How NASA’s Phoenix will land on Mars

Phoenix user’s guide for Mars arrival
The exact fate of the lost Mars Polar Lander remains somewhat uncertain because that probe had no way of communicating with Earth once it entered the Martian atmosphere. That won't be the case for Phoenix, which has a small crowd of three Mars orbiters to watch and relay information from the spacecraft throughout landing.

For Goldstein, it is the three-second communications gap between Phoenix's departure from its cruise stage and the first signals to its relay network that gives him the shivers. If Phoenix fails to land successfully, any signals just before landing will prove vital in learning its fate, he said.

"Getting that communications down is the important thing," Goldstein said. "That will be the three seconds that I'm really biting my nails over."

A wraparound antenna sits on Phoenix's back-shell, capable of transmitting an ultra-high frequency signal to Earth via NASA's Mars Reconnaissance Orbiter or Mars Odyssey spacecraft. Europe's Mars Express orbiter is also on call in case of an emergency, mission managers said.

"This is the first time for any Mars landing having orbital relay communications for both landing and being on the surface," Grover said.

Phoenix will descend and land much the same way that Mars Polar Lander was meant to, plunging into the Martian atmosphere at about 13,000 mph. That's similar to respective 2004 descents of NASA's Spirit and Opportunity rovers, though Phoenix's arrival would mark the first powered landing on Mars since NASA's Viking missions of the 1970s

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The probe combines new technology with proven methods for landing, including an Apollo-era Earth entry software algorithm to guide the spacecraft's early descent into the Martian atmosphere.

A Viking-era parachute is designed to open once Phoenix falls within 7.8 miles above Mars, creating drag to slow the spacecraft as it screams through the atmosphere at supersonic speed. The probe's landing radar should begin giving altitude and velocity of descent as Phoenix nears the surface, so that the onboard computer can make any necessary landing adjustments.

"By the time you get the parachute opening, there can be significant errors in positioning on order of kilometers," Grover said. "So that's where radar is critical, because it turns on and gets fresh knowledge of altitude."

Vertical Martian lander
Two minutes after the parachute deployment, Phoenix will have descended to approximately 0.6 miles above the surface. The lander should then jettison its backshell and freefall for half a second before lighting up its engines.

Nine of the twelve engines will pulse furiously 10 times per second — an effect Grover likened to "coming down on a jackhammer." The three non-pulsing engines should fire steadily to help ensure added stability.

"Just before touchdown, we actually pirouette the vehicle," Goldstein said, adding that the move will aid Phoenix's vital solar arrays. "We actually turn it so we maximize solar exposure."

Navigators at the Jet Propulsion Laboratory can upload fresh orders to Phoenix's guidance computer up to three hours before landing, in case course adjustments are required. However, Grover and other NASA engineers will only be able to stand by and trust in their spacecraft technology once the Mars Lander begins its descent.

"We've done all that's humanly possible," Grover said.

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