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Phoenix Mission Phases

Posted on: Thursday, 22 May 2008, 03:40 CDT

Development

The Phoenix Mars Lander is currently undergoing final testing and assembly in preparation for its August 2007 launch. Spacecraft assembly is occurring at Lockheed Martin Space Systems in Littleton, Colorado. Integrating all the subsystems (e.g., thermal, electrical, mechanical, and communications) onto the lander's structure is the first step in the assembly process. Science instruments are then installed onto the lander deck and electrically integrated to the spacecraft. Software codes that operate these subsystems and instruments for remote operations are also integrated. Testing to ensure proper operation of all the subsystems, scientific instruments, and software occurs during this assembly step. Assembly and initial performance testing occurs from April to October 2006.

From November 2006 through March 2007, the lander will be shaken, baked, frozen, zapped, and asphyxiated; to assure the entire system can survive its expected environmental conditions. Packaged into its protective capsule and bolted to a large speaker coil, Phoenix will be bombarded with strong sound vibrations simulating extreme launch stresses. The thermal and vacuum test will lay the spacecraft bare to a severe cold and airless environment similar to outer space. The spacecraft will be pelted with electromagnetic radiation to simulate the exposed conditions of outer space and the Martian surface where there is no atmosphere to protect the spacecraft from the Sun's energetics. Phoenix will also undergo a series of tests to ensure that the spacecraft properly transforms from rocket passenger to deep space traveler to Martian surface dweller. These include practice jettisons of the cruise solar panels, aeroshell, and capsule top, as well as deployments of the parachute, legs, and science instruments.

While the spacecraft is being assembled in Colorado, a parallel test effort will be occurring at the Phoenix Science Operations (SOC) Center, Tucson, Arizona. The SOC houses the Payload Interoperability Testbed (PIT), a full mock up of the lander on the surface of Mars. The PIT will feature the ability to test the landed operation sequences for digging and sample delivery to the science instruments under many different types of soil compositions and densities (hard ice through loose sand). This will provide detailed data on the digging rate, energy, time, and amount of sample that can be expected in various soil conditions that can be encountered on the surface of Mars.

In May 2007, Phoenix will undergo final assembly and shipment to Kennedy Space Center. At the cape, the spacecraft will undergo its final checkout, explosives used to support jettisons will be loaded, and hydrazine propellant will be pumped into the fuel tanks. The spacecraft will then be mated to the upper stage of the Delta II launch vehicle ready to be launched in August 2007.

The Delta II 7925 was chosen due to its successful launch history. Several previous space exploration missions have launched on a Delta II 7925, including the Spirit and Opportunity Mars Rovers launched in 2003. The launch vehicle weighs 285,228 kilograms (628,820 pounds). Getting this weight off the ground is no easy feat. A Rocketdyne RS-27A engine is used in combination with nine strap-on rocket motors. The RS-27A is capable of producing 890,000 Newtons (200,000 pounds) of thrust. As stated on the Mars Exploration Rovers webpage, this amount of thrust is equivalent to the force generated by compressing water from 2,000 fire hoses into a single hose less than 1.5 meters (5 feet) in diameter.

After launch, Phoenix will perform various maneuvers to make the transition to the cruise stage. The spacecraft will deploy its solar arrays and re-orient itself in space. A connection to the NASA Deep Space Network will be initialized which will.

allow communication with Earth. When these maneuvers are complete, the spacecraft will be generating energy from its solar panels and ready to receive further commands from Earth.

Cruise

The cruise phase lasts for approximately 10 months as Phoenix makes its way to Mars. During the cruise phase, the spacecraft verifies the health of its scientific instruments and performs trajectory correction maneuvers (TCMs). The Deep Space Network (DSN) is used to communicate with the spacecraft during these operations. The massive antennas of the DSN are also used to obtain information about the spacecraft's flight path. Often times, the spacecraft will be observed from DSN sites in different parts of the world simultaneously to find its exact trajectory. These measurements are crucial for planning (TCMs).

