"This is not a trip to grandma's house. Putting a spacecraft safely on Mars is hard and risky," said Ed Weiler, associate administrator for NASA's Science Mission Directorate at NASA Headquarters in Washington. If all goes according to plan, about 14 minutes prior to landing, the probe will swivel around in space so its heat shield faces forward. Five minutes later, the spacecraft will hit Mars's atmosphere at more than 20,000 kilometres per hour, causing the shield to heat up to more than 1400 Degree Celsius.
In another 4 minutes, with Phoenix about 13 kilometres above the Martian surface, a parachute will deploy to decelerate the spacecraft. The timing of this is critical because if it occurs too early, aerodynamic forces will rip the parachute to shreds.
A few seconds later, the heat shield will be jettisoned, the spacecraft will extend its legs, and it will begin using radar to measure its altitude and speed. With 43 seconds left to landing, the spacecraft will have slowed down to 200 kilometres per hour. With the ground 1 kilometre below it, the probe will cut loose its parachute, relying on thrusters to slow it down further.
It must also manoeuvre away from its parachute to prevent the chute from falling on top of Phoenix after landing.
Risks to the spacecraft also include large rocks, which can spoil the landing or prevent opening of the solar panels.
However, images from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter, detailed enough to show individual rocks smaller than the lander, have helped lessen that risk.
According to Ray Arvidson of Washington University in St. Louis, chairman of the Phoenix landing-site working group, "This is one of the least rocky areas on all of Mars and we are confident that rocks will not detrimentally impact the ability of Phoenix to land safely."
If it is successful, it will be the first probe since the Viking missions more than 30 years ago to land safely without airbags.