Closing in: Phoenix is ready for a risky descent to the surface of Mars. This artist's impression shows the probe jettisoning parts required for the long flight from Earth, but not the descent.
Credit: NASA
WASHINGTON DC: The first probe ever to the frigid Martian arctic is on course to land this morning as NASA scientists fret about the historically coin-flip odds of a successful mission to the Red Planet.
Phoenix Mars Lander mission officials said that they had signed off on the probe's course without any last-minute corrections and were just waiting for the first news of the landing, expected to be known at 11:53pm GMT (9:53am Sydney time), around 15 minutes after the probe makes contact with Mars' surface.
Phoenix team jitters
"This is a jittery time," Barry Goldstein, the Phoenix project manager at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, which controls the mission, told reporters, noting the less that 50 per cent success rate for all Mars missions since 1960.
In the final hours before landing the Phoenix team could only "wait and watch," he said.
But, he added, "We have a very good track record for landers" on Mars, pointing to, among others, the Spirit and Opportunity rovers that NASA safely placed on the planet in January 2004.
The US$420-million (A$438-million) spacecraft is to land near Mars's frigid north pole, where it will dig into the icy soil in search of signs of the conditions that would have made the area once habitable for forms of life.
While scientists of the U.S. space agency were confident they had minimized the risks to the probe's successful landing, they described the challenges to decelerating in seven minutes from 20,400 km/h to manage a soft landing.
The Phoenix will turn its heat shield down as it plunges into Mars' atmosphere, then after four minutes deploy a parachute to slow to about 190 kph in three minutes, and then in a split-second drop the parachute and fire thrusters to slow further to eight km/h, deploy its three legs and touch down on the surface.
Seven minutes of terror
But during that "seven minutes of terror" NASA will have no communications from the craft. It will have to wait some 15 minutes after landing for the first signal from Phoenix to cross the cosmos from Mars to Earth.
"The scariest moment for me, is if we lose the signal during the descent," said Peter Smith, Phoenix principal investigator at the University of Arizona.
The landing area is a relatively rock-free, flat region in the Mars arctic, on the circumpolar region known as Vastitas Borealis -- akin to northern Canada in Earth's latitude.
The only concern is a small two-metres high rockpile mission officials call "the hill" near the landing target, but Goldstein put the odds at hitting it at just one percent.
Goldstein said, too, that if it lands on any rocks, Phoenix is designed to operate with no problems at up to a 16º tilt.
One minute after Phoenix confirms arrival, its radio will go silent for 20 minutes to save its batteries before deploying its two solar antennas.
Climate change
Its first images will reach Earth via NASA's Odyssey Mars orbiter only after two hours – expected to be first photographs of the craft itself, to show NASA if all the equipment deployed in working order, and then, Smith said, possibly shots of the surface.
NASA wants to assess whether the Martian arctic has ever had conditions favourable to microbial life, Smith said.
Given that Mars' polar region is subject to Earth-like seasonal changes, Smith said, the scientists are looking to see whether there is a point where the region warms and changes into a water-rich soil with organic, life-supporting minerals.
"We find that the arctic region is really sensitive to climate change on a planet ... it also preserves the history of life," Smith said.
"We think that organics must have existed at least at one time" from meteorite and other impacts, he said. The presence of liquid water and organics would signify a "habitable zone," he said.
Phoenix is equipped with a camera and a 2.35-metre robotic arm that can dig as deep as one meter to find ice and heat up samples to detect carbon and hydrogen molecules, essential elements of life. It also has meteorological equipment to study the Mars atmosphere.
