbiter will be commanded to receive and transmit through its high-gain antenna. Cruise command sequences are generated and uplinked approximately once every four weeks during one of the regularly scheduled Deep Space Network passes.

The spacecraft will determine its orientation in space chiefly via a star camera and a device called an inertial measurement unit. The spacecraft will fly with its medium or high gain antenna pointed toward the Earth at all times while keeping the solar panels pointed toward the Sun. The spacecraft is stabilized in three axes and will not spin to maintain its orientation, or "attitude."

The spacecraft's orientation will be controlled by reaction wheels, devices with spinning wheels similar to gyroscopes. These devices will be occasionally "desaturated," meaning that their momentum will be unloaded by firing the spacecraft's thrusters.

During interplanetary cruise, Odyssey is scheduled to fire its thrusters a total of five times to adjust its flight path. The first of these trajectory correction maneuvers is scheduled for eight days after launch, and will correct launch injection errors and adjust the Mars arrival aim point. It will be followed by a second maneuver 90 days after launch.

The remaining three trajectory correction maneuvers will be used to direct the spacecraft to the proper aim point at Mars. These maneuvers are scheduled at 90 days after launch, 12 days before arrival and seven hours (October 24) before arrival. The spacecraft will communicate with Deep Space Network antennas continuously for 24 hours around all of the trajectory correction maneuvers. Maneuvers will be conducted in what engineers are calling a "constrained turn-and-burn" mode in which the spacecraft will turn to the desired burn attitude and fire the thrusters, while remaining in con-tact with Earth.

Navigation tracking during cruise involves the collection of two-way Doppler and ranging data. In order to provide additional information for navigation, the project has added a program of delta differential one-way range measurements, called delta DOR, that will be taken periodically during cruise and Mars approach. Delta DOR measurements are interferometric measurements between two radio sources. In this case, one of the radio sources is the DOR tones or telemetry signal coming from Odyssey. The second source will be either a known, stable natural radio source like a quasar or the telemetry signal from the Mars Global Surveyor spacecraft. Each source is recorded simultaneously at two radio antennas. The triangulation achieved through this method provides navigators with much more refined knowledge of the spacecraft's position. With this information, spcecraft operators can more precisely adjust Odyssey's flight path. Delta DOR measurements will be collected and processed for system testing during early and mid-cruise and weekly during the Mars approach phase to provide additional data to the navigation team. For the first 14 days after launch, the Deep Space Network will continuously track the spacecraft. During the quiet phase of cruise when spacecraft activity is at a minimum, only three 8-hour passes per day are scheduled. Continuous tracking will resume for the final 50 days before Mars arrival.

Science instruments will be powered on, tested and calibrated during cruise. The thermal emission imaging system will take a picture of the Earth-Moon system about 12 days after launch if the spacecraft is operating normally. Star calibration imaging is also planned 45 days after launch, while a Mars approach image is planned about 12 days before arrival if the Earth-Moon calibration image is not taken.

Two calibration periods are planned for the gamma ray spectrometer during cruise. Each of the spectrometer's three sensors may be operated during the calibration periods depending upon spacecraft power capabilities. The Mars radiation environment experiment is designed to collect radiation data constantly during cruise to help determine what the radiation environment is like on the way to Mars.

A test of the orbiter's UHF radio system is planned between 60 and 80 days after launch. The 45-meter (150-foot) antenna at California's Stanford University will be used to test the UHF system ability to receive and transmit. The UHF system will be used during Odyssey's relay phase to support future landers, it is not used as part of the orbiter's science mission.

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 maneuver so that it orbits the planet approximately once every 11 hours.

Aerobraking

Aerobraking is the transition from the initial elliptical orbit to the science orbit where Odyssey will circle Mars at a uniform altitude. It is a technique that slows the space-craft down by using frictional drag as it flies through the upper part of the planet's atmosphere.

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 circularize.

Aerobraking will occur in three primary phases that engineeers 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 low-ered to within about 100 kilometers (60 miles) above the Martian surface. As the spacecraft's orbit is reduced and circularized 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 dur-ing 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 kilometers (75 miles) and an łapoapsis˛ -- the farthest point from Mars -- near a desired 400-kilometer (249-mile) altitude. Periapsis will be near the equator. A maneuver to raise the periapsis will be performed to achieve the final 400-kilometer (249-mile) 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 longi-tude 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 (59 feet). 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 favorable 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.