After a journey of 10-months and more than 400 million miles, Phoenix is set to arrive at the Red Planet just before 8 p.m. EDT this Sunday, beginning its study of water and possible conditions for life in the Martian arctic.

Brent Shockley, Phoenix Configuration and Information Management Engineer, noted on the mission blog, that the Phoenix remains in good health and the navigation team is having daily meetings to evaluate the trajectory of the spacecraft to determine whether a final trajectory correction maneuver will be required on Saturday night.

NASA plans a press conference for tomorrow to relay the status of the spacecraft and the decision for a final trajectory correction maneuver tomorrow night.

Phoenix will land in an arctic plain comparable in latitude to central Greenland or northern Alaska. The selected landing area is centered at 68.16 degrees north latitude, 233.35 degrees east longitude. Topographical mapping by Mars Global Surveyor’s laser altimeter indicates a broad, shallow valley about 50 kilometers (about 30 miles) wide and only about 250 meters (about 800 feet) deep.

The intense period from three hours before the spacecraft enters Mars’ atmosphere until it reaches the ground safely is the mission phase called entry, descent and landing. The craft will 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 will use heat-generating atmospheric friction, then a parachute, then firings of descent thrusters, to bring that velocity down to about 2.4 meters per second (5.4 miles per hour) just before touchdown.

There is no guarantee of a successful landing, despite extensive analysis, testing and review of the entry, descent and landing system. In the international history of the space age, only five of 13 attempts to land on Mars have succeeded.

The entry, descent and landing system for Phoenix weighs 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.

Like NASA’s twin Viking landers in 1976, Phoenix will use descent thrusters in the final seconds down to the surface and will set down onto three legs. 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. Engineers for Phoenix have remedied all the vulnerabilities identified in reviews of Mars Polar Lander, and have also identified and addressed dozens of other potential issues.

Seven minutes before it reaches the top of Mars’ atmosphere, Phoenix will jettison the cruise stage hardware that it has relied on during the long flight from Earth to Mars. Half a minute later, the spacecraft will begin a 90-second process of pivoting to turn its heat shield forward. Five minutes after completing that turn, Phoenix will start sensing the top of the atmosphere, at an altitude of about 125 kilometers (78 miles). Friction from the atmosphere during the next three minutes will take most of the velocity out of the descent. Friction will heat 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 will deploy its parachute, which is attached to the back shell. The spacecraft will descend on the parachute for nearly three minutes. During the first 25 seconds of that, Phoenix will jettison its heat shield and extend its three legs.

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

Descent speed will have slowed to about 56 meters per second (125 miles per hour) by the time the lander separates from the back shell and parachute, about a kilometer (six-tenths of a mile) above the ground. The spacecraft will be in free fall, but not for long. Thrusters will begin firing half a second later and will increase their thrust three seconds after Phoenix sets itself free from the parachute. Touchdown will still be about 40 seconds away. The onboard computer will use information from the radar to adjust the pulsed firings of the 12 descent thrusters.

By the time the lander gets to about 30 meters (98 feet) above the surface, it will have 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 will also have minimized horizontal velocity and rocking. For that final piece of the journey, Phoenix will maintain a steady descent velocity with accelerometers until it reaches the surface for a soft touchdown. It will shut off the thrusters when sensors on the footpads detect contact with the ground.

When Phoenix sets its three legs onto the surface, the time of day at the landing site will be afternoon. A Martian day, or “sol,” lasts 39 minutes and 35.244 seconds longer than an Earth day.

The planned operational life of the Phoenix Mars lander after it reaches Mars is 90 Martian days. The first week or so after landing will be a characterization phase for checking and understanding the performance of the spacecraft in its new environment. That will leave the Phoenix team less then three months to use the lander’s instruments to address the water and habitat questions of the mission’s science objectives.

The team plans to conduct the research in a series of cycles of digging and analysis. Each cycle will likely take more than a week, with each Martian day, or sol, having a different principal activity. For example, one sol of the cycle would be for the medium-temperature operation of the Thermal and Evolved-Gas Analyzer, another for high-temperature operation, one sol for beginning a wet chemistry analysis, another sol for microscopy. While one cycle is being completed, the team will be refining plans for the next cycle of digging and analysis.