Mission personnel scoured massive amounts of data on the Mars surface and topography to be sure that the landing area is safe for the spacecraft. "Everything needs to be pre-programmed and autonomous onboard the lander during the seven minutes of entry, descent and landing," Spencer said. The lander also has to slow from about 13,000 mph, as it reaches the Martian atmosphere, to about 5 mph when it touches down, in a span of seven minutes.

Much of the critical data has come from a new high-resolution camera, HiRISE (High Resolution Imaging Science Experiment), that NASA launched in 2005 on board its Mars Reconnaissance Orbiter, a specially equipped science satellite designed to orbit the planet and study the history of water on Mars.

The orbiter, which has been circling Mars since arriving there in 2006, has a powerful and diverse set of science instruments. HiRISE was developed by researchers in the department of planetary sciences at the University of Arizona, which is a key partner in the day-to-day Phoenix mission as part of NASA's low-cost interplanetary exploration effort, called the Scout Program. Also involved in the mission is Lockheed Martin Space Systems.

The new camera, or "imager," is able to provide scientists with their most-detailed photographs so far of the Martian surface in preparation for the Phoenix landing.

"HiRISE can actually see the rocks on the surface of Mars, and so what we've done is selected a landing site that is as hazard-free as possible, from a rock standpoint as well as a slope standpoint," Spencer said. "HiRISE can see rocks that are on the order of a meter to a meter and half in diameter. Those rocks are a threat to us and we can also extrapolate ... the sizes to the smaller-sized rocks as well" to avoid them.

Using HiRISE, scientists were able "to pick out a 'sweet spot' within our landing latitude range," he said. Once on the Martian soil, a robotic arm on the lander will dig below the surface and access what scientists expect will be ice within about six centimeters of the surface, based on the photos provided by HiRISE. The soil samples will then be scooped up and delivered into the lander's analytic instruments that can perform chemical analysis and provide compositions of the soil and elements that may be ingrained in the ice.

As the Phoenix travels to Mars, NASA has made several scheduled midcourse trajectory corrections to get the lander to just the right spot on Sunday, but otherwise, the mission has been going very smoothly, Spencer said.

"We've had a very safe and quiet cruise from Earth to Mars," he said. "So far the spacecraft has performed very reliably. The navigation has been excellent to date. We are right on target for our desired landing at 68 degrees north latitude."

Now scientists can only wait as the landing time approaches to see if all goes according to plan.

"What I can say is that we have done everything that we can do to make sure that the system is as robust as possible," Spencer said. "There are no guarantees in this business. It's a very challenging and difficult environment to operate in, but this program expends a great deal of our resources to ensure the reliability of the system, and so we are confident that we will have a successful outcome. But there are no guarantees."