Mars

Phoenix

September 15, 2010, 9:51 pm
Source: NASA
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NASA's Mars Phoenix Lander is studying the far northern plains of Mars to analyze components of the surface, subsurface and atmosphere. It uses a trench-digging arm and a set of analytical tools to study water believed to be frozen into the soil just below the surface. It will also seek evidence of organic compounds to determine whether the site has been a favorable environment for microbial life. 

Launch
On Aug. 4, 2007, at 5:26 a.m. EST, a three-stage Delta II launch vehicle lofted the Phoenix spacecraft from pad SLC-17A of Cape Canaveral Air Force Station, Fla. into the pre-dawn eastern sky. The mission used the 7925 model of Delta ll, which has a liquid-fueled first stage with nine strap-on solid-fuel boosters, a liquid-fueled second stage and a solid-fuel third stage. With its Phoenix payload on top, it stood 39.6 meters (130 feet) tall.

Celestial motions rule scheduling for Mars launches. As Earth and Mars race around the sun, with Earth on the inside track, Earth laps Mars about once every 26 months. The two planets come relatively close together at that point, which is called an opposition because Mars is temporarily on the opposite side of Earth from the sun. The best time to launch a mission to Mars, in terms of how much energy is required for the trip, is a few months before that happens. NASA has used every one of these Mars launch opportunities since 1996. During the 2007 opposition period, the closest approach of the two planets was on Dec. 18, 2007, when they were 88 million kilometers (55 million miles) apart. That distance, the launch vehicle's power, the spacecraft's mass and the desired geometry for a high-latitude landing on Mars were all factors in determining the range of possible launch dates. The first possible date was Aug. 3. Rainstorms at the launch pad in preceding days delayed loading fuel into the launch vehicle, moving launch to Saturday morning, Aug. 4.

After separating from the third stage of the Delta II, the Phoenix spacecraft began communicating with the Goldstone, Calif., station of NASA's Deep Space Network. It unfolded the solar panels of its cruise stage, determined the direction toward the sun, and slewed to the best orientation to receive solar power and communicate with Earth.

Interplanetary Cruise and Approach to Mars
Phoenix began the cruise phase after the spacecraft established radio communications with Earth and sent information that the cruise solar panels were generating electricity and spacecraft temperatures were stable.

Phoenix used a Type II trajectory to Mars, meaning the spacecraft is flying more than halfway around the sun while in transit from one planet to the other. This takes longer than the Type I trajectories flown by Mars Odyssey, Spirit, Opportunity and Mars Reconnaissance Orbiter. During the cruise phase, the Phoenix lander remains tucked inside the aeroshell, with the aeroshell attached to a cruise stage that was jettisoned in the final minutes of flight.

Navigators' assessments of the spacecraft's trajectory used three types of tracking information from ground antennas of NASA's Deep Space Network at Goldstone, Madrid and Canberra, Australia. One traditional method is ranging, which measures the distance to the spacecraft by timing precisely how long it takes for a radio signal to travel to the spacecraft and back. A second traditional method is Doppler, which measures the spacecraft's speed relative to Earth by the amount of shift in the pitch of a radio signal from the craft. A newer method, called delta differential one-way range measurement, adds information about the location of the spacecraft in directions perpendicular to the line of sight. Pairs of antennas on different continents simultaneously receive signals from the spacecraft, and then the same antennas observe natural radio waves from a known celestial reference point, such as a quasar. European Space Agency antenna stations in New Norcia, Australia, and in Cebreros, Spain, supplement the Deep Space Network stations in providing the delta differential one-way range measurements.

The Phoenix team began cruise-phase tests of the spacecraft's science instruments on Aug. 20. By Oct. 26, initial in-flight tests had been completed on all the instruments that the mission will use at Mars. Systems such as the radar -- critical for landing -- and the ultrahigh-frequency radio -- crucial for communication relays at Mars -- were also tested. Repeated heating of the Thermal and Evolved-Gas Analyzer instrument during the cruise phase was used to drive out most water vapor carried from Earth with the instrument, making it more sensitive for studies of any water in Martian samples.

While Phoenix was in flight, the orbits of NASA's Mars Reconnaissance Orbiter and Mars Odyssey were adjusted so they will be in the right positions to relay communications between Phoenix and Earth.

Entry, Descent and Landing
The spacecraft craft hit the top of the atmosphere at a speed of 5.7 kilometers per second (12,750 miles per hour). Within the next six and a half minutes, it used heat-generating atmospheric friction, then a parachute, then firings of descent thrusters to bring its velocity down to about 2.4 meters per second (5.4 miles per hour) just before touchdown.

In the international history of the space age, only five of 13 attempts to land on Mars have succeeded. This tally does not count spacecraft that did not even get away from Earth or were intended to land on a Martian moon. It counts as three attempts a 1998 mission that attempted three separate landings.

The entry, descent and landing system for Phoenix weighed less than the systems for earlier Mars missions, such as the air bags that cushioned the impacts for Mars Pathfinder and the Spirit and Opportunity rovers. This helps give Phoenix a higher ratio of science-instrument payload (59 kilograms or 130 pounds) to total launch weight (664 kilograms or 1,464 pounds) than any spacecraft that has previously landed on Mars.

The previous three successful landings on Mars used air bags to cushion the impact. Scaling up the air bag landing system from the Mars Pathfinder mission to the larger Mars Exploration Rover mission required heavier air bags and stretched the capabilities of that type of landing. For the even larger science payload of the Phoenix mission, an air bag system was too heavy to be feasible. Compared with the lightweight landing system used by Phoenix, an air bag landing system would add much more weight to the spacecraft. That extra weight would require eliminating some of the science payload and research capabilities of the mission.