The initial launch trajectory is intentionally pointed away from Mars so that the jettisoned third stage from the launch vehicle does not impact Mars. The first TCM, performed just 10 days after launch, places the spacecraft on a trajectory towards Mars. The subsequent TCMs are planned to take place much later in cruise phase to correct small errors in the first TCM (see image at right). These errors come from a multitude of sources, including imperfections in the flight model and slight inaccuracies in the DSN measurements. This type of deep space navigation has been used by many other NASA missions, including Mars Odyssey and the recent Mars Exploration Rovers.

During the last two weeks before Phoenix enters the martian atmosphere, the DSN will be tracking the spacecraft even closer than before. Two TCMs are scheduled to be performed within the last three days. Just before entry, flight path data is sent to Phoenix that is used by the onboard computers during the descent and landing to guide the spacecraft to its landing site. The cruise assembly, which consists of solar panels and other components that are only necessary for the cruise phase of the journey, is jettisoned five minutes prior to entry.

Entry, Descent, and Landing

At 125 km (78 miles) above the surface, Phoenix will enter the thin martian atmosphere. It will slow itself down by using friction. A heat shield will protect the lander from the extreme temperatures generated during entry. Antennas located on the back of the shell which encases the lander will be used to communicate with one of three spacecraft currently orbiting Mars. These orbiters will then relay signals and landing info to Earth.

After the lander has decelerated to Mach 1.7 (1.7 times the speed of sound), the parachute is deployed. Shortly after the parachute is deployed, the heat shield is jettisoned, the landing radar is activated, and the lander legs are extended. The lander continues through the Martian atmosphere until it comes within 1 km (.6 miles) of the Martian surface. At this point, the lander separates itself from the parachute. It then throttles up its landing thrusters and decelerates. When Phoenix is either at an altitude of 12 m (39 ft) or traveling at 2.4 m/s (7.9 ft/s), the spacecraft begins traveling at a constant velocity. The landing engines are turned off when sensors located on the footpads of the lander detect touchdown.

On Mars

Surface operations are planned in relation to martian days, which are known as sols. Because Mars rotates slightly slower than Earth, sol is 40 minutes longer than our planet's 24-hour day. A strategic plan is created that outlines operations two weeks into the future. This strategic plan is used to create a more detailed tactical plan which decides surface activities that will take place for the next two sols. Daily science and engineering data is used to assess the status of the strategic and tactical plans, and the plans are updated as necessary.

Immediately after Phoenix touches down on the surface of Mars (sol 0), critical instruments such as the solar arrays and SSI mast are deployed. Later in the afternoon of sol 0, EDL data and MARDI images are sent to Earth. On sol 1, TEGA, MECA and RAC are turned on and checked out, and the RA is deployed. SSI begins taking images of the landing site and the area where the robotic arm will be digging, and MET begins to sample the weather at the landing site.

On sols 2 through 9, the instruments aboard Phoenix continue to take initial measurements. TEGA takes measurements of the martian atmosphere using its mass spectrometer. The RA aquires a sample of martian soil and delivers it to TEGA on sol 4. This sample is analyzed by the differential scanning calorimeter in TEGA on the following sol. Another sample is delivered on sol 7 for analysis using MECA.

The digging operations phase is planned to take place on sols 10-90. SSI and RAC images will be analyzed to determine where the RA should dig. Phoenix will dig for up to 2.5 hours per sol during this period. As the RA digs into the martian surface, SSI and RAC images will help determine when new samples should be delivered to the scientific instruments on Phoenix. Samples will be delivered to TEGA about every 15 cm (5.9 in) or when layering is obvious. The four MECA cells will be reserved for samples from different layers that are expected to be encountered while digging. One cell will analyze a sample from the surface, another will analyze the dry regolith overburden, and one will be kept in reserve for the icy layer. One MECA cell will be kept for a repeat measurement or to examine another layer.

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