Air bags add a safety margin for landing on slopes or rocky ground, but that advantage is not relevant for the flat and relatively unrocky terrain of the Phoenix landing site.

Like NASA's twin Viking landers in 1976, Phoenix used descent thrusters in the final seconds to the surface and set down onto three legs. However, compared to the Vikings, Phoenix uses leaner components, such as thrusters controlled by pulse firing instead of throttle-controlled and more complex interdependence among the components. The system on Phoenix resembles Mars Polar Lander's more than Viking's. Mars Polar Lander reached Mars in 1999 but did not land successfully.

The system on Phoenix was a very active one, using radar to continually assess the spacecraft's vertical and horizontal motion during the final minutes and continually adjusting the descent based on that information. Compared with Spirit and Opportunity, Phoenix separated from its parachute nearly 100 times farther from the ground. The landing system on Phoenix enabled the spacecraft to hit the ground at about one-tenth the velocity of Spirit and Opportunity's landings.

Seven minutes before it reached the top of Mars' atmosphere, Phoenix jettisoned the cruise stage hardware that it relied on during the long flight from Earth to Mars. Half a minute later, the spacecraft began a 90-second process of pivoting to turn its heat shield forward. Five minutes after completing that turn, Phoenix began sensing the top of the atmosphere, at an altitude of about 125 kilometers (78 miles). Friction from the atmosphere during the next three minutes took most of the velocity out of the descent. Friction heated the forward-facing surface of the heat shield to a peak of about 1,420 degrees Celsius (2,600 degrees Fahrenheit) at an altitude of 41 kilometers (25.5 miles).

At about 12.6 kilometers (7.8 miles) in altitude and a velocity about 1.7 times the speed of sound, Phoenix deployed its parachute, which was attached to the back shell. The spacecraft descended on the parachute for nearly three minutes. During the first 25 seconds of that, Phoenix jettisoned its heat shield and extended its three legs.

About 75 seconds after the parachute opened and 140 seconds before landing, the spacecraft started using its radar. The radar provided information to the onboard computer about distance to the ground, speed of descent and horizontal velocity. It took readings at a pace of 10 times per second until touchdown.

Descent speed slowed to about 56 meters per second (125 miles per hour) by the time the lander separated from the back shell and parachute, about a kilometer (six-tenths of a mile) above the ground. The spacecraft was not in free fall for long. Thrusters began firing half a second later and increased their thrust three seconds after Phoenix discarded its sets parachute.

The onboard computer used information from the radar to adjust the pulsed firings of the 12 descent thrusters. To dodge a chance of the parachute following the lander too closely and draping it after touchdown, Phoenix performed a backshell avoidance maneuver. It used radar sensing of horizontal motion as an indicator of which way the wind is blowing, and thrusters shoved the lander in the opposite direction.

By the time the lander got to about 30 meters (98 feet) above the surface, it had slowed to about 2.4 meters per second (5.4 miles per hour) in vertical velocity. Continuous adjustments to the thruster firings based on radar sensing minimized horizontal velocity and rocking. It shut off the thrusters when sensors on the footpads detected contact with the ground.

Mars Surface Operations
The prime mission is planned for three months of surface operations, which is expected to be long enough to dig to the icy layer and analyze material collected from it. Those three months will extend from late spring to mid-summer in the northern hemisphere of Mars.

Phoenix relied on battery-stored energy as it descended through the atmosphere until the lander's solar arrays could be opened after touchdown. The meteorology mast and camera mast was extended upwards. The stereo camera will took its first images to show that the solar arrays had deployed.

Once the lander was cleared for science operations, the center of operations switched to the University of Arizona, Tucson. The season was late spring and early summer during the lander's initial weeks at its far-northern landing site, so the sun will not set. However, the midday hours have the sun at its highest angle, and the solar-powered spacecraft is busiest. On most sols, Phoenix receives its daily commands relayed from an orbiter passing overhead in the morning, and sends its daily report back to Earth about seven hours later via another relay pass.

Spacecraft
The Phoenix Mars lander has a science payload and systems that enable the payload to do its job and send home the results. The lander's main structure was built for the Mars Surveyor 2001 program and then kept in a protective, controlled environment after the lander portion of that program was cancelled. Several modifications have been made to the inherited lander, some to meet return-to-flight recommendations from review of Mars mission failures in 1999 and some to adapt to the specific goals and plans for the Phoenix mission.

On the surface of Mars, the lander's power comes from a two-wing solar array converting solar radiation to electricity. The array is shaped as two nearly circular decagons extending from opposite sides of the lander, with a total of 4.2 square meters (45 square feet) of functional surface area on flexible, lightweight substrate. A pair of rechargeable 25-amp-hour lithium-ion batteries provides power storage.

Landing Site
Phoenix landed in Mars' Vastitas Borealis at 68 degrees north latitude, and 233 degrees east longitude in an arctic plain comparable in latitude to central Greenland or northern Alaska.

Favorable opportunities to launch missions to Mars come about every 26 months, but the 2007 launch opportunity was the best in several years for sending a surface mission so far north on Mars. NASA's Mars Odyssey orbiter found evidence in early 2002 that this region shelters high concentrations of water ice mixed with the soil just beneath the surface.

The Phoenix mission was developed to take advantage of the 2007 launch opportunity by sending a payload of science instruments particularly appropriate for examining an environment of ice and soil. The landing region has been a key factor in defining the mission. The region has expanses with little variation on the surface, but a key attraction within arm's reach underground. This stationary lander with a robotic arm was made for just such a place.

Glossary

Citation

(2010). Phoenix. Retrieved from http://www.cosmosportal.org/view/article/141446

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